Current state of anthelmintic resistance in horses with a special focus on the situation in Germany

The occurrence of anthelmintic resistance has been reported soon after the introduction of well tolerated broad-spectrum anthelmintics in the 1960s. Like in bacteria the development of resistance in gastro-intestinal parasites of equines is a consequence of repeated drug application and represents an evolutionary process. In Germany, resistance has been detected in small strongyles to benzimidazole (several studies with up to ca. 80% resistance frequency) and tetrahydropyrimidine (several studies with up to ca. 25% resistance frequency), in ascarids to macrocyclic lactones (single study on one farm) and recently in pinworms to ivermectin (macrocyclic lactone) (single study on one farm). Once anthelmintic resistance is established it remains in the worm population even if the particular drug class is not used over a longer period of time. Today different tests are available for detection of anthelmintic resistance. Despite limitations, the FECRT remains the test of choice in horses. Different worm control concepts have been established to delay the development of resistance. Any sustainable worm control strategy targets the least frequent application of anthelmintics that does not harm the health and well-being of the horse. The targeted selective treatment that has been developed for the prevention of anthelmintic resistance in small ruminants can only be used in horses with limitations and will be further discussed in this review.