Effect of selection pressure for thiabendazole resistance on fitness of Haemonchus contortus in sheep.

A strain of Haemonchus contortus showing moderate levels of resistance to thiabendazole (TBZ) (egg hatch assay LC50 2.1 microM) was subjected, in alternating sequence, to 5 in vitro and 4 in vivo selections with TBZ. This resulted in a significant increase in resistance (egg hatch assay LC50 2.8 microM). The isolates obtained after each combined in vitro and in vivo selection were compared in terms of egg hatch assay, establishment, net egg output, per capita egg output and degree of pathology caused in infected hosts. The increase in resistance was accompanied by a significant increase in larval establishment in sheep (22.8-39.2%) and was associated with more severe pathology, higher egg count and worm recovery 3 months post-infection. The resistant strains were also compared with a TBZ-susceptible strain (egg hatch assay LC50 1.0 microM). The TBZ-susceptible strain had the highest net egg output, it caused the most severe pathology and it resulted in higher worm recovery than any of the TBZ-resistant strains, although the most resistant strain was very similar to the TBZ-susceptible strain. Neither the per capita egg output nor biological features of the free-living stages were significantly affected by the degree of resistance. The implications of these observations for anthelmintic use and for management of nematode anthelmintic resistance in the field are discussed.