Pigeon fever is often found in the dry climates of the West, but Ruth Sorensen, DVM, typically doesn't see as many cases as she has this year. Her practice, which includes part of northern Colorado and southern Wyoming, has recently seen a spike in the number of horses affected by the disease, which can be tough to spot and even more difficult to treat. Recently, Western Horseman interviewed Sorensen about her experience treating horses with pigeon fever, and what owners can expect if their horses become infected with the disease.
Why is the bacterial infection called pigeon fever?
Pigeon fever is the nickname largely due to the abscess lumps that often appear on the pectorals, making the horse look like it has a pigeon's breast. But horses don't often have a fever, so the name is a bit misleading. Sometimes people call it "pigeon breast." I've also heard it called "dryland distemper" and "Colorado strangles."
How prevalent is pigeon fever?
Most Western states are affected—California, Oregon, Washington, Colorado, Arizona—but cases are also increasing in Florida and Kentucky, probably due to movement of horses in those areas.
Why do you think you've seen an increase of cases this year?
This past summer, we had an increase in flies that carry the bacteria. My own horse had an unusually bad "fly strike" area under his belly, which could be the reason he was more vulnerable. Most of Colorado has been in a drought for the past 10 to 15 years, and drier climates seem to be more affected. Other environmental factors, such as temperature, also affect the spread and viability of the organism. The disease also tends to run in seven-to-10-year cycles. I saw several cases of pigeon fever in 2000 and 2001, so maybe it's come back around again.
What causes pigeon fever?
A bacteria called Corynebacterium pseudotuberculosis.
How do horses get it?
It's thought that the bacteria enter through skin abrasions and insect bites. So, there are several different ways: a bite from an insect carrying the organism; horse-to-horse contact; horse-to-cow contact; through the soil; through a skin abrasion; or through a break in the skin. My mare got it from my stallion when he bred her, so I have to assume it can be transferred from horse-to-horse during breeding.
What are the symptoms?
This is the tricky part. The symptoms can vary widely between individuals, which is what makes it difficult to diagnose. It doesn't always start with the typical pectoral swellings, as the name implies. A swelling in the face might appear just like a tooth root abscess. A swelling in the sheath or groin might look like a kick or an injury. An abscess on the back may look like a saddle sore. But once pigeon fever is on the property, you have to be suspicious of every abnormality in a horse. A horse may appear stiff or lame, run a fever, go off feed, have a wound, have a swollen sheath or mammary glands, lose weight, perform poorly or appear depressed. You have to put the whole picture together.
When my stallion got it, he had what I would characterize as the typical signs: softball-sized abscesses on the chest, abdominal swelling and edema, and he was just not himself. My mare got it next, with an abscess on her neck. In another case I saw, a swollen sheath was the only sign initially, but after several weeks, an abscess appeared in the groin area.
Another horse I saw was just not thriving, and had been losing weight. Bloodwork showed he was anemic with low neutrophils [white blood cells], which is often seen in cases of chronic inflammation. After a few other tests that came up negative, I started to think he might have pigeon fever with internal abscesses. Sure enough, after a couple weeks he popped some abscesses on his neck.
How serious is the condition?
It's usually not serious, and is self-limiting without antibiotics or expensive treatments. However, a couple of conditions can make pigeon fever life threatening:
First, if an abscess appears in a dangerous location. For example, my 27-year-old broodmare came up sick late one night, unable to breathe with feedstuff draining from her nose. She was weak, dehydrated and her head was swollen. I wasn't sure what was wrong at first, as pigeon fever can look like many other things. It turned out she had a big abscess that popped up on her neck, partially occluding her esophagus, trachea, jugular vein and carotid artery. In that case, I used anti-inflammatories and antibiotics to reduce the size of the abscess, because in such a situation it's often difficult to lance the abscess and drain it. I couldn't go poking around with a blade with so many vital structures in the same region.
Another problem can be if an abscess internalizes. In some cases—less than 10 percent—abscesses internalize and affect a major organ, sometimes causing a horse to colic. This is one of the reasons I generally avoid using anti-inflammatories and antibiotics before the abscess comes to a head. It's thought that use of such medications may cause the abscess to internalize and become a greater concern.
The other dangerous spot is in the hind legs. Ulcerative lymphangitis—swelling and draining lesions on the hind legs—can be extremely difficult to resolve.
