Effective management

Nutrition

Healthy animals harbour worms and always will. Eradication is neither an appropriate goal nor achievable. Healthy mature animals are generally net removers of parasite larvae and can reduce the larval challenge on pasture, while also acting as a valuable source of susceptible worms. The aim should be to use nutrition in mature stock to manage their parasite challenge. Well-fed animals are less affected by worms than those under nutritional stress. The aim should be to feed animals well, particularly young stock where good post grazing residues should be left behind and parasites in the lower part of the grass sward are avoided.

Pasture contamination

Most of the year there are more worms, in the various life stages, on pasture than inside the animals. To reduce the impact of parasites on animals we need to focus on decreasing the exposure of animals to the infective larvae. Older animals are generally less susceptible to worms than younger ones, and, at times, can be used to reduce the number of infective larvae on pastures. Animals of different species can also be used to decrease pasture contamination.

Refugia

Refugia is the proportion of the total parasite population that is not selected for by an antiparasitic drug treatment – essentially, those parasites that are in “refuge” from the drug. A population of susceptible parasites (i.e. a refugia) should be maintained on farm. Therefore, avoid drenching adult animals as much as possible. Integrate undrenched adults with young stock, either within the group (e.g., cull ewes run with lambs) or in rotation (undrenched ewes following lambs). Consider not drenching all the animals in a group (e.g., targeted selection based on weight gain, body condition score, weight or random allocation).

Genetics

This is a long term aid to parasite management. Animals vary in their susceptibility to parasites and can be selectively bred for resistance and resilience to worms. Ram breeders should be recording for worm FEC or resilience. Genetic progress can still be made for production traits at the same time as these worm resistance traits.

Effective drenching program

Only use drenches that have been shown to be effective on the property. Routine drench checks or faecal egg count reduction tests (FECRT) should be used to determine what is an effective drench. Use of effective combination drenches may help to delay the onset of resistance to the individual actives, but the benefit may be lost if resistance to the individual components is already advanced.

Drenches are a finite resource. The way in which a farm uses the drench and manages parasites can change the rate at which they select for resistant worms. The effective drench should be given to the right class of stock, at the right time and only when it is required. In this way we minimise the exposure of the parasites to the drench.

Long-acting drench formulations use may hasten the development of drench resistance. Restrict the use of these products to only those animals that need it, based on age, body condition, physiological state, and larval challenge. Once present on farm, resistance to drenches can be considered permanent.