Massive use of chemotherapy influences life traits of parasitic nematodes in domestic ruminants

1. The size of nematodes is an operational indicator of fecundity. The variation in size due to chemotherapy was studied with a benzimidazole anthelmintic in susceptible and resistant nematodes. Teladorsagia circumcincta circumcincta (Stadelmann 1894), a nematode endoparasite of sheep and goats, was investigated as many components of its fitness are already known. 2. Susceptible worms submitted to increasing selective pressure by anthelmintics increased in size (by 6-10%). it was hypothesized that this was partly under the control of sheep, as treated lambs may mount and maintain a better response when infected (premunition). 3. The resistant worms, whatever the anthelmintic pressure, were always 3% larger than susceptible ones. 4. Thus, size may increase in susceptible worms and resistant worms for very different reasons. If resistance does not emerge, massive chemotherapy should lead to larger nematodes, and hence to more fertile worms.