In vitro selection and differentiation of cyathostomin L3 larvae with a reduced sensitivity to ivermectin.

Abstract

In today’s adult horse population the most pathogenic and common nematode infection is that of the cyathostomins. In the past these worms were of little concern, next to the highly prevalent and pathogenic Strongylus vulgaris. Due to the introduction of macrocyclic lactones (ivermectin and moxidectin) in the 1980’s and 1990’s, S. vulgaris is no longer considered a big risk in domestic horse populations. Instead the cyathostomins took over and are presently considered the most pathogenic horse nematode. Cyathostomins are prevalent in horse populations around the world. They can cause severe health problems in the horse. Up to 50% of larval cyathostominoses has a fatal ending for the horse despite accurate treatment. Since the introduction of anthelmintic drugs the use has increased greatly. To date, worm control in horses is mostly based on the exclusive and regular use of these drugs. Due to the increasing development of anthelmintic resistance against these drugs this approach has to be considered unsustainable. This sub research is part of an ongoing research at the Utrecht University. The purpose of this research is to determine the composition of cyathostomin populations on horse stables in the Netherlands before and after in vitro exposure to ivermectin. The species of cyathostomins will be differentiated for each stable before and after this exposure to varying concentrations of ivermectin. This might give an indication of reduced sensitivity for ivermectin in certain species. When cyathostomin populations of different stables are compared it will show if there are differences between and/or within cyathostomin species. We used an rLMIA, a reiterative Larval Migration Inhibition Assay (with two sieves), for in vitro selecting of cyathostomins with a reduced sensitivity to ivermectin. This rLMIA is an experimental method. We used a Reverse Line Blot, to differentiate the species of cyathostominae. Results of the 4 cyathostomin populations in this research differed strongly. Differences were found between cyathostomin populations, between cyathostomin species and within species. In 3 populations a reduced sensitivity was found. In two of those stables this reduced sensitivity was linked to certain cyathostomin species by differentiation of larvae.