Horses antimicrobials

Erythromycin estolate in conjunction with rifampin (both drugs administered orally) can be recommended for the treatment of Rhodococcus equi pneumonia in foals. Early diagnosis of the infection and prompt initiation of therapy considerably increase the effectiveness of treatment. Apart from this specific indication, macrolide antibiotics (including erythromycin) and lincosamides (lincomycin and clindamycin) are contra-indicated in horses. Antimicrobial agents in these classes can cause severe disturbance of the balance between commensal bacterial flora in the colon of the horse. 

The data from the 2000 FSIS/USDA monitoring program (Table 12.4) present very similar results to those found in the 1998 data (USDA, 2000). For the sake of simplicity, the data presented show the violations and the upper 95% confidence limits. These data confirm the consistently low incidence of antibiotic and sulfonamide residues in animals grown for food. Even in the worst-case situation, the upper range of the violative incidence remains relatively low, usually less than 2%. There are some hot spots, namely in veal calves, hogs, and horses. Horses appear to be a special case because these animals are rarely used for food in the U.S. It may be that horses are treated with antibiotics or antimicrobials to ensure the animals or carcasses are in the best shape for marketing. 

Age or body weight can affect the systemic availability of many antimicrobial agents. In the physically smaller animal (sheep and pig) the peak serum concentration of a drug is usually higher and is followed by a rapid decline compared with a lower peak and a slower decline of the antibiotic in seruon of the larger animal (cow and horse). The limited experimental data appear to indicate that the extent of systemic availability of IM-administored antibiotics can vary as widely between different sites as between IM and SC sites. A corollary to this observation is that the location of the extra-vascular injection site should be well-defined when determining the systemic availability of parenteral preparations (9). 

In the general screen for antimicrobials, 36 of 2112 samples from cattle, sheep, pigs, horses, deer, poultry, and ratites had residues above the MRL. Two of these samples were from cattle and the residues found were oxytetracycline and dihydrostreptomycin. The remaining 34 noncompliant samples were from 721 pig samples tested in the general screen. From tliese samples 19 were due to chlortetracycline, 14 due to oxytetracycline, and 1 to neomycin. 

In companion animals, the bacterial infections most frequently requiring treatment involve the skin, wounds, ears, respiratory tract, and urinary tract. Prevalence of resistant bacteria is difficult to assess in pets. Since empirical treatment is common, there is limited information on trends in resistance, and almost no data are available on the magnitude of antimicrobial use in these species. Increasing resistance among E. coli and Staphylococcus isolates from pets has been reported from studies in a number of countries. Methacillin-resistant Staphylococcus aureus (MRSA) have been isolated from dogs and horses, although animal outbreaks have generally been traced to infected humans. ... 

Much of the use of antimicrobial agents is irrational. Antimicrobial therapy for superficial wounds and single doses of penicillin administered after the elective castration of normal horses has no real therapeutic benefit and encourages antimicrobial resistance. 

The use of antimicrobial agents for relatively trivial infections encourages the selection of resistant organisms. Consequently, in the absence of evidence of a susceptible pathogen, antimicrobial use is irrational and may expose treated horses to unnecessary risks. 

In addition to finding a suitable product, the practitioner must also determine an appropriate withdrawal time for an antimicrobial agent used in a horse intended for human consumption or performance. 

Florfenicol is associated with producing transient diarrhea in calves and has been associated anecdotally with severe colitis in horses. In one pharmacokinetic study, all of the horses and ponies developed mild diarrhea following i.v., i.m. or p.o. administration of florfenicol. In a recent study where the commercial florfenicol formulation was administered i.m. to horses at 20mg/kg every 48 h, all of the horses remained clinically normal but showed significant changes in commensal fecal flora. High numbers of Clostridium perfringens and Salmonella spp. were isolated from some of the treated horses. Florfenicol should be used with caution in horses because of its potential to induce antimicrobial-associated colitis. 

In horses, oxytetracycline administration has been associated with proliferation of Clostridium and Salmonella spp., resulting in potentially fatal colitis. However, the cases reported typically involved stress, surgery, transportation and the concurrent use of multiple antimicrobial agents. Recent studies have demonstrated minimal changes in the fecal flora and no other adverse effects in horses given multiple i.m. doses of oxytetracycline-PEG. 

The tetracyclines are amphoteric antimicrobial agents that can form salts with bases or acids (see Chs 1 and 2). Oxytetracycline is a bitter, yellow, odorless crystalline powder. The base is slightly water soluble and the hydrochloride is readily water soluble and is typically administered to horses by slow i.v. injection. It is effective at 5.5 mg/kg once daily for 2 days or more in the treatment of B. equi but is unlikely to completely clear this infection. It is, therefore, used for premunition. Rapid i.v. injection may cause a precipitous drop in blood pressure and collapse owing to the effects of calcium chelation on the myocardium. Intramuscular injection causes objectionable local reactions in horses and should be avoided. Oral administration may be more... 

Antimicrobial agents are used frequently in horses with gastrointestinal disorders but in most cases their use is empirical. For some infectious disorders of the alimentary tract, there are specific antimicrobial treatments, including ... 

The treatment of UTIs in horses consists of proper antimicrobial therapy and, if possible, the correction of the predisposing anatomical or functional problems with urine flow (e.g. outflow... 

It warrants mention that resistance to a particular antimicrobial agent in vitro may not preclude successful treatment with the drug as long as high concentrations are achieved in urine. Similarly, demonstrable susceptibility in vitro does not always guarantee a successful response to treatment. For example. Enterococcus spp. is often found to be susceptible to the potentiated sulfonamide combinations in vitro however, this pathogen is inherently resistant to these combinations in vivo (Jose-Cunilleras Hinchcliff 1999, Schott 1998). Antimicrobial therapy should be continued for at least 1 week for the treatment of lower UTIs and for 2-6 weeks for upper UTls in horses. Ideally, a voided, midstream urine sample should be submitted for bacterial culture 2-4 days after the initiation of therapy and again 1-2 weeks after treatment has been discontinued. 

The cephalosporins and tetracyclines are commonly used for treatment of UTIs in other species. However, in horses, the cephalosporins are rarely more advantageous than the penicillins or potentiated sulfonamides. However, ceftiofur has broad-spectrum antimicrobial activity and may be indicated when urinary pathogens are resistant to... 

The treatment of endometritis should include antimicrobial therapy based on culture and susceptibility testing (see Ch. 2). The treatments used commonly for bacterial and fungal uterine infections in horses are included in Tables 11.1 and 11.2. Fungal or yeast infections often result from the extensive use of antimicrobial agents in the uterus. These infections are difficult to treat and may cause permanent damage to the endometrium. An intrinsic problem with uterine yeast or fungal infections is the prolonged and potentially expensive therapy required. 

Topical 2% povidone iodine solution and 0.2% chlorhexidine gluconate solution have broad antimicrobial profiles and may be used as cheap spectrum and effective antifungal agents in equine ketomycosis, particularly where Fusarium spp. are involved. A1% dermatological cream formulation of silver sulfadiazine has both antifungal and antibacterial properties and is reasonably well tolerated topically in the horse eye. Its use is advocated where cost restraints exist or for prophylaxis in comeal injuries involving embedded plant material (Hamor Whelan 1999). 

Antimicrobial susceptibility patterns of fungi isolated from horses with ulcerative keratomycosis. American Journal of Veterinary Research 59 138-142 Brooks D E, Andrew S E, Biros D J et al 2000a Ulcerative keratitis caused by beta haemolytic Streptococcus equi in 11 horses. Veterinary Ophthalmology 3 121-126 Brooks D E, Andrew S E, Denis H M et al 2000b Rose Bengal positive epithelial microerosions as a manifestation of equine keratomycosis. Veterinary Ophthalmology 3 83-86... 

Aerosol administration of antimicrobial drugs is technically feasible in horses and the available devices can readily deliver clinically relevant dosages to the lower respiratory tract. Controlled, prospective studies must be performed to determine appropriate indications in horses, including the need for concurrent systemic antimicrobial therapy, ideal drug concentrations and dosages and the susceptibility of specific respiratory pathogens. 

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