Five horses in Washington State have tested positive for pigeon fever since the beginning of October. The cases include two horses on the same farm in Clark county, and three isolated cases on farms in King, Kitsap and Thurston Counties, according to the Equine Disease Communication Center (EDCC).

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 horses in San Mateo County (San Francisco Bay Area) of California in 1915. For much of the 20th century, pigeon fever was most commonly found in the Southwest and in California. But changing weather conditions has lead to the spread of the disease. It’s now been documented in 25 states and has the potential to occur nearly year-round. 

Click here to read an in-depth article on Pigeon fever. 

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