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 tes t the resistance to colonization elicited by vaccination. Nine calves were vaccinated with a tissue culture-derived P haemolytica serotype-l vaccine which, in a prior study, had elicited a serotype-specific inh ibition of nasal and tonsillar colonization by the homologous serotype under field conditions. The vaccinates and 9 nonvaccinated control ca lves 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 a nd, thus, increased the sensitivity of detection of the challenge P ha emolytica by 100-fold. The challenge strain retained its plasmids duri ng the period of colonization. Inhibition of colonization was evidence d by lower frequency of isolations and fewer isolations of the challen ge strain from nasal secretion and tonsil wash specimens of the vaccin ates than from those of the nonvaccinates.