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

Johan Höglund; Katarina Gustafsson; Britt-Louise Ljungström; Moa Skarin; Marian Varady; Fredrik Engström

doi:10.1016/j.vprsr.2016.02.001

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

Vet Parasitol Reg Stud Reports. 2015 Dec:1-2:10-15. doi: 10.1016/j.vprsr.2016.02.001. Epub 2016 Feb 12.

Authors

Johan Höglund¹, Katarina Gustafsson², Britt-Louise Ljungström³, Moa Skarin⁴, Marian Varady⁵, Fredrik Engström²

Affiliations

¹ Department of Biomedical Sciences and Veterinary Public Health, Section for Parasitology, Swedish University of Agricultural Sciences, P. O. Box 7063, SE-750 07 Uppsala, Sweden. Electronic address: johan.hoglund@slu.se.
² Vidilab, P.O. Box 33, SE-745 21 Enköping, Sweden.
³ Farm and Animal Health Service, Kungsängens gård hus 6B, SE-753 23, Uppsala, Sweden.
⁴ Department of Biomedical Sciences and Veterinary Public Health, Section for Parasitology, Swedish University of Agricultural Sciences, P. O. Box 7063, SE-750 07 Uppsala, Sweden.
⁵ Institute of Parasitology, Slovak Academy of Sciences, Hlinkova 3, 040 01, Kosice, Slovak Republic.

PMID: 31018402
DOI: 10.1016/j.vprsr.2016.02.001

Abstract

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 7-10days 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 0.2mg IVM, 0.2mg doramectin (farm F) or 0.2mg moxidectin per kg body weight (farm A and B). Four flocks (farm A, D, G and I) were also treated with 4.8mg albendazole and/or 7.5mg 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 7-10days 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.

Keywords: Anthelmintic resistance; Faecal egg count reduction test; Haemonchus contortus; Haemonchus-specific quantitative PCR; Ivermectin; Nematodes; Sheep.