Drugs For The Treatment Of Horses

May. 27, 2021

New macrolide antibiotics represent another class of drugs that are being considered for horses. The prototypic macrolide antibiotic is erythromycin.


Because of poor absorption and adverse effects (primarily diarrhea), new drugs have been developed that have better pharmacokinetic properties, improved spectrum of activity, and are better tolerated. The new macrolides and derivatives include clarithromycin (Biaxin), tulathromycin (Draxxin), tilmicosin (Micotil), and azalide azithromycin (Zithromax). As previously stated, tilmicosin injection, a cattle and swine antibiotic, should not be used in the horse until safety data become available. This caution should probably also be applied to tulathromycin.


The pharmacokinetics of clarithromycin have been investigated in foals and a dose of 7.5 mg/kg orally 12h is suggested for the treatment of R. equi pneumonia.Azithromycin has been evaluated in foals in two independent pharmacokinetic studies. Azithromycin has a half-life of 11 and 16 hours in foals after oral and IV dosing, respectively. The oral absorption is similar to people at 33%, and the volume of distribution is very high at 12 L/kg. 

Tilmicosin Injection

Tilmicosin Injection 

One of the distinct advantages of azithromycin in horses is its ability to concentrate in leukocytes for extended periods. The concentration of azithromycin in neutrophils reached a level that was 200 times the plasma concentration and depleted with a half-life longer than 50 hours and was above a concentration of 5.68 μg/mL (above the MIC breakpoint) for 120 hours after administration.


Based on the pharmacokinetic data and the anecdotal clinical experience, veterinarians have used azithromycin in foals at a dose of 10 mg/kg once daily initially, followed by 10 mg/kg every other day, orally after clinical improvement is observed. Because of the difference in half-life, clarithromycin is recommended for twice-daily administration, whereas azithromycin can be administered once daily. Another difference is that clarithromycin exhibits time-dependent PK-PD characteristics, whereas azithromycin efficacy is determined by the AUC/MIC. To maintain the duration of exposure for clarithromycin, twice-daily administration is needed.


Clinical experience with azithromycin and clarithromycin in the field has indicated that it has been safe for use in foals for the treatment of R. equi infections. Although each of these drugs has activity against gram-positive cocci, efficacy was only evaluated in one report of R. equi treatment. In people, these drugs are used to treat streptococcal respiratory infections. In a retrospective study, clarithromycin-rifampin combination was compared to erythromycin-rifampin and azithromycin-rifampin for the treatment of R. equi pneumonia. The foals treated with clarithromycin-rifampin had a higher overall short-term and long-term success rate and better radiographic improvement than foals treated with either of the other two combinations studied.



Treatment of otitis media/interna is based on the institution of antimicrobial therapy. Antimicrobial drugs used for the treatment of otitis media/interna must be effective against the most frequently isolated pathogens (i.e., Mycoplasma sp.) and diffuse into the middle and the inner ear. Antimicrobial drugs that could be used to treat mycoplasma infections in cattle are tetracycline, spectinomycin, tylosin, tilmicosin, florfenicol, and fluoroquinolones. Resistance to several of these drugs has been reported with M. bovis.


Successful treatment of otitis media/interna has been reported with enrofloxacin, but that drug is only approved by the U.S. Food and Drug Administration for the treatment of respiratory disease in beef cattle. Optimal therapy of otitis media in cattle remains to be determined. Proposed duration of treatment ranges from 5 days to several weeks. The duration of the treatment is influenced by the chronicity of the disease, as well as the implicated pathogens.


Nonsteroidal antiinflammatory drugs can be administered to increase the animal's comfort. Although local treatment does not appear necessary, local irrigation of the middle ear may be performed when the tympanic membrane is ruptured. Surgical treatment of chronic cases has been reported to be successful in calvesand goats.