Controlling internal parasites without anthelmintics (a review) OF0132

Summary

1.0 Executive summary
1. Internal parasites are a major source of economic loss in grazing ruminants. To a greater or lesser degree, most farms in the UK rely on anthelmintics for control. In most situations these products continue to be highly effective, but anthelmintic resistance is increasing to the limited range of products available, raising serious concerns over the future of worm control.
2. Internal parasites are also of concern on organic farms, where the prophylactic use of anthelmintic is prohibited by the standards for organic production. Organic farms generally rely on grazing management, with restricted use of anthelmintic if required. However, also within the regulations is the absolute requirement to treat individual animals if this is necessary to avoid suffering. Organic systems should aim to eliminate the need for anthelmintic drenching, but the number of farms where this has been achieved is few.
3. This report reviews current methods of parasite control, and evaluates the potential of alternatives to the use of anthelmintic, with particular reference to organic systems.
Epidemiology
4. The biology and epidemiology of the major internal parasites of sheep and cattle are well known. These form the basis of management and biological control, including strategies for clean grazing. However, less is known specifically about the complex interactions between parasite, host, climate, management and pasture factors which give rise to the occurrence of disease.
5. Useful computer simulation models have been developed, usually related to specific locations, species of parasite or conditions of management. These are becoming increasingly complex as further variables such as host resistance, sward type and grazing behaviour are included in the model. As knowledge and computing power increase, the opportunity is there for models to become more comprehensive, and more accurate.
6. Trends in the occurrence of disease can broadly be predicted, on a national or regional basis, from climatic and management data. At the present time however, there is no single comprehensive model which can be used to adequately quantify and predict the development of disease at a farm or system level. Research work is continuing on this topic.
Maximising the host’s ability to fight disease
7. Host factors affecting the incidence and severity of parasitic attack include the development of acquired immunity, the effect of nutritional status and the presence of concurrent disease.
8. Under-nutrition has a general debilitating effect on an animal’s ability to fight infection. In terms of roundworms, the development of the immune response and the host’s ability to restore damaged tissue has been shown to be related to energy, and particularly protein, intake. A shortfall of specific minerals and trace elements e.g. cobalt or selenium will affect the host response to infection. Parasitic infection in it's own right can increase the requirement for specific nutrients, due to increased loss or less efficient use.
9. The underlying approach to disease control on organic farms is to promote the natural immune system and livestock health. Optimising management conditions to minimise stress, provide high quality, balanced diets and avoid concurrent disease is entirely consistent with an organic approach to animal husbandry, and a good basis for any disease control programme.
Clean grazing systems
10. Management strategies which optimise health, and reduce the exposure of animals to parasitic infection are a prerequisite on organic farms. Clear benefits have been demonstrated under conventional, as well as organic systems. Clean grazing systems are the favoured and most direct method of controlling internal parasites on organic farms, and wherever possible should underpin all other methods of parasite control.
11. However, the managerial discipline and farm infrastructure are not always present to operate an effective clean grazing system. On conventional farms, the perceived conflict between efficient and flexible grassland utilisation, and the availability of effective anthelmintic products, has discriminated against greater uptake of clean grazing. On organic farms the motivation is greater, but the optimum balance of livestock and arable enterprises is not always present, making management more difficult or reducing the effectiveness of a clean grazing approach. Implementation of clean grazing systems is particularly difficult on hill farms, on single enterprise units with permanent pasture, or where the ratio of cattle to sheep livestock units is wider than 35:65.
12. It is also difficult to operate current clean grazing systems without using some anthelmintic, particularly when animals are first put onto clean pasture. This is particularly critical for freshly lambed ewes, which temporarily lose their immunity and produce a high output of worm eggs during early lactation. In this case anthelmintic drenching is often required to reduce contamination of clean grazing, and to avoid the system breaking down later in the season. Systems of alternate or mixed grazing have value, but on their own are unlikely to achieve an acceptable degree of parasite control.
13. Where clean grazing cannot be set up, for example on hill farms, there is greater reliance on the extensive nature of the grazing to reduce parasitic burden, and on the sale of lambs at weaning. By means of a carefully timed move from lambing pastures to open hill, an extensively managed hill farm could theoretically avoid the need to drench lambs at least until weaning.
14. However, for organic farms in the hills/uplands trying to finish lambs after weaning, parasitic gastro-enteritis will remain a hazard, and the limited use of anthelmintic is almost inevitable for individuals, or whole groups, depending on the season. Even where lambs are sold at weaning, all or a proportion of lambs may require drenching on arrival at their new destination, particularly if going onto clean pasture.
15. On lowland organic farms supporting ‘commercial’ stocking rates, current practice is to support clean grazing by the strategic use of anthelmintic, often a single drench to ewes around lambing time, before entry to clean pastures, or as a ‘salvage drench’ should parasitic burden become too great.
Genetically resistant animals
16. Trials have shown that 10% of lambs in a given flock may contribute 50% to total faecal egg output.Ironically, one method of reducing total anthelmintic use is to target those individual lambs within the flock which, in terms of faecal egg output, appear to be the worst offenders. At a practical level this is done by drenching scouring lambs. However, scouring in not necessarily indicative of a high faecal egg output.
17. Eliminating the most contaminating individuals from a flock, can have a very significant impact on the overall level of parasitic challenge. The existence of genetically linked resistance to roundworms is well recognised in the UK and abroad. Several experimental flocks have been established in Australia and New Zealand, with high and low responder lines differing in faecal egg output for a particular parasite, usually Haemonchus contortus or Ostertagia circumcinta.
18. These traits are low to moderately heritable, and therefore can be used to increase the frequency of resistant genes in a particular flock or population. Neither does selection for increased parasite resistance necessarily reduce performance in economically traits, especially in a heavily parasitised environment.
19. Genetically resistant animals have traditionally been selected on the basis of nematode egg output in the faeces, after dosing with a known challenge of infective larvae, or from naturally acquired worm burdens. This the most direct and cheapest form of selection. Indirect methods have been sought on the basis of host physiology and blood biochemistry. So far the results have not been conclusive.
20. Some progress has been made in the identification of genes associated with the sheep Major Histocompatibility Complex (MHC) which may be involved in conferring increased resistance to internal parasites. However, there are other non-MHC genes also involved in the regulation of host responses.
21. A recent development by Glasgow University Veterinary School has been the identification of a genetic marker for increased resistance to O. circumcinta in Scottish Blackface sheep. This test is currently being validated on commercial farms where single sire mating is practised in pure-bred flocks. With continuing improvement in genetic mapping and biotechnology techniques, this type of test is likely to be used increasingly in genetic selection.
22. One of the difficulties in extrapolating from an experimental flock of genetically resistant animals, is in determining how representative this sub-group is of the sheep population as a whole. Nevertheless, commercial breed improvement programmes are now in operation in Australia and New Zealand which incorporate resistance to worms in the selection index.
23. On an individual organic farm which breeds its own replacements, selection for parasite resistance is possible. Selecting future breeding stock by the extent of dagginess (as currently practised on some organic farms) may in fact be selecting for resilience against parasite attack, rather than for resistance. Fortuitously, resilience and resistance traits are positively correlated. The accuracy of selection for resistance could be greatly improved by carrying out egg counts in samples of faeces taken over a period. Just as importantly, this strategy could be used to eliminate from the breeding flock animals inclined to have persistently high faecal egg output, thereby reducing pasture contamination and parasitic challenge to the flock as a whole.
Manipulating sward structure and composition
24. Sward structure has been shown to affect the concentration of infective larvae on the upper horizon of the sward, through effects on the micro-climate within the sward or due to the morphology of the plants themselves. These effects have been shown to modify parasite burden in the grazing animal; Yorkshire Fog for example, has been shown to result in lower rates of infection than Ryegrass swards.
25. Most of the data derives from studies in Australia and New Zealand, often with H. contortus. Significant effects remain to be demonstrated at a strategic or applied level under UK conditions. Conclusive relationships have yet to be established, which would allow this factor to be used in a management system.
26. New Zealand studies have shown that the species of plant present in the sward can also affect the worm burden acquired by grazing stock. Some of the most interesting work relates to certain plants, in particular, Lotus corniculatus and sulla (Hedysarium coronarium) which contain condensed tannins (CT’s). Several experiments have shown that CT’s can reduce the parasite load and improve the performance of parasitised lambs due, at least in part, to their protective effect on dietary protein which increases protein supply at the duodenum.
Control by vaccination
27. The host parasite relationship is very complex. A lack of detailed knowledge of the immune response is the main factor which currently limits the ability of molecular biologists to develop suitable vaccines. Initial successes against blood sucking and mucosal parasites provide some optimism for effective vaccination against gastrointestinal nematodes in the future.
28. Commercial vaccines are not yet available, but considerable progress has been made in isolating various parasite enzymes and proteins which may be used as antigens. As knowledge of the immune response increases, so too does the ability to select appropriate antigens to stimulate an immune response in the host. Improvements in adjuvants will ensure the right type of response is induced.
29. Much of the work has been undertaken with H. contortus - it’s blood sucking feeding habit providing a good means of delivering antibody to the digestive system of the parasite. However, this species is of limited importance in the UK. For practical use, control of Ostertagia in particular, will need to be incorporated in any commercial vaccine.
30. While a vaccine for the control of parasitic gastro-enteritis may be available in the future, its adoption within organic systems will depend on how its acceptance by the Organic Sector Bodies. Draft EU Standards would allow vaccination in circumstances where disease had been identified and treatment recommended.
Role of homeopathy
31. There is little evidence to confirm any direct effect of homeopathy in the treatment of parasitic gastro-enteritis. However, homeopathic preparations may have a useful role in aiding the recovery of tissues damaged by parasitic attack. An effective nosode is available to provide protection against lungworm infection.
Herbal treatments
32. Many plants and their extracts have been shown to act as mild verimfuges. However, little comparative information is available on their potential for worm control in farm animals. A herbal approach may have better longterm prospects for nematode control than homeopathy, but further screening and evaluation is necessary.
33. With both homeopathy and herbal treatment, a practical method of delivery under field conditions can be difficult. One option could be to establish monocultures of certain plants (vermifuges, or CT’s) to be grazed strategically by organic stock, for example immediately after weaning.
Biological control
34. Fungi are only one type of organism with potential for controlling the free living stages of gastrointestinal worms, but these have been the most studied. Other biological control agents are theoretically possible, particularly bacteria.
35. Initial screening work carried out in Australia identified approximately 100 species of fungi with nematophagous activity. Progress has been made under laboratory conditions in selecting fungal species which meet this criteria, and which can survive passage through the ruminant gastrointestinal tract. Some field testing has been carried out under Australian conditions with selected fungal species. Significant reductions in the number of infective larvae present on the pasture were reported, and a level of control comparable to the use of anthelmintic was claimed.
36. Research is ongoing to determine threshold dose levels and to investigate systems of sustained delivery of the fungi. Genetic typing and manipulation, while less acceptable for organic systems, could be another option to increase efficacy. The ultimate aim might be to develop a biological control system through a feed supplement, feed block or an intraruminal controlled release device.
37. New Zealand research has concentrated on understanding the role, and factors which affect the activity, of fungi already present on the pasture, rather than artificially deploy fungi onto faeces. There is little or no information on either aspect available from the UK. With a better understanding of fungal distribution on pasture, it may be possible to encourage sward conditions to maximise the impact of nematophagous fungi which are already present on the sward
Controlling internal parasites without anthelmintics - Conclusions
38. Effective systems of clean grazing have been developed in the UK. With current knowledge, these must be rigidly applied to be fully effective, often reducing their chances of being taken up on farms whose infrastructure and enterprise mix are not ideal. Furthermore, current systems of clean grazing generally depend on strategically timed anthelmintic treatment. To retain effective parasite control, while simultaneously reducing anthelmintic input and relaxing some of the more rigid features of conventional clean grazing systems, poses a considerable challenge.
39. An increasing number of factors are recognised as affecting parasitic burden. The need is to understand how collectively to harness some, or all of these, to bring practical benefits to parasite control. The best prospect is to combine several approaches in an integrated control programme, which could vary depending on individual farm circumstances. Some research has been carried out in New Zealand, but this work has been curtailed because of lack of funding. More information is required on parasite epidemiology, the effect of pasture species, grazing management and factors affecting larval movement and survival, which can be incorporated into a predictive model. Some of this basic modelling work is already taking place. However, practical evaluation under commercial conditions is still some time away.
40. Breeding for host resistance to parasites is a relatively new subject in the UK. Carried out on a flock basis, there is the advantage that selection is specific to farm conditions. However, if selection is based solely on the incidence of scouring the accuracy of selection and rate of progress will be low. Accuracy of selection may be improved by the use of faecal egg counts, replicated over time. The use of genetic markers, although promising, requires further investigation and validation in the field. Ultimately selection should be based on a multi-component selection index. The weighting of this index for disease resistance may need to be greater for organic systems, simply because the costs (economic or ideological) are higher, and the response to improvement in disease control greater in a parasitised environment, where the aim is to avoid anthelmintic treatment. Serious consideration should be given to developing a protocol for selecting more resistant animals on a within-flock basis, perhaps using existing Sire Reference Schemes as a model framework.
41. Experience of biological control outside of controlled glasshouse situations is often unconvincing. In terms of biological control of roundworms, there are reasonable prospects of developing a delivery system through the grazing animal to target the parasite in the faecal environment. Alternatively, a better understanding of how to optimise pasture conditions in favour of the control agent, probably a fungus, could significantly reduce the viability of free living stages. Further research needs to be undertaken in the UK.
42. The approach which might give the best return, could be the strategic use of specific forages. These are likely to be those containing condensed tannins, or perhaps plants with specific vermifuge qualities. Typically, these could be used for freshly lambed ewes before being moved on to clean pasture, so that periparturient worm burdens are reduced, or for lambs after weaning. Alternatively, weaned lambs from organic hill farms could be put onto such forages, before being released onto finishing pastures. There is a need to evaluate the role of these forages on organic farms, their effectiveness, agronomy, and short/longterm effects on the grazing animal.