A questionnaire-based survey on the efficacy of antiparasitic products against ectoparasites of companion and livestock animals in Denmark.

INTRODUCTION: Ectoparasitic resistance is increasingly reported worldwide, with major implications for animal welfare and economic sustainability in production and companion animals. Despite global concern, data from Denmark are lacking. This study examined national ectoparasiticide use and, based on a questionnaire survey, assessed whether veterinarians and pet owners experienced reduced drug efficacy or decreased susceptibility in ectoparasites of cattle, poultry, dogs, and cats in Denmark. METHOD: National ectoparasiticide usage data were obtained from VetStat, Denmark's veterinary drug database. A survey comprising six questionnaires (September 2024-January 2025) was conducted among veterinarians (cattle, poultry, dogs, cats) and dog and cat owners to assess perceived ectoparasiticide efficacy. RESULTS: Among respondents, perceived reduced efficacy was reported by 7/15 (46.7%) of cattle veterinarians, 0/3 of poultry veterinarians, 6/50 (12.0%) of dog veterinarians, and 6/29 (20.7%) of cat veterinarians; corresponding proportions were 12/490 (2.5%) for dog owners and 8/75 (10.6%) for cat owners. Cattle veterinarians suspected reduced efficacy of deltamethrin, primarily against stable flies and house flies. Companion animal veterinarians and cat owners suspected reduced efficacy of fipronil-containing products against fleas and ticks, while some dog owners noted concerns with imidacloprid and isoxazolines. DISCUSSION: Based solely on questionnaire data, indications of perceived reduced efficacy of ectoparasiticides in Denmark were identified, however, no resistance assessment was performed. This underscores the need for further investigation using direct measures such as field efficacy trials or molecular markers to determine whether ectoparasiticide resistance occurs, particularly in flies exposed to deltamethrin and possibly in ticks and fleas exposed to fipronil.