Minimising the development of anthelmintic resistance, and optimising the use of the novel anthelmintic monepantel, for the sustainable control of nematode parasites in Australian sheep grazing systems

Abstract

MethodNematode populations and the progression of drug resistance for a variety of treatment options and management practices in sheep rearing areas of WA, VIC and NSW were simulated. A scoring system was devised to measure the success of each option in delaying resistance to each anthelmintic and in controlling nematode populations. ResultsThe best option at all sites was combining the new anthelmintic (monepantel) with a triple mixture of benzimidazole, levamisole and abamectin (COM). The next best 2 option was: in NSW, rotation at each treatment between monepantel, moxidectin and COM; in VIC, rotation at each treatment between monepantel and COM; and in WA, rotation at each treatment between monepantel (used in winter) and COM or moxidectin (used in summerautumn). In WA, rapid selection for resistance occurred as a consequence of summer-autumn treatments; however, if a small percentage of adult stock were left untreated then this selection could be greatly reduced. Despite purposely assuming relatively high resistance to benzimidazole and levamisole, COM was still effective in controlling worms and delaying resistance.Conclusions Due to cost constraints, it may not be feasible or profitable for producers to always use the combination of all drugs. However, the second and third best options still considerably slowed the development of anthelmintic resistance.

A multi‐species model to assess the effect of refugia on worm control and anthelmintic resistance in sheep grazing systems

“…29 This system attempts to quantify, as a single pooled measure, the efficacy of nematode control and the impediment of drug resistance, which in practice are conflicting objectives. These ranks are given in the results (summarized in Figures 1-4 and described in detail in Tables A2-A5 (Appendix 1)) and are determined from the sum of the 'mean worm score' and the 'mean resistance score' for each option.…”

“…[17][18][19][20][21][22][23][24][25][26][27][28][29] The methods for the modelling are shown in Appendix 1 in the online version, or are available from the senior author. , which may have masked the negative impact of leaving some animals untreated.…”

“…These management assumptions are typical for Merino wool-producing systems but not for prime lamb enterprises, which may require some modification to the treatment times and risk weights for the output variables. 29 Selection for anthelmintic resistance Selection for anthelmintic resistance from a treatment with benzimidazole (BZ) plus levamisole (LEV) was modelled as a single gene (denoted BZLV) and initial R-allele frequency was set at 40% as resistance to this combination was assumed to be common. Macrocyclic lactone (ML) resistance was assumed to be incomplete recessive, selected for by the drugs abamectin (ABA) and moxidectin (MOX).…”

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

“…As noted above, few farmers test for AR (Lawrence et al, 2007;Dobson et al, 2011a;Morgan et al, 2012). Under these circumstances, a concern is that fixed-dose combination anthelmintic products could mask the development of resistance.…”

“…Importantly, animal welfare considerations demand prompt treatment of any animal that is ill due to parasitism. Since the drugresistance status of parasites on farms is rarely determined prior to choosing a treatment (Lawrence et al, 2007;Dobson et al, 2011a;Morgan et al, 2012), an approach of using single-constituent active products for strategic dosing to account for the species and AR status present at the time is not likely to be practical or sustainable even in smaller operations. This will be exacerbated as populations of multiply-resistant parasite populations spread, requiring multiple treatments with different products in separate dosage forms to maintain control.…”