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A study was conducted to evaluate the occurrence of resistance against, in particular, ivermectin in cyathostomins in the Netherlands. Seventy horse farms were visited between October 2007 and November 2009. In initial screening, faecal samples were collected 2 weeks after deworming with either ivermectin, moxidectin or doramectin. Pooled faecal samples from a maximum of 10 horses were examined for worm eggs using a modified McMaster technique and for worm larvae after faecal larval cultures. In total 931 horses were involved. On 15 of 70 farms eggs and/or larvae were found. On 8 of these 15 farms a FECRT with ivermectin was performed on 43 horses. Efficacy of ivermectin against cyathostomins of 93% was found in one animal on one farm. Additionally, the strategies and efforts of the horse owners to control cyathostomins, as well as risk factors for the development of macrocyclic lactone resistance were evaluated with a questionnaire. Strikingly, many responders indicated that the control of cyathostomins in horses is achieved through very frequent deworming. Fourteen percent of these owners deworm seven times per year or more. On 34% of the 70 farms treatment was repeated within the Egg Reappearance Period of a product.

Cyathostomins are considered to be the primary helminth pathogen of horses and macrocyclic lactones (ML) are the most frequently used anthelmintics. Therefore, ML resistance is a serious threat for the control of these parasites. In the present study ivermectin resistant cyathostomin L3 were in vitro selected, using a reiterative larval migration inhibition assay (rLMIA) and differentiated by reverse line blot (RLB). Larvae were obtained from two populations, one from a never treated, free-roaming horse population in the nature reserve Oostvaardersplassen (OVP) and the other from regularly ivermectin-treated ponies of Utrecht University (UU). In the rLMIA the proportion of larvae that migrated increased with each passage, demonstrating that the applied procedure indeed selects for larvae the least susceptible for ivermectin. This was further supported by the fact that glutamate addition to this procedure reversed the selection effect, which also suggests that glutamate-gated chloride channels (GluCls) play a role in the ivermectin resistance of the selected L3. In both populations the predominant species were Cyathostomum catinatum, Cylicostephanus longibursatus and Cylicocyclus nassatus. After in vitro selection in the rLMIA in the presence of ivermectin the predominant species became C. catinatum in both larval populations, while C. nassatus disappeared in the never treated OVP larval population but not in the regularly ivermectin-exposed UU population. It is concluded that the rLMIA and RLB can be used to study anthelmintic resistance in cyathostomin populations and to study differences and changes in species composition between populations with different anthelmintic exposure histories.