What factors explain the geographical range of mammalian parasites?

Host–Parasite Relationship—Nematode Communities in Populations of Small Mammals

Nematode burdens and variation in morphological characteristics were assessed in eighty-eight animals from three host species (Apodemus sylvaticus, Apodemus flavicollis, and Myodes glareolus) from eight localities in Serbia. In total, 15 species of nematodes were identified, and the overall mean parasite species richness (IndPSR) was 1.61 per animal (1.98 in A. flavicollis, 1.43 in M. glareolus, and 0.83 in A. sylvaticus). Furthermore, the studied host species significantly differed in individual parasite load (IndPL) and in the following morphological characters: spleen mass, body condition index (BCI), and body mass. We aimed to analyze the relationship between the burden of intestinal nematodes, on one hand, and the body conditions of the host and its capability to develop immune defends on the other. Spleen mass was considered as a measure of immune response. In all host species, larger animals with a better condition (higher BCI) were infected with more parasites species (IndPSR), while parasite load was not related to BCI. Only in A. flavicollis were males significantly larger, but females of the same sizes were infected with more parasite species. This female-biased parasitism is contrary to the theoretical expectation that males should be more parasitized, being larger, more active, with a wider home range. Although the spleen size was significantly correlated with body condition and body mass, IndPSR was not related to spleen mass in any studied species, but in M. galareolus, we found that a smaller spleen was related to higher infection intensity (IndPL).

Viral Hyperparasitism in Bat Ectoparasites: Implications for Pathogen Maintenance and Transmission

Humans continue to encroach on the habitats of wild animals, potentially bringing different species into contact that would not typically encounter each other under natural circumstances, and forcing them into stressful, suboptimal conditions. Stressors from unsustainable human land use changes are suspected to dramatically exacerbate the probability of zoonotic spillover of pathogens from their natural reservoir hosts to humans, both by increasing viral load (and shedding) and the interface between wildlife with livestock, pets and humans. Given their known role as reservoir hosts, bats continue to be investigated for their possible role as the origins of many viral outbreaks. However, the participation of bat-associated ectoparasites in the spread of potential pathogens requires further work to establish. Here, we conducted a comprehensive review of viruses, viral genes and other viral sequences obtained from bat ectoparasites from studies over the last four decades. This review summarizes research findings of the seven virus families in which these studies have been performed, including Paramyxoviridae, Reoviridae, Flaviviridae, Peribunyaviridae, Nairoviridae, Rhabdoviridae and Filoviridae. We highlight that bat ectoparasites, including dipterans and ticks, are often found to have medically important viruses and may have a role in the maintenance of these pathogens within bat populations.

Predictors of zoonotic potential in helminths

Helminths are parasites that cause disease at considerable cost to public health and present a risk for emergence as novel human infections. Although recent research has elucidated characteristics conferring a propensity to emergence in other parasite groups (e.g. viruses), the understanding of factors associated with zoonotic potential in helminths remains poor. We applied an investigator-directed learning algorithm to a global dataset of mammal helminth traits to identify factors contributing to spillover of helminths from wild animal hosts into humans. We characterized parasite traits that distinguish between zoonotic and non-zoonotic species with 91% accuracy. Results suggest that helminth traits relating to transmission (e.g. definitive and intermediate hosts) and geography (e.g. distribution) are more important to discriminating zoonotic from non-zoonotic species than morphological or epidemiological traits. Whether or not a helminth causes infection in companion animals (cats and dogs) is the most important predictor of propensity to cause human infection. Finally, we identified helminth species with high modelled propensity to cause zoonosis (over 70%) that have not previously been considered to be of risk. This work highlights the importance of prioritizing studies on the transmission of helminths that infect pets and points to the risks incurred by close associations with these animals. This article is part of the theme issue 'Infectious disease macroecology: parasite diversity and dynamics across the globe'.

Functional biogeography of parasite traits: hypotheses and evidence

Functional biogeography, or the study of trait-based distributional patterns, not only complements our understanding of spatial patterns in biodiversity, but also sheds light on the underlying processes generating them. In parallel with the well-studied latitudinal diversity gradient, decades-old ecogeographical rules also postulate latitudinal variation in species traits. Notably, species in the tropics are predicted to have smaller body sizes (Bergmann's rule), narrower niches (MacArthur's rule) and smaller geographical ranges (Rapoport's rule) than their counterparts at higher latitudes. Although originally proposed for free-living organisms, these rules have been extended to parasitic organisms as well. In this review, I discuss the mechanistic hypotheses most likely to explain latitudinal gradients in parasite traits, and assess the empirical evidence obtained from comparative studies testing the above three rules as well as latitudinal gradients in other parasite traits. Overall, there is only weak empirical support for latitudinal gradients in any parasite trait, with little consistency among comparative analyses. The most parsimonious explanation for the existence of geographical patterns in parasite traits is that they are primarily host-driven, i.e. ecological traits of parasites track those of their hosts, with a direct influence of bioclimatic factors playing a secondary role. Thus, geographical patterns in parasite traits probably emerge as epiphenomena of parallel patterns in their hosts.

This article is part of the theme issue ‘Infectious disease macroecology: parasite diversity and dynamics across the globe’.

Host and parasite traits predict cross-species parasite acquisition by introduced mammals

Species invasions and range shifts can lead to novel host-parasite communities, but we lack general rules on which new associations are likely to form. While many studies examine parasite sharing among host species, the directionality of transmission is typically overlooked, impeding our ability to derive principles of parasite acquisition. Consequently, we analysed parasite records from the non-native ranges of 11 carnivore and ungulate species. Using boosted regression trees, we modelled parasite acquisition within each zoogeographic realm of a focal host's non-native range, using a suite of predictors characterizing the parasites themselves and the host community in which they live. We found that higher parasite prevalence among established hosts increases the likelihood of acquisition, particularly for generalist parasites. Non-native host species are also more likely to acquire parasites from established host species to which they are closely related; however, the acquisition of several parasite groups is biased to phylogenetically specialist parasites, indicating potential costs of parasite generalism. Statistical models incorporating these features provide an accurate prediction of parasite acquisition, indicating that measurable host and parasite traits can be used to estimate the likelihood of new host-parasite associations forming. This work provides general rules to help anticipate novel host-parasite associations created by climate change and other anthropogenic influences.

Species-level drivers of mammalian ectoparasite faunas

Traditionally, most studies have described the organization of host-parasite interaction networks by considering only few host groups at limited geographical extents. However, host-parasite relationships are merged within different taxonomic groups and factors shaping these interactions likely differ between host and parasite groups, making group-level differences important to better understand the ecological and evolutionary dynamics of these interactive communities. Here we used a dataset of 629 ectoparasite species and 251 species of terrestrial mammals, comprising 10 orders distributed across the Nearctic and Neotropical regions of Mexico to assess the species-level drivers of mammalian ectoparasite faunas. Specifically, we evaluated whether body weight, geographical range size and within-range mammal species richness (i.e. diversity field) predict mammal ectoparasite species richness (i.e. degree centrality) and their closeness centrality within the mammal-ectoparasite network. In addition, we also tested if the observed patterns differ among mammal orders and if taxonomic closely related host mammals could more likely share the same set of ectoparasites. We found that ectoparasite species richness of small mammals (mainly rodents) with large proportional range sizes was high compared to large-bodied mammals, whereas the diversity field of mammals had no predictive value (except for bats). We also observed that taxonomic proximity was a main determinant of the probability to share ectoparasite species. Specifically, the probability to share ectoparasites in congeneric species reached up to 90% and decreased exponentially as the taxonomic distance increased. Further, we also detected that some ectoparasites are generalists and capable to infect mammalian species across different orders and that rodents have a remarkable role in the network structure, being closely connected to many other taxa. Hence, because many rodent species have synanthropic habits they could act as undesired reservoirs of disease agents for humans and urban animals. Considering the reported worldwide phenomenon of the proliferation of rodents accompanying the demographic decrease or even local extinction of large-bodied mammal species, these organisms may already be an increasing health threat in many regions of the world.

Effects of geographical and climatic factors on cystic echinococcosis in south-western Iran

Cystic echinococcosis (CE) is caused by the larval form of Echinococcus granulosus that can cause serious health and economic problems in the endemic foci. CE is globally distributed in various climatic conditions from circumpolar to tropical latitudes. Iran is an important endemic area with a spectrum of weather conditions. The aim of this study was to determine the effects of geo-climatic factors on the distribution of livestock CE in south-western Iran (SWI) in 2016 to 2018. Data of livestock CE were retrieved from veterinary organizations of four provinces of SWI. The geo-climatic factors, including mean annual temperature (MAT), minimum MAT (MinMAT), maximum MAT (MaxMAT), mean annual rainfall (MAR), elevation, mean annual evaporation (MAE), sunny hours, wind speed, mean annual humidity (MAH), slope, frost days and land cover, were analysed using geographical information systems (GIS) approaches. The statistical analysis showed that MAR, frost days, elevation, slope and semi-condensed forest land cover were positively and MAE, MAT, MaxMAT, MinMAT and salt and salinity land cover were negatively correlated with CE occurrence. MAE was shown to be a predictive factor in the stepwise linear logistic regression model. In short, the current GIS-based study found that areas with lower evaporation were the main CE risk zones, though those with lower temperature and higher rainfall, altitude and slope, especially where covered with or in close proximity of semi-condensed forest, should be prioritized for consideration by health professionals and veterinarians for conducting control programmes in SWI.

Drivers of compositional turnover are related to species' commonness in flea assemblages from four biogeographic realms: zeta diversity and multi-site generalised dissimilarity modelling

We investigated drivers of species turnover in fleas parasitic on small mammals in four biogeographic realms using novel methodology (zeta diversity, and Multi-Site Generalised Dissimilarity Modelling). We asked whether (i) flea turnover was better explained by host turnover or environmental variables; (ii) different factors drive the turnover of rare and widespread fleas; (iii) the factors affecting the turnover of rare or widespread fleas differ between realms; and (iv) environmental variables drive flea turnover directly or via their effects on hosts. Dissimilarity in host species composition was the most important factor affecting flea turnover in all realms. In the Afrotropics, the Nearctic, and the Neotropics, this was true mainly for rare species, whereas the zeta diversity of the Palearctic hosts exerted a strong effect on the turnover of both rare and widespread fleas. Dissimilarity in temperature contributed the most to the turnover of rare fleas in the Neotropics and the Palearctic, whereas the turnover of widespread species in these realms was strongly affected by dissimilarity in precipitation. In the Nearctic, dissimilarity in precipitation or temperature mostly affected the turnover of rare fleas or common species, respectively. In the Afrotropics, dissimilarity in the Normalised Difference Vegetation Index and temperature affected the turnover of all species, independently of their level of commonness, while dissimilarity in rainfall was important for the turnover of rare fleas. The responses of flea assemblages to environmental factors represented a combination of direct responses and responses mediated via effects on host turnover. We conclude that host turnover is a more important factor than environmental dissimilarity in its effect on flea species turnover. However, the relative effects of host composition and environment, as well as those of temperature, precipitation and the amount of vegetation, on flea turnover differ (i) between realms and (ii) between rare and common fleas.