Lack of efficacy of topical administration of eprinomectin against gastrointestinal nematode in a French dairy sheep farm: A case of underexposure of worms

Abstract

Resistance to eprinomectin was suspected in a dairy sheep farm in southwestern France. The efficacy of topical and injection formulations of eprinomectin against gastrointestinal nematodes (GINs) was compared using a faecal egg count reduction test. GIN species were identified by real-time PCR, and eprinomectin concentrations were measured in serum by High-Performance Liquid Chromatography 2 and 5 days after treatment. Efficacies were 99.6% and 86.1% for injection and topical formulations, respectively. Before treatment, the three species Haemonchus contortus, Teladorsagia circumcincta and Trichostrongylus colubriformis were identified in both groups. After treatment, H. contortus and T. colubriformis were identified in the topical group only. Two days after treatment, eprinomectin concentrations were above 2 ng/ml in the injection group and under this value in the topical group, suggesting underexposure of GIN to eprinomectin in this latter group. High levels of exposure to eprinomectin are important to avoid loss of efficacy in the field.

First report of eprinomectin-resistant isolates ofHaemonchus contortusin 5 dairy sheep farms from thePyrénées Atlantiques départementin France

“…In the study by Bordes et al . ( 2022 ) serum concentrations of EPN were below 2 ng mL −1 for all animals treated with a topical formula. These findings are in line with others (Hoste et al ., 2004 ; Hodošček et al ., 2008 ) that underline the highly variable bioavailability of the topical formula of EPN.…”

“…However, cases of underexposure of GIN to EPN when using a topical formula have previously been reported by veterinary practitioners and confirmed by 2 recent studies in France. Bouy et al and Bordes et al described cases where FECR after EPN treatment were below 95% when using the topical solution and above 95% when animals of the same flock were treated with a subcutaneous solution (Bouy et al, 2021;Bordes et al, 2022). In the study by Bordes et al (2022) serum concentrations of EPN were below 2 ng mL −1 for all animals treated with a topical formula.…”

“…Bouy et al and Bordes et al described cases where FECR after EPN treatment were below 95% when using the topical solution and above 95% when animals of the same flock were treated with a subcutaneous solution (Bouy et al, 2021;Bordes et al, 2022). In the study by Bordes et al (2022) serum concentrations of EPN were below 2 ng mL −1 for all animals treated with a topical formula. These findings are in line with others (Hoste et al, 2004;Hodošček et al, 2008) that underline the highly variable bioavailability of the topical formula of EPN.…”

“…and Bordes et al . described cases where FECR after EPN treatment were below 95% when using the topical solution and above 95% when animals of the same flock were treated with a subcutaneous solution (Bouy et al ., 2021 ; Bordes et al ., 2022 ). In the study by Bordes et al .…”

“…Ineffectiveness of the topical formula was rapidly reported in goats on the basis of fecal egg count reduction tests (FECRT) (Murri et al ., 2014 ), which prompted veterinarians to use EPN via other routes of administration that are known to be associated with higher overall exposure in plasma and tissues and potentially higher efficacy than topical administration (Lespine et al ., 2012 ). Lack of efficacy of the topical formula has also been recently described in dairy sheep (Bouy et al ., 2021 ; Bordes et al ., 2022 ).…”