Failure of ivermectin treatment in Haemonchus contortus infected-Swedish sheep flocks

- Efficacy of anthelmintics against H. contortus was tested in sheep.
- Faecal egg count reduction, qPCR, and in vitro larval development test were used.
- Ivermectin treatment failure has been confirmed in six sheep out of 11 flocks.
- This was observed after the introduction of imported dairy sheep.

Control of gastrointestinal nematodes of veterinary importance in Swedish sheep flocks is primarily based on recurring strategic anthelmintic treatments after detection of strongyle eggs in faeces samples. This study reports reduced efficacy of ivermectin (IVM) against Haemonchus contortus in naturally infected Swedish sheep flocks. Faecal egg count reduction tests (FECRT) and examinations of H. contortus-specific DNA with qPCR on larval cultures were applied to samples from 11 sheep flocks (A-K) in south-eastern Sweden between 2013 and 2014. Four of these flocks (D, E, J and K) had been in direct contact with flock H, where IVM treatment failure was first observed in October 2013, some years after the introduction of imported dairy sheep. In flock H, the resistance status to IVM was also confirmed by a larval developmental test. IVM concentrations 15-20 times higher than for susceptible strains of H. contortus were required to kill the larvae. In addition, faeces samples were obtained from 37 other Swedish sheep farms where the treatment response to IVM was screened initially in six animals using FEC and qPCR 7-10 days after administration of IVM. Six farms where the majority was identified with this pre-screening test (B, C, F, G, I and K), were also investigated in more detail with FECRT as described above after the animals had been allocated to groups and treated orally or injected with a minimum of 0.2 mg IVM, 0.2 mg doramectin (farm F) or 0.2 mg moxidectin per kg body weight (farm A and B). Four flocks (farm A, D, G and I) were also treated with 4.8 mg albendazole and/or 7.5 mg levamisole per kg body weight. Pre-treatment faeces samples were collected from 15 animals on the same day as deworming. Post-treatment samples were collected 7-10 days later, whenever possible from 10 animals per group with the highest pre-treatment egg counts. Based on FECRT results, IVM efficacy to H. contortus was reduced on six farms (C, D, E, G, H and I) out of 11 farms studied with FECRT. This is the first report of IVM treatment failure in H. contortus-infected sheep in Sweden.