Targeted treatment strategies for sustainable worm control in small ruminants.

Sustainable worm control strategies are based largely on ensuring that a source of worms not exposed to anthelmintics ("in refugia") remains after treatments are given, so that resistant worms do not become a dominant part of the total population. In environments with seasonally poor survival of worm larvae on pasture, this may require withholding treatments from a proportion of animals when the whole group would normally be treated. The "targeted treatment" approach involves using anthelmintics on an individual animal basis according to indications of parasitic effects, regardless of parasite burdens. For Haemonchus contortus, the FAMACHA system, based on the easily-visualised index of anaemia, has proved effective provided that labour is available for frequent inspections. For non-haematophagous nematodes, recent research indicates the potential of production parameters such as body weight change (sheep) and milk yield (dairy goats), providing that parasitic effects can be differentiated from nutritional and other factors. Continuing investigations are necessary to indicate the most appropriate indices for different situations, so that the refugia effect is maximized for the least risk of disease and production loss. Of prime importance, targeted treatment strategies must be practical to implement if they are to achieve widespread adoption.