Inefficacy of ivermectin and moxidectin treatments against Dictyocaulus viviparus in dairy calves.

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 the widespread use of macrocyclic lactones (MLs) in grazing cattle in the UK, there have been no confirmed reports of resistant lungworms to date, with only one case of anthelmintic-resistant (ML) 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 after treatment with either ivermectin or moxidectin.

Limitations: There are no established guidelines for the diagnosis of resistant lungworms in the field. Currently, resistance can only be diagnosed after a controlled efficacy test has been conducted. This limits the conclusions that can be drawn; however, they are highly suggestive of resistance.

Conclusion: This short report describes the inefficacy of ivermectin and moxidectin against D. viviparus and is highly suggestive of ML resistance.