Pathogenesis of fowl cholera: influence of encapsulation on the fate of Pasteurella multocida after intravenous inoculation into turkeys

The role of the capsule in the pathogenesis of fowl cholera was studied in turkeys. Avian Pasteurella multocida P-1059 was used in an encapsulated form, an enzymatically decapsulated form, and a mutant form lacking capsule-productivity (strain T-325). These forms were inoculated intravenously into normal or immune turkeys, and the numbers of viable bacteria in the blood, liver, and spleen were enumerated over a 120-minute period. Both normal and immune birds rapidly removed all three forms of organisms from the blood-stream at similar rates and trapped in the liver and spleen. In the liver of normal birds, the non-encapsulated mutant T-325 was readily inactivated, but the encapsulated P-1059 strain was not. When the decapsulated form of P-1059 was used, the bacterial counts in the liver temporarily decreased at 60 minutes PI. In immune birds, all three forms of organisms were equally inactivated in the liver. In the spleen, however, the bacterial numbers did not change throughout 120 minutes PI with all three forms of organisms in both normal and immune turkeys. The results indicated that the blood-borne P. multocida were readily trapped by reticuloendothelial phagocytes. The trapping process was not affected by encapsulation of the organism or by the immune status of turkey. Both factors, however, appeared to greatly influence the subsequent killing of P. multocida by hepatic, but not splenic, phagocytes.