The preparent period of Nematospiroides dubius was 9-10 d. Faecal parasite epg reached a peak 16-17 d after infection. The infectivity of N. dubius was not significantly influenced by larval dose. Wide variations were observed in the level of resistance attained among 17 inbred, outbred and hybrid mice against N. dubius; C3H mice were the most liable and susceptible and Quackenbush (Q) mice were the most refractory and resistant to infection. Heterosis was found in some hybrid mice. In general female mice were more resistant than males to infection with N. dubius, F1W/Q mice were heterogeneous in their level of resistance against N. dubius and were used as initial stock in a selective breeding experiment.
High- (H) and low- (L) immune-responder colonies of mice together with a randomly (Rd) bred colony of mice were established from F1W/Q mice over 7 generations through selective breeding based on their faecal parasite epg assayed 20 and 21 d after reinfection with 100 N. dubius larvae. H-mice voided significantly fewer faecal parasite eggs in each generation than did Rd- and L-mice; the reduction of faecal epg in H-mice continued from G2 to G7. Protective immunity was greater in H- than in Rd- and L-mice against N. dubius. Strong positive correlation was found between the numbers of adult N. dubius recovered from mice culled from H-, Rd- and L-mice in each generation. Female N. dubius recovered from H-mice were shorter and less fecund than parasites from L-mice, worm length was relatively uniform in Rd-mice for each generation. Selection did not significantly influence the biology of the mice but significant phenotypic and genetic correlations were found between the selected character 'faecal parasite epg after secondary infection' and innate immunity measured as faecal parasite epg after primary infection which indicated that the genetic control of innate and protective immunity was at least partially linked. The estimated heritability of the selection character was h2 = 0.56 ± 0.34 and h2 = 0.55 ± 0.39 in the Gl and G5 F1W/Q. Realized heritability for the trait was calculated from the selection experiment data at h2 = 0.41 ± 0.34. The estimated number of independent loci controlling the selected character was 2 or 3. High- (H) immune-response against N. dubius infection appeared to be inherited as a dominant character.
Frequency of infection as well as host genotype influenced the level of protective immunity in mice against N. dubius infection. H- were better protected than L-mice against infection with N. dubius.
Passively transferred immune mouse serum (IMS) from donor H-, Rd- and L-mice depressed the number of faecal parasite epg and parasites and the fecundity and length of female worms. The level of protection was greater in recipient mice injected with IMS-H than IMS-Rd and IMS-L.
The resistance of mice to N. dubius infection was not specific in relation to N. brasiliensis, H-mice were also more resistant to this parasite than L-mice, but H-mice were more susceptible than L-mice to the bacterial infection, Salmonella typhimurium. Cross-immunity was observed between N. dubius and N. brasiliensis in the selected mice.
Protective immunity against N. dubius was strongly influenced by the genetics of the host irrespective of the size of the immunizing dose of larvae.