Fate of Pasteurella multocida in the blood vascular system of turkeys following intravenous inoculation: comparison of an encapsulated, virulent strain with its avirulent, acapsular variant.

A virulent, encapsulated strain of Pasteurella multocida was compared with a spontaneously arising, avirulent acapsular variant following injection into the bloodstream of 14-week-old turkeys. Neither strain was detectable in the blood by 1 hour, but they reappeared 4 hours postinoculation in approximately equal numbers. The concentration of both strains increased with time, but the virulent strain reached concentrations 100,000-fold higher than the avirulent strain 15-24 hours after inoculation. In the liver and spleen the virulent strain reached higher concentrations than the avirulent strain, particularly 15 hours postinoculation. However, histopathological examination indicated that the difference between concentrations of the two strains was more likely due to an increased propensity for extracellular multiplication of the virulent strain rather than to greater efficiency in phagocytosis of the avirulent strain. In vitro, the two strains became associated minimally, though equally, with the mononuclear phagocytes and were destroyed. We conclude that humoral bactericidal defenses are primarily responsible for the differences in behavior between these two strains of P. multocida in vivo.