Maintaining a refugia i.e. a proportion of the population that is not exposed to selection by treatments has been suggested as an alternative to mass treatment in the management of nematode parasites of sheep. Three refugia have been identified: nematodes in untreated hosts, encysted larvae and free-living stages on pastures. Here we tested whether Targeted Selective Treatments (TST) could be relevant in controlling nematode infections and delaying anthelmintic resistance selection. We first conducted a one grazing season experiment to compare all flock treatment (the whole flock was treated monthly) and TST based on monthly excretion eggs and daily weight gain. Nematode burden was higher in TST group, but anthelmintic susceptibility of nematodes was preserved. We then used an individual-based model to evaluate the sustainability of TST on a longer period. The simulation approach indicated that TST20% and TST30% of the flock were efficient both at maintaining resistance at a low level and controlling nematode parasite infections on a mid-term period (i.e. simulation of five grazing seasons). However for an efficient TST, these percentages of treated hosts should be adapted to flock size since the number of nematode parasites exposed to selection increases in large flocks. Our results also showed a high dependence on the timing of treatment i.e. on the size of the refugia constituted by the free-living stages on the pasture.
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