Antimicrobial therapy for gastrointestinal diseases

Vet Clin Equine 19 (2003) 645–663

Antimicrobial therapy for gastrointestinal diseases Mark G. Papich, DVM, MS Department of Clinical Sciences, College of Veterinary Medicine, North Carolina State University, 4700 Hillsborough Street, Raleigh, NC 27606, USA

Problems caused by antibiotics in horses Antibiotics can cause adverse gastrointestinal effects in horses because of their activity against anaerobic bacteria or low oral absorption (a high concentration of active drug is thus retained in the intestinal lumen). Low oral absorption is not always a requirement for drug-induced diarrhea in horses. Drugs that are absorbed well orally or administered intravenously (IV) also have been associated with diarrhea because they are excreted in the bile or undergo enterohepatic recycling, whereby the intestinal lumen is exposed to high concentrations. When antibiotics decrease the anaerobes in the intestine, carbohydrate metabolism is altered, which can lead to diarrhea. A change in the population of anaerobic bacteria also favors an overgrowth of some bacteria known to be pathogenic, such as clostridia. Antibiotic therapy can select for drug-resistant strains of Clostridium difficile, and the reduced competition from other anaerobes allows the bacteria to thrive and produce toxins. This can result in severe colitis. Antibiotic-associated diarrhea in horses may carry a worse prognosis and less likelihood of survival than diarrhea from other causes. In the study by Cohen and Woods [1], horses with antibiotic-associated diarrhea, as compared with diarrhea from other causes, were 4.5 times less likely to survive. This was a limited study with only 12 cases, however. In a more recent report [2], 28 foals with C difficile and diarrhea were identified. Eighty-two percent of these foals survived treatment.

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Adverse effects caused by antibiotics Tetracyclines Tetracyclines were among the first drugs to be associated with adverse gastrointestinal effects in horses. The combination of stress and tetracycline therapy produced diarrhea, which was originally linked to the syndrome called colitis ‘‘X’’ [3,4]. Both IV and oral forms of tetracycline were implicated. Investigators proposed over 40 years ago that antibioticassociated colitis X was the result of salmonella infection [5]. The evidence now suggests that this syndrome is caused by a clostridial overgrowth and liberation of clostridial toxins [6]. The proposed mechanism is that oxytetracycline suppresses the numbers of competing organisms, allowing Clostridium spp to proliferate and release toxins. In an experimental study in horses [7], intestinal effects of oxytetracycline (10 mg/kg/d administered orally) were examined. After the first day, fecal consistency had changed, but it returned to normal by the fifth day of dosing. Oxytetracycline was associated with increased numbers of Clostridium perfringens in the feces during dosing but disappeared by the fourth day after the last dose. In an outbreak associated with tetracycline-contaminated feed, colitis in horses was also associated with an overgrowth of Clostridium spp [8]. Trimethoprim-sulfonamides Trimethoprim-sulfonamides have been associated with diarrhea in horses when administered orally, but this has not been a problem cited as commonly as for other drugs. In the early 1980s, the trimethoprim-sulfonamide paste first became available for horses, and there were many questions raised about the safety of this combination on the equine intestinal tract. In one of those studies using a dose of 30 mg/kg/d orally, except for an initial decrease in coliform counts from the feces, trimethoprim-sulfonamides did not alter the intestinal bacterial flora in horses and feces remained normal [7]. In the clinical study by Wilson and colleagues [9], they found in a two-part study that trimethoprim-sulfonamides did not statistically contribute to the occurrence of diarrhea in horses. In the first part of the study, 23% of the horses that received a trimethoprim-sulfonamide developed diarrhea compared with 20% that developed diarrhea and did not receive a trimethoprim-sulfonamide (not significant). In the second part of their study, which was the stronger cohort study, there was no difference in the horses that developed diarrhea and received trimethoprim-sulfonamides (3%) compared with those that did not receive trimethoprim-sulfonamides (3%). In both parts of the study, neither dose, frequency, nor duration of trimethoprim-sulfonamide administration was associated with the development of diarrhea. In horses that developed diarrhea, their feces quickly returned to normal when the trimethoprim-sulfonamide was discontinued.

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There were no other drug therapies (including other antibiotics) that were significant factors in the development of diarrhea. Horses may be more resistant to trimethoprim-sulfonamide–associated diarrhea problems compared with other oral drugs. For example, trimethoprim-sulfadiazine had little effect on the fecal flora of horses and did not produce diarrhea, even when the dose was increased to 120 mg/kg/d orally [7]. The safety of trimethoprim-sulfonamides in horses may be attributed to compounds like thymidine in the intestinal contents, which are known to inhibit antibacterial action against susceptible organisms [7,10]. Erythromycin and lincosamides The intestinal problems from erythromycin stem from the antibacterial spectrum and the poor oral absorption. The erythromycin base is rapidly degraded into inactive metabolites in the equine stomach and intestine [11]. If erythromycin is administered as an ester prodrug, such as erythromycin estolate, however, it is absorbed as the intact ester and converted to the active drug after absorption [12]. By comparison, the ester erythromycin ethylsuccinate is not well absorbed [13]. Oral absorption is improved if erythromycin is administered as a phosphate salt, which resists degradation in the stomach and is absorbed as active erythromycin [12]. Even under these conditions, however, the systemic availability of erythromycin phosphate or estolate is only approximately 15% to 16%. One might assume that with these formulations, the concentration of active drug in the intestine is low because of degradation in the stomach (erythromycin base) or because the drug remains in the intestine in an inactive form (ester prodrugs of erythromycin). In two studies in which erythromycin was measured in the feces, however, high concentrations of active drug were present in feces of horses treated orally with either erythromycin base or erythromycin ethylsuccinate [14,15]. To the knowledge of this author, concentrations of erythromycin have not been measured in feces of horses treated with either erythromycin estolate or erythromycin phosphate. If there is less active drug in the intestine from either of these formulations, that may represent an advantage when considering which one to administer therapeutically to horses, because these drugs may have less effect on the intestinal population of bacteria. Foals treated with erythromycin had a higher risk of developing diarrhea [16], and of 73 foals treated in one study, 36% developed diarrhea along with other signs (hyperthermia and respiratory distress). In two studies from Sweden, the horses seemed to be highly sensitive to the development of diarrhea from erythromycin. In the first study, horses developed severe colitis after administration of erythromycin, 1.25 mg/kg orally [15]. No serum levels of the drug were detected from such a low dose, but high levels were found in the feces. C difficile was isolated from the feces of some of the horses and was resistant to erythromycin. In the second study, mares

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developed acute colitis when their foals were being treated with erythromycin to treat Rhodococcus equi [14]. It is presumed that the mares were exposed orally through ingestion or contact with the erythromycin excreted in the foal’s feces. C difficile was isolated from feces of 45% of the mares with acute colitis. In the studies cited from Sweden, the horses were also administered rifampin, but horses administered rifampin alone did not develop colitis or diarrhea [15]. The authors acknowledge in this report that horses in Sweden may be more sensitive to erythromycin-induced colitis compared with other geographic locations. Horses exposed to much higher doses in the United States (eg, 12.5–25 mg/kg) have not developed diarrhea or colitis. When horses develop colitis and diarrhea from erythromycin, it seems to be secondary to disruption of the bacterial flora of the intestine and proliferation of clostridial organisms and toxins. Motility-promoting effects of erythromycin also may contribute to the diarrhea, however. Erythromycin stimulates motilin receptors in the gastrointestinal tract to induce increased motility [17]. Administration of erythromycin to dogs can cause vomiting; it also is possible that administration to horses causes diarrhea from increased intestinal motility. The lincosamides have been associated with some of the most severe cases of antibiotic-associated diarrhea in horses. The lincosamides include lincomycin and clindamycin. They may disrupt the intestinal bacteria and cause severe enteritis and even death. In people, clindamycin has been the most important cause of antibiotic-associated diarrhea. The disease in people was once called ‘‘clindamycin colitis.’’ There are not any reports of clindamycin use in horses, but a closely related compound, lincomycin, has been administered accidentally, usually in the form of contaminated feed (lincomycin premix for pigs). Clostridial enteritis in experimental ponies was induced with oral administration of lincomycin, 25 mg/kg [18,19]. In another report, the toxic dose was as low as 0.5 mg/kg administered for only 2 days [20]. Rabbits, which have intestinal tracts similar to horses, are also susceptible to adverse effects from this group of drugs. Lincomycin and clindamycin given orally have caused diarrhea and death in all rabbits at doses as low as 5 mg/kg/d [21]. b-Lactam antibiotics The b-lactam antibiotics include penicillins and cephalosporins. They have not been administered orally much in horses because of their poor oral absorption; therefore, their effects on intestinal bacteria are not really known in horses. Oral antibiotic therapy for horses is limited, because the systemic absorption is often so poor. For example, systemic availability of oral amoxicillin in adult horses is only 2% to 10% [22–24]. Cefadroxil oral absorption is also poor and inconsistent in adult horses [25]. The poor oral absorption raises concerns about the amount of active drug in the intestine

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that could potentially disrupt the intestinal bacteria. Foals absorb these drugs better than adults, but absorption is still not complete. Oral absorption of amoxicillin in foals is 36% to 42% [26], and cefadroxil had a systemic availability of 58% [27]. Since a decline in the use of clindamycin in human hospitals, the leading cause of antibiotic-associated C difficile infection is cephalosporins, particularly the second- and third-generation cephalosporins with extended spectrums [28,29]. Oral cephalosporins are not used to any great extent in horses, so the incidence of problems from these drugs is not known. Ceftiofur sodium, the only registered injectable cephalosporin for horses, has characteristics of the third-generation cephalosporins. In safety studies, at a dose of 2.2, 6.6, or 11 mg/kg (one, three, and five times the label dose), it was well tolerated, with the only adverse effect being decreased food consumption attributed to changes in bacterial flora at the highest doses [30]. When ceftiofur was given parenterally, diarrhea was reported at a dose of 2.2 mg/kg every 12 hours in surgery ponies [31]. Chloramphenicol and derivatives Chloramphenicol has been administered with relative safety in horses for many years. When the derivative, florfenicol (Nuflor), was administered to horses at 22 mg/kg IV, intramuscularly (IM), and orally [32], however, the bilirubin serum concentration increased. After administration of only a single dose by each route, the horses also developed loose feces, which raises concerns about the changes in intestinal bacteria caused by florfenicol. Fluoroquinolones Drugs like enrofloxacin, orbifloxacin, and marbofloxacin have an antibiotic spectrum that includes gram-negative aerobic bacilli and staphylococci, but they have poor activity against anaerobic bacteria. Enrofloxacin (Baytril) is commonly used orally and parenterally in horses, even though it is not registered for this use [33]. Orbifloxacin and marbofloxacin have also been administered orally to horses. One of the advantages of the fluoroquinolones is that antibiotic-associated diarrhea has been rare when they are administered at clinically recommended doses. This has been attributed to their lack of activity on anaerobic bacteria of the intestine. Ciprofloxacin has low oral absorption in horses, and there have been isolated reports of colitis caused by its oral administration, but these have been unconfirmed cases. The newer generation of fluoroquinolones, such as gatifloxacin and moxifloxacin, have a spectrum that includes grampositive bacteria and anaerobes not covered by older fluoroquinolones like enrofloxacin. This increased anaerobic spectrum of activity may cause problems in some horses. Administration of moxifloxacin to adult horses at 5.8 mg/kg orally consistently produced diarrhea [34]. When moxifloxacin was discontinued, all horses recovered. These observations illustrate that an

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anaerobic spectrum is an important feature linked to the development of antibiotic-associated diarrhea in horses.

Association of antibiotic administration and salmonella infection in horses In some of the earlier publications that associated antibiotic administration with the development of diarrhea in horses, one of the risks was development of salmonella infection. The studies that have been published since then indicate that antibiotics cannot be considered as a cause of salmonella enteritis in horses but that the administration of antibiotics may contribute to shedding and add to the risk when other factors are also in place. In a case-controlled study to examine the risk factors associated with isolation of salmonella infection in hospitalized horses [35], oral antibiotic administration was associated with a 40.4 times greater risk of developing salmonellosis. Oxytetracycline, 10 mg/kg IV every 12 hours for 5 days, also prolonged the excretion of Salmonella spp in experimentally infected ponies [36]. It is possible that antibiotic administration decreases the competitive balance in the intestine and allows latent Salmonella spp to proliferate and shed. The studies that have investigated the association of antibiotic administration and Salmonella spp shedding have not always been in agreement. In another study that was case-controlled with longitudinal follow-up, there was no association between antimicrobial treatment and shedding of Salmonella spp in horses [37]. Although antibiotic administration is frequently avoided to treat horses with salmonellosis, there may be exceptions for which antibiotic use is indicated. Fluoroquinolones are the first drug of choice for this treatment. Because enrofloxacin is the only veterinary fluoroquinolone available in an injectable form, this is the most common choice. Fluoroquinolone properties that are an advantage for this treatment include favorable pharmacokinetic features (intracellular penetration), bactericidal activity, and little effect on the anaerobic intestinal bacteria. Use of fluoroquinolones in horses is discussed later in this article. Fluoroquinolones should not be administered to young foals [38]. For treatment of foals with bacteremia caused by Salmonella spp, a b-lactam, such as an extended-spectrum cephalosporin or ampicillin-sulbactam, alone or in combination with an aminoglycoside (gentamicin or amikacin) would be preferred.

Antibiotic-associated clostridial enteritis This disease and its pathogenesis were thoroughly reviewed in a previous issue of Veterinary Clinics of North America Equine Practice and are not covered in such detail here [39]. There have been several articles and case reports that have implicated antibiotic therapy in horses as a cause, or

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contributing factor, in the development of clostridial enteritis [14,15,18,40]. Stress, such as stress from transport or another disease, also may contribute to the pathogenesis. Two organisms have emerged as the cause of this disease, C difficile and C perfringens. Both organisms are obligate anaerobic gram-positive bacilli. C difficile has been positively cultured from horses with antibiotic-associated diarrhea and colitis [14,15,40]. C difficile was not cultured from healthy horses, however, nor was it cultured from other horses with enteritis that were not treated with antibiotics. The pathogenesis of antibiotic-associated clostridial infection is related to disruption of the bacterial population in the intestine to allow Clostridium spp to proliferate. Toxins liberated from the organisms then cause the clinical signs of enteritis. The reason Clostridium spp proliferate is that drugs administered are active against the competing bacteria but not these Clostridium spp. In studies in which the susceptibility of C difficile was measured, it was resistant to the antibiotic associated with the onset of clinical problems. The antibiotics most often implicated in clostridial enteritis are tetracyclines (particularly oral oxytetracycline), oral lincomycin, b-lactams (penicillins, penicillin derivatives, and cephalosporins), oral neomycin, and erythromycin. Clostridial enteritis in experimental ponies could be induced with oral administration of lincomycin, as little as 0.5 mg/kg [20], and in horses with erythromycin, as little as 1.25 mg/kg [15]. In people, the use of clindamycin has decreased. Because of this risk, there has been increased use of cephalosporins. Now, in some human hospitals, the leading cause of antibiotic-associated C difficile infections is cephalosporins, particularly the second- and third-generation cephalosporins with extended spectrums [28,29]. Treatment of clostridial enteritis C difficile releases potent toxins that cause diarrhea, pseudomembranous colitis, enteritis, and even death. In human beings, the most common treatment is metronidazole or oral vancomycin [28]. The cost of oral vancomycin is high, so metronidazole is generally used. Metronidazole is also the treatment of choice listed by the Centers for Disease Control and Prevention (CDC) and The Infectious Disease Society of America. In horses, the most often cited treatment regimen is also oral metronidazole. In one study [41], there was a high percentage of resistance to metronidazole, but the strains were still sensitive to vancomycin, rifampin, and chloramphenicol. All the isolates in that study were resistant to bacitracin. Despite this reported incidence of resistance of Clostridium spp to metronidazole, the drug is still cited as an effective treatment by all studies reviewed for this article that discussed treatment of clostridial enteritis in horses. Another treatment cited is oral vancomycin (125 mg per horse every 6 hours orally). Because of the cost of the drug and unfamiliarity among veterinarians with the use of vancomycin, it is seldom used, however. The third treatment cited

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is oral bacitracin. This drug is not reported to be used in people, but there are published reports of its use and efficacy in experimental equine infections [19]. Methods to adsorb the clostridial toxins also have been used in people [28]. Cholestyramine is an anion-exchange resin that, given orally, adsorbs some toxins. There are no reports of its use in horses. Metronidazole Metronidazole belongs to the nitroimidazole class of drugs and is the only one in this class used commonly in veterinary medicine. (Tinidazole has been studied in horses, but is not used in the United States.) Metronidazole’s selective antibacterial action is derived from an anaerobic organism’s energy metabolism, which differs from that of aerobic cells. Therefore, metronidazole can be used in horses without disrupting aerobic or facultatively anaerobic bacteria. Although metronidazole is not registered for veterinary use, it has been an important drug for horses [42]. It has a narrow spectrum that includes protozoa and gram-positive and gram-negative anaerobic bacteria. In a clinical study [41], 19% of the C difficile organisms cultured were resistant, and in another study [2], 43% of the organisms were resistant to metronidazole. Why are there reported cure rates when the incidence of resistance is this high? Several factors may play a role in this disparity. It is possible that the in vitro tests for anaerobic bacteria in this case do not correlate with clinical use. The concentrations tested for in vitro susceptibility are based on plasma drug concentrations and may not reflect the concentrations achieved in the intestinal lumen after an oral dose. It also is possible that the effects of metronidazole on the intestine may extend beyond its antibacterial properties. Some clinicians propose that it has an anti-inflammatory effect in the intestine. The ability of metronidazole to reduce intestinal inflammation may be related to its action on other metronidazole-sensitive organisms [43]. There is extensive experience with the use of metronidazole in horses [42]. Oral doses range from 20 to 25 mg/kg every 8 hours to 15 mg/kg every 6 hours. The half-life is much shorter in horses compared with people, which necessitates more frequent administration. For clostridial enteritis, the most common dosage cited is 15 mg/kg every 8 hours orally for 3 to 5 days. Metronidazole is available in tablets (250 mg or 500 mg) or capsules (375 mg). The taste is bitter and unpleasant for some animals, and the crushed tablets or capsule contents are often mixed with a vehicle (eg, molasses) for oral administration to horses. Metronidazole also can be administered IV, but this route is expensive and not used commonly. Metronidazole for injection has a low pH and can be irritating. Rectal administration has been used in horses [44], but it is not recommended, because absorption from this route is only 30% compared with 74% to 85% for the oral route. For the treatment of intestinal disease, the oral route would seem to be more logical.

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Metronidazole is well-tolerated in horses. Rare neurologic and gastrointestinal adverse effects have been seen, which are exhibited as anorexia, depression, and neurologic deficits, but they are not of the same frequency or severity as reported in dogs and cats that received high doses. Bacitracin Bacitracin is a rarely used antibiotic in veterinary medicine, except for topical use. It is active primarily against gram-positive bacteria, and it is not absorbed from the intestine. Because of its narrow spectrum, it spares the gram-negative endogenous bacteria. Its use for treating clostridial enteritis in horses originated from studies performed in experimental animals [19]. The dose cited for oral zinc bacitracin (available as a premix feed additive) is 25 to 50 g per horse administered orally every 12 hours for the first day of treatment. Thereafter, this dose is given every 24 hours for 4 days. It has been administered via stomach tube or orally in a formulation mixed with molasses. One of the concerns about the use of bacitracin is the development of resistance. Despite the reported efficacy by Sta¨empfli and colleagues [19], they showed in a study at the University of California that all strains of C difficile tested from 105 horses with diarrhea were resistant to bacitracin [41]. If efficacy is observed, it is possible that most of the action is against C perfringens, because it is more susceptible than C difficile. Resistance of C perfringens to bacitracin has not been reported. It is also possible that measurements of resistance from an in vitro test do not correlate with clinical results, because this is a nonabsorbed antibiotic that reaches high concentrations in the intestinal lumen.

Treatment of infections associated with colic The most serious infections related to colic in horses are those associated with the gram-negative bacteria, particularly, Enterobacter spp, Klebsiella spp, and Escherichia coli. When the integrity of the bowel is compromised or when the bowel is stagnant, these bacteria can translocate from the intestine and lead to bacteremia and septicemia. Despite this possibility, the actual prevalence of bacteremia or septicemia in horses with mucosal barrier injury is actually low. Whether this reflects the effective prophylactic treatment that is often employed in these patients or another unknown intrinsic mechanism in horses is not known. Many, if not most, equine patients with colic are treated prophylactically with combinations of drugs (usually penicillin plus gentamicin), because clinicians fear that mucosal integrity has been breached or that immune compromise will lead to sepsis. Antibiotics are also used prophylactically when the surgeon anticipates an enterotomy during surgery to remove intestinal contents or perform an anastomosis.

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If a horse becomes bacteremic or septicemic associated with colic, prompt treatment is necessary. Because a culture and sensitivity result may not be available at the time treatment is initiated, antibiotic selection is empiric. Unless other organisms are identified, clinicians assume that E coli is the most likely cause of sepsis; therefore, this is the organism at which antibiotics are directed. Because many E coli infections are resistant to the commonly used antibiotics, such as penicillins, aminopenicillins, firstgeneration cephalosporins, and sometimes trimethoprim-sulfonamide combinations, other drugs should be considered. Based on susceptibility data, the gram-negative enteric bacteria are usually expected to be susceptible to fluoroquinolones and aminoglycosides. Resistance to fluoroquinolones (eg, enrofloxacin, orbifloxacin, marbofloxacin) has been documented in small animals [45] but has not been an issue in equine medicine. Gentamicin usually has reliable activity against E coli, but resistance to gentamicin among equine pathogens has been documented in veterinary teaching hospitals [46,47]. Amikacin is the most active of the aminoglycosides against gram-negative bacteria in horses. Aminoglycosides The use of aminoglycosides in horses has been recently reviewed [48]. Pharmacokinetic studies were done in horses undergoing abdominal surgery to justify the currently used doses of 4 to 6.8 mg/kg once daily [49]. In adult horses, an appropriate dose of amikacin is 7.25 to 14.5 mg/kg once daily. Because of a higher volume of distribution in foals, the dose of amikacin should be increased to 20 to 25 mg/kg once daily [50]. This class of antibiotics exhibits concentration-dependent bactericidal activity and a postantibiotic effect (PAE). Therefore, aminoglycoside dosing regimens are designed to achieve high peak plasma concentrations of the drug (CMAX) and, more specifically, a high ratio of CMAX to the minimum inhibitory concentration (MIC). These goals can be achieved by the use of once-daily dosing [48]. The longer interval between doses also provides a drug-free period that allows for reversal of adaptive resistance. Indeed, studies in human patients with gram-negative infection have provided evidence that once-daily dosing results in a more rapid clinical response than traditional dosing regimens. Once-daily dosing also may lessen the risk of nephrotoxicosis compared with more conventional aminoglycoside dosing (eg, the total daily dose divided equally into three doses and administered at 8-hour intervals). Fluoroquinolones The other drugs usually administered to patients with infections associated with colic are the fluoroquinolones, penicillins, and metronidazole. Use of metronidazole was discussed previously in this article. The most

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commonly administered fluoroquinolone is enrofloxacin (Baytril). Because enrofloxacin is available in an injectable formulation (either the 22.7-mg/mL small animal formulation or the 100-mg/mL cattle formulation), it is used in horses more commonly in colic situations than oral fluoroquinolones. Use of enrofloxacin in horses and the pharmacokinetics have been reviewed [33,51]. The dosage for horses is designed to achieve a CMAX/MIC ratio or area under the curve (AUC)/MIC ratio that has been associated with clinical cure in experimental animals. Sensitive gram-negative bacilli from horses have an MIC for enrofloxacin of 0.125 lg/mL or less, based on available information [52]. Pharmacokinetic studies of enrofloxacin in horses [33,51,53,54] showed that to achieve the goal of 10  MIC or an AUC/ MIC ratio greater than 100, IV doses of 5 mg/kg once daily and 7.5 mg/kg orally are adequate. This author has monitored plasma concentrations in many clinical equine patients after oral and injectable administration of enrofloxacin (unpublished observations) and confirmed that these doses are adequate to achieve targeted plasma concentrations. The IV injection should be administered slowly so as to avoid excitement. Studies with orbifloxacin (Orbax) have been conducted and have confirmed that oral absorption of this drug is high and that a dose of 5 mg/kg/d would be adequate for adult horses (unpublished data). A pharmacokinetic study in horses [55] at a dose of 7.5 mg/kg produced plasma concentrations sufficient for most gram-negative equine pathogens. The authors also discussed in their paper that a lower dose of 5 mg/kg/d would probably achieve targeted plasma concentrations for gram-negative bacteria in most horses. Marbofloxacin also seems to be suitable for horses with good oral absorption and favorable activity against Enterobacteriaceae. The authors of two studies concluded that marbofloxacin at a dose of 2 mg/kg once daily orally would be sufficient for gram-negative bacilli of the Enterobacteriaceae [56,57]. Neither orbifloxacin nor marbofloxacin is available in injectable formulations in the United States. The fluoroquinolones of the newer generation (eg, moxifloxacin, gatifloxacin) that have more active anaerobic activity should not be used in horses with colic until more safety data become available. As discussed previously in this article, moxifloxacin has caused diarrhea in experimental horses [34].

Proliferative enteritis Proliferative enteritis in horses, caused by the organism Lawsonia intracellularis, is a relatively new intestinal disease. There are now several published clinical reports, but definitive research on treatment is lacking. Most of the natural history of this organism comes from the experience with

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this disease in pigs [58]. L intracellularis has been identified as the cause of an important intestinal disease of pigs, and there are many reports published describing the etiology, pathogenesis, and clinical course [59]. It is also an important intestinal disease of hamsters. Regarding the equine form of proliferative enteritis, there are no published trials to guide veterinarians on the best treatment. Supportive measures, such as nutritional management and correcting fluid and electrolyte imbalances, are important. The role of antibiotics in the cure is not established, however. Antibiotics may shorten the clinical course, but this disease may be self-limiting and resolve without antibiotics. Compared with the disease in pigs, L intracellularis infection in horses is not associated with outbreaks in herds but rather seems to occur in isolated patients. There are no reports that have measured the susceptibility of equine Lawsonia organisms to antibiotics. Susceptibility tests cannot be performed using standard methods, because this is an obligate intracellular pathogen with specialized growth requirements. Susceptibility tests conducted on the organism isolated from pigs reveal patterns of susceptibility that vary widely among drugs, even among drugs within the same class [60]. The susceptibility report of L intracellularis from pigs [60] showed that the most active drugs were penicillin and ampicillin, erythromycin, difloxacin, virginiamycin, and chlortetracycline. Susceptibility to tilmicosin was intermediate. Resistance was measured for enrofloxacin, cephalosporins (ceftiofur), aminoglycosides (neomycin and gentamicin), lincomycin, vancomycin, and tylosin. Although breakpoints for susceptibility have not been determined for L intracellularis, the sensitive and resistant breakpoints used for other animal pathogens were considered for this classification [61]. The puzzling aspect of this susceptibility pattern is that this organism shows divergent susceptibility to drugs within the same class that act on the same bacterial target. For example, it is unusual that an organism would be resistant to tylosin and lincomycin but sensitive to erythromycin. Likewise, it is unusual that this organism was sensitive to difloxacin but resistant to enrofloxacin and sensitive to penicillin and ampicillin but resistant to ceftiofur. These drugs generally share similar bacterial targets. There usually is at least some degree of cross-resistance among drugs that share the same mechanism of action. These patterns of susceptibility suggest that other studies of susceptibility are needed to gain a better understanding of the in vitro activity of drugs against this organism. Until more reports become available, one should be cautious about selecting drugs against L intracellularis on the basis of in vitro susceptibility tests alone. One of the survival features of this organism is that it is a fastidious obligate intracellular pathogen. It survives in the cytoplasm of enterocytes. Therefore, it is reasonable to assume that drugs will be ineffective in vivo if they cannot reach the intracellular site. Drugs that accumulate in leukocytes, fibroblasts, macrophages, and other cells are fluoroquinolones, rifampin, lincosamides (clindamycin, lincomycin), macrolides (erythromycin, clarithromycin), and

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the azalides (azithromycin). The b-lactam antibiotics and aminoglycosides do not reach effective concentrations within cells and probably should not be used for this disease. In addition, aminoglycosides are not active in the low oxygen environment of the intestine and colon. Clinical cases of L intracellularis have been treated successfully with erythromycin [62]. Published review articles on this disease also recommend oral erythromycin as the drug of choice, with the possible addition of rifampin. Doses cited are erythromycin, 25 mg/kg every 8 to 12 hours orally, alone or in combination with rifampin, 10 mg/kg every 24 hours orally. This treatment is recommended for 21 days. The degradation of erythromycin in the stomach and systemic absorption of the various dose forms were discussed earlier in this article. Because of concerns regarding erythromycin-induced colitis as discussed earlier in this article, the risks of using erythromycin in horses should be considered. The formulations of erythromycin that seem to achieve the best oral absorption and are the best tolerated are either erythromycin estolate or erythromycin phosphate. Erythromycin estolate is not active in the intestine but is absorbed as the intact ester and converted to active erythromycin in the blood. It is not known how much active drug is in the intestine or how critical high intraluminal drug concentrations are for the treatment of proliferative enteritis. The erythromycin derivative azithromycin (Zithromax) achieves particularly high concentrations of active drug intracellularly, but its activity or efficacy against L intracellularis has not been reported. In studies in horses [63], the oral absorption of azithromycin in foals was 33% and the concentrations achieved in phagocytes were 200 times the corresponding plasma concentrations. Although its efficacy has never been tested, this drug may have potential for treating intracellular infections, such as L intracellularis. There are anecdotal reports of the successful use of azithromycin for the treatment of R equi in foals at a dose of 10 mg/kg orally once per day to every other day, without reports of drug-induced colitis. Chloramphenicol also has been administered orally at a dose of 50 mg/kg every 6 hours, but the availability of chloramphenicol is decreasing among pharmacies. There are no reports on the efficacy of chloramphenicol for treating L intracellularis infections. As discussed earlier in this article, florfenicol should be avoided in horses until reports of its safe use are available.

Ehrlichial colitis A new organism that has emerged as a cause of colitis in horses is Neorickettsia risticii. This agent was formerly called Ehrlichia risticii but recently underwent a name change. This is one of the ehrlichial diseases of horses, and it also has been called Potomac horse fever (PHF) and equine

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monocytic ehrlichiosis (EME). The major clinical signs are fever, depression, decreased feed intake, and diarrhea. The clinical course of the disease and clinical manifestations have been discussed in review articles [64]. This organism is an intracellular pathogen and survives inside macrophages by inhibiting phagosome-lysosome fusion unless drugs used for treatment have good intracellular penetration. Treatment recommendations for ehrlichial colitis are based on treatment of experimentally induced infections and experience of equine clinicians. In small animals, it is the consensus of clinical experts that the treatment of choice for Ehrlichia spp is a tetracycline, specifically doxycycline [65]. Other drugs that have been successful for treatment of ehrlichiosis in small animals are chloramphenicol, imidocarb, and amicarbalide. Enrofloxacin was successful in small animals for the treatment of other rickettsial diseases but not effective for experimentally induced ehrlichial infection in dogs [65]. In human beings, doxycycline is the drug of choice for infections caused by the Ehrlichia spp, including human granulocytic and monocytic ehrlichiosis. Antibiotic activity has been compared for treatment of PHF in mice. In one study, doxycycline was compared with demeclocycline and rifampin [66]. When these antibiotics were given after development of clinical signs in mice infected with N risticii (E risticii), the best clinical response was seen with doxycycline. The authors speculated that doxycycline performed better than other tetracyclines because of its higher lipophilicity, longer half-life, and intestinal secretion as a route of elimination. Although intestinal secretion of doxycycline has been shown for other animals, to this author’s knowledge, this has not been demonstrated for horses [67]. Although doxycycline is the drug of choice in other animals and human beings for the treatment of ehrlichial diseases, caution is advised when administering doxycycline to horses. A letter to the editor [68] warned about IV administration to horses after sudden death was observed after IV infusions of only 0.18 to 0.44 mg/kg. Doses of only 0.3 mg/kg caused near collapse and atrial tachycardia. The mechanism of the cardiotoxicosis is not understood. Calcium concentrations were not affected from the infusion, and similar reactions were not observed from the drug vehicle. This unfortunate observation led the same investigators to a more in-depth report in which cardiovascular effects were investigated [69]. The horses in this report were given IV administration (doxycycline, 100 mg/mL, in a vehicle of ethanolamine/ethylene glycol diethyl ether/water) at a dose of 0.18 to 10 mg/kg or doxycycline in a vehicle of 0.9% saline solution at doses of 0.3 to 3 mg/kg IV. These studies concluded that IV administration of doxycycline is to be avoided in horses. Oral doxycycline has been administered to horses for five consecutive treatments at a dose of 10 mg/kg every 12 hours, however [67]. The dose was administered by dissolving doxycycline hyclate in 40 mL of water and administering it via stomach tube. In these horses, there were no adverse effects observed with five consecutive doses. The authors [67] also reported the observation that

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doxycycline has been administered in their practice at this dose for as long as 4 weeks without consequences. In horses, treatments have been examined in experimentally induced infections [70–72]. Although oral doxycycline remains a possibility for treatment, current regimens are based on studies performed with oxytetracycline, which is frequently the first drug of choice for the treatment of equine ehrlichiosis. The fact that tetracycline administration to horses can cause adverse gastrointestinal effects has already been discussed in this article. Therefore, for equine ehrlichiosis, treatment is limited to a short course, doses are kept low, and tetracycline is administered IV rather than orally. These precautions limit the risk of adverse changes in the intestinal flora caused by tetracycline administration. Current doses are based on experimental studies in ponies. Ponies that were experimentally infected with N risticii were pretreated with IV oxytetracycline, 6.6 mg/kg every 12 hours, 14 hours before inoculation and were treated for 5 and 10 days thereafter at this dosage [72]. Treatment delayed but did not prevent clinical signs. There was a concern expressed that if treatment is initiated too early, relapses may occur when treatment is discontinued because it may interfere with the development of an effective immune response. When treatment was given during the acute stages of experimentally induced ehrlichial colitis with IV oxytetracycline at a dose of 6.6 mg/kg IV every 24 hours for 5 days, it did not interfere with the development of an immune response [71]. Treatment at this dose was effective when administered during the acute stages just once per day. Nevertheless, the most commonly cited treatment regimen for ehrlichial colitis is oxytetracycline, 6.6 mg/kg IV every 12 hours. The duration of treatment is usually no longer than 5 days. The formulation of oxytetracycline used is one of the cattle formulations available in a solution that can be administered IV. IM administration of oxytetracycline is usually avoided in horses because of the problem of injection site myositis. Treatment of experimental infections emphasized the need for early recognition and prompt treatment of the disease. Treatment was initiated within 24 hours after the first appearance of clinical signs. If treatment is initiated early, a rapid response (within 12–24 hours) is expected. If one wishes to avoid administration of tetracyclines to horses for the treatment of equine ehrlichial colitis, the combination of erythromycin and rifampin is another alternative. Results of a study in experimental animals [70] showed that rifampin and erythromycin are effective if given early in the course of the clinical disease. The response to rifampin and erythromycin was prompt but not as rapid as with IV oxytetracycline. The response to rifampin and erythromycin is better than what would be expected on the basis of in vitro susceptibility results [66]. It was proposed that this response may be attributed to the synergism of the combination and to the high intracellular penetration of these drugs. Both of these drugs can attain high intracellular/extracellular drug concentration ratios [70]. Based on this

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assumption, it is possible that other drugs with high intracellular penetration, such as azithromycin [63], might also be effective for this treatment, but evaluation of other drugs in horses has not been performed.

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