Pyrrolizidine Alkaloid Toxicity

Geoffrey W. Smith

■ Etiology Pyrrolizidine alkaloid (PA) toxicity is a chronic, progressive, often delayed toxicity that results when animals consume plants containing PAs.

PA toxicity is seen wherever plants containing the alkaloids are found. The plants are not very palatable and in most cases are not readily eaten, but animals may eat them when the growth of the toxic plant is so thick the animal cannot separate it from normal forage or when other forage is sparse. The plants are toxic in hay, including pelleted and cubed hay. PAs also survive ensilage. For example, oat silage contaminated with Amsinckia or other PA-containing plants has been respon­sible for disease in dairy cattle. Seeds of Heliotropium plants have also poisoned feedlot cattle.⁴

The approximate toxic dose of dried Senecio as a percentage of body weight for each species is horses, 5%; cattle, 2% to 5%; goats, 125% to 400%; and sheep, over 150%. Most tansy

■ TABLE 33.4

Common Plants Containing Pyrrolizidine Alkaloids (PAs)

ragwort contains less than 0.2% PA by weight, but there are reports of much higher levels in some of the other plants. Horses were consistently poisoned by greater than 250 mg of total PA per kg body weight.⁵ The PA in houndstongue (Cynoglossum officinale) is extremely toxic to horses.⁶ The dose does not have to be eaten all at once, since the effects are cumulative. Because signs often are delayed, some animals may not become ill until a year or more after removal from feed sources containing the toxins.^7,8 Signs do not occur until hepatocyte loss and replacement by fibrous tissue have caused failure of liver function.

■ Pathophysiology There are a number of PAs, and many of the poisonous plants contain four to six different alkaloids. About 50% of the alkaloids are toxic, and some are more toxic than others. The alkaloid concentrations vary slightly among plants from different areas, but there is greater variation in concentrations from different parts of the same plant.⁹ PAs are not directly toxic but must be bioactivated to toxic alkaloids (pyrroles) in the liver. After absorption, the portal circulation carries the alkaloids to the liver, where they are metabolized by microsomal enzymes of the hepatocyte to more toxic pyr­roles.^3,10 The pyrroles may crosslink double-strand DNA in a dose-dependent manner.¹¹ The degree of DNA crosslinkage depends on the concentration of the pyrrole but not on the base sequence of the oligonucleotide target.¹² The crosslinking of DNA produces an antimitotic effect.¹³ The hepatocytes cannot divide and often become megalocytes as cytoplasm expands without nuclear division. As cells die, they are replaced by connective tissue rather than new hepatocytes. This anti­mitotic effect may explain why megalocytosis (large hepatocytes and large nuclei) is commonly seen with PA poisoning.

With progressive death of hepatocytes and subsequent fibrosis, liver function begins to fail. Blood supply through the hepatic lobule is disrupted by fibrosis, making regeneration impossible. This becomes a veno-occlusive disease, resulting in marked portal hypertension. The increased portal hydrostatic pressure leads to diarrhea and ascites in ruminants. It is unclear why diarrhea and ascites are seen infrequently in horses with PA toxicity.

■ Clinical Signs The clinical signs of PA poisoning are basically those of liver failure. The most common signs of PA toxicity in the horse are weight loss, slight to moderate icterus, and abnormal behavior (e.g., wandering, ataxia). Signs seen less frequently in horses include photosensitization of the white areas and, in rare cases, diarrhea.

Sheep and goats are more resistant to PA toxicosis but can be affected by certain alkaloids at doses 30 times or more than the dose that affects cattle and horses. A consortium of ruminal microbes from sheep degrades PAs found in Senecio jacobaea to less toxic metabolites.¹⁷ Liver microsomal enzymes in sheep may also play a role in detoxifying PAs, but this alone does not seem to account for the differences in susceptibility.¹⁸

■ Diagnosis Hepatic enzyme activities are generally elevated during periods of active hepatocyte destruction caused by PA poisoning. Although the dehydrogenases (e.g., SDH, GDH, LDH) are elevated initially, they may have returned to normal by the time the animal first shows clinical signs of functional failure. Because lesions are largely in the portal region, GGT and ALP tend to be consistently elevated.^7,19 In a study of sublethally poisoned horses, AST and the ratio of branched- chain to aromatic amino acids were persistently elevated.¹⁵ Bile acid concentrations are increased, and levels above 50 μmol∕L would be a poor prognostic indicator in the horse.⁵ Serum protein concentration is usually normal, and only terminally does albumin decrease or blood clotting become altered.

Although liver biopsy can be very useful in the diagnosis of PA toxicity, other causes of chronic hepatitis (e.g., aflatoxins) may produce a similar histologic appearance. The triad of fibrosis, bile duct proliferation, and megalocytosis is charac­teristic of PA poisoning. Modest changes in hepatocytes and biliary hyperplasia are reversible. Fibrosis bridging portal areas indicates an eventually fatal condition, as does the extensive fibrosis of an end-stage liver.

Cattle dying of PA poisoning have ascites and prolapsed rectums and are thin and emaciated. Horses are usually thin and may be icteric. In all species, the liver tends to be small, pale brown to yellow, and firm and may appear to be scarred. Hepatic megalocytosis is considered the hallmark of PA poisoning, but it has also been seen in aflatoxicosis and is not always apparent in the earliest stages of PA toxicity. Biliary hyperplasia may occur early in the disease. Nonspecific nuclear changes, such as invaginations of cytoplasm into the nucleus, have been seen.^7,9 Isolated hepatocyte necrosis is seen later, and finally portal or massive generalized fibrosis develops. Once bridging of connective tissue between the portal areas occurs, the disease is fatal.

■ Treatment Liver failure may occur with either acute or chronic liver disease. The two must be differentiated for prognosis because end-stage fibrosis caused by PAs has virtu­ally no chance for regeneration and recovery. No satisfactory treatment for PA poisoning exists. Once obvious clinical signs of liver failure develop, the animal usually dies within 5 to 10 days. Horses with mild clinical signs and reversible histologic lesions survived if they retained an appetite and were not exposed to any more PA-contaminated feed.¹⁵ An accurate prognosis concerning mildly affected cases may best be obtained from a combination of consecutive liver biopsies and liver-derived serum enzyme activity, as well as from SBA concentrations.^5,15 Preventing further exposure to the toxic plants is indicated and may delay or stop the progression of the liver lesions, particularly if an uncontaminated feed source is provided before clinical signs develop.

Therapy of liver failure is inappropriate if severe fibrosis has occurred because regeneration is impossible. If the animal still has a reasonable appetite and only modest degrees of fibrosis histologically, treatment may be attempted by providing a low-protein, high-energy diet.

■ Prevention and Control PA poisoning is prevented by keeping susceptible animals from pasture, hay, cubed hay, or seeds that contain PA plants. Because most poisonings are attributed to contamination of forages or feed, careful inspection of feed is recommended. Senecio vulgaris tends to contaminate mainly first-cutting alfalfa, whereas Amsinckia intermedia is often found in planted fields of oat hay. Senecio can be controlled by cultivation because it is a biennial or by herbicide spraying in the early rosette stage. Species-specific herbicide regimens have been developed for most plants containing alkaloids and are widely available through state weed and extension services. Sheep are sometimes used to graze Senecio-infested pastures to control the weed, because they are less susceptible to PA poisoning.

Other Hepatotoxins

The liver is particularly vulnerable to toxic insults because it is the first organ to receive toxins absorbed from the GI tract. Enzymes in the hepatocyte may either activate a toxin or in some cases metabolize it before it can cause damage. Most hepatotoxic agents are described in more detail in Chapter 54. Some of the more common liver toxins are listed in Tables 33.5, 33.6, and 33.7, but these are not complete lists because many other plants and chemicals can damage the liver under the right conditions. Once a toxic liver disease has been diagnosed, possible sources of the toxin might be identified from these tables. A more detailed description of toxins affecting the liver can be found in Chapter 54.