Pigeon Fever Update

There are hundreds of cases of pigeon fever each year in California, said Nathan Slovis, DVM, Dipl. ACVIM, from his experience working at the University of California, Davis. Slovis, who currently is an internal medicine specialist at the Haygard-Davidson-McGee medicine clinic in Lexington, Ky., presented a lecture at the Gluck Equine Research Center on Jan. 27 that covered several topics, including pigeon fever.

Pigeon fever is caused by the bacteria Corynebacterium pseudotuberculosis, which can exist in the environment for weeks or months, said Slovis. The term pigeon fever arises from the fact that the majority of the abscesses arise underneath the pectoral musculature. When the lymph node becomes enlarged and an abscess forms, the pectorals become enlarged and can look like a “pigeon breast.” He said that the disease is “here to stay” in Kentucky, and that veterinarians and horse owners need to become educated and aware of the symptoms in order to get immediate and appropriate treatment for horses.

He said this disease is seen in horses, sheep, goats, cattle, and occasionally affects humans. There is a difference in strains. There are two species-specific strains of pigeon fever based on the difference of nitrate reduction. Horses are affected by nitrate+ strains, while that of small ruminanats (goats and sheep) are affected by nitrate– strains. Cattle can be infected with both strains. Therefore horses cannot be affected by the same strains as the small ruminants. In regards to cattle, there is a slight chance that a nitrate+ strain infecting a cow could infect a horse. Humans are not affected by nitrate+ strains that the horses are exposed to, so there is no need to worry about being infected from a horse that has a draining abscess.

Because of the huge swelling that can occur, “A common complaint from the horse owner is that the horse got kicked,” commented Slovis. “But if you get out the ultrasound machine and look at it, it’s an abscess.”

Slovis noted an increase in the disease throughout the United States last year, especially in Colorado, Kentucky, Montana, and California. Coincidentally, Colorado, Montana, and Kentucky had more cases of West Nile virus last year, especially in the first two-named states.

A retrospective study of 528 pigeon fever horses by Dr. Monica Aleman and Dr. Sharon Spier from UC Davis showed that during the months of September, October, and November, the largest number of cases occurred when dry months were followed by heavy rainfall. Slovis said this is suggestive of a vector (such as the horn fly), but that has not been proven even though Corynebacterium pseudotuberculosis has been detected in horn flies.
Slovis said the study had a higher incidence of disease in younger horses (less than two years of age) as opposed to mature horses. The study also showed that 496 of the 538 horses in had external abscesses; 42 had internal abscesses.

“This bacteria can enter the lymphatics and bloodstream,” said Slovis in explaining the spread of the disease in the horse.

Clinical signs include edema (swelling), fever, lameness, non-healing wound, lymphangitis (swelling of the  lymphatics often seen in the hind limbs of horses), anorexia (unwilling to eat), and mastitis (infection in the mammary glands).

Diagnosis is based on clinical signs, time of year, culture, and a rise in titer (from blood samples) for internal abscesses.

Treatment for external abscesses usually calls for allowing the abscess to mature without antibiotic use, which seems to prolong the maturation of the abscess to drainage while not “curing” the disease; lancing the abscess (being careful not to contaminate the environment with the exudate); and hot packs. Treatment for internal abscesses and lymphangitis includes long-term antibiotics (possibly including Rifampin, TMS, or Chloramphenicol).

“If internal abscesses have not affected the internal organs, then you have a good chance in saving  the horse,” said Slovis.

Prevention of the disease centers on insect control, use of fly sheets on the horses, and decreasing environmental contamination.

“You don’t usually see an ‘outbreak’ on a farm,” noted Slovis.

In 2002 at Hagyard-Davidson-McGee, there were 23 cases of pigeon fever seen by the veterinarians starting in September. Thus far in 2003, there have been three cases at Hagyard-Davidson-McGee, including one that caused an abortion. The organism was not able to be grown from that fetus, but cytology was positive for the disease.

 “The Kentucky Diagnostic Lab usually sees only two or three cases a year,” said Slovis. “Hagyard-Davidson-McGee only had one in the five years prior to 2002.”

One of the points Slovis reiterated was that this disease occurs after dry weather and followed by a wet season. His impression was that there was a higher number of horn flies in Kentucky in the fall of 2002 that coincided with the increased number of pigeon fever cases. “It’s something to watch out for next year,” he said.

In discussing pregnant mares and pigeon fever and the potential for the fetus to have the disease, Slovis said he still would not recommend treating an affected mare with antibiotics unless she is showing clinical signs of illness (i.e., anorexia, fevers, depression, lethargy) or has an internal abscess. Even though it has been shown that the fetus can be affected by C. pseudotuberculosis, the incidence is very low.


About the Author

Kimberly S. Brown

Kimberly S. Brown was the Publisher/Editor of The Horse: Your Guide To Equine Health Care from June 2008 to March 2010, and she served in various positions at Blood-Horse Publications since 1980.

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