By Richard Ehrhardt
A central concept in the development of a sustainable parasite management plan is the maintenance of a refugia parasite population within a given farm. A refugia population is one less exposed to anthelmintic drugs (dewormers). This refugia population acts as a reservoir of parasites that contain resistance alleles at a lower gene frequency than those that survive dewormer treatment. These populations (refugia and non-refugia) then mix and mate, effectively diluting the resistance genes within a particular parasite population. This dilution effect helps to maintain dewormer efficacy. A major challenge faced in modern parasite management is how to maintain refugia without compromising flock/herd health and well-being. This requires an understanding of which animals are most susceptible as well as where the parasite populations are living within the farm (in the animals vs. on pasture).
The good news is that host susceptibility to parasite infection, which is a key factor in maintaining this balance, can be predicted relatively accurately by examining both the physiological state of the animal (stage of production) and its plane of nutrition (how well an animal is fed). Both of these characteristics impact host susceptibility separately, but they can also have profound additive effects. By taking host susceptibility into account, one can target animal classes of greatest susceptibility for anthelmintic treatment while maintaining refugia in those classes with lower susceptibility. This is a central concept of selective treatment, or smart drenching, which is especially important when parasitic contamination of pasture is low and therefore most of the parasite population is in the sheep potentially receiving treatment. When parasite contamination of a pasture is high, the need for selective treatment is far less, as the large population of parasites present in the pasture will act as refugia. Treatments given during periods of high pasture contamination are known as targeted treatments, and in these instances the entire flock, or a high proportion, can be treated without compromising refugia of the farm’s parasitic population.
Source : msu.edu