Refugia-based strategies for sustainable worm control: factors affecting the acceptability to sheep and goat owners

Sustainable nematode management programs aim to minimise animal production loss and prevent parasitic disease, without increasing the level of anthelmintic resistance. Resistance management strategies are now largely based on the "refugia" concept, by which populations of nematodes not recently exposed to treatment are deliberately allowed to survive. Progeny from the unselected parasites provide a source of less-resistant worms which can dilute resistant worms surviving anthelmintics, and hence reduce the rate of resistance development. This can be achieved by either modifying strategic treatment regimens to ensure the survival of infective worm larvae on pasture, or by avoiding treatments to individual animals identified as best able to cope with parasites. These strategies include "targeted treatment" (based on estimates of worm-burdens) and "targeted selective treatment" (based on indications of parasitic effects). However, the departure from conventional anthelmintic approaches represents a major conceptual challenge to many livestock owners. Factors that may affect the wide adoption of refugia strategies include the increased risk of parasitism and production loss, the effectiveness of reducing the development of resistance, the practicality of implementation, and the direct effects on costs and labour efficiency. The acceptance of particular strategies is likely to vary considerably according to environmental effects, nematode species, animal production aims and resource availability. However, recent indications that comparatively small changes to present practices can provide substantial refugia benefits suggest that appropriate resistance management approaches can be developed for different situations.