First report on reduced efficacy of ivermectin on Oesophagostomum spp. on Swedish pig farms.

Anthelmintic efficacy was investigated in nine sow herds that had been identified with high faecal egg counts in a prevalence study. Faecal samples were collected from a total of 104 individual sows, and analysed using a centrifugal flotation McMaster technique. Samples positive for strongyle eggs were cultured to third stage larvae (L3) for genus identification and then further identified to Oesophagostomum species by sequencing. Following the initial sample collection, the sows were treated with either fenbendazole (FBZ, n = 5 farms) or ivermectin (IVM, n = 4 farms) at the recommended dosing and sampled again 14 days post treatment. Faecal Egg Count Reduction (FECR) was used to determine the treatment efficacy. With respect to Ascaris suum, the anthelmintic treatment was successful (FECR >90%) on the five farms where this parasite was detected, regardless of what drug had been used. In contrast, 4/9 farms were positive for Oesophagostomum spp. post treatment, out of which three had a FECR of <90%. These three herds had all been treated with injectable IVM. Out of the six farms where treatment showed good efficacy (FECR 95-100%), five herds had used FBZ and one herd IVM. This study is the first to recognise reduced efficacy to IVM on Oesophagostomum spp. in Swedish pigs. Sequencing of the Oesophagostomum L3 showed that both O. dentatum and O. quadrispinulatum were present on 5/9 farms pre-treatment and on 2/9 farms post-treatment, unrelated to what anthelmintic had been used. Given these findings we could not correlate the reduced efficacy by the species of Oesophagostomum present in the herd. Prolonged usage of only one class of anthelmintic may predispose selection of resistance and has been suspected as a cause of treatment failure of porcine Oesophagostomum spp. in other studies. On all three farms showing reduced efficacy, IVM had been used as the sole anthelmintic drug for several years, and two of the farms also used IVM twice or more per year to control sarcoptic mange. A reduced efficacy to the available anthelmintic drugs used in the control of Oesophagostomum spp., may result in a subsequent surge of the possible negative effects caused by this parasite.

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