Parasite control in sheep: A risky business

Sheep are infected with a range of parasites, for which appropriate and effective control strategies are required. Changing patterns of disease and the inability to control parasitic infections effectively require the implementation of risk-based approaches to parasite control for many of the major parasitic diseases affecting sheep. Developing nematode resistance to anthelmintics necessitates changes to conventionally worming programmes that reduce the risk of further development of anthelmintic resistance through incorporation of new anthelmintic classes and products, along with ‘best practice’ guidelines for anthelmintic usage. Climatic changes pose a greater risk to sheep flocks through spread of parasites, such as Haemonchus contortus and Fasciola hepatica, which are parasites inextricably linked to high rainfall and increasing temperatures. Intensification increases the risk from diseases such as coccidiosis and it is now advocated that a risk-based approach to the control of coccidia infection in lambs should be based on strategic, treatments interventions linked to identified periods of risk. Concerns over the risks from the use of ectoparasiticides, have seen the demise of sheep dipping in many countries leading to increasing levels of ectoparasitic infections. At the same time, greater reliance has been placed on the use of injectable macrocyclic lactones, which inadvertently may also increase selection pressure for anthelmintic resistance. As a consequence of all the above factors, risk-based approaches to parasite control need to consider the strategic use of antiparasitics for individual target parasites, as well as needs to integration and rationalisation of all the component strategies.