Antimicrobial Stewardship

The dearth of antimicrobial drug development with the advent of increasingly resistant bacteria necessitates that we use these drugs in a judicious manner that prolongs their effectiveness.

Antimicrobial stewardship is defined as “an activity that includes appropriate selection, dosing, route, and duration of antimicrobial therapy” so that a positive clinical outcome can be achieved with a minimum of unintentional consequences.^1,2 The American Veterinary Medical Association (AVMA) defines antimi­crobial stewardship for the veterinarian as follows: “Antimicrobial stewardship refers to the actions veterinarians take individually and as a profession to preserve the effectiveness and availability of antimicrobial drugs through conscientious oversight and responsible medical decision-making while safeguarding animal, public, and environmental health.”³ Although a number of other definitions exist, the central concept of antimicrobial stewardship is to use antimicrobial drugs in a rational and responsible manner to have a positive clinical outcome while minimizing the emergence of resistance and adverse reactions.^4-7 The details regarding implementation of an antimicrobial stewardship program will vary according to an individual practice or hospital needs; however, certain elements should be incorporated.

Many antimicrobial stewardship guidelines describe the use of the “3 Rs” for antimicrobial stewardship.

The AVMA has established core principles for antimi­crobial stewardship and judicious use of antimicrobials that incorporates many of the ideas found in the 5 Rs.^3,9 Judicious use principles can include attention to disease prevention strategies and biosecurity to prevent introduction or spread of pathogens, veterinary oversight of antimicrobial use, use of antimicrobials only after careful review including evi­dence provided by culture and susceptibility results, use of narrow-spectrum drugs when possible, and use of regimens of antimicrobial treatment based on evidence and scientific principles including pharmacokinetic data. Other veterinary associations and government agencies have advocated or pro­vided guidelines for antimicrobial stewardship or prudent use of antimicrobials, including the American Association of Bovine Practitioners, American Association of Equine Practitioners, American Association of Swine Veterinarians, and the Center for Veterinary Medicine (CVM) of the U.S. Food and Drug Administration (FDA).^3,9-12

Antimicrobial therapy is the cornerstone for treatment of infectious diseases.

■ BOX 45.1

Considerations for Antimicrobial Choice

Agent present

Agent factors

Antimicrobial susceptibility of the agent

Compliance with the regimen prescribed Cost

Extralabel use

Host factors

Penetration of antimicrobial drug to the site of infection Route of administration

Severity of the infection Withdrawal time

drugs and raise important considerations for the veterinarian regarding their use.^13,14

One recommendation is that veterinarians develop standard­ized antimicrobial drug use guidelines for different types of cases (e.g., wounds, respiratory infections) that are appropriate for their hospital or clinical practice.^13-15 Often this requires some knowledge of likely pathogens and their antimicrobial susceptibility.

Antimicrobial therapeutics may also be categorized into tiers for use: primary, secondary, and tertiary drugs and those that are either restricted or have been elected to not be used by the practice. Primary drugs are those that are reached for most commonly, often either before or while awaiting culture and susceptibility testing results. Often these are older antimicrobials that may have a narrower spectrum of activity for pathogens (e.g., penicillin, older tetracyclines, sulfonamides). Secondary drugs generally are reserved for more serious infections or for when susceptibility data indicate resistance to primary drugs. Frequently these second-line drugs are also important in human medicine, thus avoidance of resistance development is important. Examples of these drugs include fluoroquinolones and third-generation cephalosporins. Tertiary drugs are those that are often effective against resistant bacteria and are also important drugs used for human patients, such as carbapenems. These tertiary drugs should be reserved for serious infections with confirmed resistance to all primary and secondary drugs. Thus use of higher-level drugs should always be directed by antimicrobial susceptibility testing. Finally, an example of a drug that may be used only after approval by an institutional committee (i.e., restricted) or prohibited is vancomycin.

In addition to establishing tiers of drugs for antimicrobial use, it is important to follow a logical approach when deciding on the type of antimicrobial drug to use. The goal is to use an antimicrobial drug to which the organism is susceptible and that will reach adequate concentrations at the site of infection, has little to no host toxicity, and is economically suitable for the owner and/or producer.

Is There an Infection?

This is a frequently overlooked step when determining what type of antimicrobial therapy should be instituted. Too often, antibacterial drugs are used “just in case” there is an infec­tion, without proper examination for the signs of infection. A number of clinical signs and clinical laboratory findings can be evaluated that may point to the presence of an etiologic agent. The old saying that redness, swelling, and heat are cardinal signs of infection and inflammation still holds true. Certainly, one frequently encountered finding is elevation of body temperature. However, inflammation for reasons other than the presence of an infectious agent may result in fever. Swelling and redness are frequent indicators of a localized infection. Development and appearance of purulent material is further evidence of infection.

Clinicopathologic findings are also important supportive evidence for an infectious disease. Elevated or depressed periph­eral white blood cell counts frequently accompany significant infections. In large animal species, fibrinogen elevations are observed in bacterial infections. Serum biochemistry values may also inform the decision to use antimicrobial therapy. For example, elevations in globulin may indicate production of excess immunoglobulin as seen in subacute to chronic infectious, primarily bacterial, processes. Decreases in serum albumin may develop as a negative acute phase protein in response to inflammation accompanying an invasive pathogen.¹⁶ If possible, cytologic examination of material from the site of suspected infection should be performed.

What Organism Is Present?

Once the presence of an infection is determined, the next step is to decide what organism might be present. Ideally, samples should be collected prior to initiating therapy so that an etiologic diagnosis can be made and susceptibility testing can be performed. In the case of a serious or life-threatening infection, therapy must be initiated prior to receipt of culture and susceptibility results. Empirical therapy often relies on clinical judgment and knowledge regarding the likely organ­isms present, depending on the body system affected and its likely susceptibility patterns. Additional findings such as results from cytologic examination help to inform the antimicrobial choice.

Sample Selection and Transport to the Laboratory

Appropriate collection and transport of samples to the micro­biological laboratory are essential to detection of the causative agent. The site of the infection should be sampled. This may seem obvious; however, a surprisingly large number of inap­propriate samples are submitted to the microbiology laboratory. In general, sterile sites are the best regions to sample. For example, sampling of nasal discharge is almost never the appropriate choice when faced with purulent nasal discharge. Identification of the large number of normal bacteria and interpretation of the findings are difficult when normally colonized sites are sampled. The actual source of the discharge, such as the sinus, guttural pouch, or lung, should be determined and sampled directly. An exception to this rule is in the case of equine Streptococcus equi subsp. equi infections. Lesions such as a draining tract are problematic. Frequently they have superficial contamination of the opening of the tract that is inadvertently sampled. When possible, underlying soft tissue or bone should be collected rather than inserting a swab into the tract. Drains or urinary catheter tips are also inappropriate for samples. Too often they carry a biofilm of contaminating bacteria that may or may not be indicative of the disease process.

If anaerobic culture is deemed necessary, it is important that adequate volume of tissue or fluid be collected to preserve the anaerobic environment. If possible, at least a milliliter of fluid or a square centimeter of tissue should be collected. Swabs are inappropriate for anaerobic culture because air can be trapped between the fibers, leading to death of nonaerotolerant obligate anaerobes. If the sample is smaller and not taken immediately to the laboratory, it should be placed in some sort of anaerobic transport medium that will maintain an anaerobic environment. These anaerobic systems will maintain the presence of aerobic and facultative anaerobes as well.

Samples for microbiological culture should be transported to the laboratory as soon after collection as possible. The sample should be refrigerated if it cannot be delivered quickly, and it should be kept cool or cold but not frozen for transport to the laboratory. An exception to this rule is when anaerobic transport medium (e.g., Anaerobe Systems, Morgan Hill, Calif.) is used. These vials should be maintained at room temperature. It is possible that samples that are not kept adequately chilled can have overgrowth of contaminating bacteria, which can obscure detection of the “true” pathogen.

It is best if samples are collected prior to initiation of therapy. However, if antimicrobial treatment has already been instituted, the clinician should consider withdrawing therapy for a short period before sampling. Waiting between 24 and 36 hours after the last administration of drugs is advisable, and 99% of the drug should be eliminated after 7 half-lives. In seriously ill animals such as the septic foal, delaying antibiotic therapy for any amount of time to collect samples may be detrimental. In such cases, samples should be collected just prior to the next administration of drug. Some blood culture systems contain resins that bind or inactivate antimicrobial drugs and have proven to increase the ability to detect bacteria within the sample.¹⁷

Microscopic Examination of Sample

Initial examination of a sample provides important clues to the type of organism present. A Wright-Giemsa-type stain is frequently used to examine cytologic preparations made from samples. This staining technique can provide much information on the bacterial or fungal organism present. Identification of coccoid or rod-shaped bacteria along with the site of infection will often result in a differential diagnosis list of organisms to consider. An examiner with a practiced eye may be able to make an educated guess about the genus of the organism observed. A Gram stain provides additional information. Although many veterinarians do not use this stain in their clinical practice, the technique is rapid and, with a little practice, easy to perform and interpret. Identification of an organism as gram-negative or gram-positive further informs the prac­titioner of the likely organisms present and helps to guide empirical antimicrobial choice (Box 45.2).

What Is the Severity of the Infection?

One of the first decisions to be made when considering antimicrobial therapy is the severity of the infection. The clinician needs to decide if therapy must be initiated immediately or whether the patient can wait for results of culture and susceptibility testing before initiating therapy. Obviously, life-threatening infections require immediate and aggressive antimicrobial therapy. Infections of sites such as joints or other orthopedic structures may necessitate empirical therapy while awaiting the results of microbiological testing. In these cases, samples should be taken for identification and susceptibility testing before initiating therapy. In cases of serious infection, empirical therapy may include use of a bactericidal drug or a combination of antimicrobial agents (see section on Combina­tion Therapy). On the other hand, an animal with a chronic, mild, or moderate infection may wait to receive antimicrobial drugs until testing has been completed.

■ BOX 45.2

Gram-Stain Reaction and Morphology of Commonly Encountered Large Gram Stain of Animal Bacterial Pathogens

Gram-Positive Cocci

Enterococcus spp. Staphylococcus spp.

Streptococcus spp. Peptostreptococcus anaerobius (anaerobe)

Gram-Positive Rod or Filamentous

Actinomyces spp. Corynebacterium spp.

Listeria spp.

Trueperella pyogenes (formerly Arcanobacterium pyogenes) Clostridium spp. (obligate anaerobe)

Gram-Negative Rods

Actinobacillus spp. Brucella spp.

Bordetella spp. Escherichia spp.

Haemophilus spp. Histophilus spp.

Klebsiella spp. Mannheimia spp.

Pasteurella spp.

Pseudomonas spp. Bacteroides spp. (obligate anaerobe) Fusobacterium spp. (obligate anaerobe)

What Is the Likely or Confirmed Antimicrobial Susceptibility of the Organism?

In a perfect world, the clinician should have laboratory­generated antimicrobial susceptibility results to guide therapy. However, frequently therapy must be initiated prior to receipt of culture and susceptibility results. Empirical therapy should be based on knowledge of the inherent resistance of suspected agents and the antimicrobial susceptibility patterns observed in the practice region. Susceptibility data should be compiled regularly by the regional microbiology laboratory and used by the practitioner in the hospital setting.

The microbiology laboratory should be proficient in antimicrobial susceptibility testing and follow guidelines established by the Clinical and Laboratory Standards Institute (CLSI) or similar institution.¹⁸ Quality assessment testing should be performed regularly. A number of methodologies are used to examine susceptibility to different antimicrobials. Two frequently used methods for antimicrobial susceptibility testing are disk diffusion, also known as Kirby-Bauer test, and broth dilution or broth microdilution, which uses a 96-well plate.

Disk diffusion is a relatively straightforward testing meth­odology that can be performed with little specialized equipment. It involves creating a lawn of bacteria using a standardized concentration, placing disks containing a set amount of antimicrobial drug on top of the bacterial lawn, incubating, and measuring the zone of inhibition around the disk. This zone diameter determines whether the bacterium should be considered susceptible, intermediately susceptible, or resistant to a particular drug. The advantages of this method are that it is rapid and easy to perform provided strict protocols are followed. The disadvantages are that it cannot be used for slow-growing bacteria and it provides only an interpretation rather than information about the specific concentrations of drug that inhibit growth. Consequently, knowledge about the pharmacokinetics of a particular drug cannot be fully used to develop a treatment regimen to deliver optimal dose of the drug to the site of infection.

The broth dilution or microdilution method involves inoculation of a standard amount of bacteria into tubes or wells containing an antimicrobial drug. Most clinical laboratories use twofold dilutions of drug centered on the concentration of drug found systemically using typical dosing strategies. The inocula are subsequently incubated, and then the wells are examined for visible growth. The lowest concentration of drug that inhibits visible growth is called the minimum inhibitory concentration (MIC). Sometimes the wells are subcultured to a nonselective agar and then incubated, and this agar is examined for growth. The lowest concentration of drug that results in no growth is the minimum bactericidal concentration (MBC). For cost reasons, most clinical laboratories do not determine the MBC for each organism.

The MIC is unique for each bacterium-drug combination. The resulting MIC guides interpretation of the bacterium as susceptible, intermediately susceptible, or resistant to a particular drug. The concentration of drug at which the interpretation of testing changes from susceptible to intermediate or resistant is called the breakpoint. For example, Enterobacteriaceae isolated from adult horses are considered susceptible to amikacin if the MIC is 4 μg7mL or less; they are deemed resistant if the MIC is 16 μg∕mL or greater. If the MIC is 8 μg∕mL, the isolate is considered intermediately susceptible.¹⁸ Generally, the clinician needs to use the interpretation only when deciding which drug to choose for therapy. However, the MIC can be used to refine therapy, by altering the dose of drug or frequency, to optimize treatment.

Interpretation breakpoints for susceptibility testing of a bacterium as susceptible or resistant are set by the CLSI. This body bases interpretation on three major factors: the epidemiologic breakpoint of a population of bacteria, phar­macokinetic data, and (when available) clinical outcome from treatment of animal or human patients.¹⁹ A graph showing MICs of a population of a single species of bacterium frequently has a bimodal distribution. The trough between the two peaks is the epidemiologic breakpoint; those bacteria with higher MICs have acquired some sort of resistance determinant. The pharmacokinetic information used to assist with determination of the breakpoint includes the peak concentration of drug, the area under the pharmacokinetic curve, and time the drug is above a particular MIC. A simplified way to think about how these data are used is to look at a bacterium with a high MIC. It would be considered resistant because the amount of drug that would have to be administered to exceed the MIC would be considered toxic to the animal host. Generally, the highest concentration of drug tested is near or a bit above the highest concentration of drug that can be achieved with recommended dosages.

Antimicrobial therapy should be directed to achieve levels that are above the MIC for a particular isolate. Ideally, the “best” antimicrobial drug to use depends on many factors other than the susceptibility result. Obviously, the organism should be susceptible to the drug chosen, but other considerations should be included, such as route of administration, penetration to the site of infection, toxicity, cost, and convenience. All of these factors being equal, the optimal drug is one for which the organism’s MIC is several dilutions below the breakpoint. However, further refinement is possible when one understands the pharmacodynamic properties of the drug and bacterium.

Bactericidal and Bacteriostatic Antimicrobial Drugs

The terms bactericidal and bacteriostatic refer to the in vitro observation that a drug kills or merely inhibits growth of a particular bacterium, respectively. If one examines the relation­ship between the MIC and MBC, these two values are close or nearly identical for bactericidal drugs. In contrast, the MBC is several twofold concentrations above the MIC for bacteriostatic drugs. To be clear, it takes a higher drug concentration to kill bacteria with a static drug. The in vivo consequence of this relationship means that more drug must be delivered to the site of infection to kill the bacteria with a bacteriostatic drug. Often this is not achievable given the toxicity and pharmacokinetics of the drug. In the case of bacteriostatic drugs, the clinician must rely on the host immune responses to clear the inhibited organisms. Accordingly, bactericidal drugs should be used in an immunocompromised patient. Some drugs are concentrated in areas of infection. For example, drugs undergoing renal clearance are found in high concentrations in urine. Thus drugs that show in vitro bacteriostatic activity may achieve concentrations high enough to kill bacteria in the bladder. The converse is also true; use of a bactericidal drug at inadequate dosages or that has poor penetration to the site of infection may not achieve high enough concentrations to kill the offending organism. Thus some drugs can be considered either bactericidal or bacteriostatic. In practical terms, in an immunocompetent host without an overwhelming bacterial burden, the differing impact of a bacteriostatic versus bactericidal drug is minor provided that adequate concentration of the drug reaches the site of infection.

Concentration and Time-Dependent Antimicrobial Drugs

Pharmacokinetic (PK) and pharmacodynamic (PD) principles can also be used to guide antimicrobial therapy. Pharmacokinet­ics refers to drug absorption, distribution, metabolism, and elimination. Pharmacodynamics examines the effect of the drug concentration on the microorganism at the site of infection over time. The relationship between PK and PD principles describes the relationship between the drug concentration and the effect on the pathogen and clinical response.²⁰ Parameters that explore relationships between PK and PD principles, such as the peak concentration relative to the MIC, are reflected in the PK/PD index.²¹ The relationship between the MIC and PK parameters may determine the outcome of therapy.

There are two categories of antimicrobial drugs: concentration-dependent drugs and time-dependent drugs. This categorization is primarily based on in vitro work but is supported by clinical data in animals and humans.²² Concentration-dependent drugs are those whose antimicrobial activity is best predicted by the relationship between peak drug concentration and the MIC of the organism. Drug classes that are considered concentration-dependent drugs include ami­noglycosides and fluoroquinolones. The ratio between the peak drug concentration and the MIC should equal at least 8 to 10.²3 Using the example of amikacin given earlier, if the bacterium had an MIC of 2 μg∕mL, the drug should be dosed to reach a peak concentration of 20 μg∕mL. The time that the drug concentration is above the MIC appears to be less important for concentration-dependent drugs. Accordingly, many concentration-dependent drugs can be administered infrequently (e.g., once daily). This dosing strategy can be advantageous with certain drugs with toxicity. For example, nephrotoxicity of the aminoglycosides is best determined by trough concentrations of drug. These antimicrobials may be administered at high dosages once daily to achieve the high peak concentration, well above the MIC, while having prolonged low trough levels. Many concentration-dependent drugs have prolonged postantibiotic effect, meaning that the effect of the drug to inhibit growth of the organism is apparent for some time after the drug is removed.

Fluoroquinolone efficacy is best predicted by the area under the concentration curve (AUC) in relation to the bacterial

MIC. The AUC/MIC ratio should be 125 or greater for gram­negative organisms.^22-25 This ratio should be 60 or greater for gram-positive bacteria.²² Some studies indicate that ratios that approach 250 or greater be used to avoid development 22

of resistance.²²

Time-dependent drugs are those whose antimicrobial activity is best predicted by the time the drug concentrations are above the MIC. Many drug classes are considered time dependent, including the β-lactam drugs and tetracyclines. These drugs should remain above the MIC for at least 50% of the time. These antimicrobial drugs generally are dosed more frequently or via continuous rate infusion for those with short half-lives.

Knowledge of the PK and PD principles can guide the practitioner in appropriate dosages and frequencies for an individual drug. Monitoring of drug levels can further aid development of the therapeutic plan. Table 45.1 summarizes the PK and PD parameters and activities of some antimicrobial drugs.^26,27

Where Is the Infection?

Too often the question of where the infection is located is not considered when choosing an antimicrobial drug. Penetration of the drug to the site of infection is essential for successful therapy. Remember that the MIC and interpretation of the MIC are focused on systemic (plasma) levels of drugs and rarely include information on drug penetration to various anatomic locations; CLSI is increasingly providing site-specific interpretations of MICs.¹⁸ Some anatomic locations are par­ticularly difficult to enter, such as the central nervous system, ocular fluid, and bone. Abscesses are another structure that is difficult for antimicrobial drugs to penetrate; they are sur­rounded by a thick capsule with poor blood supply and lack of oxygen, which can limit penetration into the abscess and offer poor entry into the bacteria. The pH is generally low, which may inactivate some agents, and there is abundant protein present to bind antimicrobial drugs. A further complication is the presence of purulent debris that can provide metabolic precursors for bacteria to bypass inhibitors of metabolism, such as trimethoprim-sulfonamide combination drugs. In general, lipophilic drugs will be better able to penetrate these difficult-to-reach anatomic locations. Highly polar or charged molecules have difficulty reaching these sites.

If the causative organism is found intracellularly, as seen with rhodococcal pneumonia or Corynebacterium pseudotuberculosis infections, the host cell membranes are additional impediments to drug penetration. Fortunately, some drugs penetrate cells well, such as macrolides, rifampin, chloramphenicol, fluoro­quinolones, and trimethoprim-sulfonamide combinations.

Empirical Versus Definitive Therapy

Empirical therapy is used for severe infections prior to obtaining culture and susceptibility results. The choice of antimicrobial drug depends on the clinician’s knowledge of the likely agents present at the site of infection and their predicted susceptibili­ties. Knowledge of the Gram stain results can further guide therapy. Generally, empirical therapy is broad spectrum to cover multiple possible etiologies. Eventually, once the etiologic agent and its susceptibility have been identified, definitive therapy, using a drug or drugs with narrower spectrums, can be initiated. Definitive therapy should be directed to best result in a cure, using the narrowest spectrum of drug therapy, and have limited toxicity for the patient. The goal is to de-escalate the spectrum of activity of the drug as much as possible with definitive therapy in order to minimize the impact on commensal bacteria and development of resistance.

Combination Therapy

Many infections can be treated effectively with a single antimicrobial drug; however, in some instances, combination therapy may be chosen. Some drug combinations have been demonstrated to be synergistic based on both in vivo and in vitro studies. Synergy occurs when the ability of the combina­tion to kill bacterial growth is greater than the mere additive effect of each drug. Drug combinations can also be indifferent, where each drug’s effects are merely additive. Finally, a drug combination may be antagonistic, where the drug combina­tion’s effectiveness is less than the sum of the independent effects. Synergistic antimicrobial drug combinations are best used to treat resistant organisms. A common synergistic pair of antimicrobial drugs is the combination of trimethoprim and a sulfonamide. Either agent alone would be considered bacteriostatic, but since each of these drugs inhibits sequential steps in folic acid metabolism, they are considered bactericidal.

■ TABLE 45.1

Antimicrobial Drugs, Classifications, Activities, Postantibiotic Effect, and Pharmacodynamic Index

^aYes, Prolonged effect longer than 6 hours; no, less than 1 hour.

^bAUC24∕MIC, Area under the concentration curve over 24 hours/minimum inhibitory concentration; Cmax/MIC, maximum concentration/minimum inhibitory concentration; T >MIC, time drug concentration is greater than the MIC.Adapted from Martinez M, Toutain PL, Walker RD: The pharmacokinetic-pharmacodynamic (PK/PD) relationship of antimicrobial agents. In Giguere S, Prescott JF, Baggot JD, et al., editors: Antimicrobial Therapy in Veterinary Medicine, ed 4, Ames, IA, 2006, Blackwell Publishing, pp 81-106; and Martinez M, Toutain PL, Turnidge J: The pharmacodynamics of antimicrobial agents. In Giguere S, Prescott JF, Dowling PM, editors: Antimicrobial Therapy in Veterinary Medicine, ed 5, Ames, IA, 2013, John Wiley and Sons Inc., pp 79-103.

A combination of antimicrobial drugs might be chosen for empirical therapy before the agents and their susceptibilities are known in order to provide broad-spectrum therapy. The combination of a β-lactam drug with an aminoglycoside is frequently chosen as empirical therapy in serious infections in equine patients. This combination is synergistic as well as broad spectrum. It is important to remember that in these cases, therapy should be reduced to a single agent or a less broad spectrum of activity as soon as testing results become available.

The presence of a polymicrobial infection is another reason why combination therapy may be used when a single agent will not be able to treat all bacteria present. For example, intraperitoneal infections are frequently caused by more than one agent. It is difficult to find a single antimicrobial drug to treat all possible bacteria present, such as Enterobacteriaceae, gram-positive bacteria, and anaerobes. Individual drugs that are sufficiently broad spectrum are available for use in humans; however, many of these would be cost prohibitive in large animal patients. The clinician should be cautious, however, of isolation of multiple agents from a single site such as a wound. Superficial samples taken from these sites may have contaminat­ing bacteria, often with highly resistant antimicrobial patterns, that are not truly part of the infectious process.

Combined antimicrobial drug therapy can be used to prevent emergence of resistant bacteria. Certainly, for serious or difficult-to-treat infections such as those caused by Pseudo­monas spp., avoiding development of resistance is important not only to the patient receiving treatment but also to others who come in contact with the bacteria. The best illustration of this principle is in the treatment of human patients for Mycobacterium tuberculosis, in which combination therapy is always used to prevent development of resistance.²⁸ Resistance to certain antimicrobial drugs, such as rifampin, can emerge quickly when they are used alone; thus these drugs are best used in combination.

There are some drawbacks to using combination therapy. One is the possibility of antagonism between antimicrobial drugs. Most instances of antagonism have been demonstrated in vitro, although in vivo reports have appeared.^28,29 Interestingly, combination therapy of penicillin with chlortetracycline was found to be inferior to penicillin alone in survival rates in human pneumococcal meningitis.²⁹ Antagonism was also seen with the combination of chloramphenicol with gentamicin used to treat immunocompromised mice with a Proteus mirabilis infection and in a rabbit model. Interestingly, this antagonism was not evident in immunocompetent mice, suggesting that the effect of antagonism was evident only with a diminished immune response.^28,30 Use of antimicrobial drug combinations also represents an increased cost to the client. Finally, there is an increased possibility of adverse effects of and/or hyper­sensitivity reactions to the drugs when used together. Antimi­crobial drug combinations that may be detrimental include penicillin with tetracyclines and chloramphenicol with erythromycin. Combination therapy should not be chosen merely to provide comfort to the clinician, but rather clinicians should have specific reasons for their choice.

Drug Incompatibility

Drug incompatibility or antagonism can be the result of direct incompatibility, meaning that the drugs should not be mixed together for administration or that they should not be used together therapeutically. For example, certain penicillin or cephalosporin drugs should not be mixed with an aminogly­coside in the same vial because of inactivation of the amino­glycoside and formation of precipitates. Examples of therapeutic antagonism in vivo are harder to find. The few that have been well documented have been described in humans or animal models of disease rather than in spontaneously occurring veterinary patients. Still, these antagonistic effects are worth noting, as they may someday be demonstrated to be important in veterinary patients.

Host Factors That Influence Antimicrobial Choice or Dosage

Age

The age of the animal may influence many aspects of antimi­crobial therapy. Bioavailability of certain drugs varies with age where some drugs can be used orally in neonatal or young animals but are poorly absorbed in mature animals.³¹ The drug distribution may also vary with age because young animals generally have a larger volume of distribution than adults due to their larger percentage of total body water.³² Drug biotrans­formation in neonates may vary from that in adults as well.³² Therefore drug dosages and/or frequencies may have to be adjusted in young animals to provide adequate levels of drug at the site of infection and to minimize toxicity.

Fluoroquinolone use in young and adult animals should be considered carefully. This class of antimicrobial drug has been documented to cause cartilage damage in several species of young animals, and ciprofloxacin has been associated with tendinopathies in humans.^33-35 Of interest is the observation that fluoroquinolones are capable of having chondrotoxic effects on equine and canine chondrocytes in vitro,^36,37 whereas a lamb model failed to demonstrate chondrotoxicity.³⁸ Unfortunately, specific guidelines for the safe use of fluoroquinolones in young animals are lacking. If fluoroquinolone use is necessary in young animals, it is important to limit the duration of therapy in order to minimize the chance of damage to developing cartilage. Along the same line, tetracyclines should also be avoided in young patients to avoid discoloration of enamel of developing permanent teeth.

Reproductive Status

Antimicrobial drugs may have deleterious effects on reproduc­tion and the developing fetus. Depending on the drug, type of placentation, and stage of gestation, some drugs are able to reach the fetus. For example, use of potentiated sulfonamides in pregnant mares has been associated with anemia, fever, and abortion.³⁹ Administration of pyrimethamine and trimethoprim­sulfamethoxazole to stallions has been associated with weakness and abnormal copulatory form and function.⁴⁰

The lactating large animal must also be considered. Many antimicrobial drugs can be found in milk and may be transferred to the lactating animal or human who consumes the milk. Some drugs reach the milk in high concentrations, whereas others are found in low amounts relative to plasma. Regardless, almost all antimicrobial drugs may be found in milk. Such adulteration may make the milk unsuitable for human consump­tion, and appropriate withdrawal times should be adhered to before milk enters the human food chain.

Treatment of bacterial mastitis presents several challenges and is covered in more detail elsewhere in this book. Treatment should be initiated early and be based on susceptibility of the causative agent. If individual milk culture and susceptibility testing are not feasible, treatment should be based on historical information from the affected herd. Penetration into milk is also an important consideration when using systemic drugs to treat mastitis. Many drugs, such as the β-lactams and sulfon­amides, are found in much lower concentrations in milk. Others, such as the macrolides, can be concentrated into milk. Because of the low concentrations found in milk for many of the drugs used systemically in food production animals, intramammary administration is preferred, although systemic administration can also be used and is probably indicated in the systemically ill animal or when intramammary abscesses preclude delivery of the drug via the intramammary route. As for systemic administration, withdrawal times should be closely followed with intramammary infusions.

Renal and Hepatic Function

As for all drugs, metabolism of antimicrobial drugs takes place in the liver and/or kidney, and excretion follows these routes. Therefore compromised renal or hepatic function should be considered when choosing an antimicrobial drug. Aminoglycosides are well known for their nephrotoxicity and should be avoided in cases with elevations in serum blood urea nitrogen or creatinine. If their use is still warranted despite renal dysfunction, the frequency of administration or, in some cases, the dose should be altered. Therapeutic monitoring of aminoglycoside levels should be performed to guide therapy. Macrolides and rifampin are metabolized and excreted via the hepatic route, and liver enzymes should be monitored for elevation during use. Knowledge of the route of excretion of active drug can be used to concentrate drugs at the site of infection. In the case of bacterial pyelonephritis or cystitis, renal excretion of many antimicrobial drugs can be used to the clinician’s advantage during treatment.

Immune Status

The effectiveness of the immune response is important for successful antimicrobial therapy. Many drugs only reach bacteriostatic levels at the site of infection, therefore only inhibiting growth of the offending agent. In these cases, the immune response is essential for clearing the infection. Bac­tericidal concentrations of drugs at the site of infection should be used in cases of immunocompromise. Immunodeficiency, whether primary or secondary, may be an indication for use of combination therapy.

Duration of Therapy

Unfortunately, there are few guidelines in veterinary medicine that direct duration of therapy. Certainly, serious or deep-seated infections such as endocarditis, pyelonephritis, and osteomyelitis warrant prolonged antimicrobial administration. However, a meta-analysis of human infection outcomes in critically ill patients demonstrated that shorter duration of therapy for infections such as urinary tract infections, ventilator-associated pneumonia, or community-acquired pneumonia had no impact on mortality.⁴¹ In addition, prolonged antimicrobial use can be associated with adverse effects and emergence of resistant secondary infections. In veterinary medicine we must frequently rely on response to therapy. Resolution of pyrexia, decreases in white blood cell count or fibrinogen, or resolution of abnormal radiographic and ultrasonographic findings are important parameters to monitor when deciding to cease therapy. In some cases, sampling and culture of the previously infected site may be warranted to demonstrate microbiological cure. For simple infections, treatment for a few days past resolution of clinical abnormalities and clinicopathologic findings may be adequate. For complicated infectious processes or chronic infections, treatment may be needed for weeks or longer after apparent cure.

Route of Administration

The route of administration varies depending on the patient species, age, severity of the infection, bioavailability of orally administered drugs, and site of the infection. Intravenous delivery of antimicrobials is indicated for serious life-threatening infections in which high levels of drug at the site of infection are indicated. Many drugs with good oral bioavailability in small animals, such as β-lactams, cannot be administered orally in large animal patients, necessitating parenteral administration. Oral administration of antimicrobials should be used cautiously in large animal species to minimize disruption of ruminal or colonic microflora in ruminants and horses, respectively. The route of administration may also be influenced by client or patient compliance and the drug label. Intramuscular administration may be difficult for some clients and patients, resulting in inadequate duration of therapy and possible development of resistance by microorganisms. Guidelines for therapeutic use of antimicrobial drugs in feed are provided later in this chapter.

Monitoring of Therapy

The response to treatment should always be monitored throughout antimicrobial drug administration. It helps to guide decision making regarding duration of therapy and effectiveness of the chosen antimicrobial regimen. As mentioned, clinical and clinicopathologic findings are useful guides to judge therapeutic success. Failure to respond to therapy could indicate that the organisms present in the infection are resistant to the chosen drug, particularly if the drug was chosen empirically. It may also indicate development of resistance during therapy or that there is poor penetration of drug to the site of the infection. Monitoring drug levels may be indicated to determine whether dosage and/or frequency is adequate in cases of treat­ment failure. A positive response to therapy may be an indication that antimicrobial treatment can be de-escalated in the case of broad-spectrum administration or that the patient can be switched from parenteral to oral administration.

Prophylactic and Metaphylactic Use of Antimicrobial Drugs

Antimicrobial drugs for prophylactic or metaphylactic use should be used only for cases or situations that are deemed high risk for infection and in which clinical data have demonstrated that this use is effective for preventing infection or decreasing morbidity or mortality.¹³ Prophylactic use of antimicrobial drugs is therapy directed toward preventing infection in which the risks or consequences of development of an infection are great and outweigh the risks of adverse outcome from the drug used. Its most frequent application is in the surgical setting to prevent surgical site infections. The decision to use prophylactic antimicrobials depends on the type of surgery and the possibil­ity of entering nonsterile sites such as the gastrointestinal or genitourinary tract, the length of the surgical procedure, the use of surgical implants, and the degree of tissue damage. It is important that the antimicrobial drug be present at therapeutic levels at the time of contamination to be effective in preventing surgical site infections.⁴² The therapy should be directed toward the pathogens likely to cause the infection. In the case of surgical site infections of large animals, these include Enterobacteriaceae, Staphylococcus, and Streptococcus.⁴"⁴ Generally, the drug should be administered 30 to 60 minutes before the surgery begins.^46,47 If the surgery is prolonged, repeated administration of the antimicrobial drug should be performed. However, the timing of surgical antimicrobial prophylaxis should be tailored to the drug used to ensure adequate concentrations at the surgical site at the time when contamination might occur.⁴⁸ Perioperative administration is what is important to prevent surgical site infections, and continued use after completion of surgery is unnecessary as determined in human patients.^48,49 Therapy beyond this point will not appreciably reduce the incidence of surgical site infections and may predispose to development of resistance or superinfections and increase the likelihood of adverse reactions.⁴²

Prophylactic antimicrobials should be drugs other than those used therapeutically so that treatment options remain available should an infection develop. It has been recommended that older drugs be used so that newer drugs may be used therapeuti­cally.⁵⁰ Prophylaxis should never be used in the place of good aseptic technique and is not indicated for use to prevent infection of intravenous catheters or other similar indwelling devices. Antimicrobial use in these instances only increases the likelihood that if infection develops, it will more likely be resistant.

Metaphylaxis is use of antimicrobial drugs after exposure to an infectious agent but before development of clinical signs. Examination of parenteral treatment of calves at high risk for infection in the bovine respiratory disease complex prior to or upon entry to the feedlot has shown decreased morbidity and mortality.^51,52 However, more recent studies have demon­strated either no benefit or mixed results.^53,54 Furthermore, multidrug-resistant Mannheimia haemolytica has been isolated from calves receiving prophylactic or metaphylactic antimicrobi- als.⁵⁵ It is important to note that metaphylactic use of antimi­crobial drugs will not eliminate all infections and may have a greater impact on the severity or duration of the illness. Metaphylactic administration may not always be practical for large groups of animals, and labeling may preclude antimicrobial use in this manner.

Biosecurity and Infection Control

A cornerstone of effective antimicrobial stewardship is to prevent infections in the first place through biosecurity and infection control measures on the farm and in the veterinary hospital or clinic. Segregating animals coming onto a farm, eliminating predisposing factors for bacterial disease, and vaccinating for viral diseases can limit infections and the need for antimicrobial drugs. The single most important step to control spread of disease in the clinic or hospital or from farm to farm is frequent hand washing and attention to personnel hygiene. Isolation or segregation of animals with contagious or highly resistant organisms and control of movement of animals can go a long way toward decreasing infection rates. Infection control and hygiene plans are important for any premises where animals are managed, not just for veterinary hospitals or clinics.^13,14

Microbial Factors Affecting Therapy

Resistance

A number of microbial factors may influence the success of therapy. Resistance of the agent to the chosen therapeutic is a common occurrence in therapeutic failure. Resistance can result from the inherent properties of the microbe. For example, mycoplasmas lack a cell wall and are impervious to the effects of cell wall-targeting drugs like the β-lactams. Other drugs may not be able to penetrate the bacteria, as is seen in the inability of anaerobes to take up aminoglycosides in the anaerobic environment. A major contribution to resistance is acquisition of resistance determinants. Resistance may result from alteration of the antimicrobial target, gain of genes capable of degrading the drug (e.g., β-lactamases), or acquisition of genes encoding efflux pumps (e.g., tetracyclines, aminoglyco­sides, fluoroquinolones) that pump antimicrobials out of the bacterial cell. Bacteria can develop resistance through mutation of the gene encoding the protein target of the drug (e.g., fluo­roquinolones and DNA gyrase). These events are not frequent, occurring once every 10¹⁰ to 10¹⁴ cell divisions. However, the rapid multiplication rate of a large number of bacteria makes this a real possibility. These events are often random, and their appearance is generally not influenced by the presence of a particular antimicrobial drug. However, if a drug is present, bacteria with the mutation will have a selective advantage and will rapidly outcompete nonresistant bacteria.

Alternatively, bacteria can acquire genes encoding a resistance determinant. Bacteria can obtain resistance genes through several mechanisms. Movement of plasmids, a type of extrachromosomal DNA, through conjugation between bacteria of the same or different species is a major means by which resistance develops. Other mobile DNA elements can move resistance genes between bacteria. Resistance genes are frequently associated with transposons, DNA capable of excision from host genomic DNA to plasmids and subsequent transport to other bacteria. Bacteriophages (bacterial viruses) may also move resistance genes between bacteria through a process called transduction. Finally, resistance genes can be taken up as naked DNA in the bacterial environment through a process known as transformation.

The simple act of using antimicrobial drugs will result in selection of bacteria that are resistant. Survey of gastrointestinal microbes, such as Escherichia coli, after initiation of antibiotic therapy has shown that resistance will develop in normal flora.^56-59 The most effective way to avoid resistance is to minimize the use and duration of antimicrobial drugs and, when they are used, to be sure that they are delivered to the site of infection at adequate levels to kill the bacteria.

Biofilms

Bacteria can move from existing as individual cells, also known as planktonic growth, to develop into a population that functions more as a multicellular organism known as a biofilm. The definition of a biofilm is “aggregated microbial cells surrounded by a polymeric self-produced matrix, which may contain host components.”⁶⁰ Biofilms are notoriously difficult to treat with simple antimicrobial therapy and are known to form on a variety of surfaces, including catheters, orthopedic implants, and bone. Biofilm bacteria are different from planktonic organisms in several ways. First, they are less susceptible to antimicrobial agents. Second, they persist in the animal despite a vigorous immune response. Unfortunately, effective treatment of the biofilm-associated infection necessitates removal of the infected implant or bone.

Adverse Reactions

Adverse reactions to antimicrobial drugs certainly occur, although hundreds or thousands of doses may have to be given before they are observed. A report from Switzerland indicated that 20% of adverse reactions to drugs were to antimicrobial agents.⁶¹ Adverse events associated with antimicrobial use can range from a hypersensitivity reaction to direct toxicity to disruption of normal microbiota. Fortunately, antimicrobial agents are directed toward the prokaryotic structures of bacteria that generally differ to some degree from the eukaryotic equivalent of the host. Consequently, they are less likely to result in direct toxicity in the patient. Some drug toxicities are predictable based on their mechanism of action or accumulation in certain cells of the host. Hypersensitivity reactions are the result of immunologic stimulation by the drug or carriers in the drug formulation. One frequently observed adverse event associated with antimicrobial use is the development of severe gastrointestinal disturbances. These may be the result of changes in the gut microbiota and/ or proliferation of pathogenic bacteria such as Clostridioides difficile or Salmonella. Adverse reactions to antimicrobial drugs should be reported to the manufacturer and to the Food and Drug Administration Center for Veterinary Medicine.

Adjunctive Therapy

In some cases, antimicrobial drugs alone are unable to clear the infection. In these cases, adjunctive therapy should not be overlooked. Surgical debridement of necrotic material and reestablishment of an adequate blood supply help to remove bacteria and allow better antimicrobial penetration. Sometimes simple drainage of an abscess is all that is necessary to treat certain infections, obviating the use of antimicrobial drugs.

Topical treatment may be possible in some instances. Superficial skin infections may be treated with either topical antimicrobial drugs or antiseptics. Deep or severe infections may need both systemic and topical treatment. Most ocular infections are successfully treated topically where high drug concentrations can be delivered.

Local treatment of severe internal infections can also be used as an adjunct to systemic therapy. Intraarticular administration of antimicrobials and/or regional perfusion are frequently used to treat joint and bone infections in large animals. These routes deliver high levels directly to the site of the infection. Although efficacious, adjunctive therapy, including debridement and lavage, are important to remove the bacterial burden and inflammatory substances that may be detrimental to joint structures. In these instances, systemic administration is indicated.