Some evidence of an age susceptibility to contagious bovine pleuropneumonia

In two separate experiments it was shown that cattle over three years of age were more resistant to contagious bovine pleuropneumonia (CBPP) than younger animals. A statistical analysis of the results of one of the trials indicated that the difference in the response to challenge between calves less than one year old and animals between two and three years old was not significant. However, when these two groups were compared with animals over three years old the results were highly significant (P less than 0.001). When death was used as the sole criterion of response to challenge a similar picture was seen: nine 14 calves died, four of 15 animals aged between one and two years died, five of 11 animals aged between two and three years died, none of 16 animals aged over three years died. No attempt is made to explain this increase in resistance with increasing age. It is suggested that a better knowledge of the factors that influence resistance and susceptibility would be of value in understanding the pathogenesis of CBPP. It is recommended that to ensure maximum susceptibility of control animals in vaccine trials, all animals used should be less than three years old.