SEROTYPE-SPECIFIC INHIBITION OF COLONIZATION OF THE TONSILS AND NASOPHARYNX OF CALVES AFTER PASTEURELLA-HAEMOLYTICA SEROTYPE A1 AFTER VACCINATION WITH THE ORGANISM

Vaccination of cattle with a tissue culture-derived Pasteurella haemol ytica serotype 1 vaccine elicited a serotype-specific inhibition of na sal and tonsillar colonization by the homologous serotype under field conditions. Calves (n = 101) originated from a single farm, where half the calves were vaccinated. The calves were delivered to an order-buy er barn 105 days later, and given a second dose of vaccine. At the ord er-buyer barn, calves were mixed with 27 calves, some of which had cli nical signs consistent with respiratory tract disease. Also 12 of the original calves were infected with P haemolytica serotype 1 by tonsill ar instillation. After 6 days at the order-buyer barn, calves were shi pped 1,600 km by truck to a feedyard, and arrived the next day. Tonsil lar wash and nasal secretion aspiration specimens were collected for c ulture of P haemolytica on days 1, 8, and 29 at the feedyard. Inhibiti on of colonization was evidenced by lower frequency of isolations from the vaccinates than from the nonvaccinates after transport to the fee dyard. Selectively lowering the frequency of colonization by P haemoly tica serotype 1 could reduce losses attributable to pneumonic pasteure llosis.