LEVAMISOLE RESISTANCE IN HAEMONCHUS-CONTORTUS SELECTED AT DIFFERENT STAGES OF INFECTION

Resistance to the anthelmintic levamisole is common amongst most gener a of trichostrongylid nematodes of sheep, but is relatively rare in th e species Haemonchus contortus. The level of resistance in at least on e strain of this species varies during the life cycle, Investigation o f this phenomenon, especially in parasitic stages of the parasite may reveal information on resistance mechanisms and factors affecting the selection of resistance. Compared with adult worms of the same resista nt strain, immature worms are more susceptible to levamisole both in i n vitro contraction assays and an in vivo efficacy experiment, To asce rtain if immature parasitic worms of a susceptible strain could be sel ected for resistance, 3 lines of worms were selected with levamisole f or 9 generations. Lines were selected with the same dose rate (2 mg kg (-1)) at the 4th and 24th day of infection, In addition, because levam isole is less active against immature worms a further line was selecte d at the 4th day with a higher dose (8 mg kg(-1)) but similar efficacy to the selection at day 24. In in vitro assays the line selected at d ay 4 with the lower dose of levamisole showed no resistance compared w ith an unselected line. The other 2 lines developed high levels of res istance indicating that resistance can develop after selection at eith er stage. In addition these data directly show that selection pressure is important in determining the rate of development of resistance.