Actinobacillus lignieresii infection in two horses.

A 10-year-old pregnant Norwegian Fjord horse was examined for gross swelling of the muzzle of 2 years' duration. Examination of biopsy specimens revealed diffuse dermal fibrosis, micropustule formation, and vascular thrombosis; large numbers of Actinobacillus lignieresii were isolated in pure culture. Prolonged treatment with i.v. administration of sodium iodide and oral administration of trimethoprim-sulfamethoxazole caused regression of the swelling and did not induce abortion. A 5-month-old American Paint filly was examined for swelling in the udder region. Bacteriologic culture of purulent material obtained from the left teat revealed A lignieresii. Treatment with oral administration of rifampin and trimethoprim-sulfamethoxazole resulted in complete resolution of clinical signs. To the authors' knowledge, these findings represent the first report of mastitis and chronic nasal cellulitis caused by A lignieresii infection in horses.