Monitoring macroparasites in wild ruminants populations: utility and limitations

Although the role of macroparasites in regulating wildlife populations is generally accepted (Tompkins DM et al, 2002. In: The ecology of wildlife diseases. Oxford University Press: 45-62), their sanitary impact is difficult to detect, due to numerous factors that can influence and/or mask host-parasite interactions. This is particularly evident in the case of the helminths, which are consistently present in wild ruminant populations worldwide and have in general a subclinical impact. Notwithstanding, sanitary evidences observed in the past and recent years suggest an importance of macroparasites even from a sanitary perspective. In our opinion, monitoring macroparasites in wild animal populations might have two basic aims: i) to enlighten the role and/or possible impact of such agents on their host populations and ii) to detect if selected parasitological indices can be useful as indicators of the population health status. The former scope involves both ecological and sanitary implications, while the latter appear mainly involved in a management perspective. Some experiences performed in alpine wild ruminant populations have suggested that helminths could exert a negative impact in host populations. In particular, negative correlation between abomasal parasitic burdens and body condition indices was found in roe deer Capreolus capreolus (Zaffaroni E et al, 1997, Parassitologia 39: 313-317). Moreover, investigations in alpine chamois Rupicapra rupicapra showed negative correlation between selected abomasal helminth species and blood protein level (Sala M et al, 2000, Parassitologia 42 (S1): 76). These findings suggest the hypothesis of a role of parasites mainly in reducing body resources. Actually, it appears that macroparasites could participate in making the population less able to cope with different pathogens and disease complexes (Wobeser GA, 1994, Investigation and management of disease in wild animals, Plenum Publishing Corporation, 265 pp.; Citterio CV et al, 2003, Vet. Rec. 153: 592-596). However, the impact of parasites cannot be considered as a constant in every situation, since it is mediated by different factors which can vary among different contexts and even in the same context in relation to the ecological and epidemiological correlates of parasitic patterns, to the effective pathogenic potential of parasites and to the factors that can influence it. In order to evaluate the factors above, a wide sanitary monitoring appears to be necessary, and informations about parasite and host's life histories are needed. Moreover, selected risk factors, such as interactions between different host populations and environmental factors should be considered. As an example, interactions for selected abomasal helminth species, such as Haemonchus contortus, between domestic and wild ruminants have been evidenced as a sanitary risk factor for alpine wild ruminant populations, due to the lack of an inherent equilibrium in the parasite communities of these hosts, which appear to be isolationist (Zaffarom E et al, 1996, Suppl. Ric. Biol. Selvaggina 24: 53-68; Zaffaroni E et al, 1999, Parassitologia 41: 567-572), and to the low host-specificity and high pathogenic potential of such species (Zaffaroni E et al, 2000, Vet. Parasitol. 90: 221-230). Moreover, the life history and pathogenic action of gastrointestinal helminths can be related to other factors, such as food shortage and/or low diet quality (Gulland FMD, 1992, Parasitology 105: 493-503; Ezenwa VO, 2004, Int. J. Parasitol. 34: 535-542). Considering all above, a wide-range monitoring could lead to a better understandig of the parasitic impact on hosts. At the same time, the simple parasitic counts appear inadequate as indices of population health (Pérez JM et al, 2003, J. Parasitol. 89: 315-318), since they can widely vary, even not depending on the health of their host population and without any sanitary impact. Thence, it arises that an evaluation of such an impact should imply considerable efforts in collecting and analyzing data, which should be extended in time.