A Consideration of the Relation of Host-specificity of Helminths and other Metazoan Parasites to the Phenomena of Age Resistance and Acquired Immunity

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Abstract
1.The relation of host-parasite specificity to age resistance.Conclusive demonstration of a substantial resistance developing concomitantly with the ageing of the host has been made with reference to four helminths, namely: Ancylostoma duodenale andA. caninumin the dog andAscaridia lineataandSyngamus tracheain the chicken.A. duodenalein the dog andS. tracheain the chicken are manifestly in abnormal hosts. In contrast with this, it is to be noted that the normal hosts of these parasites, respectively man and the turkey, exhibit no appreciable age resistance. With regard to the remaining two cases, those ofAncylostoma caninumandAscaridia lineata, the association of age resistance and a specific host-parasite mal-adjustment is not nearly so clear. In nature, both parasites enjoy a polyxenous distribution, neither having acquired the need for strictly specific host conditions for development. As a result of recent researches withA. caninumit has been brought out that the species is comprised of strains better adapted to one species of host than to another, and it has been demonstrated that a higher degree of age resistance is exhibited in the host parasitised with a foreign strain. Although no information on the matter is available, it is quite possible that age resistance will be found to be less markedly expressed in other species or races of suitable hosts, when the bionomics of bothAncylostoma caninumandAscaridia lineataare further investigated.