Known as pigeon breast, dryland distemper, and Colorado strangles, this disease is unpredictable, contagious and, thankfully, curable in most cases.
You go out to the pasture to check on some horses you haven’t looked at for a few days and notice one of them has swelling over the pectoral muscles (front of the chest) or along the underside of the belly, and is walking stiffly. Your first assumption is bruising from a kick, since horses at pasture are always playing around, and the swelling is in a location for a possible kick wound. But when your veterinarian examines the horse, the diagnosis is not trauma; instead, it’s an infection deep in the tissues, commonly called pigeon fever – since swellings on the front of the horse look like the protruding breast of a pigeon. Originally thought to be a disease that only crops up sporadically in a few dry regions of the West, it has been quietly expanding its territory. The past five years have seen an alarming increase in cases, in regions where it was never seen before.
For instance, Fred Robinson, D.V.M, of Riverside Veterinary Clinic, Pendleton, Ore., says his county had an extensive outbreak in 2005, with cases showing up over a large area of northeastern Oregon – a state that historically had not experienced this disease.
“For some reason, we seem to be a hot spot,” Robinson said.
This bacterial infection is caused by Corynebacterium pseudotuberculosis and characterized by deep intramuscular (and sometimes internal) abscesses in horses. It has also been called pigeon breast, dryland distemper, and Colorado strangles. According to Sharon J. Spier, D.V.M, Ph.D., Dipl. ACVIM, and professor in the Department of Medicine and Epidemiology at UC Davis School of Veterinary Medicine, this disease was first reported in horses in San Mateo County of California in 1915. Since then, it has been recognized in many Western states and, in recent years, has spread to other areas, including Kentucky and Florida.
Spier has been doing research on this disease for more than 20 years. There are actually three forms. Most common is the appearance of external abscesses in the pectoral region (breast muscles) or ventral abdomen (along the midline of the belly). Much less common is involvement of internal organs, with abscesses appearing in lungs, liver, kidneys or spleen. Least common in North America is infection of limbs (ulcerative lymphangitis, with swelling and multiple draining lesions on the hind legs). In a study of infections in California, Spier found that external abscesses made up about 81 percent of cases, internal abscesses about 8 percent, and ulcerative lymphangitis about 10 percent of infections. Some veterinarians who deal with pigeon fever have never seen the lymphangitis form of the disease, however, even though they occasionally see cases with internal abscesses.
Sheep and goats are sometimes infected with the same organism, but it’s a different biotype; horses can’t get pigeon fever from sheep or goats. But cattle can be infected with either type, and horses could theoretically get the disease from cattle and vice versa, particularly if they live in the same pastures. These bacteria occur worldwide, and live in the soil. The organism has been shown to survive for up to two months in hay or bedding (straw, shavings) and more than eight months in soil samples.
Incidence of disease fluctuates from year to year, possibly due to herd immunity and environmental factors such as temperature and rainfall, with the highest numbers of cases occurring during dry months – especially fall and early winter. Since flies are thought to be involved in the spread of the disease, and since it takes awhile for abscesses to develop after a horse is infected, most cases start showing up several weeks after fly season begins, and may keep cropping up for weeks after flies are gone in late fall.
Horses kept outdoors or with access to an outdoor paddock seem to be at higher risk than stabled horses, and foals less than 6 months old seem at least risk, which suggests that passive transfer of antibodies via the dam’s colostrum may offer protection to foals born in regions where the disease exists. Horses with internal abscesses are generally diagnosed one to two months after the peak number of cases with external abscesses, according to Spier.
Strong evidence suggests the bacteria can be transmitted via horse-to-horse contact or from infected horses to susceptible horses via insects, or by contact with contaminated soil. The portal of entry is thought to be abrasions or breaks in the skin or mucous membranes. Insects are probably one of the primary ways infection is transmitted, and Spier was involved in studies looking at this possibility.
“We went to farms that were experiencing outbreaks, several different years, and also in years they had no cases,” Spier said. “We were interested in transmission and learning about the reservoir – how the bacteria persist in the environment. Some of that research is still going on, but what we found with flies was quite interesting. When farms were having outbreaks with infected horses on the premises, we could easily find the bacteria in three different species of flies.”
Spier worked with entomologists from UC Davis – Dr. Scott Carroll and Dr. Jenella Loye. They trapped flies using drift traps that capture all the insects that fly through, and they also netted flies from horses that had abscesses. The entomologists identified all the species of flies, and they tested those insects for presence of the bacteria and found them in three common species – the typical housefly, the stable fly and horn fly. Spier noted that the horn fly feeds on the ventral midline, which is where a lot of abscesses start.
“On farms where horses had the disease, we found that as many as 20 percent of houseflies were carrying the bacteria, which means one out of every five houseflies that lands on your horse could possibly transmit the disease,” Spier said. “We went back to the farms on years with no cases of pigeon fever, and found the fly populations negative. This tells us the reservoir is not the fly, but most likely in the soil. We conducted soil studies and found the bacteria can survive in a wide range of soil conditions.”
Spier said that bacteria penetrates the skin though abrasions and then enters the cells. Until the horse can develop an immune response, the infection travels along the lymphatic system and deeper into the lymph nodes, where it can survive in the white blood cells.
External abscesses can be quite deep, in lymph nodes under the muscles in the pectoral (breast) region or the forearm. Due to variable incubation periods, which may be three to four weeks, there may no longer be a break in the skin (such as ventral midline dermatitis from fly bites) by the time the abscesses mature enough to be seen.
Flies, and movement/transport of horses can probably spread the disease, and Spier recommends protecting your horse against flies to prevent the disease.
“There have been episodes as far east as Kentucky and Florida. Though it still has a stigma as being a California disease, it is now present in Texas, Oklahoma, Colorado, New Mexico,
Arizona, Wyoming, Idaho, Oregon and has appeared in several Eastern states,” She said. “It is more prevalent when drought conditions exist, due to characteristics of the bacteria. We are conducting a study right now and don’t have all the results yet, but we know the bacteria survive very well when soils go from moist to dry conditions—which probably allows the bacteria to spread more readily.
“My own theory is that when weather is hot and dry, horses are standing in shade in preferred areas and pass manure where they are standing. These organisms thrive in soil mixed with manure. In dry conditions, the soil underfoot becomes dusty and blows around. The horses are stomping their feet, fighting flies, defecating, and churning the soil into dust, so bacteria could potentially flourish in the environment.”
External abscesses can occur anywhere on the body, but most often develop in the muscles of the breast and along the underside of the belly – and sometimes in the armpit or groin.
“If bacteria enter the midline [where flies often feed], swellings may move forward toward the pectoral region or back toward the sheath or udder. These are the most common areas, but abscesses may appear in other areas where flies feed on the horse, especially if there are abrasions,” Spier said.
The horse may have a single abscess or multiple abscesses involving several regions of the body. Most horses with external abscesses do not develop signs of systemic illness, such as weight loss, but about one-fourth of cases will have a fever for a day or two or even longer. Many affected horses will be lethargic for a while, or stiff and lame from the pain and swelling. In addition, fever may compromise fertility in a stallion or pregnancy in a mare.
“I’ve only had one pregnant mare with pigeon fever in the last three years that developed fever, but she and the foal were both fine,” said Dr. Fred Robinson of the Riverside Veterinary Clinic in Pendleton, Oregon “Fever can definitely affect fertility in a stallion, but fortunately, pigeon fever doesn’t occur in our region until after the breeding season,” he said.
An abscess in the udder can cause mastitis, however, and abscesses on the legs can cause lameness, chronic scarring, and joint problems. There may be swelling at the chest and/or under the belly or between the front legs, or at the sheath or udder. Swelling in the groin or any hind leg lameness may be indicative of infection that could progress to ulcerative lymphangitis, which is a very serious form of the disease. In this instance, one hind leg usually swells, the horse is ill, and the infection is very difficult to resolve. Horses with this problem are severely lame, off-feed and feverish, with swelling, cellutitis and draining tracts along the leg.
If the horse is ill, which can happen without hind leg involvement, further diagnosis is needed to determine if there is internal infection. Internal abscesses may cause death of the horse in 30 to 40 percent of cases. An abdominal ultasound is useful to identify abscesses in the liver, lungs, kidney or spleen. The horse with internal abscesses is usually off-feed, lethargic, with fever and weight loss, and may show signs of respiratory disease or abdominal pain (colic) depending on location of the abscesses.
“Some of the more unusual cases we’ve seen this year were horses with abscesses in their ears,” Robinson said. “The abscesses ruptured on the back of the ears, which is a challenge for medicating them. We had several at the base of the ear, where it ties into the jaw. This is a tough area to drain safely because there are large veins and nerves in that location. Swelling in this area may make it hard for the horse to chew and eat. Abscesses can appear nearly anywhere, and may cause a functional problem just from the swelling and pressure,” he said.
Controlling an outbreak
If you have a case on your property, insect control is important to make sure other horses do not become infected. Horse owners should practice good sanitation, disposing of contaminated bedding (that may contain pus from an abscess), or material from draining abscesses, and clean out a stall that was occupied by an infected horse. Any equipment or blankets, etc. used on the horse should be disinfected. Wash hands after handling an infected horse, and change clothing before handling a healthy horse. Don’t share tools (pitchforks, rakes, manure carts) between stalls if they were used in the stall of an infected horse. Since the organism lives in the soil, however, it is impossible to get rid of it completely.
Pay attention to any wounds or abrasions to make sure they heal quickly and don’t serve as portals of entry for bacteria. Postpone elective procedures that require opening the skin, like certain types of surgery or gelding a horse. Even though the site may heal nicely, a pigeon fever abscess may develop in those tissues several weeks or even months later, if bacteria gained entrance.
If a horse develops pigeon fever, consult a veterinarian for treatment.
“Two forms of the disease [internal infections and infection of the limbs] definitely require antibiotics, but uncomplicated external abscesses usually do not,” Spier said. “Use of antibiotics for external abscesses may actually prolong the time to resolution in some horses.”
Robinson uses ultrasound 90 percent of the time, to identify any deep abscesses and determine their maturity to see if they are ready to drain.
“About the only ones I don’t use ultrasound on are horses whose abscesses are right at skin surface and can be easily seen and felt. With many horses, however, we use ultrasound as a guide to get at the abscess and do as little damage as possible when trying to drain them. Some patients have an initial abscess we drain, and we may start the horse on antibiotics to try to prevent secondary infection in the tissues we open up.”
Yet Spier maintained that every case is different and that the best course of action would be to have your horse examined by a veterinarian.
Draining an Abscess
Deep abscesses should be surgically drained by your veterinarian, since it may take ultrasound to determine the location and extent of the abscess and proper time to drain it after it has matured. In some instances, the vet may recommend hot packing a swollen area to help bring an immature abscess to a head.
Abscesses are usually walled off within a thick capsule containing many separate pockets of pus. Using local anesthetic, often with the horse sedated, the vet may use a long needle (guided by ultrasound) to find the abscess, leaving the needle in place to guide surgical opening with a scalpel. Once opened, pus flows freely from the abscess, sometimes as much as a quart or more of discharge material. This should be collected in a container, for disposal. After draining, the abscess should be flushed with 10 cc of Betadine added to a liter of saline, with flushing repeated daily until there’s no more pus.
Take care to clean up the pus and flushing fluid so it won’t contaminate the stall or pen and infect other horses. Whenever an external abscess breaks open on its own or is surgically drained, there’s risk of the pus spreading the disease to other horses.
“You might have only one or two affected horses, and the rest of the herd is fine, so always handle infected horses last, to avoid spreading the bacteria,” Robinson said. “If you are draining/flushing an abscess, clean up the stall afterward. If it’s an outdoor pen, do your best to clean up the ground where you treated the horse.”
Hope for a Vaccine
The fact that horses tend to have immunity for five to seven years after experiencing pigeon fever abscesses shows that a vaccine could probably be developed. There is currently a vaccine for the strain that infects sheep and goats, but no vaccine for horses.
“We need a vaccine, but a big problem is lack of interest from vaccine companies because they think it’s just a local/regional disease, and no one wants to fund studies and work toward a licensed product,” Spier said. “Most people don’t realize how widespread it is. It’s not highly fatal like West Nile virus or tetanus, but it’s a serious disease. We are just waiting to have a company take it up as a project.”
However, there must be enough economic incentive for a vaccine company to do the necessary research and efficacy testing. At this point, limited work has been done in producing the disease experimentally.
“We have a mouse model of disease, and others have induced the disease in horses, but until we have a vaccine product to test, I don’t want to infect horses with this,” she said. “A protective vaccine is one of my goals. Even though autogenous bacterins are available from a few companies, they can’t be used in the general population of horses.”
Reach Heather Smith Thomas at email@example.com