Managing anthelmintic resistance: Is it feasible in New Zealand to delay the emergence of resistance to a new anthelmintic class?

Abstract

The recent registration in New Zealand of the first new class of broad-spectrum anthelmintic, for use against nematode parasites of ruminants, in nearly three decades has raised the possibility that parasite management practices could be improved to minimise the emergence of resistance to the new drug. A review of knowledge pertaining to the selection of anthelmintic resistance in nematode parasites of sheep highlights a number of management practices which could be altered to achieve this. A number of previously common practices such as whole-flock treatment of adult ewes around lambing, and treatment of lambs as they are moved onto pastures with low parasite contamination have been clearly identified as high risk for selecting resistant parasites. Once high-risk practices have been identified steps can be taken to either eliminate their use or mitigate the associated risk. Much of the focus on the management of resistance around the world is on the retention of susceptible genotypes in refugia. While approaches to retaining unselected parasites are likely to vary around the world, empirical studies indicate that the practice is likely to be effective at slowing the development of resistance. The challenge for farmers and advisors will be to strike a balance between retaining sufficient susceptible parasites to usefully delay the development of resistance while not unduly compromising animal performance and farm profitability. The merits of combining different classes of anthelmintic in order to slow the development of resistance remains somewhat contentious in some countries. However, the attributes of oral anthelmintics are such that they seem likely to meet most, if not all, of the criteria for combinations to be highly effective at slowing the build-up of resistance in nematode parasites. It is evident that considerable progress has been made in understanding the factors involved in selecting anthelmintic-resistant nematodes since the last broad-spectrum anthelmintic class was released in the early 1980s. Therefore, it should be possible to manage a new class of anthelmintic in such a way as to significantly extend its effective life. The challenge is likely to be in convincing farmers of the merits of adopting such pro-active strategies.

Animal health aspects of adaptation to climate change: beating the heat and parasites in a warming Europe

“…At present, an increasingly sound evidence base for sustainable general practices is being developed (e.g. Leathwick et al, 2009 and2012), which will provide a platform for context-specific approaches. Considerable local adaptation to take account of climate and management will be needed.…”

World Association for the Advancement of Veterinary Parasitology (W.A.A.V.P.) Guideline: Anthelmintic combination products targeting nematode infections of ruminants and horses

“…Mottier and Prichard (2008) have questioned the use of anthelmintic combination products containing a BZ and a ML on the basis that repeated exposure of H. contortus to ML anthelmintics promoted allelic changes in the ␤-tubulin isotope 1 gene, the key locus involved in the mechanism of BZ resistance. This observation may have implications for the use of anthelmintic combination products containing constituent actives in these classes, but it is not clear that it represents true cross-resistance (Leathwick et al, 2009). Leathwick et al (2009) further point out that, while Mottier and Prichard (2008) suggest that MLs may act as modifiers of BZ resistance, no pharmacological evidence has been advanced to show that nematodes resistant to MLs will also be resistant to BZs.…”

“…This observation may have implications for the use of anthelmintic combination products containing constituent actives in these classes, but it is not clear that it represents true cross-resistance (Leathwick et al, 2009). Leathwick et al (2009) further point out that, while Mottier and Prichard (2008) suggest that MLs may act as modifiers of BZ resistance, no pharmacological evidence has been advanced to show that nematodes resistant to MLs will also be resistant to BZs. Additionally, it is not clear that a limited degree of cross-resistance, should it exist, would be sufficient to nullify the benefits of administering the anthelmintics in combination (Leathwick et al, 2009).…”

“…Theoretical considerations of the value of combination anthelmintic products have recently been discussed (Leathwick et al, 2009;Bartram et al, 2012), and the benefits of such products were further demonstrated in a simulation study using an Australian sheep model with multiple parasite species. A key outcome from this modeling was that a (theoretical) 4-way combination of BZ-LEV-abamectin-monepantel (aminoacetonitrile derivative class) was the best option for delaying the development of AR while achieving effective nematode control despite the presence of resistance to the first three drugs (Dobson et al, 2011a,b).…”

“…Full consideration should be given to refugia when designing and implementing nematode control programs. With heightened scientific interest in maintaining adequate refugia as a means of slowing the development of AR, considerable improvements have been made in recent years to our understanding of the concept (Kenyon et al, 2009;Leathwick et al, 2009;Bartram et al, 2012).…”