Diminished immunological responsiveness to helminth parasites. The effect of repeated reinfection of rats from an early age with Nippostrongylus brasiliensis.

An unusual form of immunological unresponsiveness peculiarly suited to the propagation of nematode parasites has been revealed by a study of the effect of repeated reinfection of rats from an early age with Nippostrongylus brasiliensis. In essence such rats, while showing other signs of a functional immunity to the parasite, are unable to eliminate a threshold population of approximately 200 worms from the intestine, maintaining a patent infection beyond the time when their immunological competence would otherwise be fully developed and in the face of repeated challenge infections in adult life. A short worm-free period following anthelmintic treatment allows a transition from this incomplete response to the type of response typically seen in adult rats. The importance of the results is the demonstration that the normal expression of the immune response of an adult animal, with respect to a particular parasite, can be substantially modified as a result of early and continuous exposure to infection. It is suggested that a similar form of unresponsiveness may explain epidemiological features of many helminth infections of animals and man.