Biological control of gastrointestinal nematodes and liver fluke in sheep and cattle

Summary

Gastrointestinal nematode parasitism and liver fluke infection continued to be major problems to sheep and cattle producers in Australia due to their detrimental effects on animal production and the high cost of control measures. The rising incidence of resistance to anthelmintic chemicals and the increasing demand by consumers for livestock products free of chemical inputs revived interest in the identification and application of non-chemotherapeutic means of control.

With this in mind, options for the biological control of nematode parasites and liver fluke in sheep and cattle were reviewed. Natural pathogens of the parasitic stages of nematodes of sheep and cattle had not yet been identified. A proposed novel approach to the control of parasitic stages by biological means required the genetic manipulation of pasture plants or gut micro-organisms to deliver toxins, such as nematocidal Bacillus thuringiensis crystal proteins, to predilection sites within the host.

Ethical and regulatory concerns were addressed for this approach to succeed. Numerous antagonists of the free-living stages of nematode parasites have been identified in pasture systems that favour nematode survival. Dung beetles, earthworms, predacious free-living nematodes, microarthropods and fungi have been shown to reduce numbers of nematode larvae emerging from faeces and infecting pasture and agricultural practices that conserve or enhance natural populations of these predators and pathogens will continue to assist in controlling larval populations on pasture. Methods for the applied biological control of nematode parasites of livestock are currently being investigated.

For the foreseeable future, the greatest potential seems to be through the inundative release of specific pathogens to reduce or eliminate larval stages in or near deposited faeces. Of the possible pathogenic agents identified in the review, the following three offer the greatest potential for development and application in Australian livestock systems. Bacillus thuringiensis genetic manipulation of bacteria that colonize the gut and/or faecal material to include nematocidal B. thuringiensis toxins should enable reduction of the free-living larval stages of parasitic nematodes that feed on bacteria in faeces. Unfortunately, this approach may encounter problems similar to those faced by any other genetically modified organism proposed for use in agriculture. Arthrobotrys spp. recent research overseas suggests that this aggressive and effective trapper of parasitic nematode larvae deserves detailed assessment and laboratory selection of isolates with increased ability to survive gut passage. Selected isolates should then be evaluated for production of conidia/fungal biomass using methods established for other Arthrobotrys spp.

In addition, investigation of methods developed for protecting other microbiologicals and nutrients from digestion and degradation should indicate their potential for application to Arthrobotrys spp. (and possibly other fungi like the egg-parasitic Paecillomyces spp.) for use in biological control of nematode parasites of livestock. Duddingtonia flagrans - The concentrated effort currently being directed towards D. flagrans will probably lead to availability of products containing this fungus in Australia within the next 2 to 3 years. Critical to the application of D. flagrans under industry conditions will be testing and development of strategies to maximise the effectiveness of this means of control including studies of duration and timing of application and integration with other control alternatives. Determination of the environmental impact of inundative application of D. flagrans will also be essential before widespread use can be recommended. It appears likely that there will be at least one agent for biological control of nematode parasites of sheep and cattle in the near future, i.e. D. flagrans.

The other potential biological control agents described require further development, and research should continue on these to increase the options available for application in Australian livestock production systems. It should also be emphasised that successful biological control of nematode parasites may not require elimination of the entire parasite population, although this could be possible over time if a highly effective agent was identified. A more realistic scenario would include the strategic use of the biological control agent to lower nematode populations below the threshold where livestock production is affected by parasitism while retaining sufficient larval challenge to stimulate host immunity to infection.

Careful selection and integration of all appropriate options for control (chemotherapy, grazing management, enhanced immunity by improved nutrition or genetic selection) are essential elements in the production of sheep and cattle in areas where nematode parasites are a problem. The successful development of biological control agents will add to this arsenal and may reduce the current reliance on anthelmintic chemicals. No options for the biological control of fluke within the host have been identified and possibilities for the control of fluke eggs and larvae in faecal material remain speculative at the present time.

The search for biological agents for potential application to control Fasciola spp. infection in livestock therefore rests with identified pathogens of the intermediate host Lymnaeid snails. Many of these pathogens have been observed to occur in native snail populations and would already exert some regulatory effect on those populations. Elimination of Lymnaeid snails may be possible within a limited area through inundative release of specific pathogenic organisms in fluke prone areas once methods for mass rearing are developed but environmental consequences on non-target snail species will need to be closely monitored. Overall, the prospect for the biological control of liver fluke infection in livestock is poor and it is expected that livestock producers will need to rely on currently recommended strategies for control for the foreseeable future.