Pigeon fever becomes more prevalentWritten by Heather Smith Thomas
By Heather Smith Thomas, WLR Correspondent
Pigeon fever is an infection caused by Corynebacterium pseudotuberculosis, which creates abscesses in the chest/breast area and underline horses. On occasion, it can also cause internal abscesses.
Ken Mills, PhD, Professor of Microbiology at the University of Wyoming and head of the microbiology section at the Wyoming State Veterinary Diagnostic Laboratory, says the number of samples sent to the lab are small, compared to the actual number of cases in the state on any given year. Some veterinarians, he explains, don’t send in samples. “Once they’ve seen a few cases, they just diagnose them on clinical signs and don’t send them in. We’ve only seen a smattering of samples this year, from the Bighorn Basin and a few other areas where we were surprised to see it. The most cases have been in the Cheyenne area,” he says.
“I don’t think horse owners can do much to prevent this disease, especially the way horses are moved around. The main thing, if you have a horse with an abscess (and huge volumes of infectious material when the abscess is draining), is to dispose of that material properly so it won’t infect other horses. In some instances it’s possible to have as much as a gallon of pus. You should try to collect that material and disinfect it. The easiest way to do that is mix bleach with it. Bleach is a good disinfectant. If the pus is still liquid you can dump it down the drain, but it’s wise to try to sterilize it with bleach. Sunshine is also one of the best sterilizers for contaminated dirt. A moist and shady environment (such as where there’s straw that hasn’t been cleaned up) is ideal for maintaining the bacteria,” explains Mills.
“Try to reduce the amount of contamination that might be spread from the abscesses. When you see a horse with this problem, move it to an area where you have a better chance of cleaning it up if it breaks and drains or if you decide to drain it. If you have other horses, you want to reduce the amount of exposure. As with most bacteria, the rate of infection is tied to the amount of exposure,” he says.
“This bacteria is genetically not the same as the one with the same name that infects sheep and goats. There’s no transmission from sheep and goats to horses or vice versa; it’s not the same organism,” he says.
Historically, Corynebacterium is fairly hardy, due to the way it grows, so it can last a fairly long time in the environment if it’s not in direct sunlight. “These bacteria have an exterior similar to that of the microbacterium that causes tuberculosis. It’s a waxy exterior and it’s difficult for disinfectant to get into the organism. It’s also difficult for the immune system to clear it from the body because it can’t get at it,” he says.
“Even though it’s called pigeon fever because it causes swelling in the breast area, it’s certainly not limited to that site. It can occur on the legs, or internal organs. Regarding antibiotic therapy, I doubt very much that this would help retard a developing abscess on the chest. But I can certainly see the reason to treat a horse if you are not sure what’s going on inside. If you can stop development of abscesses in organs, this may be crucial, because horses can certainly die from this,” says Mills. “When you get abscess formation close to a major blood vessel and it ruptures, the horse dies. If the infection gets into the bloodstream, it’s serious.”
Regarding external abscesses, the best treatment is just to lance and drain them, to get rid of them. Antibiotic therapy is more to prevent crops of abscesses that might get into the vital internal organs. “I’ve seen reports where horses develop abscesses and seem to be recovering and all of a sudden they tip over, due to a ruptured internal abscess.” The big ugly abscess on the outside may be the tip of the iceberg.
Many horses with external abscesses do fine even without treatment; the disease runs its course, the abscesses break and drain and heal. But a small percent of cases have problems with internal abscesses that may cause death of the horse unless treated.
There are still many things we don’t know about this disease, such as how it is transmitted. “It crops out in certain areas and not others. They had it in Colorado and it moved up through the Front Range, and now we have it in many areas of Wyoming. Southern California has had it for a long time. We’re still not sure about the conditions that cause it and spread it. Some people think it’s spread by insects, and that seems logical because it often occurs along the horse’s midline, where insects feed. The other thing I always wonder about, since it seems to be tied to drought conditions, and horses are reaching through barbed wire trying to get a little bit of grass on the other side of the fence, is if small injuries from the barbed wire might be a factor.” This could break the skin in the breast region and allow bacteria to enter. There are certainly other possibilities for injuries to the skin, but barbed wire could be one factor, he says. And the fact that cases tend to diminish or cease after advent of cold weather lends credence to the idea about insects helping spread it.