Pigeon fever is a disease of horses that was originally seen in California over 100 years ago, and then it began spreading to other western states.  A survey in 2012 indicated the disease has occurred in all areas of the U.S. and that the highest number of cases was in Texas with a couple of cases reported in Mexico.  The disease is caused by the bacterium Corynebacterium pseudotuberculosis and it is not known why the number of cases is increasing, although I have seen the disease for 25 years in central Texas and the number of cases is definitely increasing. 

This bacterium also affects sheep and goats but it is a different strain than the one that causes disease in horses. 

It is generally believed that the environment is a factor in survival of the organism as dry soil with fecal contamination promotes survival of the bacteria in the soil.  Also, insect populations are believed to be a major transmitter of the organism, especially house flies, horn flies and stable flies.  The disease is characterized by external abscesses in the pectoral region that cause a large amount of swelling. The swelling resembles a pigeon’s breast.  Also, infection can develop on the horse’s ventral abdomen, and internal abscesses, can also be seen, which are much more difficult to treat.  The external abscesses should not be treated with antibiotics but allowed to mature, and then lanced and drained by your vet.  The internal abscesses can be diagnosed with blood testing and ultrasound and require long-term antibiotic therapy.  The same organism can also cause ulcerative lymphangitis on the horse’s lower legs and antibiotics are required to treat this condition.  If your horse develops a swelling in the pectoral region, call your vet as pigeon fever may be the problem.