Anthelmintic resistance in Cyathostominae
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The most common endoparasites of a horse are cyathostominae. It is worrying that resistance against two groups of anthelmintics has been found in various countries. There are also indications of resistance development against the last active group of anthelmintics, the so-called macrocyclic lactones (such as ivermectin) used in the control of cyathostominae in the horse. Aim of this study to determine if there are differences in the characteristics of cyathostominae in horse populations that differ in frequency of deworming with ivermectin. This is done by selecting larvae with a reiterative Larval Migration Inhibition Assay (rLMIA) and differentiation of larvae by reverse line blot (RLB). In the rLMIA larvae migrate through 2 sieves. The larvae were incubated and migrated in a solution with different concentrations of ivermectin per batch. Larvae were counted before and after migration: an increase in percentage of larvae after migration indicates a reduced susceptibility in this population. After differentiation of the RLB larvae, the percentages of a species per fraction (before sieve 1, after sieve 1, after sieve 2) per concentration ivermectin were compared: an increase of the percentage of a species after sieve 2, compared to after sieve 1, implies an increase in heterogenicity for ivermectin susceptibility in vitro in a population of worms. Prior to this study two other studies were done. In this study some previously tested horse populations like CAS 1 were investigated again and two new horse populations which are treated with ivermectin at least four times per year were included. The three populations in this study are called CAS 2, CRU and BOE. All populations, except CRU, showed a reduced susceptibility in migration in 0.24 μg/ml ivermectin. In CAS 2, CRU and BOE also a reduced susceptibility in 30 μg/ml ivermectin was found. In CAS 2, CRU and BOE were not found enough larvae per species for further analysis. In CAS 1 there was found a reduced susceptibility in migration at 0.24μg/ml ivermectin, and except an increase in heterogenicity of C.longibursatus in 0.24μg/ml ivermectin there was also a significant increase in heterogenicity of Cyathostomum catinatum at 0.24μg/ml and 30μg/ml ivermectin.