Pigeon Fever

by Heather Smith Thomas of Salmon, ID

Pigeon fever is a condition in horses that is also called pigeon breast, dryland distemper, and Colorado strangles. The abscess-producing infection is caused by Corynebacterium pseudotuberculosis and is most common in arid western regions. Some years are worse than others for numbers of cases and some years it doesn’t appear at all. In the summer of 2007 there were quite a few cases in the Northwest. Dr. Debra Sellon, Associate Professor of Equine Medicine, Washington State University, says there have been cases in Oregon and the university has had a few calls from Washington as well, from horse owners and veterinarians. “We’ve been getting quite a few calls from around the Pacific Northwest, particularly northern Oregon and central Washington, the past few months,” says Sellon.

“We have cold enough winters here that it usually goes away by winter, however,” she says. “The organism is in the soil and may be spread by insects, but we don’t know the exact route of transmission. Some people think the horse may get tiny cuts or abrasions in the skin that allow bacteria to enter, especially along the midline of the abdomen, as when the horse is lying down in the dirt. In winter the bacteria are not proliferating in the soil or being spread by insects,” she says.

“This disease used to be something we only heard about in California but it’s cropping up all over the country now in different places. We don’t know why it’s showing up more. It fluctuates every year, but the environmental factors that aid or hinder its spread or that make one year a bad year or not a bad year have not been determined. The highest numbers seem to occur in the dryer months of the year, however, which is late summer and fall, but you could find a case any time,” says Sellon.

“Most of the time, we treat the uncomplicated cases just like any other abscesses. When they are ready, they can be opened and drained, and the area kept clean, and we let it heal on its own. The abscess must come to a head, but if it’s almost ready you can help it out by opening it up. A poultice can sometimes be used to bring it to a head quicker, but the front of the horse is not an easy place to apply a poultice,” she says.

The pus is infective and should be cleaned up so it won’t contaminate the environment and spread the infection to other horses. “We’re not as concerned about isolating these horses as we are the horses with strangles abscesses, but while they are actively draining we try to have them in an area where there are no other horses, or where we want to put other horses, so the environment won’t become heavily contaminated.” A horse with a pigeon fever abscess should be an area that’s fairly easy to clean up after you’ve worked with the horse.

“Every horse is different, and we may or may not use antibiotics to treat this, depending on how bad the infection is, where the abscess is, how well it’s draining, and whether the horse is systemically ill (if he’s not eating or has a fever). We don’t use antibiotics in many cases, but most of the cases that come here to the university are pretty serious and end up on antibiotics,” says Sellon.

Horses with internal abscesses are generally put on antibiotics, but they are harder to diagnose, especially if there is nothing showing externally. “They often come in with weight loss and fever and we can tell there’s probably an infection somewhere, based on the blood work and fever, but sometimes it can be hard to find the cause. The blood work suggests there’s an active infection, so we start looking for it in the abdomen or chest. We had a stallion here a few years ago that had a pigeon fever abscess in his testicle. That took awhile to find because the testicle was not swollen when he first came in. Eventually they had to remove the testicle surgically, to get rid of the infection,” she says.

“We use ultrasound to find the internal abscesses; it helps us see how big an abscess is or whether there are multiple abscesses. It’s particularly helpful for looking at internal or very deep abscesses. Sometimes the abscesses are very deep in the muscles and not very obvious, and ultrasound helps us identify them and can guide the therapy, to decide where to drain or if it’s possible to drain safely,” explains Sellon.

On occasion the infection can affect the legs, if it gets into the muscles around the front legs. “Sometimes lameness is the first sign people see. The abscess is so deep that you don’t see the swelling externally, but the horse feels it and is lame. There is pain and pressure. So you have to figure out where it is and whether or not to drain it.” Every case is individual and the owner may need help from a veterinarian to figure it out.

“There is a blood test available that can be used if you are suspicious there’s a possibility of an internal abscess but can’t find it. This may help tell you if the horse has been exposed. The general blood work like a CBC or white blood count can tell you if there’s a bacterial infection, and then you can do a more specific test, which is a serum antibody titer to this organism. This could help you decide if it’s this problem,” she says.

Just because there’s a titer is not in itself definitive; it may just mean the horse was exposed at an earlier time, but if the titer is above a certain level it probably means an active infection. “The labs have gotten better at interpreting how high is too high. The titer is usually very high with internal abscesses or active infection,” she explains.

The type of antibiotic used will depend on the horse and how sick it is. “For many of them, if they are not terribly systemically ill we’ll use an oral antibiotic. The ones we’ve used the most here lately are trimethoprim sulfa and rifampin. Rifampin is not a drug we use a lot in horses; it’s used most frequently in foals with Rhodococcus pneumonia. We usually don’t give it by itself; it’s almost always given in combination with something else. The two antibiotics complement one another. Also, if rifampin is given by itself the organism can often rapidly develop resistance, but if you give it with another antibiotic it keeps resistance from occurring,” she explains. “If the horse is really sick, we may still use rifampin, but combine it with something we can give intravenously, and hospitalize the horse (for more supportive treatment).”

Pigeon fever is not something that clears up very fast. You’re usually looking at weeks of treatment/recovery, depending on how bad it is. “Some horses will get multiple abscesses along the ventral midline. These can take quite awhile to clear up,” she says.

A bacterium by the same name causes a common problem in sheep, called caseous lymphadenitis. “Every now and then we have people ask if their horse got pigeon fever because of exposure to sheep. The strain that affects horses is different from the one that affects goats and sheep; it does not get passed back and forth between them,” she says. They are different biotypes.

Originally pigeon fever was only seen in the western states, but in recent years has also turned up in the East. “I’ve heard of cases in Kentucky and elsewhere east of the Mississippi,” says Sellon. “Here in the West we see a lot of cases some years, and other years we don’t see any,” she says. Horses with serious cases are often brought to WSU for treatment, and come from Washington, Oregon and a few from northern Idaho. One year a horse with an internal abscess was brought to WSU from southern Idaho.

“It’s not highly contagious, like strangles or flu, where a horse might get it if it’s next to an infected horse. We might have more than one case of pigeon fever on a premise but we don’t have herd outbreaks like we do with strangles or flu,” she says.

“There is another form of pigeon fever that we don’t see very often in North America but is seen quite commonly in other parts of the world, and it is called ulcerative lymphangitis. There’s infection of the lymph vessels in the legs. One or two of the legs are swollen, with draining tacts from the lymph vessels. Those horses need pretty aggressive treatment, so it’s good we don’t see this form very often,” says Sellon.