R. Equi Foal Pneumonia: Control and Prevention (AAEP 2010)

For years foal owners have struggled with controlling the bacterium Rhodococcus equi and the infections it causes--some farms manage cases annually, despite following strict farm management strategies to reduce risk of infection. Resulting pneumonia and diarrhea can be deadly for foals, and treating survivors is costly and labor-intensive.

According to Noah Cohen, VMD, MPH, PhD, Dipl. ACVIM, preventing and controlling R. equi infections might be the ideal way to deal with some farms' recurrent R. equi problems. However, during a presentation at the 56th Annual Convention of the American Association of Equine Practitioners, held Dec. 5-8 in Baltimore, Md., he said that finding the means to prevent and control R. equi infections is still a challenge that researchers face.

Cohen said that two options for controlling and preventing R. equi infections were recently the subjects of several peer-reviewed studies: chemoprophylaxis and immunoprophylaxis.

Chemoprophylaxis (the use of antimicrobial agents to prevent foal pneumonia), was considered as an option for preventing R. equi infections, Cohen said. Cohen cited two studies in which researchers used azithromycin in an attempt to prevent infections, one of which showed a 76% reduction in the risk of contracting an infection. The other study, however, showed little difference between the infection risk of foals treated with azithromycin and those untreated. Cohen said the reason for the discrepancies in the results is unknown. But the reason for the discrepancy is moot; using azithromycin for preventing R. equi infections should not become common practice because it could create microbial-resistant bacteria in the foal and its environment. If the foal subsequently contracts an R. equi infection, the prognosis would be worse due to the resistant bacteria (which can be shed by carriers and picked up by other foals, resulting in them contracting a resistant form of infection).

Cohen also discussed the option of immunoprophylaxis (immune system modifiers) to prevent R. equi infections. He said that despite extensive research into developing a vaccine, no such product has been effective against R. equi infections.

The only clinically acceptable prevention method, Cohen said, is the administration of hyperimmune plasma (which contains high levels of antibodies against R. equi) to young foals via transfusion shortly after birth and possibly again at 3 to 4 weeks of age. Hyperimmune plasma is thought to provide passive immunity to treated foals against R. equi and reduce the incidence of pneumonia caused by this bacterium.

Cohen adds that although observational studies haven't produced uniform evidence of effectiveness, the cumulative interpretation of published studies indicates that plasma transfusion has reduced the risk of foals contracting R. equi infections.

"There is tremendous need for the development of a highly effective preventative strategy," Cohen said in his study. "In the absence of a preventative strategy, further application, evaluation, and development of screening tests are greatly needed to address this important health problem of foals."

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