Anthelmintic Resistance in Sheep: Surveillance and Characterisation Studies
Research at Moredun has shown that anthelmintic resistance (AR) to one or more of the available broad spectrum anthelmintic compounds is extremely common, particularly in the most prevalent and economically significant nematode of small ruminants, Teladorsagia circumcincta. Moredun surveys show that around 80% of lowland Scottish sheep farms had worms that were benzimidazole resistant, whilst 30% had worms with ivermectin resistance. The project has also looked at ways of controlling populations that are resistant to multiple families of anthelmintic through multi drug combinations and has provided vital information on biosecurity strategies for new and re-introduced stock.
At present little is known about the current efficacy of anthelmintics against the parasites of cattle and which, if any, worms are implicated in anthelmintic resistance. Isolated reports of ivermectin resistance in the cattle worms Cooperia and Ostertagia have occurred, but as yet no large scales surveys have been conducted in the United Kingdom. The parasitology department is planning a survey to determine the efficacy of ivermectin in first season grazing calves from across Scotland.
At present we have a relatively poor understanding of the importance of the various mechanisms that underpin parasite resistance and act either to affect target site sensitivity, and/or drug metabolism/transport. In vitro and in vivo studies at Moredun have demonstrated the important role that oxidative enzymes (cytochrome P450; CYP) and drug handling membrane proteins (P-glycoproteins;PgP) can play in macrocyclic lactone resistance. It is hoped that a detailed understanding of these mechanisms will provide new drug targets and ways of optimising macrocyclic lactone treatments.
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