Principles of helminth control

Abstract

In this review of recent research on prophylactic control, the discussion primarily refers to gastro-intestinal nematodes of sheep and cattle. Eradication of most helminth infections is not practical. Rather the aim of control is to ensure that parasite populations do not exceed levels compatible with economic production. At present most parasite control is ‘protective’ in orientation and is based almost entirely on the regular use of anthelmintics. While usually affording protection against serious disease and mortality, such treatments are frequently not effective in preventing the exposure of animals to high levels of pasture infestation. Consequently, production losses still occur as a result of re-infection in the interval between treatments. In contrast, recently developed preventive control programmes against parasitic gastro-enteritis emphasise the principle that effective control must be based on measures designed to prevent or limit contact between parasite and host. The strategies of such preventive control are: (1) to prevent the build-up of dangerous numbers of larvae on pastures; (2) to anticipate the periods during which large numbers of larvae are likely to occur and to remove susceptible animals from heavily contaminated pastures before these periods. These aims can be achieved using three interrelated approaches: by grazing management, by the use of anthelmintics and by dependence on the acquisition of immunity. Potentially, the most efficient control requires the complete integration of all three facets. Effective, integrated control is dependent upon a detailed understanding of the sequential interrelationships between the various sources of pasture contamination, the availability of infective larvae and the build-up and decline of infections; a knowledge of the time course of events is also of paramount importance. The essential requirement of integrated control is the provision of ‘safe’ pasture for susceptible animals at appropriate times. Safe pasture can be produced by a variety of stock and pasture management manipulations and maintained by the use of strategic anthelmintic treatments prior to the anticipated occurrence of conditions favourable for the free-living development of the parasites. In some climatic areas, an alternative to a change to safe pasture is the use of ‘critical’, strategic anthelmintic treatments at times when re-infection is negligible. Systems of parasitological monitoring (e.g. faecal egg counts or pasture sampling), and forecasting on the basis of meteorological data and computer simulation provide a different approach to preventive control. Advocates suggest that such techniques allow the selection of appropriate treatment and management strategies. However, the practical value of such aids, in some circumstances, has been questioned. Because of the requirement for planned grazing in modern parasite control systems, decisions on the implementation of these can no longer be separated from other management considerations. It is essential that the methods of disease eradication, the costs and their benefits, be analysed in the broadest context and in relation to whole husbandry systems. This will involve a trans- or multi-disciplinary approach and will require the closest co-operation between helminthologists and various other specialists.