Resistance to a triple combination of broad-spectrum anthelmintics in naturally-acquired Ostertagia circumcincta infections in sheep.
OBJECTIVE
To measure the efficacy of two macrocyclic lactone-levamisole-benzimidazole combination drenches against naturally-acquired abomasal nematode infections on a sheep farm in the North Island, New Zealand.


METHODS
Eighteen lambs carrying naturally-acquired worm burdens were removed from pasture and randomly allocated to one of three equal-sized groups, consisting of an untreated control group and two treatment groups. One treatment group was given a single oral dose of a triple-combination anthelmintic administered at the manufacturer's recommended dose rate of 0.2 mg/kg ivermectin, 7.5 mg/kg levamisole and 5.0 mg/kg albendazole. The other treatment group received a similar dose of another triple-combination drench consisting of 0.2 mg/kg abamectin, 8.0 mg/kg levamisole and 4.5 mg/kg oxfendazole. Worm counts were carried out post mortem on the abomasa of all animals in all groups, 10 days after treatment.


RESULTS
While the abamectin-levamisole-oxfendazole combination was highly effective against all three abomasal nematode species present, the ivermectin-levamisole-albendazole combination was not. In the latter instance, a reduction of only 78% was achieved against infections of Ostertagia (=Teladorsagia) circumcincta.


CONCLUSIONS
These results provide evidence of resistance to an ivermectin-levamisole-albendazole combination drench by O. circumcincta.


CONCLUSIONS
Besides representing the first confirmed report of resistance to a combination drench consisting of three broad-spectrum anthelmintics, the present results provide further evidence of the continuing escalation of multiple anthelmintic resistance in sheep nematodes in New Zealand.
Wrigley
J
J
Ancare New Zealand Limited, 17 Shea Terrace, Takapuna, Auckland, New Zealand.
McArthur
M
M
McKenna
P B
PB
Mariadass
B
B
D015766:Albendazole / Q000494:pharmacology / Q000627:therapeutic use; D000818:Animals; D000871:Anthelmintics / Q000494:pharmacology / Q000627:therapeutic use*; D001562:Benzimidazoles / Q000494:pharmacology / Q000627:therapeutic use; D004351:Drug Resistance; D004359:Drug Therapy, Combination; D007559:Ivermectin / Q000031:analogs & derivatives / Q000494:pharmacology / Q000627:therapeutic use; D007978:Levamisole / Q000494:pharmacology / Q000627:therapeutic use; D008297:Male; D009520:New Zealand; D010029:Ostertagiasis / Q000188:drug therapy / Q000662:veterinary*; D021261:Parasitic Sensitivity Tests / Q000662:veterinary; D011897:Random Allocation; D012756:Sheep; D012757:Sheep Diseases / Q000188:drug therapy*; D016896:Treatment Outcome
D000871:Anthelmintics; D001562:Benzimidazoles; D007978:Levamisole; C048324:abamectin; D007559:Ivermectin; D015766:Albendazole; C011030:oxfendazole