Tyzzer’s disease affects many species of animals. It was originally described as an illness of mice, but has also been reported to cause disease in dogs, cats, rats, gerbils, rabbits, guinea pigs, monkeys, muskrats, hamsters, and foals. Tyzzer’s disease is caused by a bacterium named Clostridium piliforme (Bacillus piliformis). This organism is an obligate intracellular pathogen and can be grown only in tissue cultures.

The disease in horses occurs as an acute, typically fatal disease of foals around one month of age. The foal is usually found dead but clinical sighs of depression, fever and jaundice may be seen. When examined pathologically, the liver is often swollen with areas of hemorrhage and necrosis. Microscopically, the liver lesions consist of areas of necrosis with little inflammatory response. The bacteria can be seen inside hepatocytes at the periphery of the necrotic zones.

Over the last 5 years, 27 cases of Tyzzer’s disease have been diagnosed in foals at the Livestock Disease Diagnostic Center. This number represents 2% of the total foals in this age range that were necropsied. There were 13 colts and 13 fillies and one case where the sex of the foal was not noted. Of the total 27 cases, 25 were Thoroughbreds with the other 2 cases being a Standardbred and a Quarter horse. The high percentage of Thoroughbreds is consistent with the overall breed distribution seen in this facility. The ages of the foals ranged from 7 to 92 days old with an average age of 25 days. Only 3 foals were greater than 30 days of age. All 27 cases died in the month of February through June. No case was received after June 10th.

Cases were from different farms with one exception. This farm was the only one with cases during the several years, 1 in 1993, 2 in 1996 and 3 in 1997