Biology of the Rat Nematode, Nippostrongylus brasiliensis (Travassos, 1914). III. Characteristics of N. brasiliensis after 30 to 120 Serial Passages in the Syrian Hamster
An average of only 2.8% of the infective larvae of a rat strain (RS) of N. brasiliensis developed to maturity in adult Syrian hamsters. Prolonged serial passage of the rat parasite in hamsters resulted in a population, called hamster strain (HS III) which had an average developed of 44.9% in hamsters. This level of development was reached by 30 passages and was maintained through 90 additional generations in hamsters. HS HI was still highly infective for rats, although perhaps slightly less so than RS for rats. Furthermore, after 39 generations in hamsters followed by 40 passages through rats, the high infectivity of HS III for hamsters was unchanged. Development of both strains was greater in mature male than female hamsters but with the RS the difference was 16.7-fold as compared to only 1.2-fold with HS III. No sex difference was apparent in immature hamsters infected with either strain. Mature hamsters of both sexes were significantly less susceptible than immatures to RS, whereas HS HI appeared to develop equally well in hamsters that ranged in age from 2 to 44 weeks. Substantial numbers of larvae were found in the lungs of RS-infected rats and HS-infected hamsters on the 2nd day of infection, but on the 4th, 6th, and 10th days most of the worms recovered were in the gut. By comparison, the vast majority of worms found in RS-infected hamsters, at all times, were third-stage larvae in the lungs. Female worms were significantly more numerous than males in rats that received RS, but in hamsters given RS the reverse was found. However, in hamsters, as well as rats infected with HS III, female worms predominated. It is suggested that the adaptation of N. brasiliensis of rats to the hamster occurs through the process of natural selection.