Development of a sheep model of mastitis caused by Mycoplasma bovis

Mycoplasma bovis (M. bovis) is an important pathogen of cattle, causing pneumonia, arthritis and mastitis. The study described here aimed to develop an ovine model of mastitis caused by M. bovis to facilitate studies of the pathogenesis of this disease. The left mammary glands of five lactating ewes were inoculated with M. bovis, and the left glands of another five lactating ewes were inoculated with culture medium. From day 2 after inoculation, milk production in the infected left glands dropped significantly, from an average of 250 mL/day to 40 mL/day. The mean somatic cell counts (SSC) of the milk from the infected glands were significantly higher than those of milk from the uninfected glands from day 1 to day 20 after inoculation, with the mean SCC of the milk from infected glands increasing from 64 cells/µL to > 4000 cells/µL. At necropsy on day 21 after inoculation, the infected glands were reduced in size, had lost most of the glandular tissue, and were filled with caseous material. Microscopically, the normal architecture of the mammary tissue was lost, with alveolar tissue and lobular ducts filled with inflammatory cells. Antibodies against M. bovis were detectable in the serum and milk of all the infected sheep by day 14 after inoculation. Thus, infection of the mammary gland of sheep with M. bovis induced mastitis typical of the naturally occurring disease associated with M. bovis in cattle, indicating that it could be used as a model to advance understanding of the pathogenesis of this disease.