Control of gastrointestinal nematodes in Australia in the 21st century.

Because the Australian livestock industries rely overwhelmingly on year-round grazing of pastures, nematode parasites are of major concern. Nematode parasitism is recognized as the cause of the most economically important diseases of sheep, while in cattle, the relative importance of nematode parasitism will probably increase following the eradication of diseases such as brucellosis and tuberculosis. The extraordinarily high prevalence of resistance to benzimidazoles, levamisole and morantel in nematodes parasitising sheep has stimulated the formulation and, to a surprisingly large extent, adoption by farmers of strategic control programmes aimed at preserving the useful life of ivermectin. Even so, it is highly likely that we will enter the 21st century with ivermectin resistance. Current research aimed at control in the next century is progressing in three broad directions. First, towards better control using existing anthelmintics through modifications to pharmacokinetics, together with integrated management aided by computer simulation models of nematode population biology. Secondly, towards immunological control arising from a better understanding of the ruminant immune response to helminths, particularly in young animals, and finally, a coordinated investigation of genetic resistance, within breeds, of sheep to nematode parasitism. The appearance in the 21st century of novel anthelmintic compounds, most probably as a result of research by the international pharmaceutical industry, may well reduce the sense of urgency driving current Australian research. Nevertheless, success in one or more of these three lines of endeavour will mean a longer and more profitable life for the next generation of anthelmintics marketed in Australia.