Cercarial production related to miracidial dose variation with Schistosoma bovis strains from Sudan and Spain in Bulinus truncatus from Tunisia was studied. Results showed that an increase in the miracidial dose proposed to the host-mollusc (1 and 5 miracidia) gave rise to a decrease in cercarial productivity of Sudanese S. bovis and to an increase for Spanish s. bovis. It is concluded that this response difference tothe miracidial dose variation depends on genetic characters peculiar to the parasite strain and represents a new aspect of genetic variability of schistosome populations.

Mongolian gerbils were infected with either a benzimidazole-susceptible or a benzimidazole-resistant strain of Trichostrongylus colubriformis larvae. The comparative pathogensis of both strains of T. colubriformis was investigated for 50 days post-infection. The gerbils infected with the drug-susceptible strain of the parasite showed lower survival rates, reduced body weights, higher parasite egg output and higher worm burdens than animals intected with the drug-resistant stram of parasite. There was also evidence of an enhanced hypoalbuminaemia and a more marked gastrointestinal plasma leak in the gerbils infected with the drug-susceptible strain of patasite. Scanning electron microscope examination of the small intestine showed signs of villus atriphy in both groups of ingected grebils.