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 vari ation in size due to chemotherapy was studied with a benzimidazole anthelmi ntic in susceptible and resistant nematodes. Teladorsagia circumcincta circ umcincta (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 anthelmi ntics increased in size (by 6-10%). it was hypothesized that this was partl y under the control of sheep, as treated lambs may mount and maintain a bet ter 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 ver y different reasons. If resistance does not emerge, massive chemotherapy sh ould lead to larger nematodes, and hence to more fertile worms.