Pigeon fever was first reported in California in 1915. It occurs sporadically in arid regions of the West and seems to be expanding in territory. In recent years it has cropped up as far east as Kentucky and Florida. This disease appeared for the first time in eastern Oregon a few years ago, says veterinarian Chris McIlmoil, Country Animal Clinic, Island City.
"The first cases we saw in our area were in September 2004. My colleague, Dr. Mark Omann, has practiced here since 1988 and had never seen a case here in the valley. In 2004, we saw more than 20, in 2005 we had a few less, then in 2006 we had more, and this year we've seen nearly 50 horses with it. Most of these have had external abscesses, but a few have also had internal abscesses," he says.
"When there are abscesses the main question is whether to treat or not. We have established a treatment pattern. If the horse has one or two external abscesses along the underline of the belly and they are small, crusty abscesses, we can open them up and they drain out thick pus. If that's all there is, we just flush those out with dilute Betadine and have the owner continue watching the horse. A lot of those horses clear up and are fine. Another aspect of the disease is when there are, in addition to the abscesses, some swellings due to fluid in the tissues," says McIlmoil. These swellings may go up to the muscles in the breast or chest region or clear back to the udder or sheath.
"They may be any size from a couple centimeters up to 10 or 12 centimeters of edema along each side, spreading in long strips along the belly. We may also see swollen pectoral muscles in the front. This is where pigeon fever or pigeon breast gets its name," he says. The horse's front is sticking out like the breast of a pigeon.
"When we see the swollen chest we often cannot drain the abscess right away. It may take seven to 30 days to get to the point it can be drained. The infection is presumed to travel in a certain type of white blood cell and is spread via the lymph vessels," he says. This is why it can be so deep in the muscle. Inflammation can block the vessels and this leads to the edema, he explains.
"There's a very thick capsule around the abscesses in the chest, thicker than a typical abscess. These must be opened surgically, with a scalpel blade. Abscesses may appear in other areas as well. We've seen abscesses in the udder, in the foreskin of male horses, abscesses on the withers, hips, or just about anywhere on the horse," says McIlmoil.
"Now when we see a horse with any sort of swelling or abscess on the body, we become suspicious and do a culture. We take a sample of the material and send it to a lab to determine if this particular bacterium is present. There is also a blood test that measures antibody level. When we see a certain number, this is a high indicator for internal abscesses," he explains.
"If a horse has one abscess under the belly that we can just drain, flush and have the owner monitor, and we don't see any other signs, we don't feel there's a need for antibiotics. If there's anything more than that, we do use antibiotics on that horse along with draining and flushing, as well as using anti-inflammatory medication, because it can be quite painful to the horse to have that much swelling in the chest," he says.
If a horse owner sees swelling or an abscess on a horse, they should be in contact with a veterinarian and describe the condition.
"If someone calls and tells me the horse has one abscess and it's open and draining and wonders if they have to bring the horse in, I usually tell them to continue watching, and keep me posted. Most of the time, however, I want to see the horse, to fully answer the owner's questions. My responsibility as a veterinarian can't be fully completed without examining the patient. If a horse owner suspects a problem, they need to consult a veterinarian," he says.
There are puncture wound abscesses, for instance, that can look just like a single pigeon fever abscess. It's best to know exactly what you are dealing with, McIlmoil says.
Pigeon fever infections generally appear during fall and early winter months then subside after cold weather, with highest number of cases in September, October and November.
"By December last year, for instance, we just had a couple lingering cases that were internal abscesses. The fact that incidence diminishes after cold weather lends credence to the idea the bacteria may be spread by flies, but since the incubation period is not known — and could be weeks to months — you could still have abscesses appear even after the insects are gone," he says. This is why cases may continue into early winter.
"In summer, fly control might be helpful. There was a study at UC-Davis, looking at risk factors. They recommended keeping your pastures clean and practicing insect control, since these are logical things that could help control the disease," he says.
"Everyone who has a horse should be checking that horse daily, looking closely at the underside checking for swelling because these are the common areas where you will see abscesses. Every horse on your place should be looked at daily, and this time of year it's not always easy because people are feeding morning and night in the dark. If you do find swelling, contact your veterinarian, rather than just trying to guess at what's going on," advises McIlmoil.
NOTE: Corynebacterium pseudotuberculosis can be spread to humans. Potential mechanisms of transmission include drinking milk from an infected goat or cow, handling contaminated equipment, exposure of wounds to exudates/pus from abscesses.