Field parasitological studies consistently demonstrate the reality of polyparasitism in natural systems. However, only recently, studies from ecological and evolutionary fields have emphasised a broad spectrum of potential multiple infections-related impacts. The main goal of our review is to reunify the different approaches on the impacts of polyparasitism, not only from laboratory or human medical studies but also from field or theoretical studies. We put forward that ecological and epidemiological determinants to explain the level of polyparasitism, which regularly affects not only host body condition, survival or reproduction but also host metabolism, genetics or immune investment. Despite inherent limitations of all these studies, multiple infections should be considered more systematically in wildlife to better appreciate the importance of parasite diversity in wildlife, cumulative effects of parasitism on the ecology and evolution of their hosts.

No abstract

Rodents are recognized reservoir hosts for many human zoonotic pathogens. The current trends resulting from anthropocene defaunation suggest that in the future they, along with other small mammals, are likely to become the dominant mammals in almost all human-modified environments. Recent intricate studies on bat-borne emerging diseases have highlighted that many gaps exist in our understanding of the zoonotic transmission of rodent-borne pathogens. This has emphasized the need for scientists interested in rodent-borne diseases to integrate rodent ecology into their analysis of rodent-borne pathogen transmission in order to identify in more detail the mechanisms of spillover and chains of transmission. Further studies are required to better understand the true impact of rodent abundance and the importance of pathogen sharing and circulation in multi-host- multi-pathogen communities. We also need to explore in more depth the roles of generalist and abundant species as the potential links between pathogen-sharing, co-infections and disease transmission.

We summarize the current knowledge on parasitism-related invasion processes of the globally invasive Rattus lineages, originating from Asia, and how these invasions have impacted the local epidemiology of rodent-borne diseases. Parasites play an important role in the invasion processes and successes of their hosts through multiple biological mechanisms such as "parasite release", "immunocompetence advantage", "biotic resistance" and "novel weapon".Parasites may also greatly increase the impact of invasions by spillover of parasites and other pathogens, introduced with invasive hosts, into new hosts potentially leading to novel emerging diseases. Another potential impact is the ability of the invader to amplify local parasites by spill-back. In both cases, local fauna and humans may be exposed to new health risks, which may decrease biodiversity and may potentially cause increases in human morbidity and mortality. Here we review the current knowledge on these processes and propose some research priorities.

Summary1. We tested how habitat structure and fragmentation affect the spatial distribution of common murine rodents inhabiting human-dominated landscapes in South-East Asia. The spatial distribution patterns observed for each rodent species were then used to assess how changes in habitat structure may potentially affect the risk of several major rodent-borne diseases. 2. For this analysis, we used an extensive geo-referenced data base containing details of rodents trapped from seven sites in Thailand, Cambodia and Lao PDR. We also developed land-cover layers for each site. Results from published studies that screened for five major rodent-borne pathogens in rodents were used to estimate how these pathogens would likely be impacted by these alterations in habitat structure and composition. 3. Our results confirmed the specialist and/or synanthropic status of several rodent species, although the majority of species studied demonstrated some degree of low level of habitat specialization. 4. Habitat diversity and its alteration (decreasing forest cover, increasing fragmentation, increasing urbanization) were found to favour the presence of synanthropic rodent species such as Rattus tanezumi, known to damage crops and host important rodent-borne diseases. 5. Synthesis and applications. The five major rodent-borne pathogens were linked to ongoing changes in habitat structure. In particular, the presence of Bartonella spp. and hantaviruses seemed to be favoured in wooded landscapes affected by ongoing fragmentation and human encroachments. Rodents also pose significant problems for crop production in South-East Asia. Our results showed that the structure of the landscape affects the likely presence of rodent species considered as agricultural pests. The patchy structure of a landscape can either enhance, such as B. indica, or decrease, such as B. savilei, the presence of rodents that may cause serious damage to crops.

The risk of parasitism is considered to be a general cost of sociality and individuals living in larger groups are typically considered to be more likely to be infected with parasites. However, contradictory results have been reported for the relationship between group size and infection by directly transmitted parasites. We used independent contrasts to examine the relationship between an index of sociality in rodents and the diversity of their macroparasites (helminths and arthropods such as fleas, ticks, suckling lice and mesostigmatid mites). We found that the species richness of directly transmitted ectoparasites, but not endoparasites, decreased significantly with the level of rodent sociality. A greater homogeneity in the biotic environment (i.e. a reduced number of cohabiting host species) of the more social species may have reduced ectoparasites' diversity by impairing ectoparasites transmission and exchange. Our finding may also result from beneficial outcomes of social living that include behavioural defences, like allogrooming, and the increased avoidance of parasites through dilution effects.

International audienc

The reduction in biodiversity from land use change due to urbanization and agricultural intensification appears to be linked to major epidemiological changes in many human diseases. Increasing disease risks and the emergence of novel pathogens result from increased contact among wildlife, domesticated animals, and humans. We investigated the relationship between human alteration of the environment and the occurrence of generalist and synanthropic rodent species in relation to the diversity and prevalence of rodent‐borne pathogens in Southeast Asia, a hotspot of threatened and endangered species, and a foci of emerging infectious diseases. We used data from an extensive pathogen survey of rodents from seven sites in mainland Southeast Asia in conjunction with past and present land cover analyses. At low spatial resolutions, we found that rodent‐borne pathogen richness is negatively associated with increasing urbanization, characterized by increased habitat fragmentation, agriculture cover and deforestation. However, at a finer spatial resolution, we found that some major pathogens are favored by environmental characteristics associated with human alteration including irrigation, habitat fragmentation, and increased agricultural land cover. In addition, synanthropic rodents, many of which are important pathogen reservoirs, were associated with fragmented and human‐dominated landscapes, which may ultimately enhance the opportunities for zoonotic transmission and human infection by some pathogens.