Changes in the response of Haemonchus contortus eggs to the ovicidal activity of thiabendazole during the course of infection.

Abstract

Hatching rates and susceptibility to thiabendazole were studied on Haemonchus contortus eggs collected after either a single, a challenge or a trickle infection of sheep with a non-resistant strain. Considerable changes were observed both in hatching rates and LC50 values for thiabendazole during the course of infection. Peaks in LC50 suggested that a susceptible strain can produce eggs at least tolerant and perhaps slightly resistant at some days pi even without any previous contact with anthelmintic. The peak obtained after a single infection was more transient than in the other cases thus suggesting that the immune response of the host may facilitate the selection of tolerant or resistant sub-populations of worms or of eggs. The inheritance of this feature was not clearly demonstrated. Nevertheless, the LC50 of a selected sub-population collected from an immune lamb and given to another lamb was higher and more constant than that of the original strain.