Abstract
Sixty-four, 10-week-old turkeys were inoculated with a highly virulent field isolate (86–1913) of Pasteurella multocida serotype A:3,4 by an oculo-nasal-oral route. Inoculated turkeys were examined at 4, 8, 16, 20, and 24 hours post-inoculation for bacteremia and histologic lesions. Bacteremia was detected in one of six turkeys 8 hours after inoculation and in four of six turkey poults at 16 hours post-inoculation. Pasteurella multocida was isolated from the spleens of two turkeys at 8 hours and from the spleens of all six poults 16 hours after inoculation. Peak concentrations of P. multocida reached 10° colony forming units per ml of blood. At 4 to 8 hours post-inoculation, isolate 86–1913 produced a fibrinopurulent bronchopneumonia followed by severe pulmonary necrosis, pleuritis, vasculitis; and, at 16 to 24 hours post-inoculation numerous extracellular bacteria were observed. Hepatic lesions included focal heterophil aggregates 8 hours after inoculation; these progressed to hepatic necrosis. Numerous extracellular bacteria within sinusoids were present 16 to 24 hours after inoculation. At 16 to 24 hours post-inoculation, there was degeneration of periarteriolar reticular cells in the spleen; these cells progressed to coalescing coagulative splenic necrosis with extracellular bacterial colonies. A second group of 41, 10–week-old turkeys, previously vaccinated with the Clemson University strain of P. multocida serotype A:3,4, were challenged with isolate 86-1913. When compared to the nonvaccinated group, vaccination with the Clemson University strain resulted in significantly fewer vaccinated turkeys with bacteremia. Although histologic lesion scores were not significantly different for vaccinated and nonvaccinated turkeys, the vaccinated turkeys had a significantly lower mortality rate. These findings suggest that respiratory infection of susceptible turkeys with virulent P. multocida serotype A:3,4 is followed by septicemic disease characterized by progressive bacteremia with massive tissue necrosis and extracellular bacterial replication in vascular and tissue spaces. In addition, vaccination may reduce mortality rates by lowering the incidence of bacteremia.