Anthelmintic resistance in the equine roundworm
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Short summary
The equine roundworm is the horse's largest intestinal parasite. Equine roundworm infections are very common in foals and young horses. Older horses develop immunity to the worm, and in horses older than 4 years, equine roundworm infections are very uncommon.
The roundworm, Parascaris equorum, is the main intestinal parasite of horses. Fluke infection is very common in foals and young horses, after which horses develop immunity to the worm, and in horses over 4 years of age, fluke infection is very uncommon.
Background
Roundworm eggs are highly resistant and can remain infective on pasture for many years. The foals ingest the eggs on the pasture and the eggs then hatch in the gut. Symptoms of roundworm infection include reduced appetite and general condition, nasal discharge, coughing, shaggy coat and poor growth. If the foal has a very high parasite burden, it can lead to small intestinal obstruction/ constipation resulting in severe colic and possible death.
Resistance to deworming agents reported
In general, foals are dewormed against roundworm several times during their first year of life. In the past, all available deworming agents have been effective against roundworm, but about 15 years ago there were international reports of resistance to ivermectin (Ivomec® and others). In 2005, the first cases of resistance to ivermectin were detected on Swedish farms. Currently, benzimidazole (Axilur®) or pyrantel (Banminth® and others) are used for deworming against roundworms, but resistance to these drugs has also been detected internationally.
Resistance means that the parasite survives a dose of drug that would normally be lethal. It is clear that excessive use of deworming agents has led to increased drug resistance in parasites, posing a major threat to both animal and human health. In order to reduce this alarming development of resistance, the Danish Medicines Agency decided in 2007 that deworming agents for horses and other animals must be prescribed by a veterinarian.
About this research project
In the first part of our study, the aim is to investigate the resistance status of the equine roundworm to pyrantel and benzimidazole on Swedish farms today. In the second part of the project, we will investigate the underlying resistance mechanisms in the horse roundworm. Today, there is very little knowledge about how resistance to deworming agents arises. Among other things, we will identify and study the genes in the horse roundworm that are probably involved in the uptake and degradation of deworming agents. The expression of these so-called candidate genes in the worms will be studied before and after treatment with deworming agents to try to find markers that are important for the development of resistance in the roundworm.
Aim and objective
This project will give us a picture of the resistance situation in the horse roundworm today, and may influence future recommendations on deworming and parasite control. Hopefully, we will also gain new knowledge about how resistance develops in the roundworm. We also hope to find markers to identify incipient resistance, and that the project will help advance research in the field of drug resistance in parasites.