Azithromycin Protects Foals Against R. equi in New Study

Texas A&M researchers have discovered they can reduce the incidence of foal pneumonia caused by Rhodococcus equi by giving the antimicrobial drug azithromycin during foals' first two weeks of life.

Rhodococcus equi (R. equi) is a pathogenic (disease-causing) bacterium that causes pneumonia in foals. It is a common soil-dwelling bug that results in important economic losses within the breeding sector of the equine industry.

"Identifying ways to prevent R. equi pneumonia is important because diagnosis can be challenging, particularly early in the course of the disease, and treatment of R. equi is expensive, time-consuming, and can be associated with serious side effects," said Keith Chaffin, DVM, MS, Dipl. ACVIM, professor of equine internal medicine at Texas A&M.

There is growing evidence that foals are infected with R. equi within a few days after birth. The infection subsequently grows until overt signs of R. equi infection (fever, cough, nasal discharge, lethargy, progressive respiratory distress) are noted, typically around 30 to 90 days of age.

The purpose of this study was to determine if administering azithromycin during the first two weeks of life to foals born on endemic breeding farms could decrease the proportion of foals that develop R. equi pneumonia .

In this controlled and randomized study, 338 foals on 10 farms with a history of R. equi infections on the premises were included. Researchers randomly assigned 168 foals to the control group (receiving no antimicrobial), while they treated the remaining 170 foals with 4.5mg/lb of azithromycin orally every other day for the first two weeks of life.

20.8% of foals in the control group were affected by R. equi pneumonia, compared to 5.3% in the azithromycin-treated group.
"The proportion of foals affected by R. equi pneumonia in the control group was (20.8%), compared to only 5.3% in the azithromycin-treated group," explained Chaffin. "These results suggest that azithromycin may have prophylactic effects against the development of pneumonia attributable to R. equi."

Researchers did not identify resistant bacteria in this study and do not currently recommend azithromycin prophylaxis on a wide-scale basis because of the potential for development of resistance to this type of antibiotic.

While these pilot study results are exciting, the researchers state that the study has limitations and further research is needed to evaluate the impact of azithromycin used in this setting, particularly considering that antimicrobial chemoprophylaxis has potential for the development of drug resistance in bacteria.

These study results are an important contribution to the expanding body of information on R. equi (see archived articles Virulent Rhodococcus equi in Soil Not an Indicator of Pneumonia Problems, and Are Mares a Source of R. Equi For Their Foals?).

This study was supported by the Morris Animal Foundation and the Link Equine Research Endowment.

The study, "Chemoprophylactic effects of azithromycin against Rhodococcus equi-induced pneumonia among foals at equine breeding farms with endemic infections," was published in the April edition of the American Journal of Veterinary Research.

About the Author

Stacey Oke, DVM, MSc

Stacey Oke, MSc, DVM, is a practicing veterinarian and freelance medical writer and editor. She is interested in both large and small animals, as well as complementary and alternative medicine. Since 2005, she's worked as a research consultant for nutritional supplement companies, assisted physicians and veterinarians in publishing research articles and textbooks, and written for a number of educational magazines and websites.

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