Variability in susceptibility to anthelmintics of the lungworm Muellerius capillaris first-stage larvae. Relationship to dairy-goat farms and previous exposure to febantel treatment.

Seven dairy-goat farms, located in central, western France, were studied in order to assess the variability in susceptibility of the lungworm Muellerius capillaris first-stage larvae (L1) to 3 different anthelmintics in relation to farm origin, by means of motility tests. The motility tests were performed by mixing larval suspensions with pyrantel (PYR), thiabendazole (TBZ) or ivermectin (IVE) solutions (3 concentrations x 3 incubation durations). The same anthelmintic tests were repeated 7 and 21 d after febantel (probenzimidazole) treatment of the goats. Before the treatment of goats, the average ratio (x 100) of L1 motility compared with control were very different for the 3 anthelminthics: 44, 30 and 59, respectively for TBZ, PYR and IVE. The ratios of L1 motility were the same at days 7 and 21 after the treatment of goats. The susceptibility of L1 to anthelmintics varied a great deal from 1 farm to another when goats had not been previously treated, whereas L1s from the same farms were the same after treatment of goats. The occurrence of different motilities of L1 with the anthelmintic tests from 1 farm to another should be related to the existence of different populations of M capillaris. The L1 motilities were reduced after anthelmintic treatment of goats suggesting a temporary effect on populations of M capillaris females shedding L1.