Control by management

It is currently possible to reduce significantly the number of anthelmintic treatments required by grazing ruminants by employing some relatively simple management procedures, enlightened by knowledge of the epidemiology of the parasites in the climatic region and production system in question. Some examples of these procedures, such as the Michel dose and move system for nematode control in cattle, strategic treatments at times of low larval availability for sheep and alternate grazing of sheep and cattle will be considered. While such procedures can have a major impact on frequency of anthelmintic treatment, they rarely have a commensurate effect on reducing selection for anthelmintic-resistant worms. Only in control systems that avoid the use of anthelmintics altogether is there any certainty of avoiding selection for resistance. The nearest approaches to this ideal goal are some alternate grazing schemes and rotational grazing systems currently being tested in tropical environments. Even if these anthelmintic-free production systems are successful, the worms are still being selected for attributes that may ultimately render these control procedures ineffective. Examples of this are the propensity for alternate grazing systems to select for reduced host specificity, or increased numbers of parasite species able to infect both host species. Similarly, rotational grazing systems could be subverted by selection for enhanced larval survival or faster development from egg to infective larva. Nevertheless, it is hoped that selection for such fundamental changes as those presumably required to affect survival, development or host specificity will be slower than selection for anthelmintic resistance.