Sheep worm infections increasingly threaten the profitability of the Australian sheep industry as the prevalence and severity of anthelmintic resistance continues to rise. Field experiments have demonstrated that the use of drenches that are of reduced efficacy due to resistance can reduce wool production by 10% and significantly reduce sheep sale value. The major factor in benzimidazole (BZ) and levamisole (LV) resistance, and in part macrocyclic lactone (ML) resistance in Haemonchus contortus, is considered to be the excessive frequency of treatment. In Ostertagia circumcincta, ML resistance appears to be related chiefly to interactions between the environment and the time of treatment, where resistant worms surviving drenches are not significantly diluted by worms acquired after treatment. Resistance to the BZs and LV affects almost all Australian sheep farms, and on most drench efficacy is now below a useful level. ML resistance is common in O. circumcincta in Western Australia, and increasingly prevalent in H. contortus in summer rainfall regions. Closantel resistance is a major threat to H.�contortus control in northern New South Wales and southern Queensland. The organophosphate naphthalophos, especially when used in combination with other compounds, has proved useful although variable in efficacy. It is essential that the sheep industry adopts approaches that minimise reliance on chemical control, such as the breeding of worm resistant sheep, use of specific grazing strategies for worm control and apply flock treatment tactics to minimise further resistance development. New non-chemical technologies under development are also expected to contribute to more sustainable worm control. Nutritional regimens that minimise the impact of worm infections and enhance the immune response require investigation and integration into sustainable control practices.

The primary recommendation is to use a mixture of effective drugs when treating sheep. However, unless the combination treatment is highly effective it is unlikely to delay selection for ML resistance if sheep are being treated and moved to a clean or safe pasture. Where possible, reliance on the ML anthelmintics should be reduced by not using them every year, not using them in low risk stock or by using narrow spectrum and low efficacy drugs such as naphthalophos when appropriate. Anthelmintic treatment should be given as part of a strategic worm control program. It is suggested that IVM-oral and IVM-capsules should not be used when ML resistance is present. In this situation MOX or ABA should be used in combination with other drugs, provided that the chosen ML is effective against the resistant parasite. It is essential to monitor the efficacy of ML and drug combinations by post-treatment worm egg counts, particularly when ML resistance has been detected.

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1A multi-species model to assess the impact of refugia on worm control and anthelmintic resistance in sheep grazing systems. RJ ObjectiveDevelop a computer simulation model that uses daily meteorological data and farm management practices to predict populations of Trichostrongylus colubriformis, Haemonchus contortus and Teladorsagia (Ostertagia) circumcincta and the evolution of anthelmintic resistance within a sheep flock. Use the model to explore if increased refugia, provided by leaving some adult sheep untreated, would delay development of anthelmintic resistance without compromising nematode control. ProceduresCompare model predictions with field observations from a breeding flock in Armidale, NSW. Simulate the impact of leaving 1-10% of adult sheep untreated in diverse sheep grazing systems. ResultsPredicted populations of T. colubriformis and T. circumcincta were less than those observed in the field. Attributed to nutritional stress experienced by sheep during drought, and not accounted for by the model. Observed variation in faecal egg count explained by the model (R 2 ) for these species was 40-50%. H. contortus populations and R 2 were both low.Leaving some sheep untreated worked best in situations where animals were already grazing or were moved on to pastures with low populations of infective larvae. In these cases anthelmintic resistance was delayed and nematode control was maintained when 1-4% of adult stock remained untreated. ConclusionsIn general, the model predicted that leaving more than 4% of adults untreated did not sufficiently delay the development of anthelmintic resistance to justify the increased 2 production risk from such a strategy. The choice of a drug rotation strategy had an equal or larger impact on nematode control, and selection for resistance, than leaving 1-10% of adults untreated. The multi-species model described here allows for these phenomena and estimates host mortalities and inappetence based on the total worm populations. To determine the utility of the model, worm populations were monitored in flocks of breeding ewes and weaners that were managed under typical grazing systems in north-eastern Australia. Daily meteorological and management records were kept so that worm populations could be predicted using the multispecies model. Keywords 3In this simulation study, the model was used to: a) estimate the consequences of leaving a proportion of adult stock untreated (as a source of refugia) on the development of anthelmintic resistance and effectiveness of parasite control; b) determine how best to use a new anthelmintic class (amino-acetonitrile derivatives (AAD), represented by monepantel; Zolvix ® , Novartis Animal Health Inc., Switzerland) to control gastro-intestinal nematodes while minimising selection for drug resistance; and c) examine these issues in four environments to determine if there are common principles that can be applied and/or how they need to be modified to suit local environments. The aim was to fully explore the interaction between the fundamen...

This compilation of results of FECRTs conducted by Australian parasitology laboratories in 2009-12 showed widespread AR of the common sheep nematodes (Teladorsagia, Trichostrongylus and Haemonchus) to all broad-spectrum anthelmintics, with the exception of monepantel, whether used singly or in combination.