Influence of Age and Sex on Susceptibility of Dogs to Primary Infection with Ancylostoma caninum

Abstract

Natural age resistance, in absence of previous exposure to hookworm, commences to operate against the intestinal establishment of Ancylostoma caninum at an earlier age in the bitch than in the dog. There is a marked age resistance in the adult animal and adult bitches are significantly less susceptible to primary infection than are adult dogs. During investigations of the immune responses of dogs of various ages to experimental vaccination with irradiated Ancylostoma caninum larvae (Miller, 1965a) discrepancies were noted in the susceptibility of some of the groups of control animals to primary infection with normal larvae. Reduction in susceptibility was attributed to natural age resistance in absence of prior exposure of the control animals to hookworms (including Uncinaria). Closer examination of the results indicated that adult bitches might be more resistant than adult dogs. Further examination of the susceptibility of control dogs and bitches of various ages was undertaken by the analysis of the results of four different experiments. MATERIALS AND METHODS These have been described previously (Miller, 1964). Dogs and bitches of mixed breed and of various ages, without prior exposure to hookworm (including Uncinaria), were infected with third-stage A. caninum larvae by subcutaneous inoculation and their worm burdens recorded at necropsy at 21 to 26 days after infection. The weights of the experimental animals varied among the different age groups and within the adult groups, and in view of the variations with age in the susceptibility of dogs to the pathogenic effects of hookworm (Miller, 1965a), the infections were computed at an approximate rate of between 50 and 100 larvae per lb body weight. Whenever older dogs and bitches were infected a group of young pups aged 8 to 12 weeks were also infected, their necropsy worm burdens serving as control for the infectivity of the larvae used in each experiment. The results were grouped according to the age and sex of the experimental animals and the necropsy worm burdens were calculated in terms of percentage hookworm establishment of the dose of infective larvae (i.e., per cent take). Mean group per cent takes and standard deviations of these means were calculated for each different Received for publication 13 November 1964. 701 group of animals. The results from the different groups were compared statistically where possible using Student's t test. For the purpose of determining susceptibility to infection, the mean group per cent takes (combined dog and bitch pup results) for the groups of 8to 12-week-old control pups were taken as representing 100% susceptibility in each of the four experiments, to facilitate comparison of the results of different experiments in which larval infectivity might vary. A h stogram was prepared to facilitate this comparison. There were no apparent differences between the types or breeds of puppies, dogs, and bitches used for the different experiments. All the experimental animals originated from pet dealers in the more crowded areas of the city of Glasgow. Adult animals were known to be considerably more than 1 year of age and most of the bitches had whelped at least once before commencing the experiments.