THERE are many different species of nematode parasites that infect livestock but only few parasite species cause major problems, notably Haemonchus, Teladorsagia, Trichostrongylus, Nematodirus and Cooperia spp. The conventional method to control is with the use of synthetic chemotherapeutic drugs (anthelmintics).
Consumers now demand that the agricultural products should be both “clean” and “green”. The demand for “clean” livestock products has followed adverse publicity about impact of agro-chemicals on human health, and the development of super-resistant human microbial pathogens, caused by the use of antibiotics in intensive livestock production systems.
The term ‘green’ refers to low-input operations based on grazing animals on pasture against feedlot or housed systems of production. The move back to pasture-based livestock management is driven by public concerns over hand feeding of animals and the bovine spongiform encephalopathy (BSE; ‘mad cow’) disease scare; the emergence of multi-resistant microbes derived from selection by ‘growth promoters’ commonly used in the intensive poultry industry, and the entering of chemical residues into the human food chain.
There is a downside to organic/green farming in the livestock. Difficulties have arisen in adequate control of pasture-borne infectious diseases, particularly those due to nematode parasite infections.
Investigations have concluded that nematode parasitism is the greatest economic constraint of grazing livestock production, whether in the industrialized or the developing countries. The most profound effects of parasitism are on sub-clinical production loss (i.e., not obvious by visual appraisal), of which farmers or their advisers – are unlikely to be aware of.
The assessment of animal health issues associated with organic farming are often based on farmers’ perceptions in questionnaires or surveys, rather than on detailed veterinary investigation. As a consequence, new and serious animal welfare issues might emerge in organic farming that are caused by distress suffered by animals as a result of uncontrolled parasite infections.
To counter this, the move to green and organic livestock production has also been accompanied by an increase in research aimed at exploring non-chemical approaches to parasite control.
Genetic resistance is ultimate in parasite control. It is economical, permanent solution requiring no extra resources and no additional costs. However, for most species of ruminant livestock, animals that have evolved to be highly resistant to parasite infection are not generously endowed with desirable productivity traits for wool or meat production.
These innately resistant breeds are found in the tropics, where the formidable combination of malnutrition, environmental stress, long-term and often massive larval challenge and limited relief by way of effective anthelmintic treatment have imposed the harshest conditions for selection, resulting in survival of the fittest. However, attempts are being made to identify those genes that encode parasite resistance in laboratory animal models.
With the aid of comparative genomic maps, the aim is to identify the locations of similar genes in ruminants and to develop transgenic animals in which genes for resistance are inserted into economically productive breeds. Pakistan has a galaxy of different breeds of livestock but none of them has been exploited for their potential of resistance against nematode parasite. This could be a very good research issue based on future demands.
To date, vaccines against nematode parasites have had very limited commercial success. Early forays into this area were made using attenuated whole parasites and, although these showed some promise and marketing opportunities (notably the irradiated larval vaccine of the cattle lungworm, Dictyocaulus viviparus).
Better nutrition could reduce worm burden in different livestock species. Due to internal parasite infection there is increasing endogenous loss of protein and a reduced efficiency of the animals.
The increase in parasite challenge associated with the contemporary livestock production systems must come at a price with regards to animal productivity, particularly at times of sub-optimal nutrient supply, when the animal has to prioritize the allocation of scarce nutritional resources.
Strategic feed supplementation, particularly to young and peri-parturient animals, can have long-term benefits, and research is now targeted at fine-tuning the ways, means and timing of doing this that would be practical, profitable and, if needed, acceptable to the organic standards.
Another important component of green ruminant production system could be the use of herbal drugs for the treatment of parasitic diseases. Anthelmintic medication has its origin in the use of plant preparations.
In general, these were hazardous concoctions with low anthelmintic efficacy, especially in ruminant species, and they rapidly disappeared from human and veterinary use with the discovery of synthetic anthelmintic compounds.
Although a large and diverse range of herbal de-wormers is used throughout the world, particularly in Asian and African countries, generally there is a lack of scientific validation of the purported anthelmintic effects of these products. In ruminants, the claimed efficacy is often associated with farmers observing the occasional elimination of tapeworm segments, which has little bearing on production, let alone parasite control.
There is considerable and apparently expanding interest worldwide in traditional health practices in both the industrialized and developing countries of the world, including herbal de-wormers. However, for resource-poor farmers in developing countries, traditional herbal remedies based on local plants offer an alternative to the expensive and often inaccessible commercial anthelmintics.
The use of plant/crops containing secondary metabolites (or nutricines) can also be a good method of parasite control in “green” system. The crops are either grazed or fed after preservation, with the main purpose of reducing parasite infections, and ideally they can be incorporated into crop rotation schemes. A specific group of plant polyphenols, the condensed tannins, has attracted attention in recent years. When animals are grazed on the leguminous crops which are rich in these compounds exert great effect to reduced GI nematode.
For example, the administration of quebracho, an extract of condensed tannins, might reduce nematode burdens of the small intestine (Trichostrongylus colubriformis), but not those of the abomasum (Haemonchus contortus; Teladorsagia circumcincta). The epidemiological importance of reduced faecal egg counts continues to be investigated. Grazing of chicory (Cichorium intybus) by infected sheep has shown some promising results, in particular with regard to reductions in abomasal worm burdens.
Keeping in view the world’s trend about tannin containing plants, the authors have also screened various plants containing condensed tannins and out of these a few showed very good results.
For decades, various grazing management practices have been the cornerstone of epidemiologically based parasite control strategies in the temperate regions of the world. Not only were they cost efficient and highly effective, particularly when combined with anthelmintic treatment, but they also provided the opportunity for dual livestock species parasite control, such as with sheep/cattle interchange grazing. These concepts became established in the applied veterinary parasitology jargon, with the epithet of ‘dose-and-move strategies’.
Another concept is the biological control. In the simplest words, biological control is to kill life with life and biological control of nematode parasites of livestock is almost exclusively associated with the nematode-destroying microfungus Duddingtonia flagrans.
The microfungus has three very important attributes: (i) the ability to survive gut passage of livestock; (ii) the propensity to grow rapidly in freshly deposited dung; and (iii) the possession of a voracious nematophagous capacity. This fungus thus breaks the lifecycle by capturing infective larval stages before they migrate from dung to pasture, where they would otherwise be acquired by grazing animals.
Field evaluation of this concept for a range of livestock species, in a variety of geo-climatic regions, has been under way for the past decade. At the same time, several potential stumbling blocks on the path towards product registration have largely been overcome.
First, it is now possible to produce large quantities of D. flagrans spore material; second, long-term field trials using D. flagrans have shown no adverse effects on the environment; and third, it has been established that the D. flagrans is ubiquitous and that very close genetic similarity exists between isolates from all regions of the world.
The commonly used means of deployment of D. flagrans spore material is by a feed additive. To achieve optimal results, the fungal spores need to be continuously shed in the dung of animals at the same time that contamination of pasture with parasite eggs occurs.
Thus, daily supplementation of fungal material is recommended during the predetermined period of time that biological control is to be effected. Clearly, much greater opportunities for this innovation would occur if effective methods of D. flagrans depot delivery were available.