The impact of multiple infections on wild animal hosts: a review

Bordes, Frédéric; Morand, Serge

Multiple infections (simultaneous infections with multiple parasite species in an individual host), also called polyparasitism, are the rule, rather than the exception, in the wild. These simultaneous infections are however part of a more broad pattern of parasite assemblages. In fact, if parasites of different species may be encountered in the same host individual (i.e. infracommunities), infections by different and often numerous parasite species are also observed in a host population (called parasite species richness at population level) or in a host species (called parasite species richness at host species level). Moreover, infections with different parasite species are observed at all geographical scales: local (i.e. a village, a forest), regional (i.e. a country, a state) or global scales (i.e. biogeographic realm, continent), generally with parasite species increasing with the geographical scale (5) . Despite being recognised for many years (1, 6) , investigations on the impacts of multiple infections on individual hosts or higher parasite species richness on host populations or species are still scarce in disease ecology and in evolutionary ecology (7, 3) . Our main goals with the present review are to introduce the factors that may explain higher parasite species richness (often investigated as parasite species diversity) in wild animals and to reunify observations and data from various fields (clinical, immunology, ecological parasitology, genetics, theoretical evolutionary epidemiology) to assess the reality of the impacts of multiple infections in natural individual hosts, their potential underlying mechanisms and evolutionary implications (Fig. 1 ). theoretical ecology (with determinants such as latitudinal gradients, geographical range, home range) and epidemiological theory (with determinants such as population size, population density, population longevity). More recently, behavioural ecology has inspired the search of new determinants (sociality, grooming and preening behaviour). Although there is a need for a comprehensive analysis of all published studies, two major determinants seem to emerge consistently from the studies. Host geographical range, an ecologically linked factor, and host population density, an epidemiologically linked factor, appear to be the two most important drivers of parasite species richness, and hence for polyparasitism (Table 1) . A host species living on a large geographical range in high population densities generally harbours a higher diversity of parasite species than a host species living in a more restricted geographical area and in low population densities.