The bacterium Rhodococcus equi has been a known cause of life-threatening pneumonia in foals for many years. But the ideal treatment for R. equi infection remains debatable because of the lack of research comparing the efficacy of each possible treatment in foals. However, according to Steeve Giguère, DVM, PhD, Dipl. ACVIM, a University of Georgia College of Veterinary Medicine professor and the Marguerite Thomas Hodgson Chair in Equine Studies, current evidence suggests that the most successful treatments are a combination of the drug rifampin and a macrolide (an antimicrobial drug). He presented a review of treatments for R. equi foal pneumonia at the 2010 American Association of Equine Practitioners convention, held Dec. 4-8 in Baltimore.
Giguère said that veterinarians have used the combination of rifampin and erythromycin (a macrolide) since the 1980s, and with this strategy they have drastically reduced the number of fatalities resulting from R. equi pneumonia, at least compared to historical data.
"The combination of a macrolide and rifampin is synergistic both in vitro (in the laboratory) and in vivo (in a live animal), and the use of the two classes of drugs reduces the likelihood of R. equi resistance to either drug," he said. Giguère added that rifampin and macrolides are liquid-soluble, a trait that "allows the drugs to penetrate cell membranes."
In addition to erythromycin, veterinarians have begun using two more recently developed macrolides to treat R. equi infections. Both azithromycin and clarithromycin have more modern chemical properties, meaning a smaller amount of drug is required to be effective, and the drug can be administered less frequently. Both characteristics result in fewer doses for the horse.
Giguère mentioned that there is a need for newer antimicrobial agents that are long-acting and require less frequent administration. However, the two long-acting macrolides currently available in the United States (tulathromycin and tilmicosin) are poorly active against R. equi. As a result, their use for the treatment of infections caused by R. equi is not recommended.
In addition to the macrolide antimicrobial agents, Giguère mentioned a few alternative classes of drugs that can be used for foals with R. equi pneumonia. One treatment option that might be successful in some foals is an oral dose of doxycycline in combination with rifampin. Finally, he said the antibiotic drug chloramphenicol has some activity against many R. equi strains, but it carries with it a health risk to the humans administering it.
Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.