Cases of pigeon fever are on the rise and no longer limited by geographic region or season, according to a study from the University of California, Davis.

An infection caused by Corynebacterium pseudotuberculosis bacteria, pigeon fever is spread via stable flies, horn flies and houseflies. The most common clinical expression of the disease is abscesses in the pectoral or ventral abdomen. It is often referred to as pigeon fever because the swelling of the horse’s pectoral region resembles a pigeon’s breast. Two other clinical forms of the disease are less common: in one, abscesses form in internal organs, including the liver, kidneys or spleen; and in the other, ulcerative lymphangitis, the infection affects the limbs.

Pigeon fever was first reported in 1915, and for decades it was primarily limited to hot, arid regions and occurred mainly during the summer months, when insects are most active. However, after unprecedented pigeon fever outbreaks occurred in Kentucky and Colorado in 2002, researchers decided to begin tracking the incidence of the disease. Using data from January 2003 to December of 2012 provided by veterinary diagnostic laboratories in 23 states, researchers were able to piece together a very different picture of the disease.

A total of 2,237 pigeon fever culture- positive cases were identified by the laboratories over the study period. Although 46 percent of those cases were diagnosed between 2003 and 2010, 54 percent of all confirmed cases occurred during the last two study years (2011 and 2012). Texas and California, states where outbreaks of the disease are common, had the highest number of cases, but the researchers discovered that pigeon fever was also diagnosed in Michigan, North Carolina, Vermont and Wisconsin, states where it had never been identified before.

In addition, the number of cases of ulcerative lymphangitis related to pigeon fever jumped fivefold during the last two years of the study period: Four cases were reported from 2003 to 2010, but 20 were diagnosed in 2011 and 2012. “One theory is that there may be a different insect population, such as Culicoides, that might be causing the higher incidence of ulcerative lymphangitis,” says Isabelle Kilcoyne, MVB. “I think more research on the insect vectors needs to be done to investigate this and potentially to look at virulence factors too.”

The study also revealed that even though pigeon fever is associated with insect activity, it can occur year-round. In fact, the greatest proportion of cases (35 percent) were diagnosed from November to January. Kilcoyne says this shift may be due to several factors: “The incubation period can be long for the bacteria and also with milder winters we are seeing changes in the insect population. The important point is that the disease isn’t necessarily associated with a single season anymore.”

Overall, says Kilcoyne, the study may also underscore one of the effects of weather trends. “We think climate change is contributing to changes in insect vectors, resulting in a higher incidence of disease,” she says. “Fly generation times are inversely correlated with environmental temperatures. To take one example, house fly populations are projected to double in this century, which could significantly increase the rate of spread and incidence of diseases like pigeon fever.”

No vaccine against pigeon fever is currently available, but Kilcoyne says owners can reduce the risk of pigeon fever by protecting horses from insect exposure, managing healing wounds carefully and isolating affected horses.

Reference: “Frequency of Corynebacterium pseudotuberculosis infection in horses across the United States during a 10-year period,” Journal of the American Veterinary Medical Association, August 2014

This article first appeared in EQUUS issue #445,

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