Colonization of the tonsils and nasopharynx of calves by a rifampicin-resistant Pasteurella haemolytica and its inhibition by vaccination.

A rifampicin-resistant Pasteurella haemolytica serotype 1 with 2 added plasmids was used as a colonization-challenge strain in calves to test the resistance to colonization elicited by vaccination. Nine calves were vaccinated with a tissue culture-derived P haemolytica serotype-1 vaccine which, in a prior study, had elicited a serotype-specific inhibition of nasal and tonsillar colonization by the homologous serotype under field conditions. The vaccinates and 9 nonvaccinated control calves were exposed by tonsillar instillation with the challenge strain. The P haemolytica were enumerated in nasal secretion and tonsil wash specimens collected biweekly for 3 weeks. Rifampicin-supplemented agar medium inhibited growth of other bacterial species in the specimens and, thus, increased the sensitivity of detection of the challenge P haemolytica by 100-fold. The challenge strain retained its plasmids during the period of colonization. Inhibition of colonization was evidenced by lower frequency of isolations and fewer isolations of the challenge strain from nasal secretion and tonsil wash specimens of the vaccinates than from those of the nonvaccinates.