Field experiments on the Immunity of Lambs to Parasitic Gastritis caused by a mixed infection of Trichostrongylid Nematodes

Abstract

Four lambs introduced to infective pasture early in life acquired an immunity to parasitic gastritis, the immunity being sufficiently strong to maintain them in good health under conditions of repeated reinfestation, heavy enough to lead to the death of ten lambs of a like age, but not so protected, within periods varying from weeks to weeks. At post-mortem examination the protected lambs were found to carry more worms than many of the lambs which died, although they had been observed to disseminate only one-thirteenth the number of eggs of some of those fatally affected. The infestation of individual lambs in the various groups showed marked differences, amounting in some instances to the presence of a large number of worms of one species in some lambs and the total absence of worms of that species from others.The observations lead to the following conclusions:—1. Lambs are able to acquire an immunity against parasitic gastritis powerful enough to protect them against a rate of reinfection at pasture that is sufficiently great to result in the death of unprotected lambs in less than seven weeks.2. This immunity operates against the acquiring of infection, but also in enabling the lambs to resist the injurious effects of the infection. Resistant animals may carry more parasites than are carried at the time of death by non-resistant animals.3. The immunity is acquired slowly and does not appear to become firmly established for 18 weeks. A gradually acquired infection leads to immunity while a quickly acquired infection leads to death where the final number of parasites is the same in both instances.4. The immunity is specific and operates more powerfully on N. fillicolis and H. contortus than on Ostertagia, Trichostrongylus and Cooperia.5. The immunity has an inhibitory influence on egg laying and on the development of young worms; the egg output of the worms may be reduced to one-thirteenth of the normal.6. There are marked differences in the reactions of individual lambs, instances of marked resistance and of marked susceptibility having been observed.7. The symptoms of parasitic gastritis are due to something more than the abstraction of blood by the worm.8. The part of immunity which inhibits the production of eggs by the parasites develops earlier than the resistance to the harmful effects of the infestation.