THE PROSPECTS FOR BIOLOGICAL-CONTROL OF THE FREE-LIVING STAGES OF NEMATODE PARASITES OF LIVESTOCK

Control of nematode parasites of livestock is focused almost entirely on the parasitic stages within the host. Current methods rely on anthe lmintic drugs, but these are under increasing threat with the developm ent of resistance covering the ,whole spectrum of anthelmintics amongs t the important nematode species of a range of livestock. However, inv ariably the greatest proportion of the parasite biomass resides not wi thin the animal hosts, but in the external environment - commonly on p asture. It is in this environment that the free-living stages are vuln erable to a range of abiotic factors (extremes in temperature and desi ccation) and biotic factors (macro- and micro-organisms) that may deci mate their numbers. Of the latter, there are organisms, which exert th eir effects either indirectly by rendering faecal deposits inimical fo r the development of nematode eggs through to infective larvae, or dir ectly by acting as pathogens or by exploiting the free-living stages a s a food source. Within this vast assemblage of organisms, which inclu de microarthropods, protozoa, viruses, bacteria and fungi, could well emerge a variety of biological control agents of nematode parasites. A t present, greatest interest lies with the nematode-destroying fungi. Work has progressed from Petri dishes, to plots, to paddocks with seve ral species of the genus Arthrobotrys and Duddingtonia flagrans. These studies indicate that the voracious nematophagous capabilities of the se fungi, clearly demonstrated in vitro, translate to reductions in th e number of infective larvae on pasture and indicate that levels of co ntrol, comparable to conventional schemes using anthelmintics, can be achieved. The challenge now lies in developing methods of administrati on of fungi to animals which can be applied under practical farm condi tions. However, the pursuit of candidates for biological control of ne matode parasites of livestock should not involve just a few species of nematophagous fungi. More than 100 species of fungi have been identif ied as possessing nematode destroying capabilities. These need to be m ore specifically investigated for their effects on free-living stages of nematode parasites of livestock, together with other classes of org anisms, particularly bacteria, which have proved successful as biologi cal control agents of arthropods. Copyright (C) 1996 Australian Societ y for Parasitology. Published by Elsevier Science Ltd.