Gastrointestinal parasites in ruminants: ecological approach in wildlife as a ground for a possible control in domestic animals.

The study of host-parasite relationship in wildlife frequently follows an ecological approach (Hudson PJ et al., 2002, The ecology of wildlife diseases, Oxford University Press, Oxford, UK). This approach is seldom applied to study the parasites of farmed animals. Ecological approach to host-parasite relationship was applied in domestic ruminants, for example, by Coyne and Smith (Scott M.E., Smith G., 1994, Parasitic and infectious diseases. Epidemiology and ecology, Academic Press, San Diego, California). More recently ecological aspects of domestic ruminants-parasites relationship was considered principally for organic farming (Thamsborg SM et al, 1999, Veterinary Parasitology, 84: 169-186; Cabaret J et al., 2002, Veterinary Research, 33: 625-640; Hoste H et al., 2002, Veterinary Research, 33: 531-545). One of the main purposes of parasitology applied to wildlife is to understand the mechanisms that lead to a stable relationship between host and parasite populations. It is important, in wildlife management, in order to maintain or promote stability. This ecological approach is, moreover, particularly useful in order to evaluate the possibility of parasites control, for example in the case of parasitic zoonoses or when parasites have a heavy impact on host population dynamic. In household conditions, host-parasite stability is almost invariantly lost because of human management. Breeding practices tend to maintain, with repeated measures such as pharmacological treatments, an instable situation, namely a situation that would not be maintained in nature and that would be probably lost as the above-mentioned measures cease to be applied. In addition, breeding conditions are usually very different from natural ones: animals often live in high-density groups, frequently homogeneously assembled, and parasite communities are usually poorer when compared with wild ones. Moreover, the aim of parasite management in domestic animals is generally to maximise productions, which is quite different from the purpose of maintaining viable wild populations. Ecological parasitology considers host-parasite association as an ecological relationship between populations in a given environment. Moreover, it deals with the intricate network involving the whole biocoenosis, including all host and parasite populations, i.e. host and parasite communities. The ecology of parasites is not really different from that of free-living organisms, but it is characterised by peculiar patterns. Considering in particular gastrointestinal helminths, it is important to remember the presence of external and internal environmental factors, in relation to the free-living and parasitic stages, respectively. Internal factors are clearly influenced by host response to parasites, above all immune response. Moreover, internal environment is not a continuum, but it is fragmented among each individual host. A great number of ecological features usually studied in wildlife parasitology, open interesting possibilities for the application to domestic hosts. Above all, the frequency distribution of parasites within host population, the mechanisms involved in parasite population dynamics and particularly in parasites regulation (either mediated or not by the host), and the factors influencing the composition and stability of parasite communities. These ecological processes, even if undervalued in domestic circumstances, are anyway continuously working on parasite populations. For example, just like any population, parasites tend to increase their fertility or decrease their mortality when their number decreases. It could lead to paradoxical consequences when parasite control is carried out, especially if control itself affects host population immunity to parasites. Moreover, competition between parasites should be taken into account while controlling or eradicating a particular parasite species, as it could lead to an unexpected increase of other species.