The First Report of Macrocyclic Lactone Resistant Dictyocaulus Viviparus in the UK.

Abstract Background: The bovine lungworm Dictyocaulus viviparus negatively impacts bovine health and leads to substantial economic losses. Lungworm infections can be difficult to manage due to the unpredictable and severe nature of clinical outbreaks. Despite widespread use of long-acting macrocyclic lactones in grazing cattle in the UK, there have been no confirmed reports of resistant lungworm to date, with only one case of anthelmintic resistant lungworm confirmed worldwide. Methods: Lungworm Baermann filtrations were conducted on first season grazing dairy calves as part of a wider study investigating anthelmintic resistance in gastrointestinal nematodes in Scotland using the faecal egg count reduction test. Results: Clinical signs and significant numbers of lungworm larvae in faeces were observed post-treatment with either ivermectin or moxidectin. Resistant gastrointestinal nematode populations were also detected in both treatment groups. Conclusion: This short report describes the first evidence of macrocyclic lactone resistant D. viviparus in the UK.