Broad spectrum anthelmintic resistance of Haemonchus contortus in Northern NSW of Australia.

On a sheep farm in Northern New South Wales (NSW) of Australia a degree of anthelmintic resistance was suspected. With noticeable clinical signs of infection and sheep not responding to treatment, a faecal egg count reduction test was conducted to ascertain the broad spectrum of anthelmintic resistance at this farm. A number of classes of anthelmintics were assessed including organophosphate, macrocyclic lactone (ML) and in combination an ML, benzimidazole, levamisole and salicylanilide. In addition, the more recently registered classes of anthelmintics, monepantel (amino-acetonitrile derivative) and derquantel/abamectin combination (spiroindole+ML) were included. Ninety merino sheep naturally infected with a field strain of Haemonchus contortus were randomly allocated to 6 treatment groups (15 animals/group). Sheep were subsequently treated based on label recommendations and individual bodyweight. Faecal samples were collected post-treatment on Days 7, 14 and 21 to conduct faecal egg counts and group bulk larval cultures. Broad spectrum anthelmintic resistance was confirmed at this site with treatment efficacies ranging from 21.3% (monepantel) to 93.8% (derquantel/abamectin combination) against the H. contortus strain. Furthermore, resistance to the multi-combination anthelmintic containing 4 active ingredients was evident (52.5%). This broad spectrum of resistance highlights the need for integration of alternative sustainable methods in parasite control in order to slow development of resistance and increase the life time effectiveness of anthelmintics.