Anthelmintic resistance in sheep nematodes in Australia: the need for new approaches

Abstract

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.

Genotyping of benzimidazole resistant and susceptible isolates of Haemonchus contortus from sheep by allele specific PCR

“…The trend continued with the introduction of a new generation of drugs in the same class as well as other classes like imidathiazole and macrocyclic lactones. This has led to the development of multiple drug resistant (MDR) strains of H. contortus in many parts of the world viz., South America (Nari et al 1996), South Africa (van Wyk et al 1999, Malaysia (Chandrawathani et al 2003), Southeast USA (Mortensen et al 2003), Australia (Besier and Love 2003), New Zealand (Leathwick et al 2001) and India (Gill 1996;Swarnkar et al 1999;Easwaran et al 2009). In Tiwari et al (2006) reported the prevalence rate of BZ resistance in H. contortus of Indian isolates by RFLP-PCR technique.…”

The role of targeted selective treatments in the development of refugia-based approaches to the control of gastrointestinal nematodes of small ruminants

“…Effective targeting should also lead to an increased interval between treatments, allowing susceptible genotypes a better opportunity to establish on pasture and so reduce the risk of heterozygotes mating and producing homozygous resistant offspring. Recent examples of the use of targeted treatments include studies conducted in Western Australia (Besier and Love, 2003) and Italy (Cringoli et al, 2008) where treatments were stopped during summer months when few parasites were present on pasture and given instead at other times when more parasites were present in refugia.…”

Genome-wide association and regional heritability mapping to identify loci underlying variation in nematode resistance and body weight in Scottish Blackface lambs

“…Costs arising from these components have been quantified for the UK sheep to be approaching d100 million per year (Nieuwhof and Bishop, 2005). Control is largely based around anthelmintic usage; however, the development of anthelmintic resistance in nematode populations has fast become an international problem (Besier and Love, 2003;Bartley et al, 2004). This has led to the need for new control measures, such as selection for increased nematode resistance, with available field data (for example, Kemper et al, 2010), suggesting that such selection should be successful.…”