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 haemolytica serotype 1 vaccine elicited a serotype-specific inhibition of nasal 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-buyer barn 105 days later, and given a second dose of vaccine. At the order-buyer barn, calves were mixed with 27 calves, some of which had clinical signs consistent with respiratory tract disease. Also 12 of the original calves were infected with P haemolytica serotype 1 by tonsillar instillation. After 6 days at the order-buyer barn, calves were shipped 1,600 km by truck to a feedyard, and arrived the next day. Tonsillar wash and nasal secretion aspiration specimens were collected for culture of P haemolytica on days 1, 8, and 29 at the feedyard. Inhibition of colonization was evidenced by lower frequency of isolations from the vaccinates than from the nonvaccinates after transport to the feedyard. Selectively lowering the frequency of colonization by P haemolytica serotype 1 could reduce losses attributable to pneumonic pasteurellosis.