Ascaris: development of selected genotypes in mice

Using nucleotide variation in the first internal transcribed spacer of nuclear ribosomal DNA, five different genotypes (designated G1-G5) have been identified and the preponderance of genotype G1 in humans and of genotype G3 in pigs led to the proposal that parasites bearing the two genotypes have an affinity for a particular host species. A subsequent study using eggs of genotype G1 from humans and G3 from pigs to infect pigs and mice indicated that there is a significant difference in the ability to infect and establish as larvae in mice and as adults in pigs between the two genotypes. Extending previous investigations, the present study investigated whether there are differences in development as designated by egg hatching, larvae migration and distribution in the mice between the Ascaris strains with known genotypes. Ascaris eggs of genotypes G1 (predominating in human-derived worms) and G3 (predominating in pig-derived worms) were used to infect C57BL/6 mice orally. Eggs/larvae were examined from the small and large intestines, thoracic and abdominal cavities, peripheral blood, livers and lungs at intervals of 2h until 12h post-infection, then periodically until 34 days of infection. Results showed distinct differences in egg hatching (the timing and location of hatching, and the numbers hatched), and in larvae migration and distribution (the means and constituent ratios, the time of peak recovery, and larvae reappearing in intestines) between the two strains. The results can explain the findings of significantly higher larval recovery of genotype G1 than G3 in the mice, and may shed some enlightenment to understand the difference in host affiliation of Ascaris of different genotypes.