In vivo efficacy of macrocyclic lactones on goat farms - pour-on vs injectable application.

Introduction: Pour-on eprinomectin (EPN) is the only anthelmintic currently available that is approved for use in lactating goats with a zero-day milk withdrawal period. The aim of this study was to compare the efficacy of the pour-on form of EPN with the efficacy of the injectable version of ivermectin (IVM) on goat farms in Slovakia.

Material and methods: The study was conducted from September to December 2023 on eight dairy-goat farms. The goats on six farms were divided into two groups of 10 animals each. The first group was administered pour-on EPN at the recommended dose for sheep and goats, and the second group was treated with injectable IVM at a dose 1.5-fold higher than the recommended dose for sheep. The goats on the remaining two farms were treated only with EPN. Efficacy was determined using the in vivo faecal egg count reduction test and the in vitro larval development test (LDT). Infectious third-stage (L3) larvae were harvested before and 14 days after treatment and were assigned to species/genus based on morphology.

Results: The percent reductions for IVM and EPN ranged from 80.8 to 93.6 and 51.0 to 96.6, respectively. A substantially higher egg reduction on the 14^th day post treatment of 20 to 30% was noted in the groups on three farms treated with injectable IVM. The reduction differed little between the drugs on the other three farms (1-3%). Infectious L3 larvae in the LDT were observed at concentrations equal to or higher than the minimum inhibitory concentration of 21.6 ng/mL on all farms. Haemonchus contortus dominated on the 14^th day post treatment with both drugs on all farms.

Conclusion: This study is the first to report reduced efficacy of the pour-on form of EPN in goat herds in Slovakia.