Pneumonic pasteurellosis of cattle: microbiology and immunology.

Pasteurellosis was investigated under natural conditions by comparing bacterial and viral nasal flora and levels of bacterial and viral antibody in sera and nasal secretions between animals sick with the disease and those that remained well. The animals were classified sick or well on the basis of the levels of body temperature and plasma fibrinogen. The most significant feature of the bacterial flora was the higher frequency of isolation and the numbers of Past. hemolytica in the nasal flora in the first two weeks after shipment. As indicated by the number of animals with serum antibody to PI-3 virus, infection with this virus was active in both sick and well animals, and serologically, the incidence of infection was higher in animals that remained well. Nasal antibody to PI-3 virus was slightly lower in incidence than serum antibody. Examination of untreated fatal cases and animals killed during the experiment suggests that in some animals there may be a relationship between the high numbers of a bacterial species in the nasal passage and infection by that organism in the lung.