RESISTANCE STUDIES WITH THE NEMATODE, NIPPO-STRONGYLUS MURIS, IN LABORATORY RATS*

Abstract

Following initial infestation, rats re-exposed dermally to infective larvae exhibited acquired resistance as measured by egg production of resulting infestations as compared with that of controls. When graded nos. of larvae (50-6400) were used for initial infestation, followed by re-exposure to 600 larvae per rat after the 1st infestation had ceased to produce eggs, greater resistance was manifested in groups initially exposed to 800-6400 larvae per rat, than in groups initially exposed to 50-400 larvae. In groups of comparable age, initially exposed to a small no. (50) of larvae and re-exposed at weekly intervals to a no. equal to twice the total of the previous nos., resulting egg-count maintained a fairly low level as compared with that of groups introduced into the series at each weekly infection period where heavy egg production followed exposure to the larger number of larvae. Egg production resulting from initial infestations from exposure to graded nos. of larvae was not strictly proportional to number of larvae used, but showed little evidence that egg production was reduced markedly in heavier infestations.