Anthelmintic-resistant strains of Haemonchus contortus and Trichostrongylus colubriformis isolated from an organic sheep and goat farm in Denmark

A suspected case of anthelmintic resistance (AR) was investigated in an organic dairy sheep and goat farm. The herd was established in 2007 by purchase of animals from a number of other farms. Selection for the study was based on history of anthelmintic-treatment failure. Forty-eight lambs and 48 kids were selected for faecal egg count (FEC) reduction tests.

Animals were allocated into one of 5 treatment groups, or 1 untreated control group, for each species. Lambs were treated with 5 mg/kg fenbendazole (FBZ), 0.2 mg/kg moxidectin (MOX), 7 mg/kg levamisole (LEV), 0.2 mg/kg ivermectin (IVM) or 10 mg/kg closantel (CLO). Kids were treated with 10 mg/kg FBZ, 0.3 mg/kg MOX, 14 mg/kg LEV, 0.2 mg/kg IVM or 10 mg/kg CLO.

FECs were performed at day of treatment and 14 days post treatment. In a subsequent investigation, faeces from adult goats were cultured to obtain 3rd-stage nematode larvae (L3) for a controlled efficacy test (CET). The resulting isolate was composed of 77 % Trichostrongylus/Teladorsagia spp. and 33 % Haemonchus spp.

Eighteen parasite-naïve lambs were infected with 9,000 L3/lamb of this isolate. At day 35 post-infection (p.i) lambs were allocated in 3 groups and treated with 5 mg/kg FBZ, 0.2 mg/kg IVM or left untreated as controls. At day 42 p.i. lambs were euthanized and worms recovered for species identification.

In the field study, FBZ reduced the FEC of lambs and kids only by 27 % and 56 %, respectively, and IVM reduced the FEC in lambs and kids by 71% and 81 %, correspondingly. In the CET, FBZ reduced H. contortus worm burdens by 52 %, whereas T. colubriformis was not affected by the treatment, and worm counts were not significantly different from the untreated control group (p>0.05).

IVM removed 100 % of adult H. contortus and reduced T. colubriformis worm counts by 84 %. This is the first isolation of BZ-resistant H. contortus and T. colubriformis in Denmark and highlights the need for continuous surveillance of AR in conventional and organic farms.