America has an outbreak of pigeon fever. This infection, which historically been limited to arid areas, has been surfacing all over the country as far East as Florida. As it spreads into areas not usually affected new manifestations of disease have been seen and possibly new strains have developed. Anytime disease spreads faster than fact, misinformation and panic often cloud barn and internet discussions. As with many equine diseases the list of fact about pigeon fever is small compared to the list of opinions.
Pigeon fever, sometimes called dryland strangles, is caused by Corynebacterium Pseudotuberculosis. This bacteria can live in the soil but is generally thought to be transmitted to horses by fly bites or cactus punctures. Once the skin has been penetrated the bacteria travels through the lymphatic system to regional lymph nodes. Because the flies tend to affect horses along their bellies where the skin is more thin, and serum more easily obtained, the bacteria once across the skin often collect in lymph nodes deep within the pectoral muscles of the horse causing them to enlarge ‘like a pigeon’s breast’. The time between inoculation of the bacteria under the skin to the development of abscesses is unknown but it could take months.
As with any abscess forming disease the use of antibiotics is controversial. Generally excepted medical practice is to use them only if the patient is “sick’ (ie: the disease is life threatening), prior to the development of abscesses, or only after the abscess is draining. The concern is that the bacteria ‘will be driven deeper’ causing a more serious form of internal involvement with a much worse outcome. My personal experience has been that antibiotics are useful only with the “sick” horses to help them stabilize. Unfortunately in most cases antibiotics do not seem to shorten the course of the disease process, and may even lengthen it.
Although the disease is often reported on the internet as contagious, personally I prefer to use this term for a disease that can be transmitted directly from horse to horse more easily like true strangles caused by Streptococcus equi equi. While true strangles will travel through a barn and affect basically all horses not previously exposed, like the flu or chicken pox, pigeon fever typically affects only the occasional horse in a geographical area. This is typical for a disease that requires a vector of transmission, like the fly, and a naive immune system. This is not to say that one shouldn’t employ good hygiene during an outbreak. Reasonable disinfection to minimize transfer of exudate (pus) between horses is key but unless a stable can sterilize the soil and eliminate all flies completely breaking the pigeon fever cycle is unlikely.
As the consulting veterinarian for Equine Voices Rescue & Sanctuary through this outbreak I have recommended conservative and holistic management for the effected horses. Because flies have been shown consistently, through research and clinical observation, to be the primary source of transmission, efforts to stop the spread of the disease are as successful as stopping the spread of flies. Other research has supported the theory that the soil is the primary reservoir for the bacteria Corynebacterium. Imagine trying to eliminate the flies and soil in Tucson! Through the years I have treated many inflicted horses and have found that antibiotics tend to complicate and lengthen the natural course of the disease and, therefore, reserve them for compromised horses. Instead our recommended approach has been to support the immune system. Hundreds of dollars have been spent on immune system support supplements and to date all effected horses at Equine Voices have responded and have recovered well.
Horse rescues do such an important and often thankless job, and many of the horses that arrive at sanctuaries arrive with depleted immune systems. Equine Voices has rescued hundreds of horses that would otherwise have gone to slaughter, and although pigeon fever is at the sanctuary, it has affected less than ten percent of their herd. Their care and management of the effected horses has been excellent and within the standard guidelines of current veterinary medicine.
Michael D. Hutchison, DVM, cVMA
Pegasus Equine Veterinary Service, PC
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