Efforts to Transmit and to Enhance the Virulence of an Avirulent Strain of Pasteurella multocida in Turkeys

Abstract

Nonvaccinated turkeys were added to groups of turkeys that had been recently vaccinated with the avirulent Clemson University vaccine strain of Pasteurella multocida by the drinking water route. Separate control groups were nonvaccinated. Five weeks after treatment, all groups were challenged with a highly virulent P-10S9 strain. None of the turkeys added to the vaccinated groups developed clinical evidence of fowl cholera disease during the 5-weeks exposure period. After challenged with the highly virulent P-1059 culture, the total number infected was 95% in the nonvaccinated groups, 12.5% in the turkeys that had received the avirulent vaccine strain, and 85% in the nonvaccinated turkeys added to the vaccinated groups.These results suggest, under the conditions of this experiment, (1) that if the vaccine strain is transmittible by cohabitation, then this transmission is not easily accomplished and (2) that the unvaccinated turkeys remained susceptible to fowl cholera infection.In a second experiment, the vaccine strain was injected intravenously into turkeys, was isolated from them after death and was then injected into additional turkeys, etc., for a total of 10 turkey passages. Separate groups of turkeys were then exposed by the drinking water route to the 5-passage isolate, to the 10-passage isolate, and to the original vaccine strain. Other groups were not exposed. Two weeks post treatment, all groups were then challenged with a highly virulent P-1059 strain of P. multocida.Under the conditions of the second experiment, one of the 20 turkeys exposed to the 10-turkey passage isolate developed an infection of the tibiometatarsal area. None of the turkeys exposed to either the 5-turkey passage isolate or to the vaccine strain developed clinical evidence of fowl cholera disease. After challenge with the highly virulent P-1059 strain, the total number infected was 5% in the turkeys that had been exposed to the 10-turkey passage isolate and 10% in those that had been exposed to the 5-turkey passage isolate. None of the turkeys that had received the vaccine strain developed evidence of infection and the infection rate in those not vaccinated was 85%.The results of the second experiment suggest that there was little or no increase in the virulence of the avirulent vaccine strain after either 5-turkey passages or after 10-turkey passages and, also, that the ability of the vaccine strain to immunize did not appear to be seriously impaired, if at all, by these passages.