R. Equi: The Hunt for New Antibiotics Continues (AAEP 2010)

It isn't for lack of effort that the equine industry still doesn't have new options for treating Rhodococcus equi pneumonia in foals. According to Noah Cohen, VMD, MPH, PhD, Dipl. ACVIM, professor of equine medicine at Texas A&M University, he and his colleagues are well aware that veterinarians are in dire need of better antibiotic alternatives. "Treatment of foals with R. equi pneumonia is generally prolonged, making treatment both expensive and labor-intensive," said Cohen in his presentation at the 2010 American Association of Equine Practitioners Convention, held Dec. 4-8 in Baltimore, Md. "Currently, the treatments of choice are a combination of the drug rifampin with azithromycin, clarithromycin, or erythromycin."

The latter three drugs are members of a family known as macrolides. Cohen explained the significance of macrolides, noting, "To date, alternatives to macrolides for effective treatment of R. equi pneumonia in foals have not been identified. Thus, when new macrolide treatments are developed, there is considerable interest among equine practitioners and farm managers about the prospects of using these new macrolides to treat foals with pneumonia."

One of the disadvantages of azithromycin, clarithromycin, or erythromycin is that these must be administered at least every 24 hours. Because treatment is generally prolonged, availability of a macrolide that could be administered less frequently to foals is desirable.

Tulathromycin is a long-acting injectable macrolide antibiotic, and data from Germany has suggested tulathromycin was useful for managing abscessing pneumonia in foals at a large breeding farm.

Cohen and colleagues therefore tested tulathromycin and 14 other antimicrobials on 98 different types of R. equi bacteria grown in laboratory culture dishes (they tested these drugs in vitro, or in the laboratory, rather than the live horse). They wanted to determine if any of these drugs might be potentially beneficial in live foals with R. equi infections.

"Unfortunately, we found that tulathromycin had poor activity against R. equi," Cohen reported. "It appeared that it would be impossible to achieve therapeutic concentrations of tulathromycin in blood or tissues."

Steeve Giguère, DVM, PhD, Dipl. ACVIM, a University of Georgia College of Veterinary Medicine professor and the Marguerite Thomas Hodgson Chair in Equine Studies is also well aware of the need for new drugs for treating R. equi infections.

“There are other long-acting macrolides with good in vitro activity against R. equi that are currently available for use in cattle in other countries,” said Giguère, who presented the in-depth session on treatment of R. equi in foals at this year’s AAEP conference. “These agents are currently being studied in foals here in the U.S. Some viable options do seem to be upcoming.”

Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.