Emerging Multidrug Anthelmintic Resistance in Sheep Gastro-Intestinal Nematodes in Sweden

Gastrointestinal nematodes are clinically and economically important parasites of small ruminants worldwide which are frequently controlled with anthelmintics. The efficacies of benzimidazoles (i.e. albendazole or fenbendazole, n=30), ivermectin (n=47), levamisole (n=2) or moxidectin (n=2) were examined between 2015 and 2021 in 81 treatment groups on 49 non-randomly selected sheep farms from south central Sweden. For this we used the recommended faecal egg count reduction test, which in general was paired with the determination of the composition of Haemonchus, Teladorsagia and Trichostrongylus, based on droplet digital PCR assay results in pre- and post-treatment coprocultures. On each of the two farms tested for levamisole and moxidectin both drugs proved to be 100% effective. In contrast, efficacies of less than 95% for ivermectin or benzimidazoles were identified in total on 26 (n=46, 57 %) and 11 (n=28, 39%) of the tested farms, respectively. On 26 farms where both ivermectin and benzimidazoles were tested, multiple resistance to both drugs was found on 6 (12%). The larvae detected both in pre- and post-treatment cultures were mainly from the genus Haemonchus suggesting that this parasite was primarily involved in the issues associated with anthelmintic resistance. This study indicates that the situation regarding the anthelmintic efficacy has deteriorated dramatically for both ivermectin and benzimidazoles compared with the latest nationwide study conducted on Swedish sheep farms between 2006 and 2007. However, as the present study mainly included farms with a recognized parasite problem, i.e. the selection was not strictly randomized as in a previous nationwide study conducted by us fifteen years ago. Thus, there is a need for a randomised study to get an update on the extent of the problem at a national level as well as to identify risk factors involved in the selection for AR.