Selection for thiabendazole resistance in Ostertagia spp. by low efficiency anthelmintic treatment

Thiabendazole (TBZ) treatment of grazing weaners infected with a predominantly susceptible strain of Ostertagia reduced worm burdens by 90%. The surviving sub population had a level of resistance four times greater than the original population when compared to similarly infected, but untreated, weaners. TBZ treated and control weaners were relocated to worm-free pasture so that the contamination on the plots grazed by TBZ treated weaners was from the survivors of the drench. The anthelmintic treated groups were subsequently managed under a two summer drench programme while control groups remained so throughout the experiment. Monitoring both parasite population size and the degree of resistance over the following 2 years indicated that after three anthelmintic treatments, resistance developed to a level where TBZ was ineffective. When compared to control groups, a mean reduction in population size of around 80% was maintained in the TBZ treated groups for 1 year. This difference, however, disappeared by the second year. A similar study over 1 year with a moderately TBZ resistant strain of Ostertagia indicated that TBZ treatment produced no significant reduction in population size but more than doubled the level of resistance. The study indicated the critical need for highly efficient anthelmintic treatment, particularly in situations where, through ecology or management, post treatment contamination is likely to contribute substantially to future generations of worms.