Pigeon Fever- Making its Rounds

Classic chest abscess from pigeon fever.

Two years, ago Horse Journal reminded readers about Pigeon Fever- a disease that traditionally has been linked to dry, arid conditions prevalent in the western and southwestern United States. Flies carry and transmit the bacteria that causes it, called Corynebacterium pseudotuberculosis. Coryne loves warm, dry conditions and can exist in soil for quite a while. One would think that these factors would inhibit Pigeon Fever from affecting horses in certain parts of the country, but this year is different- Pigeon Fever is popping up all over the United States. At the height of the season, I thought it would be a good idea to review some of the common information.

1) What is Pigeon Fever?

Pigeon Fever, rather ironically, does not involve pigeons at all, and only occasionally involves a fever. It earned its name because many horses that contract it have their chest swell up to resemble a “pigeon breast.” Also called Dryland Distemper (enter the drought conditions!), this disease results from infection with bacteria called Corynebacterium pseudotuberculosis.

2) Is Pigeon Fever contagious?

The good news is that humans cannot contract this infection from horses. The bad news is that it is highly transmissible from horse to horse.

3) How is Pigeon Fever spread?

Pigeon fever is spread primarily through fomite transmission. Fomites are things like flies, human hands, grooming equipment and tack- any object that can carry an infectious pathogen from one animal to another. Among the fomites, flies are the most common culprit thought to spread Pigeon Fever. The organism prefers to live and multiply in dry soil or manure. Flies can pick it up on their mouth-piece and then move to the horse where they can bite and inoculate it into the skin. If an infection takes off, the horse will begin to shed the bacteria from its body (nasal secretions, feces and pus from abscesses.) Pus from abscesses is the most potent source of bacteria, and flies that come into contact with the pus present a significant risk to non-infected horses. The most common fly species to carry and spread Pigeon Fever is the Horn Fly (Haematobia irritans.) You might recognize them from observing cattle- they are the tiny, black flies that swarm all over cattle. They bite and feed on blood, and in some circumstances, carry the Pigeon Fever bacteria on their mouthpiece. Often these fearless little pests sit in blankets on cow’s backs and will quickly resettle right back in the same spot when the animal attempts to swat them away. In horses, they commonly hang out not only near the withers, but also on the ventral midline under the belly. Does your horse have flaky, itchy skin on his ventral midline? Horn flies are likely to blame!

1) What are the signs of Pigeon Fever?

Pigeon Fever can manifest itself in one of three ways. The first, and most common, is an external infection that results in the formation of one or more abscesses. An abscess is a pus pocket created when the body attempts to wall off and encapsulate a foreign invader (such as bacteria). The bacteria grow and divide inside the abscess until so much pressure builds up, it bursts. Just think of a Pigeon Fever abscess as a gigantic zit on the horse! In the early stages of this form of Pigeon Fever, most horses will be lethargic and have a slight fever (usually between 101 and 102o F.) The fever usually goes away after a few days, but then owners begin to notice a swelling in their horse’s chest. Some owners mistake this swelling for a kick wound, but it progresses rapidly and within days, becomes hard. Some owners will notice edema between the legs, or in some cases, a swollen sheath since this infection can manifest anywhere along the ventrum of the horse. In rare cases, the infection will take off in the fold of the tail or even on the back!

Early stages of abscess formation

Mature abscesses about to burst.
Fully open and draining abscesses.
Abscesses in the healing stage.

The abscess grows and matures rapidly and within days to weeks, will burst (or be cut open by the veterinarian). Horses will very commonly be lame during the maturation phase of the abscesses, since the pressure that builds up can be painful. After the abscess bursts and drains, it will then heal over a period of weeks.

In the second form of the disease, which is far less common than the first, the horse can develop an internal infection (usually in the form of an internal abcess.) Internal abscesses commonly occur in the liver and spleen, but have also been reported in the lungs, uterus, and on the kidney. In this form of the disease, the horse will display generalized lethargy, recurrent fever, loss of appetite and weight loss. Veterinarians diagnose it by blood testing and also by ultrasounding the abdomen.

In the third, and rarest form of the infection, horses can get a condition in the leg called Ulcerative Lymphangitis. One leg will become extremely swollen and edematous – sometimes two to three times its regular size! The swelling can extend all the way up to the stifle. The edematous leg will usually begin to leak serum directly through the skin or out of multiple ulcerative wounds that extend up and down the leg. Horses may or may not have a fever with this form of the infection. Surprisingly, they are not always lame with this ailment.

Ulcerative Lymphangitis

4) How will I know for sure if my horse has Pigeon Fever and not just a random abscess?

If you have a horse that has a swelling that is growing rapidly and becoming hard, you should be suspicious of an abscess. However, not all abscesses are Pigeon Fever. The only way to know for sure is to have your veterinarian do blood testing and/or get a sample of the pus inside the abscess and turn it into the lab for culturing to determine if it is Corynebacterium pseudotuberculosis or not. It is very important, for biosecurity of the property your horse is on, as well as for the safety and health of surrounding horses, that you identify the bacteria in an abscess.

5) Is there a common time of year that Pigeon Fever pops up?

Because the bacteria that causes Pigeon Fever prefers a dry, warm environment and climate, the disease occurs primarily in mid-summer to early fall. In some cases, Pigeon Fever can last up to a year in a horse- with wave after wave of abscess.

6) Why do some horses get Pigeon Fever and not others?

We do not fully understand why Pigeon Fever seems to be problematic for only some horses. It is thought that inherent factors such as the horse’s immune competence may play a role. For instance, if a horse was exposed to Pigeon Fever previously in its life and developed antibodies to the bacteria through exposure, that horse will be less likely to come down with an infection because he may have some degree of immunity to the bug. Also, it is thought that nutrition status of a horse, including trace mineral and vitamin levels (such as selenium, copper, zinc, and others) will influence the strength of the horse’s immune system. Finally, if a horse is stressed (such as with training, showing or travel), or if the body is affected by a chronic disease which can impair immune function (such as Cushing’s Disease), it may be more likely to contract the disease.

7) How is Pigeon Fever treated?

Surprisingly, only some cases of Pigeon Fever are treated with antibiotics. It seems counter intuitive since this is a bacterial infection that we are dealing with. However, in most cases of external abscesses, the infection will be self-limiting. This means that the infection will run its course and resolve on its own without a need for antibiotics. In the early stages of the abscess maturation, giving anti-inflammatory medications such as bute can be very helpful not only because they help control fever, but also because they mitigate pain caused by the pressure that builds in the abscess. Many owners try to accelerate the maturation of an abscess and bring it to a head by applying warm compresses to it several times per day. The warmth from the compress more or less incubates the bacteria, thus accelerating their maturation in order to get the abscess to burst.

In the rare cases involving internal abscesses or ulcerative lymphangitis, treatment with multiple types of antibiotics will be necessary, as will long term use of anti-inflammatories. Owners that have horses with either of these two conditions should be prepared for long term, expensive treatments that will often require medications to be given two to three times per day. Also, prognosis for survival with these two forms of the disease is much lower than the first and most common form described above.

In all cases, horses being treated should be provided comforts such as shelter, fly control, bedding to lie down in, feed that is hung at chest level (to make it so that the horse doesn’t have to stretch down and compress a chest abscess to get to it), and clean water. Protection from the weather will be very helpful for horses that are sick- especially ones with fevers.

8) How can I protect the other horses on the property?

Prevention of new infection is definitely key to managing Pigeon Fever. No barn wants to be labeled as a “Pigeon Fever” barn. Since it can exist for months in the soil, biosecurity measures are crucial to avoid it spreading through a barn and reinfecting horses year after year.

Tips for Preventing the Spread of Pigeon Fever

  • ü First and foremost cleaning up manure on a routine basis will lower the amount of flies in the environment as well as reduce the opportunity for flies to pick up and spread the bacteria.
  • ü Fly control is essential to prevent the spread of Pigeon Fever. Using copious amounts of fly spray, and apply SWAT or War Paint to the ventral midline of all horses. Hang fly traps and consider ordering natural fly predators to help cut down on the fly population.
  • ü Segregate infected horses from the healthy population.
  • ü Make sure to limit the number of people that come into contact with the horse and do not bring any horses over to visit during the course of the infection.
  • ü Stall cleaners should clean stalls of Pigeon Fever horses last so that other horses cannot sniff the manure cart and come into contact with the bacteria. Also, stall cleaners can carry the bacteria on their hands and feet, so having them work on the Pigeon Fever stalls last will minimize the spread of the disease.
  • ü Do not use manure carts to carry hay for feeding since this can cause all horses on the property to come into contact with the bacteria.
  • ü Hands should be washed immediately after touching a Pigeon Fever horse or leaving its stall. Use of latex gloves when handling these horses is recommended as well.
  • ü Use a shallow rubber feed bucket as a boot bath outside each stall of a horse that has Pigeon Fever. Use dilute bleach (1 part bleach to 10 parts water) to dip feet in when leaving the stall.
  • ü Feces from infected horses should either be removed from the property or buried at least four feet beneath the ground surface. This will help prevent recurrence of the disease in the future.
  • ü Pus from the Pigeon Fever abscesses is the most potent and highly virulent source of the bacteria. All pus, contaminated bedding or soil, articles of clothing, towels, etc. should be placed in a garbage bag and disposed of in a timely fashion.

9) Is a vaccine available for Pigeon Fever?

Horses that become infected with Pigeon Fever usually have immunity to reinfection for a period of 5 to 7 years. In addition, foals less than six months old do not commonly get the disease (suggesting that maternal immunity from the antibodies in their mother’s colostrum is protecting them). Both of these findings suggest that horses with an antibody reserve (or titer) will be less likely to become infected with Pigeon Fever. So… how come there isn’t a vaccine? Currently, vaccine companies do not see the economic logic in producing a vaccine because they view Pigeon Fever to be a regional problem that does not affect that many horses relative to the overall U.S. horse population. Traditionally, Pigeon Fever was thought to mainly affect horses in California, and only on a sporadic basis from year to year. Now, however, it appears to be spreading across the United States (especially now with drought conditions), so perhaps a vaccine will be forthcoming?