R. Equi Pneumonia Linked to Large Farms, Lots of Horses

Farms with large acreage and many mares and foals are more likely to have cases of R. equi foal pneumonia than smaller farms with fewer horses, according to a recently published Texas A&M University (TAMU) study. Additionally, farms that are intensively managed and use what are considered desirable practices to prevent disease are more likely to have R. equi cases.

R. equi is the most common cause of severe pneumonia in foals. Many breeding farms in Central Kentucky and other states are R. equi endemic, meaning the disease is present at a given farm on a recurrent basis. The study examined variables on 138 farms (65 affected and 73 unaffected) and appeared in the Feb. 1 issue of the Journal of the American Veterinary Medical Association.

Noah Cohen, VMD, PhD, Dipl. ACVIM, professor of equine medicine at TAMU's College of Veterinary Medicine and lead author in the study, said the associations with farm size and population numbers weren't that surprising, but the health management factor is puzzling. "This idea of more intensive management increasing the risk of disease was suggested to us by some individuals as resulting from farms where they are interacting a lot with the foals and moving stall-to-stall handling the horses: this activity could provide  greater opportunity to spread the infection from animal to animal," said Cohen. "Though plausible and possibly true, we think it is unlikely to be the case, because R. equi cases at a farm don't appear to be clustered in such a way that we'd think it's highly transferable between foals.

"Our current thinking is that it (the association of disease with intensivity of management) probably indicates that those practices, while desirable for other infectious diseases, are not adequate for resolving this problem," he added.

The current study did not determine that a high density of horses was a risk factor for R. equi, but another study conducted in Texas with similar numbers of farms and horses determined that density was a factor. Cohen explained, "One study won't yield the definitive answer to such a question. Given that the association of the number of mares and foals was consistently associated with disease in both studies and based on clinical experience, I'm more inclined to believe that horse/foal density is a risk factor. " The role of density is something he would like to investigate more thoroughly. Given the reality of large horse breeding operations today, Cohen would like to be able to offer preventive strategies to large farms with many foals to help decrease the incidence of R. equi pneumonia.

Another risk factor identified in the study was concrete flooring in foaling stalls, but Cohen thinks this might be a marker for larger, well-managed farms, so it should be "interpreted with considerable caution." In a prior study, the opposite was shown: dirt flooring increased the risk of R. equi relative to concrete and other flooring in foaling stalls. From a health standpoint, Cohen said concrete floors are easier to disinfect than dirt floors, and he does not think he would be doing a service to the industry if he recommended avoiding foaling in stalls with concrete flooring.  

Other factors were identified that Cohen believes to  be markers of the larger operations' intense management programs, including regular deworming, administration of R. equi hyperimmune plasma (considered to be a result of the disease rather than a cause), and using a manure removal program. Also, the farms tended to be Thoroughbred operations.

Current & Future Work
A research team led by Ronald Martens, DVM, involving TAMU scientists and individuals from other universities and veterinary practices, is working on many aspects of R. equi. The primary emphasis of the TAMU program is development of strategies for preventing the disease, using either "chemoprophylaxis" (drugs and other chemical substances for prevention) or "immunoprophylaxis" (immune system modifiers for prevention), and identification of factors that render some foals inherently more susceptible to the infection. Cohen said, "We want to determine what characteristics render some foals more susceptible (or resistant), and we're trying to look at strategies to modify these characteristics in order to control and prevent this devastating disease.

"To date, our work suggests that no matter how well we manage the environment or how well we provide health care for mares and foals, we may not be able to alter the innate susceptibility of some foals," added Cohen. A TAMU study presented at the 2004 AAEP Convention by Keith Chaffin, DVM, MS, Dipl. ACVIM confirmed that a subset of foals studied were relatively immunosuppressed compared with other foals, and thus were more susceptible to R. equi infection. Cohen said TAMU researchers are tracking down a possible genetic cause of this susceptibility.

"This is a disease of large, well-managed farms, and the management practices that are desirable for the prevention of a number of other infectious diseases are not sufficient to prevent R. equi pneumonia," concluded Cohen. "Given our previous evidence that there don't appear to be farm-specific strains of R. equi, we think strategies for control and prevention will have to target foal-level factors. "When you think about the triad of contributors to infectious diseases of agent, environment, and host, we feel that we should be paying a lot of attention to the host," he added.

About the Author

Stephanie L. Church, Editor-in-Chief

Stephanie L. Church, Editor-in-Chief, received a B.A. in Journalism and Equestrian Studies from Averett College in Danville, Virginia. A Pony Club and 4-H graduate, her background is in eventing, and she is schooling her recently retired Thoroughbred racehorse, Happy, toward a career in that discipline. She also enjoys traveling, photography, cycling, and cooking in her free time.

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