Feed Additives

Robert H. Poppenga

Inclusion of a variety of chemicals in livestock feed is an efficient and cost-effective mechanism to improve animal performance and prevent disease. Livestock feed additives include growth promotants such as ionophores, partitioning agents such as ractopamine, antiparasiticides and larvicides such as tetrachlor- vinphos, antibiotics, hormones such as melengesterol acetate, buffers, probiotics, nutritional agents, dietary supplements, and adsorbents such as sodium bentonite.

Urea and Other Nonprotein Nitrogen Sources

Nonprotein nitrogen (NPN) is converted by ruminal bacteria to ammonia by the enzyme urease. Ammonia is subsequently used by the bacteria to form amino acids and, ultimately, protein. Use of ammonia for bacteria protein synthesis depends on the availability of a highly fermentable carbohydrate source.

NPN can replace a portion of more expensive natural protein in ruminant rations, providing up to 40% of their nitrogen requirement. Rumen bacteria can effectively use ammonia nitrogen in limited quantities. However, when ammonia is produced beyond the capacity of the bacteria to use it, it will accumulate in the rumen at potentially toxic concentrations. The most commonly used form of NPN is urea, although other forms such as ammonium salts or ammoniated feed are also used. NPN is available either in a granular form that is incorporated into concentrates or range blocks or as a liquid that is incorporated into palatable vehicles such as molasses or encased (as urea) in a polymer network designed to release urea through pores in the polymer. Urea is typically recom­mended at a maximum rate of 3% of the grain ration or 1% of the total ration.¹

The toxicity of NPN for ruminants depends to a certain extent on the adaptation of rumen bacteria to NPN; properly adapted animals can tolerate higher concentrations of NPN than naive individuals.

Ammonia not used by rumen bacteria is absorbed systemically and can result in hyperammonemia within 30 minutes to 2 hours if it is not detoxified to urea by the liver for excretion. The amount of ammonia absorbed depends on the pH of the rumen. Normally, when rumen pH is below 7, most ammonia is in the form of the charged ammonium ion (NH₄+ versus the uncharged NH₃). As more ammonia is produced in the rumen, the pH of the rumen becomes more alkaline, with a resultant dramatic shift in the relative amounts of NH₄+ and NH₃ in the rumen. Unionized NH₃ readily crosses the rumen wall; in blood, ammonia is primarily in the form of ammonium ion.

NPN-induced ammonia toxicosis usually results from one of the following situations: (1) inadequate or improper feed mixing, (2) error in calculating the amount for inclusion in feed, (3) inadequate adaptation to NPN, (4) inclusion in carbohydrate-deficient rations, or (5) unrestricted consumption of palatable liquid NPN formulations.²

Clinical signs of ammonia toxicosis can occur within 30 minutes to several hours after consumption of excessive NPN. Signs include weakness; dyspnea; salivation; bruxism; bloat; excitation; belligerence; vocalizations; muscle tremors beginning with eyelids, lips, and neck and progressing posteriorly; and convulsions. Due to the rapid onset of clinical signs, it is common to find animals dead without signs being noted.

As evidenced by typical clinical signs, the CNS is a primary target of NPN intoxication. Mechanistically, ammonium ion causes a number of changes in the CNS, including amino acid disturbances, alterations in neurotransmission, cerebral energy deficits, and alteration of nitric oxide synthesis. Brain edema due to hyperammonemia is thought to occur through increased astrocyte osmolality and cytotoxic oxidative and nitrosative damage.³

A diagnosis of intoxication relies on a history of NPN supplementation, compatible clinical signs, and ideally measure­ment of elevated ammonia concentrations in rumen contents, whole blood, or ocular fluid. Unfortunately, ammonia is volatile, so appropriate sample handling and storage are critical for ammonia measurement. If urea is the NPN source, it can be measured in feed or, less usefully, in rumen contents. An antemortem rumen pH value greater than 8 is consistent with NPN intoxication. Postmortem pH values can be misleading depending on the time lag between death and necropsy.

Treatment is designed to decrease the amount of unionized ammonia absorbed from the rumen. Adult cattle should be given 20 to 30 L of cold water orally to reduce the ability of rumen bacteria to convert urea to ammonia by inhibiting and diluting urease. In addition, decreasing rumen pH shifts the form of ammonia to the unabsorbable ammonium ion; this is accomplished by administering 2 to 6 L of vinegar.

A number of strategies can minimize the risk of intoxication. These include a gradual adaptation to NPN, feeding readily fermentable carbohydrate sources, ensuring uniform feed mixing, maintaining a regular feeding schedule, feeding liquid supplements with phosphoric acid to maintain an acidic rumen, coating urea with fat and extruding urea with concentrates such as grain, using urease inhibitors to decrease its hydrolysis, and ensuring that mineral nutrition is adequate.²

Ammoniated Feed

Ammoniated feed intoxication is a hyperexcitability syndrome called “bovine bonkers” and is reported in cattle and calves fed ammoniated molasses, hay, straw, and silage.¹ The ammonia­tion process improves the palatability and digestibility of otherwise poor-quality roughages.

Hyperexcitability, either spontaneous or induced, is a hallmark of the syndrome. Affected animals will suddenly stampede and run in circles or a straight line until they collide with other animals, buildings, or fences. Other clinical signs include ear twitching, mydriasis, trembling, salivation, increased urination and defecation, and bellowing. The only postmortem lesions noted are secondary to trauma. The imidazoles can be excreted via milk, so nursing animals are at risk of intoxication.^4,5 Neurologic signs have been reported in nursing calves whose dams had been fed ammoniated barley greenfeed; dams were unaffected.⁴

A diagnosis of intoxication depends on a history of feeding ammoniated forages and consistent clinical signs. Treatment is nonspecific and typically limited by difficulties in handling affected animals. Sedation can be tried; acepromazine has been suggested.⁵ Animals often recover once exposure is halted.

Prevention involves restricting ammoniation to poor-quality roughages that contain low concentrations of soluble sugars. The amount of ammonia used should not exceed 3% of the dry weight of the forage. In addition, the processing temperature of the forage should not exceed 70°C (158° F).¹

Ionophores

Ionophores are commonly added to cattle and poultry feed for their growth promotant and anticoccidial properties. A number of ionophores, including narasin, salinomycin, lasalocid, monensin, semduramycin, laidlomycin, and maduramycin, are commercially available. In cattle, ionophores result in more propionic acid production in the rumen, which improves feed efficiency.

Ionophores are potentially toxic to a variety of mammalian and avian species, although sensitivities vary between species and among different ionophores.^6,7 Intoxications have been documented for horses, cattle, goats, pigs, camelids, chickens, turkeys, quail, and ostriches. Ionophores bind and transport ions across cell and organelle membranes; the direction of movement depends on the concentration gradient of a particular ion. This results in disruption of multiple ion gradients as one or more ions move into or out of cells. Monovalent ionophores such as monensin, salinomycin, and narasin preferentially complex with either sodium (monensin) or potassium (salino- mycin and narasin) ions. Divalent ionophores such as lasalocid and laidlomycin are capable of binding and transporting divalent cations such as calcium and magnesium across membranes. At a cellular level ionophores affect mitochondrial function and result in loss of aerobic energy production.

■ Horses Horses are the most sensitive livestock species with reported median lethal doses (LD₅₀s) of 0.6 mg/kg for salinomycin, 2 to 3 mg/kg for monensin, and 21.5 mg/kg for lasalocid.⁶ Clinical signs in horses are similar irrespective of the ionophore ingested, although signs can vary among individu­als within an affected group. Signs most commonly occur with 24 hours of exposure, but delayed onset can occur in some cases. Acute clinical signs are primarily due to effects on muscle cells and the nervous and digestive systems. Feed refusal is one of the earliest signs noted in horses. Other signs include weakness, ataxia or incoordination, tremors, stumbling, exag­gerated stepping, hesitance to move or turn, tachycardia, congested mucous membranes, hypotension, dyspnea, hyper- pnea, sweating, and recumbency.

A variety of clinicopathologic changes can occur, including elevated ALP, AST, lactate dehydrogenase (LDH), creatine phosphokinase (CPK), creatinine, indirect bilirubin, BUN, glucose, hematocrit, serum osmolality, and phosphorus. Decreased concentrations of serum Ca⁺⁺ and K⁺ and urine osmolality occur. Myoglobinuria is often noted. Unfortunately, the usefulness of such changes is limited by significant variation among affected individuals and the timing of their occurrence. Measurement of serum cardiac troponin I has been suggested to help with the early identification of horses with cardiac damage.⁸

The most significant necropsy findings are due to direct or indirect myocardial damage. Acutely affected horses develop degeneration and necrosis of cardiac muscle, whereas horses dying later have marked cardiac myopathy and fibrosis. Lesions can be absent in horses dying peracutely.

A diagnosis of intoxication relies on a combination of consistent clinical signs, necropsy findings where death occurs, and analytic proof of ionophore exposure. Because medicolegal considerations are often encountered in cases involving horses, it is critical to try to quantify ionophore feed concentrations.⁶ This is difficult if representative feed samples are not available. Testing several feed samples can be important to confirm non­uniform feed mixing. In the absence of feed testing, stomach contents and/or tissue testing (preferably heart tissue) can be done to confirm exposure but not necessarily intoxication.⁹

There is no specific treatment for ionophore poisoning.^10,11 It is critical to quickly remove access to suspect feed and initiate decontamination (e.g., gastric lavage and/or activated charcoal) as soon as possible. The efficacy of decontamination in most cases is questionable. Supportive therapy is directed toward restoration and maintenance of cardiac output and tissue perfusion, stabilization of cell membranes, and treatment with anti-arrhythmics if unstable, life-threatening arrhythmias are present.¹⁰ Affected horses should be rested for up to 3 months after exposure, and echocardiograms (echos) and exercising echos performed to identify evidence of chronic cardiomyopathy. Reported mortality is 60% or greater.¹⁰

■ Cattle Cattle are less sensitive to ionophores than horses. At prescribed use levels, ionophores are safe. The LD₅₀ for monensin in cattle is reported to be 26.4 mg/kg, and lethal doses of lasalocid range from 50 to 100 mg/kg.

In cattle, signs of acute intoxication include early anorexia, rumen atony, CNS depression, muscle tremors, watery diarrhea, tachycardia, ataxia, recumbency, and death. Animals can be found dead without clinical signs being noted. Animals surviving acute intoxication can subsequently develop congestive heart failure characterized by brisket edema, prominent jugular pulses, ascites, fluid feces, dyspnea, and tachycardia. Deaths can occur weeks to months after exposure and be precipitated by exertion such as calving. In dairy cattle, decreases in milk production are likely, with production requiring several days to a week to return to previous levels.

Clinical pathologic changes are similar to those described for horses. On necropsy, lesions include myocardial degenera­tion, pulmonary edema, enlargement of the heart and liver, hydropericardium, hydrothorax, and ascites.

Elements necessary for a diagnosis in cattle are similar to those for horses. However, in contrast to cases involving horses in which confirming exposure is likely to be significant, in cattle it is important to confirm exposure to feed concentrations higher than expected concentrations. Although, similar to horses, tissue testing can identify exposure to an ionophore, tissue concentrations associated with intoxication have not been definitively determined.¹² Treatment is similar to that for horses, although it is less likely to be as extensive in cases involving cattle.

■ Poultry Both salinomycin and monensin intoxication have been reported in turkeys.¹³ Sensitivity to salinomycin is age dependent, with younger birds being less sensitive. Clinical signs in affected birds include dyspnea, drowsiness, sternal recumbency with rear legs extended posteriorly, inability to stand, stiffness, and weakness. Extensive fragmentation and necrosis of skeletal muscle fibers occurs, and myocardial damage can also be noted.

Acute onset of feed refusal, decreased water consumption, and severe paralysis ranging from an abnormal gait to complete inability to move have been described in a chicken broiler breeder flock after monensin was included in their feed at approximately seven times the intended concentration.¹⁴ The flock experienced high morbidity and mortality. Postmortem lesions were similar to those reported for intoxicated turkeys. Although the mortality rate returned to normal 21 days after feed removal, loss of productivity resulted in elimination of surviving birds.

β₂-Adrenergic Agonists

In livestock, β₂-adrenergic agonists have been used as repar­titioning agents (i.e., repartitioning nutrients away from fat in favor of muscle). This results in increased carcass weight and ratio of muscle to fat and improved feed efficiency. β₂-adrenergic agonists approved in one or more countries for use in livestock feed include ractopamine (finishing beef cattle, swine, and turkeys) and zilpaterol (finishing beef cattle). Clenbuterol is approved for treating recurrent airway obstruction in horses but is not approved for use in food animals.¹⁵ Due to the anabolic effects of β₂-adrenergic agonists, there is the potential for their use as doping agents in racehorses.

Horses appear to be more sensitive to adverse effects than cattle following exposure to zilpaterol. In a limited number of horses, adverse effects were noted soon after a single zilpaterol dosage of 0.17 mg/kg BW PO; this is the recommended dosage for cattle.¹⁶ Signs included nervous behavior, profuse sweating, muscle tremors, and tachycardia within 20 to 60 minutes post dosing. Clinical pathologic changes were consistent with muscle damage and included increased serum activity of LDH, CK, and AST. Myoglobin-induced nephropathy is possible in some individuals. The duration and severity of adverse effects were unanticipated, with some signs persisting for up to 2 weeks. Affected horses recovered completely without treatment. Widespread accidental contamination of horse feed with zil- paterol resulted in violative urine residues in 48 horses from four California race tracks (http://www.chrb.ca.gov/Board/ committee_packages/Apr-2013MC.pdf). No horses were reported to be symptomatic, but the case illustrates that nontoxic concentrations of feed additives can potentially result in adverse consequences unrelated to an adverse health effect. Fortunately, because of the widespread nature of the feed contamination, no regulatory action was taken against any positive horse.

Based on human data, clenbuterol is approximately 19 times more potent than zilpaterol, and zilpaterol is approximately 125 times more potent than ractopamine. Dosages of racto­pamine of up to 10 times the dose of zilpaterol given to horses have not been associated with adverse effects.¹⁷

Antibiotics

The erroneous addition of antibiotics to feed is of particular concern in horses. Antibiotics such as lincomycin, clindamycin, tetracycline, and erythromycin have been associated with lethal colitis in horses.⁶ Adverse effects are believed to be due to disruption of the normal microbial flora, resulting in overgrowth of pathogenic bacteria such as Clostridium spp., Salmonella spp., or Escherichia coli. Subsequent production of endotoxins or enterotoxins causes digestive tract damage and systemic clinical effects. Treatment primarily involves IV fluid therapy and correction of acid-base and electrolyte abnormalities.

Pesticides

Tetrachlorvinphos (TCVP) is an organophosphorus insecticide fed to horses, cattle, and swine as a pass-through fly larvicide. It is a cholinesterase inhibitor, and intoxication is associated with overstimulation of muscarinic and nicotinic cholinergic receptors. TCVP has low mammalian toxicity, with an LD₅₀ in laboratory animals of greater than 1 g/kg BW. However, anecdotal evidence suggested that horses receiving TCVP at recommended levels were more likely to exhibit GI problems and increased reactivity, leading to handling difficulties. In one study, six horses given the recommended dose of TCVP for 30 days were noted to have heightened reactivity to an external stimulus that correlated with a reduction in whole blood cholinesterase activity.¹⁸ Another study demonstrated that TCVP at the recommended dose depressed plasma butyrylcholinesterase activity in horses but did not disrupt cholinergic neurotransmission in target tissues.¹⁹ Although TCVP at recommended doses does not cause typical signs associated with organophosphorus insecticide overexposure, some clinicians have suggested that alternative methods for fly control be used for horses due to potential adverse effects on behavior.

Pesticides and Rodenticides

■ BOX 54.1

1. Type and number of animals exposed (age, weight, dairy or beef, pregnant, lactating, etc.).

2. Location of exposure (pasture, dry lot, aerial drift, etc.) or crops/pasture/hay land.

3. Clinical signs of exposed animals (consider taking whole blood for cholinesterase activity). If animal deaths are involved, perform necropsies and take samples for routine necropsy along with brain (cholinesterase activity and chemical analysis), fat, liver, kidney, lung, urine, and skin (if dermal exposure) for possible chemical analysis.

4. Try to obtain trade name of pesticide, particularly if accidental exposure such as aerial drift, and the wind direction if sprayed onto animals.

5. If a licensed applicator accident, the applicator will have pesticide label data and concentration, wind speed, and direction.

6. With a chemical name identified, you can access information on toxicity by several routes:

a. Contact a veterinary diagnostic lab and toxicologist for toxicity data.

b. Locate data on the EPA website, including “How to Search for Information about Pesticide Ingredients and Labels” (https://www.epa.gov/ingredients-used-pesticide-products/ how-search-information-about-pesticide-ingredients-and- labels).

c. Search for chemical toxicology data, which can often be located on the U.S. National Library of Medicine website, specifically the Hazardous Substances Data Bank (HSDB). Enter the chemical name in the HSDB and data on human and animal toxicity, emergency medical treatment, pharmacology, chemical safety and handling, and environment fate can be obtained (https://toxnet.nlm. nih.gov/newtoxnet/hsdb.htm).

7. If questions on grazing restrictions or crop use for livestock, check the pesticide label for data. In addition, some state extension bulletins can provide grazing restriction information on some chemicals.

8. Data on pesticide chemical tolerances for residues in crop commodities, meat, and meat by-products can be found in the Code of Federal Regulations (4θ CFR Part 180) (https:// www.ecfr.gov/cgi-bin/text-idx?c=ecfr&tpl=/index.tpl).

9. Check the electronic Code of Federal Regulations (eCFR) for updated data. Instructions for conducting an eCFR search for tolerances can be found on the EPA website: “How to Search for Tolerances for Pesticide Ingredients in the Code of Federal Regulations” (https://www.epa.gov/pesticide-tol- erances/how-search-tolerances-pesticide-ingredients -code-federal-regulations).

10. Treatments for animals may be only supportive care for clinical signs. Consider using activated charcoal to bind pesticides by oral exposure; check individual chemical information for specific medications. If dermal exposure occurred, wash the animal with soap and water to decrease absorption. Clipping hair should be considered in long-haired livestock.

11. Chemical analyses for all pesticides, metabolites, and possible inert ingredients are not readily available (especially herbicide analysis in biological tissues). Chemical standards may not be available or could be expensive; laboratory methods in crop or biological matrices may have to be developed. Check with a veterinary toxicologist regarding costs and the pos­sibility of chemical analyses.

or toxicosis in large animals generally occurs due to misuse or overdosage with a pesticide, malicious use of pesticides or baits, or occasionally from accidental exposure (e.g., drift from aerial spraying, cleanup of buildings with old stored products).

All pesticides sold and distributed in the United States must be registered by the EPA. Registration is based on scientific data demonstrating that the chemicals can be used without posing unreasonable risks to people or the environment. With advancement of science, all pesticides that were first registered before November 1, 1984, were required to be reregistered to meet the more stringent standards. General scientific data on the toxicity of pesticides can be located on the EPA website (www.epa.gov) and in numerous databases supported by the U.S. National Library of Medicine, such as ToxNet (http://toxnet.nlm.nih.gov), using search terms for the chemical, Chemical Abstract Service (CAS) number, trade name, or clinical signs. The product label can also provide data on potential toxicosis.

Unfortunately for large animals, a limited number of antidotes is available for treatment of pesticide and rodenticide toxicoses.^1,2 For example, atropine sulfate, epinephrine, and vitamin K₁ do not have New Animal Drug Approval and are not formally approved by the Food and Drug Administration/ Center for Veterinary Medicine (FDA/CVM), but the labels are reviewed and on file with the FDA/CVM.³ These drugs may be used as antidotes in food animals, but the veterinarian should contact the Food Animal Residue Avoidance Databank (FARAD; www.farad.org) regarding withdrawal times if animals or milk will be put into the food chain. Because toxicoses involving pesticides often occur in nontarget species or at high concentrations, withdrawal intervals for these chemicals in food animals are often not readily available. A reasonable place to start locating this information is through contact with your state veterinarian, the District Office for the FDA in your region, and the U.S. Department of Agriculture Food Safety and Inspection Service (USDA FSIS; www.fsis.usda.gov). The state veterinarian may also assist with recommendations for proper disposal of chemically contaminated carcasses.

Box 54.1 outlines suggested steps to handle unknown exposure of livestock to pesticides.

Organophosphates and Carbamates

The introduction of newer types of external parasiticides and chemical products has significantly reduced the incidence of these toxicoses in livestock. Both organophosphate (OP) and carbamate insecticides inhibit acetylcholinesterase (AChE) at cholinergic nerve synapses and at neuromuscular junctions, causing acetylcholine to accumulate at nerve junctions and cause excessive synaptic action. They also inhibit other cholinesterases in the body (e.g., red blood cells, serum, liver, pancreas, nervous tissue). These insecticides lack species selectivity and may pose a hazard to domestic and companion animals, wildlife, and aquatic species. The anticholinesterase compounds may adversely affect the cardiovascular, respiratory, ocular, reproductive, endocrine, dermal, and immune systems but primarily act on the nervous system and skeletal muscles.⁴ Death is usually the result of respiratory failure, often with cardiovascular involvement.

Approximately 200 OPs and several dozen carbamate compounds have been synthesized for numerous purposes. They are incorporated into various products, including baits, dips, dusting powders, sprays, pour-ons, and oral anthelmintics. The OPs used as defoliants and herbicides, such as glyphosate and gluphosinate, have low acute mammalian toxicity but could induce delayed polyneuropathy.⁴ Aldicarb, sold under the trade name Temik, was synthesized to mimic acetylcholine and is considered to have maximum potential for mammalian toxicity.⁴

Both OPs and carbamates inhibit the metabolism of ace­tylcholine at cholinergic sites. Acetylcholine is a neurotransmitter at postganglionic parasympathetic neurons in smooth muscles, cardiac muscles, or exocrine glands; at neuromuscular junctions of the somatic nervous system; at cholinergic synapses in the CNS; and between preganglionic and postganglionic neurons of the autonomic nervous system. The enzyme AChE normally breaks down acetylcholine at these cholinergic sites, but when bound by OPs or carbamates it is inactive. During toxicity, acetylcholine maintains depolarization of the postsynaptic membrane, which is initially stimulatory but may progress to paralysis. The OPs have high affinity for the cholinesterase enzyme and are considered irreversible inhibitors of AChE. Organophosphates can undergo an “aging” process, which stabilizes the OP-AChE bond and cannot be broken. Carba­mates do not undergo “aging” and can be dislodged from AChE, resulting in spontaneous enzyme regeneration. These insecticides also induce toxic effects through noncholinergic mechanisms that may involve glutamate release and activation of N-methyl-D-aspartate receptors and the γ-aminobutergic, adenosinergic, and monoaminergic systems that are involved in seizures, which are reviewed by Gupta and Milatovic.⁴

The toxicity of OPs and carbamates varies widely, from highly toxic compounds (OP nerve gases such as Sarin, Soman, and VX, and carbamates such as aldicarb and carbofuran) to minimally toxic ones (e.g., malathion, tetrachlorvinphos). The very young, very old, and weakened animals are more susceptible to poisoning from these compounds. Following exposure, OPs and carbamates are rapidly distributed throughout the body and can undergo oxidation, hydrolysis, and conjugation. They are excreted in the urine and feces. The OPs tend to be more lipophilic than carbamates and are more likely to cross the blood-brain barrier; however, carbamates may cause marked CNS signs. Clinical signs can occur within minutes to hours of exposure depending on the dose, route of exposure, and toxicity of the insecticide. Death can occur within minutes with highly toxic chemicals or when large amounts are ingested. Repeated exposures can be clinically important. The insecticides may be metabolized and excreted within days of exposure, but the regeneration of AChE may require 2 weeks.⁵ Clinical signs of acute toxicity due to OP and carbamate exposures result from overstimulation of muscarinic and nicotinic acetylcholine receptors because of the buildup of acetylcholine from inactiva­tion of AChE. The clinical effect is excessive synaptic neurotransmitter activity and depolarization of effector organs.⁶ Muscarinic signs include ptyalism, excessive tracheobronchial secretions and sweating, bronchospasms and laryngospasms, lacrimation, nausea, diarrhea, miosis, and bradycardia. The muscarinic clinical signs are frequently described by the term SLUD (salivation, lacrimation, urination, and defecation). Nicotinic clinical signs include muscle fasciculations, tremors, weakness, ataxia, flaccid paralysis, vomiting, and paralysis of respiratory muscles. Muscarinic signs generally appear first, followed by nicotinic signs, and finally by CNS effects (hyper­activity and seizures).

Not all clinical signs are observed in an OP or carbamate toxicosis. The clinical appearance of an AChE toxicosis may vary with the species exposed, toxic compound, dose, route of exposure, and timeline in exposure-response. For example, the use of methiocarb as a snail bait in Australia uncommonly causes problems in horses but can result in AChE inhibition, clinical signs, and death. A 400-kg gelding developed clinical signs of profuse sweating, hypersalivation, lateral recumbency, paralysis of a distal hindlimb, and generalized muscle tremors within a few hours after consuming almost 20 g of methiocarb in bait form (1 kg of 2% weight/volume).⁷ The horse had signs of colic, ileus, and a significant amount of nasogastric reflux (total reflux volume after presentation at a veterinary clinic was 55 L) but never developed diarrhea. Because of the large volume of nasogastric reflux, activated charcoal was not given nor was atropine administered due to the lack of GI borborygmi. Gastric lavage had been performed about 6 hours post ingestion but was probably ineffective to recover ingested bait; the authors speculated that the worsening clinical signs were due to continued absorption of methiocarb, and the horse was eutha­nized about 54 hours post ingestion of the bait.

Delayed toxicosis has occurred with the use of chlorpyrifos, a chlorinated OP, as a pour-on in bulls and some exotic breeds of cattle. The toxicity appeared to be related to testosterone levels in cattle, with larger and older bulls affected. Clinical signs appeared 2 to 7 days post exposure and included weakness, muscle fasciculation, anorexia, rumen stasis and distention, and depression.⁸ Decontamination of the skin and rumen, activated charcoal, and symptomatic treatment are recom­mended for treatment of chlorpyrifos poisoning in bulls.

Diagnosis of OP or carbamate toxicosis includes (1) history and clinical signs consistent with the toxicosis; (2) response of animals to treatment with atropine; (3) determining cholinester­ase inhibition (whole blood, retina, brain), corresponding with adverse clinical effects; and (4) detection of the insecticide in biological tissues (GI contents or tissues). The diagnosis can be challenging. Clinical pathologic changes are not consistent and postmortem lesions are nonspecific. Often pulmonary edema is evident on postmortem, along with tracheal fluid and diarrhea, but these changes are not unique to AChE toxicosis.

Measurement of cholinesterase activity in whole blood and brain is commonly used to establish an OP or carbamate diagnosis. Generally, AChE activity less than 50% of normal level indicates exposure, and activity less than 25% of normal level in an animal with compatible clinical signs indicates toxicosis from an OP and/or carbamate. Use of AChE activity alone is not always diagnostic. The use of OP anthelmintics in horses at therapeutic doses decreased AChE activity to less than 25% of normal level, while no horses displayed clinical signs.⁹ Interpretation of AChE activity is difficult in a carbamate toxicosis. Carbamates can spontaneously dislodge from the cholinesterase enzyme before or after sample collection; therefore AChE activity may be in the normal range even though the animal is experiencing toxicity from a carbamate compound.

Check with your diagnostic laboratory for proper collection of specimens (whole blood, brain, plasma, serum, retina) for AChE activity. With an OP exposure, AChE activity in whole blood is fairly stable for about 7 days after collection if the blood is stored under refrigeration.⁹ For determination of AChE activity in the brain, submit a chilled whole or half brain because cholinesterase activity varies in different regions of the brain and the laboratory may have a preference for the brain region tested.

Identification of the insecticide can often be made on postmortem specimens, including rumen, GI contents, liver, fat, brain, and perhaps kidney. Samples should also be taken of the suspect source (e.g., bait, feed, water) for identification and concentration. It is not unheard of to have acute death losses in animals; finding depression of AChE in brain, whole blood, and retina; detecting the insecticide in rumen contents; but locating no source of insecticide on the farm.¹⁰ With suspect insecticide toxicosis, check with the diagnostic laboratory for proper collection and shipment of samples. Generally, the samples are kept chilled or frozen before analysis. Management of OP or carbamate toxicosis depends on the clinical signs and time from exposure. Treatment should be initiated quickly. If the animal is having seizures, stabilize the animal and attempt to control the seizures with diazepam, which may need to be supplemented with methocarbamol, barbiturates, or general anesthesia. Oxygen therapy and ventilatory support may help with severe dyspnea. The noncompetitive antagonist for OPs and carbamate is atropine, which blocks excessive acetylcholine at synapses. It can control muscarinic signs and some CNS signs, but not nicotinic signs (e.g., muscle fasciculations, weak­ness, paralysis). The therapeutic range for atropine sulfate is between 0.25 and 1 mg/kg, with one quarter of the dose given intravenously and the remainder administered IM or SC.^5,6 It may be necessary to use the high end of the range in toxic situations. The dose should be repeated as needed (usually a lower dose and every 2 to 4 hours) to control bradycardia and secretions into the lungs. Avoid overtreatment with atropine, especially during prolonged treatment. Note that atropine has a low margin of safety in horses and causes GI stasis, which should be carefully monitored during use. It is recommended not to exceed a total dose of 65 mg of atropine in the average horse.⁵ When atropine is used in multiple doses as an antidote for a toxicosis, check with FARAD (www.farad.org) for milk and/or meat withdrawal recommendations. Note that veterinar­ians must also consider the residue-withdrawal interval for the insecticide in tissues.

Acetylcholinesterases can be reactivated with oximes, such as pralidoxime chloride (2-PAM), if aging has not occurred or with the irreversible binding of an OP to AChE. The oxime attaches to the OP insecticide and dissociates the OP- cholinesterase bond. Response to oxime therapy is better when oximes are used within 24 hours after exposure to OP insec­ticides. Oximes ameliorate to some degree muscarinic, nicotinic, and CNS signs and work best with atropine. The dose, fre­quency, and route of administration of 2-PAM vary in the literature. A dose of 20 mg/kg IV q4-12h has been recommended in ruminants; its use should be stopped if no response occurs within 36 hours.¹¹ Excessive doses of oximes have the capability to inhibit cholinesterase.⁴ The use of 2-PAM in treatment for a large animal OP toxicosis would be very expensive; in addition, 2-PAM is not approved for use in large animals and should be used with oversight under FDA/CVM guidance. In some cases, the use of 2-PAM in carbaryl (carbamate) toxicity has been considered detrimental.

Once the animal is stabilized, decontamination should be considered. Oral dosage of activated charcoal (1 to 2 lb/500 kg in a slurry) can bind the insecticides in the GI tract and prevent absorption; this treatment may also be helpful with topical application of some insecticides. The use of activated charcoal is particularly recommended for ruminants, which have the capacity for large retention of GI contents. With topical exposure to insecticides, animals should be washed with a liquid dishwashing detergent and rinsed off to reduce absorption. Gastric lavage with water may be indicated in high-dose exposures where only a few hours have passed since ingestion. Additional therapy may include fluids to treat dehydration and electrolyte imbalances.

Several drugs are contraindicated in OP and carbamate toxicosis (with cholinesterase inhibition), including opiates, physostigmine, phenothiazine tranquilizers, neostigmine, succinylcholine, levamisole, antihistamines, aminoglycoside antibiotics, and theophylline.

Glyphosate

Although glyphosate can be chemically described as an OP compound, it is a phosphanoglycine and not recognized as a cholinesterase inhibitor. It is usually formulated as an isopro­pylamine, sodium, ammonium, or timesium salt and is active as a broad-spectrum, postemergent, nonselective herbicide. Glyphosate is one of the most used herbicides worldwide. It interferes with the production of essential aromatic amino acids in plants.¹² Glyphosate is not metabolized efficiently in mammals and is mainly excreted unchanged into the urine; however, it can undergo GI microbial metabolism in humans and rats.¹² Recently, the International Agency for Research on Cancer (IARC) classified glyphosate as “probably carcinogenic to humans (Group 2A)” based on limited evidence in humans and with a positive association for non-Hodgkin's lymphoma and with “sufficient evidence in experimental animals for carcinogenicity.”¹² Limited data are available in livestock. In a field study of Danish dairy cows, all cows on eight dairy farms excreted glyphosate, in varying concentrations, in their urine.¹³ The authors reported increased activity of several serum enzymes, glutamate dehydrogenase, glutamate oxaloacetate transaminase, and creatinine kinase in cows on all farms, which could indicate a possible effect of glyphosate on liver and muscle cells. Low serum concentrations of manganese and cobalt were observed in all animals, which may be explained by the strong mineral chelating effect of glyphosate; however, the mean levels of copper, zinc, and selenium were within the normal reference range. In a recent study of glyphosate in lactating dairy cows, glyphosate residues in foodstuffs (0.8 to 84.5 mg glyphosate/ day average intake in the experimental groups) were fed to cows during a 16-week trial.¹⁴ No glyphosate residues were detected in the milk, and no adverse effects were detected in body condition score, net energy intake, net energy balance, or milk performance parameters. Given the health controversies surrounding the widespread use and exposure to glyphosate, additional animal studies are warranted.

Pyrethroids

Pyrethroids are active synthetic derivatives of natural toxins in the plants Chrysanthemum Cinerariaefolium (or Tanacetum Cinerariaefolium) and Chrysanthemum cineum. These chemicals are important insecticides used in crop protection to control insect vectors carrying infectious diseases of animals and humans and to control insects on premises. They are available as wettable powders, granules, and emulsifiable concentrates in sprays, dusts, dips, foggers, ear tags, pour-ons, and back rubbers. Pyrethrins are the naturally active insecticidal compounds from the plants; pyrethroids are synthetic analogs of the pyrethrin esters and tend to have more potent insecticidal activity, to be more stable in the environment, and to be more toxic. The pyrethroids are categorized as type I pyrethroids (a cyano group is absent) or type II pyrethroids (an alpha-cyano group that increases the insecticidal potency is present). Allethrin, bifen- thrin, permethrin, resmethrin, and tefluthrin are examples of type I pyrethroids. Cyfluthin, cypermethrin, deltamethrin, and flumethrin are examples of type II pyrethroids. Often synergists such as piperonyl butoxide, propyl ethers, and sesamin are combined with pyrethroids to inhibit mixed-function oxidase and esterase enzymes in the target species and reduce detoxifica­tion of the insecticide and prolong its action in insects. Syner­gists may also enhance toxicity in animals. Pyrethroids interfere with conductance of nerve membranes by binding to the membrane and prolonging the sodium current. The pyrethroids act on axons in the central and peripheral nervous systems. Gamma-aminobutyric acid-gated chloride channels may also be affected by high doses of type II pyrethroids. Compared with insect sodium channels, mammalian sodium channels are much less sensitive to pyrethroids and recover more quickly from depolarization.

Most pyrethroids are applied dermally, and there is limited absorption through this route. Pyrethroids are absorbed more efficiently across GI and pulmonary membranes. Pyrethroids are lipophilic in nature and distribute into fat, central and peripheral nervous systems, milk, and other organs. Once absorbed, pyrethroids can undergo metabolism by nonspecific esterases and microsomal mixed-function oxidases. Pyrethroids and their metabolites are excreted in feces and urine, with a majority of pyrethroids excreted within 12 to 48 hours.¹⁵

Following application of pyrethroid pour-on products, cattle can exhibit a paresthesia-like syndrome, thought to result from direct action of pyrethroids on sensory nerve endings.¹⁵ Cattle may be restless and uncomfortable, and twitch the skin on their backs. Young animals are more susceptible due to a lack of metabolism of pyrethroids; death can occur due to respiratory failure and convulsions leading to paralysis. No specific clinical pathologic or histopathologic changes have been reported with pyrethroid applications. In a toxicosis, check with the veterinary diagnostic laboratory regarding analytic testing on appropriate tissues. Pyrethroids concentrate more in nervous tissue than serum or plasma.

Pyrethroids have been implicated in disrupting the endocrine system, adversely affecting reproduction and sexual develop­ment, and interfering with the immune system. Pyrethroid treatment of laboratory animals and in vitro samples has induced a decrease in epididymal sperm count and sperm motility, as well as an increase in abnormal spermatozoa.^16,17 Pyrethroid treatments of mice and rats disrupt testosterone synthesis and downregulate 17β-hydroxysteroid dehydrogenase (convert androstenedione to testosterone) and biosynthetic enzymes that have a critical role in the synthesis of testosterone in Leydig cells.^18,19 In addition, pyrethroid exposure has been shown to disrupt luteinizing hormone-responsible ovulatory genes in rats and prostaglandin synthesis in granulosa cells in vitro.²⁰

Exposure of bulls in a bull stud to bifenthrin was associated with poor semen quality, including (1) decreased motility, (2) reduced ejaculate volume, and (3) spermatozoa abnormalities with a high percent of distal midpiece reflexes.²¹ At 64 days post exposure, semen quality recovered to levels observed before exposure in these bulls. In this case, the use of bifenthin was apparently an excessive use of product in a building (bull stud) with the animals present, so that excessive exposure was dermal, inhalation, and oral; in addition, these animals were being con­tinually monitored for semen quality for cryopreservation, and therefore clinical abnormalities in semen were easily detected (Evans, T: personal communication, August 2012). Field cases of bulls exposed to pyrethroids that subsequently developed poor sperm quality and motility, with gradual recovery to normal after about 2 to 4 weeks, have been reported.^22,23 There are few scientific data, however, with controlled studies to support observations that use of pyrethroids will alter sperm quality in bulls.

Accidental pyrethroid exposure in bulls may result in inadequate semen quality (particularly for cryopreservation) but may not impair bull fertility when bulls are used in natural settings, such as pasture breeding where several males are available to breed female cows during a breeding season. The important considerations when using pyrethroids on pastured cattle are to exactly follow the application recommendations on the label, to use a product that the cattle producer has experience with for insect control, and to avoid using close to breeding if possible. Another recommendation is to use only one method of insect control with pyrethroids on breeding cattle at a time (e.g., only spraying, only ear tags).²³

With dermal exposure, wash and scrub the exposed area using liquid dishwashing soap and rinse well. Avoid using power washers because they may cause dermal trauma.¹⁵ If seizures occur, the use of diazepam, methocarbamol, and barbiturates may be effective.

Neonicotinoids

A relatively new class of insecticides are the neonicotinoids, which are heterocyclic nitromethylenes used primarily in crop protection against sucking and certain chewing pests. The neonicotinoids include imidacloprid, acetamiprid, dinotefuran, thiamethoxam, and clothianidin.

The neonicotinoids act on postsynaptic nicotinic receptors located in the CNS. They act as agonists against nicotinic acetylcholine receptors in insects and mammals.²⁴ The insec­ticides initially increase the frequency of spontaneous discharge, which is followed by a complete block of nerve propagation. Neonicotinoids have been considered relatively low risk for nontarget organisms and the environment.²⁵ The activity in vertebrates is much lower due to different binding properties to the nicotinic receptors, and they primarily act on the α7 nicotinic receptor subtype. However, with the widespread use of neonicotinoids, an increase in acute neonicotinoid poisoning cases has been reported worldwide.²⁴ Increasing evidence of a variety of toxic effects in animals and humans has been identified, including altered thyroid and reproduction hormones in birds and mammals; upregulation of cytokines and suppressed cell-mediated immunity identified in rat experiments; and oxidative stress-related damage to lipids, DNA, and proteins in vertebrates and invertebrates.²⁴ Use of neonicotinoids can represent a health risk to wild bees and honey bees, with the European Union considering a complete ban on the outdoor use of three neonicotinoids: imidacloprid, clothianidin, and thiamethoxam.²⁶

Imidacloprid has widespread use as an insecticide for dermal application on animals, as well as an insecticide for crop protec­tion. Imidacloprid is primarily metabolized in the liver.²⁷ Metabolism of imidacloprid in mammals is by two routes: (1) oxidative cleavage and glutathione conjugation and (2) hydrox­ylation. Most of the imidacloprid is rapidly excreted in the urine. Imidacloprid is not considered carcinogenic and has a high margin of safety due to low mammalian toxicity. The neonicotinoids do not readily pass the blood-brain barrier.

Adverse effects noted in laboratory animals dosed with imidacloprid were decreased activity and tremors, impaired pupillary function (dilated or pinpoint), and uncoordinated gait; higher doses were associated with hypothermia. The no-observed-effect level (NOEL) for chronic imidacloprid exposure in dogs was 15 mg/kg PO.²⁷

Treatment for overdoses of imidacloprid is symptomatic. With dermal exposure, wash the animal with soap and water; with oral exposure, use activated charcoal for adsorption and a cathartic. Generally, imidacloprid absorption and elimination are rapid, and supportive care should provide recovery.

Organochlorines

Organochlorines are chlorinated hydrocarbons. These com­pounds include lindane, mirex, kepone, dichlorodiphenyltri­chloroethane (DDT), aldrin, dieldrin, heptachlor, chlordane, toxaphene, and endosulfan. The persistence of some organo- chlorines in the environment and concern for neurotoxicity and endocrine disruption have terminated the use of most organochlorines. These compounds have limited to no distribu­tion in the United States but may occasionally cause toxicoses when stored chemicals contaminate feed or premises. Organo- chlorines are available in other countries.

The organochlorines have several mechanisms of action. The DDT-type organochlorines affect the brain and peripheral nerves by slowing sodium influx and inhibiting potassium outflow, partially depolarizing the cell and increasing firing of the neuron.²⁸ Several organochlorines may act by inhibit­ing attachment of GABA to its receptor, which stimulates the neuron. The organochlorines are absorbed orally and dermally. These compounds are highly lipid soluble and distribute to adipose tissue, brain, liver, kidney, and milk. Metabolism of organochlorines can be through dechlorination and oxidation, glucuronidation, and sulfation. More toxic epoxides can be formed by the mixed-function oxidases. The organochlorines are excreted into bile and the GI tract. They can undergo enterohepatic recycling, prolonging exposure for weeks. These compounds bioaccumulate in fatty tissue and persist in the environment. Organochlorines can act as endocrine disruptors.

Acute clinical signs include salivation, vomiting, and nervous system signs of agitation, hyperexcitability, incoordination, apprehension, nervousness, and tremors. As clinical signs pro­gress, animals may display clonic-tonic seizures, opisthotonus, paddling, and clamping of the jaw. Cattle may walk backward, lick excessively, or have abnormal postures.²⁹ Seizures can persist for several days. Between seizures the animals may be depressed or normal in appearance. Clinical pathologic changes and histopathology are nonspecific.

Diagnosis is based on history, clinical signs, and analytic determination of organochlorines in blood, fat, liver, brain, or milk. Contact the veterinary diagnostic laboratory for sample submission instructions. Generally, samples for organochlorine analysis should not be submitted in plastic but wrapped in aluminum foil and/or placed in clean glass containers. Note that tissue levels of organochlorines do not correlate well with severity of clinical signs or prognosis.

Detoxification is a crucial step in treatment. Animals with topical exposure need to be washed with soap and water, and animals with long hair should be clipped to reduce organo- chlorine exposure. Treating personnel should wear gloves and aprons to reduce human exposure. With oral exposure, activated charcoal (1 to 2 g/kg body weight in a slurry) should be administered orally. Feeding activated charcoal, at 500 to 1000 g/day for a limited time, to large animals will reduce enterohepatic cycling.²⁸ General supportive care, such as the use of diazepam or barbiturates to control seizures and placing the animals in an environment to prevent trauma and keep comfortable, are important. Organochlorines typically have a prolonged residue time in body tissue. Animals that are lactating or losing weight generally eliminate more organochlorines due to loss of milk fat or body fat, respectively.

The state veterinarian should be contacted regarding exposure of animals to organochlorines and proper disposal of contaminated tissue or materials. The long persistence of organochlorines in tissue prevents immediate placement of food animals or milk into the food chain; proper state or federal agencies will need to be contacted for withdrawal intervals.

Paraquat

Paraquat is a bipyridyl herbicide used as a desiccant on crops. Most questions about paraquat exposure in large animals relate to aerial spray drift onto animals and/or their pasture and application to forages that will be used for livestock. Paraquat absorbed onto soil particles is biologically inactive. Animals that have access to concentrated paraquat formulations or to wet forages following paraquat application are at risk for developing clinical signs.

Paraquat is poorly absorbed (5% to 10%) from the intestinal tract but has prolonged residual time in the digestive tract. Emulsifiers or other chemicals in paraquat formulations may enhance absorption and toxicity. Paraquat is concentrated in renal tissue and is eliminated in the urine.

Paraquat forms free radicals in the body and has special affinity for the lung. The production of superoxide radicals and hydrogen peroxide results in damaged cellular membranes and tissue necrosis.

Following ingestion, the initial signs reflect the irritating action of paraquat, resulting in abdominal pain, vomiting, anorexia, and depression. By the second to third day post exposure, the onset of renal failure appears along with liver damage (hepatocellular necrosis), which is followed by delayed development of pulmonary fibrosis, respiratory dyspnea, cyanosis, and pneumomediastinum.³⁰ Death may occur several weeks after exposure. With topical exposure to paraquat, erythema, blistering of skin, and corneal irritation can occur and may resolve without additional complications.

Histopathologic changes from paraquat toxicosis include apoptosis or necrosis of alveolar epithelial cells, denuded alveolar basement membranes, and with a chronic response, hyperplasia of type II alveolar epithelial cells and pulmonary fibrosis.³⁰ Hepatic and renal lesions may be present. The diagnosis is based on history of exposure to paraquat and chemical analysis of paraquat in urine, plasma (up to 30 hours post exposure), and lung tissue (after 30 hours post exposure).

Treatment should be initiated as soon as possible (within 24 hours) using activated charcoal, bentonite, or fuller's earth. Supportive care includes monitoring vital signs frequently, using fluids to maintain renal perfusion, treating secondary infections, and pain management. If treatment is delayed, the prognosis is extremely poor.

Rodenticides

Zinc and Aluminum Phosphide

Zinc phosphide is a rodenticide used since the 1930s. Zinc phosphide is a gray, crystalline powder that is relatively stable under dry conditions. Rodent formulations contain 2% to 5% zinc phosphide mixed with grains; therefore the baits are attractive to most species. Under wet or acidic conditions, zinc phosphide degrades to phosphine gas. Phosphine gas has the odor of acetylene or rotten fish, and poisoning can occur in people at detectable concentrations of 2 ppm. Acute, oral lethal doses of zinc phosphide were reviewed in various species.³¹ The lethal dose of zinc phosphide in ruminants was approxi­mately 60 mg/kg body weight.

Aluminum phosphide is a fumigant used in stored agricultural products to control rodents and insects. It has a high potential of acute toxicity with inhalation exposure. After application of aluminum phosphide tablets to grain, phosphine gas is released for up to 5 days. A general recommendation is that grain treated with aluminum phosphide should not be handled or fed for at least 10 days after treatment.

Phosphine gas is readily released in acidic environments and can block cytochrome c oxidase, resulting in disruption of oxidative phosphorylation within mitochondria of cells.³² Blocking energy-producing pathways leads to cell death and eventual multiple-organ failure. Brain, heart, liver, and kidney are most affected by phosphine gas due to high metabolic rates and high demands for oxygen. Because of the higher pH of the rumen, as compared with an acidic monogastric stomach, ruminants should be more resistant to phosphide toxicants.

The onset of clinical signs occurs within 15 minutes up to 12 to 18 hours, depending on the dose and stomach contents (the more acidic, the quicker the release of phosphine gas). Death can occur within several hours after appearance of clinical signs. Ruminants show tympany and bloat, whereas horses can display signs of colic and abdominal pain. Horses fed a pelleted ration recently treated (within 14 hours) with aluminum phosphide showed rapidly developing clinical signs of profuse sweating, tachycardia, tachypnea, pyrexia, ataxia, seizures, and widespread tremors.³³ Clinicopathologic changes include hypoglycemia, high serum lactate and ammonia concentrations, and elevated activity of gamma-glutamyl transpeptidase, aspartate aminotransferase, and alkaline phosphatase. Several horses in this case developed clinical signs consistent with hepatic encephalopathy, and eventually 27 of 28 clinically affected horses died within 2 days after exposure. Postmortem changes were petechial and ecchymotic hemorrhages in numer­ous organs, widespread vascular congestion, hepatic lipidosis, pulmonary edema, and neuronal necrosis in the cerebral cortex.

Phosphine gas can be detected in gastric contents, vomitus, and possibly liver and kidney. The samples need to be packed in airtight containers and frozen immediately to prevent phosphine gas dissipation. Check with the veterinary diagnostic laboratory for analytic capability to detect phosphine gas.

The progression of clinical signs often limits treatment options. Early decontamination of the animal by emesis (if the animal vomits), gastric lavage (especially with 5% sodium bicarbonate), and products to increase gastric pH for zinc phosphide exposure is critical. Activated charcoal may be beneficial when mixed with an antacid and a cathartic (e.g., aluminum or magnesium hydroxide). No specific antidote is available for phosphine gas. Supportive therapy should be aimed at correcting acidosis, treating shock and liver failure, and controlling seizures. Horses that developed clinical signs after phosphine gas exposure were treated by gastric lavage followed by di-tri-octahedral smectite, atropine, fluids, 10% dextrose, flunixin meglumine, and sedatives.³³

Metaldehyde

Metaldehyde is a molluscicide, usually sold in less than 5% concentrations as pelleted baits, granules, liquids, and wettable powders. Feed ingredients such as apples, molasses, rice, oats, and soybeans have been added to the baits to increase palatability to nontarget species. Most toxicoses occur in the coastal areas of the United States, where snail and slug populations can be high.

The mechanism of action of metaldehyde is unclear but may be related to alterations of neurotransmitters in the brain. Metaldehyde is toxic to all known animals. Oral ingestion is the most likely route of exposure. Acute, oral lethal doses of metaldehyde were reviewed in various species.³⁴ The lethal dose ranges for metaldehyde were 200 to 300 mg/kg body weight in ruminants and 60 to 360 mg/kg body weight in horses. Young animals appear to be more susceptible than adults.

Clinical signs of toxicosis usually occur within several hours of exposure and include salivation, restlessness, anxiety, hyperpnea, tremors, colic, ataxia, nystagmus, and convulsions. Blindness and loss of a menace reflex have been reported in cattle with metaldehyde toxicosis.³⁵ No specific changes in clinical pathology or histologic lesions occurred in animals poisoned with metaldehyde. Gross lesions have included congestion in renal, hepatic, pulmonary, and GI tissues and petechial and ecchymotic hemorrhages throughout the body. Demonstration of metaldehyde in GI contents (possibly serum/ plasma and urine) confirms exposure. Samples should be submitted frozen to the diagnostic laboratory.

Initial therapy may involve control of seizures and tremors with diazepam and barbiturates; xylazine and acepromazine have been used in horses.³⁴ Decontamination by gastric lavage and activated charcoal, orally at 1 to 2 g/kg body weight, followed by mineral oil has also been recommended. Supportive care with intravenous fluids and a quiet environment may be beneficial.

Anticoagulant Rodenticides

Cases of hemorrhagic disorders reported in cattle generally involve repeated exposure to moldy sweet clover hay or sweet vernal hay (dicoumarol) over a prolonged period. Accidental ingestion of anticoagulant baits over a brief period may result in potential toxicosis in large animals. The anticoagulant roden­ticides are divided into first-generation coumarin derivatives (e.g., coumachlor, coumatetralyl, warfarin), second-generation coumarin derivatives (e.g., brodifacoum, bromadiolone, difenacoum, flocoumafen), and the indanedione derivatives (e.g., chlorophacinone, diphacinone, pindone). Generally, the second-generation coumarin compounds are more toxic than the first-generation products and can poison with one feeding.

All anticoagulant rodenticides interfere with vitamin K epoxide reductase, which converts inactive vitamin K to the active vitamin. A depletion of active vitamin K may affect vitamin K-dependent clotting factors in the intrinsic, extrinsic, and common coagulation pathways. Anticoagulant rodenticides are well absorbed on oral exposure and are highly bound to plasma proteins. Oral availability of warfarin, chlorophacinone, and bromadiolone in sheep was estimated to be more than 79%.³⁶ Anticoagulants are metabolized in the liver, and some compounds may undergo enterohepatic recirculation. The half-life of anticoagulant rodenticides varies greatly with the compound and the species. The half-life of brodifacoum in horses was given as 1.22 ± 0.22 days, as compared with that of warfarin in horses at 16 hours.^36,37 The half-life of broma- diolone in cattle was reported to be approximately 20 hours.³⁶ As a general observation, the duration of action of anticoagulant rodenticides in large animals is about 14 days for warfarin, 21 days for bromadialone, and 30 days for brodifacoum and other second-generation coumarin derivatives. Ruminants are thought to have lower susceptibility to anticoagulants; however, a study in anticoagulant-dosed sheep revealed that the compounds were not degraded in the rumen and their bioavailabilities remained high after oral dosing.³⁷ The authors speculated that several factors may lower ruminant susceptibility, including the presence of a large forestomach (diluting the compound) and ruminal production of vitamin K₁, which may partially counteract the toxic effects.

Clinical signs may become apparent 2 to 10 days post exposure. Affected animals may display melena, epistaxis, hematuria, or excessive bleeding from a wound. If bleeding occurs within a body cavity, the animal may exhibit pallor, weakness, dyspnea, fever, and depression. Animals may die suddenly from excessive hemorrhage and shock. Generally, gross and histologic lesions are consistent with hemorrhage. Diagnosis is based on history, clinical signs, prolonged clotting times and anemia, and response to the antidote, which is vitamin K₁. Analytic detection of the anticoagulant rodenticide in whole blood, liver, and perhaps GI contents can confirm exposure.

Vitamin K₁ is the antidote for anticoagulant rodenticides; the duration of therapy varies with the specific compound causing the toxicosis and may last several days to several weeks. The recommended dose of vitamin K₁ in the adult horse is 2.5 mg/kg IM/SC once to twice daily.³⁸ The prothrombin time should return to normal within 24 hours. Vitamin K₃, although less expensive to use, is not as effective as vitamin K₁, will cause renal disease in horses,²⁹ and is not recommended.

Packed cell volumes should be closely monitored during clinical signs, and whole blood transfusions may be required if the animal is anemic. Decontamination of the GI tract with activated charcoal may lower absorption of the anticoagulant. After the animal has stabilized and the appropriate time has passed, vitamin K₁ should be discontinued for 48 hours and prothrombin time retested.

1080

Sodium monofluoroacetate, fluoroacetate, or 1080 is a nonselec- tive rodenticide that is toxic to animals and humans. The compound 1080 has been available in the United States since the 1940s but now is restricted for use as a predacide in the United States. It is used in Australia and New Zealand to kill invasive, unwanted species. Fluoroacetate naturally occurs in several plants in Africa, Australia, South America, and perhaps southern Florida. Sodium monofluoroacetate also appears to be available over the counter in the United States as foreign rodenticides or “herbal” pesticides. This compound is colorless, odorless, and water soluble.

The mechanism of action of 1080 involves a critical cycle, the Krebs or tricarboxylic acid cycle, in cellular metabolism. Many of the effects of 1080 toxicosis result from impaired oxidative metabolism and reduced energy production.³⁹ The oral LD₅₀ for monofluoroacetate in ruminants and equines ranges from 0.22 to 0.55 mg/kg body weight.⁴⁰ Sodium monofluoroacetate is quickly absorbed from the GI and respira­tory tracts and through mucous membranes but slowly absorbed through intact skin.

In general, herbivores experience cardiotoxic effects, omnivores experience cardiac and CNS effects, and carnivores experience CNS effects. Clinical signs occur within 30 minutes to several hours post ingestion. Cattle may stagger, tremble, sweat, and display signs of GI irritation; they can die of heart failure.⁴¹ Horses can display hyperesthesia to external stimuli and have marked cardiac arrhythmias and ventricular fibrillation from 1080 poisoning. Sheep and pigs may show a combination of cardiac and CNS signs. Animals may have decreased GI movements, salivation, restlessness, apparent blindness, ataxia, frequent urination, muscular tremors, polypnea, and tachycardia. Pigs may have intermittent vomiting. Clinical pathology changes may include hyperglycemia, increased serum citrate, hypocal­cemia (ionized), and acidosis. Electrocardiograms on 1080-poisoned sheep revealed S-T segment elevation and a marked increase in T-wave amplitude, suggesting cardiac ischemia.⁴² Severely affected animals generally die within hours to a few days post exposure. Postmortem lesions in ruminants may involve endocardial and epicardial petechiae, edema of the brain, and lymphocytic infiltration of perivascular tissue. A diagnosis of 1080 toxicosis is based on clinical signs, history of exposure, and detection of 1080 in the bait, vomitus/gastric contents, kidney, and liver. Few veterinary diagnostic labora­tories in the United States analyze for 1080 in tissues. Generally, the assay is by gas chromatography/mass spectrometry with a detection limit of 10 μgZkg (or parts per billion).

In a recent case of multiple episodes of 1080 intoxication in a calf-raising operation, calves were found dead or displayed a stiff gait, dribbled urine, and became recumbent and died with ecchymotic and suffusive hemorrhages on the epicardial surface with moderate to large amounts of fluid in the pericardial sac.⁴³ Another case of 1080 toxicosis involved high, acute death loss in ewes and lambs following transfer of the flock to a new pasture over a municipal landfill site.⁴⁴ Clinical signs observed in affected animals were disoriented running and breaking through fences, apparent blindness, weakness, and ataxia. Gross lesions included pulmonary congestion and edema and epicardial petechiae and ecchymotic hemorrhages on the ventricles. Histologic changes were acute to subacute myocardial degenera­tion and necrosis with neutrophilic and lymphohistiocytic myocarditis. Compound 1080 was detected in several kidney samples, but the source of 1080 was not determined. No long-term effects on health or reproductive performance were observed in ewes that survived experimental dosing with a single, near-lethal amount of 1080.⁴²

Compound 1080 has no known antidote, and the rapid onset of clinical signs may prevent effective, quick decontamina­tion of the GI tract with activated charcoal. A treatment protocol for 1080 toxicosis in dogs involves gastric lavage followed by activated charcoal, control of seizure and convulsions with barbiturates or gas anesthesia, fluid therapy including sodium bicarbonate, monitoring serum calcium and potassium, and use of acetamide if available.⁴⁵ The prognosis for most cases of 1080 toxicosis is poor.

Strychnine

Strychnine is a rodenticide often sold at 0.5% to 1% concentra­tions in a powder, pellets, or with grain bait for below-ground application. Strychnine-treated seeds are often dyed red or green. In several states, the use of strychnine is restricted. Storage of old containers of strychnine-laced grains can be sources of accidental toxicoses. Accidental and malicious toxi­coses continue to occur regularly.

Strychnine is an alkaloid from the seeds of Strychnos nux- vomica and Strychnos ignatii. Strychnine blocks inhibitory action of glycine at the inhibitory neurons, Renshaw cells, spinal cord, and medulla. The lack of inhibition leads to rigidity and tetanic convulsions. Strychnine is quickly absorbed from the GI tract and widely distributed in the body. A small portion of the dose is excreted into urine. Hepatic enzymes can readily metabolize strychnine.

Clinical signs can appear within a few minutes to a few hours and include sweating, nervousness, incoordination, recumbency, and tonic-clonic seizures. The seizures can be induced by stimuli such as bright light, loud noises, and touch. No specific clinical pathology or histologic changes occur in strychnine toxicoses. Analytic determination of strychnine in gastric contents (and to a lesser extent in urine, serum/plasma, liver, and kidney) confirms a strychnine toxicosis.

Decontamination by gastric lavage followed by oral admin­istration of activated charcoal, which binds strychnine, is important; however, the degree of sensory stimulation may require initial treatment to control seizures and support respira­tion. The use of barbiturates along with diazepam and metho­carbamol may control seizures. Severe respiratory dyspnea may require mechanical ventilation. In addition, intravenous fluid therapy and a quiet environment may be beneficial. Prognosis for treatment of large animals with strychnine toxicosis is poor because of the difficulty of treating animals in the field. In one case report, several horses had access to an old bag of strychnine-contaminated barley. Clinical signs of CNS stimulation were observed in five of six horses, with two horses dying. Treatment in the field consisted of activated charcoal and mineral oil to assist decontamination of the gut and xylazine, diazepam, and pentobarbital for CNS signs.⁴⁶