Pigeon fever in horses on rise across the US, study confirms

A horse with Pigeon Fever. © University of Florida
A horse with pigeon fever. © University of Florida

Researchers have confirmed a growing number of cases of pigeon fever among horses in the United States in the last 10 years, with a particularly notable surge in 2011 and 2012.

They say further studies are warranted to determine changes in annual incidence and to identify potential climatic conditions or vector populations associated with its transmission.

Pigeon fever, so named because of the basketball-sized pectoral abscesses it can create, is caused by the bacterial organism Corynebacterium pseudotuberculosis. It is considered endemic in California and some other western US states.

While infection results in abscesses typically in the pectoral region of horses, they can occur in other sites as well. They can develop along the belly and the lower neck region, or on a front or rear limb.

Isabelle Kilcoyne and her colleagues at the University of California, Davis, set out to quantify the number of horses that had suffered the infection in the US from January 2003 till December 2012.

In all, 44 state veterinary diagnostic laboratories throughout the US were invited by mail to participate in the study.

Data requested included the number of positive samples each year, the geographic location of the cases, the month and year of sample submission, the breed and age of horses, and the category of clinical manifestation (that is, internal infection, external infection, or ulcerative lymphangitis).

Of the 44 invited laboratories, 15 agreed to participate and provided data on affected horses from 23 states.

In all, the researchers assessed the state veterinary diagnostic laboratory records of 2237 C. pseudotuberculosis culture-positive samples from horses.

The proportion of C pseudotuberculosis culture-positive samples submitted during 2011 through 2012 – 1213 of the 2237 samples – was significantly greater than that from 2003 through to 2010, during which 1024 were reported.

C. pseudotuberculosis was recovered from horses in states where the disease has not been previously recognized as endemic, the researchers noted in their findings, which have been published in the Journal of the American Veterinary Medical Association.

Affected horses were identified year-round, they noted.

The greatest proportion of positive samples was identified during November, December, and January – some 35 percent of all cases.

No significant association between the clinical form of disease and age or breed of horse was observed.

The researchers said the occurrence of the infection in horses rose during the 10-year period, and affected horses were identified throughout the US.

“Further studies to determine changes in annual incidence and to identify potential changing climatic conditions or vector populations associated with disease transmission are warranted to help control the occurrence and spread of C. pseudotuberculosis infection in horses,” they concluded.

Scientists believe the bacterium that causes the infection can be carried in dust, hence the tendency toward more cases in dry conditions. Flies also carry the bug, and can infect open sores or wounds if they come into contact with them.

Kilcoyne I, Spier SJ, Carter CN, Smith JL, Swinford AK, Cohen ND.
Frequency of Corynebacterium pseudotuberculosis infection in horses across the United States during a 10-year period.
J Am Vet Med Assoc. 2014 Aug 1;245(3):309-14. doi: 10.2460/javma.245.3.309.

The study abstract can be read here.



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