EXPERIMENTAL MYCOBACTERIUM-BOVIS INFECTION OF CATTLE - EFFECT OF DOSEOF M-BOVIS AND PREGNANCY ON IMMUNE-RESPONSES AND DISTRIBUTION OF LESIONS

Groups of 18-month-old cattle were inoculated intratracheally with 5 x 10(5) colony forming units (high dose) or 500 colony forming units (l ow dose) of Mycobacterium bovis to determine an appropriate dose to in duce lesions similar to those seen in the natural disease. An addition al group of 21-28 weeks pregnant cattle were inoculated with the high dose of M. bovis to determine if pregnancy increased the susceptibilit y of cattle to M. bovis infection. By 23-24 weeks after challenge, the high dose of M. bovis' had induced extensive lung lesions, and tuberc ulous lesions were observed in the lymph nodes of the head, neck, and thoracic and abdominal cavities. In contrast, the low dose of M. bovis induced predominantly small lesions (< 1 cm diameter) which were loca lised to the lungs and pulmonary lymph nodes. The lesions induced by t he low dose were similar to those seen in the natural disease in cattl e. The majority of the high dose group cattle produced strong antibody responses to M. bovis culture filtrate, while only one low dose anima l produced a detectable response. All of the M. bovis-inoculated cattl e produced strong cellular immune responses to bovine PPD (skin test a nd interferon-gamma responses). Pregnancy did not appear to affect the susceptibility to M. bovis infection, and immune responses of the cat tle in this group at the end of the study were similar to those in the high dose non-pregnant group. However, from the first test after calv ing, the interferon-gamma responses of peripheral blood cultures to bo vine PPD were low compared with the responses prior to calving.