Towards a Unified Functional Trait Framework for Parasites

Functional similarity affects similarity in partner composition in flea-mammal networks

Functional signal in an interaction network is a phenomenon in which species resembling each other in their traits interact with similar partners. We tested the functional signal concept in realm-specific and regional flea-host networks from four biogeographic realms and asked whether the species composition of (a) host spectra and (b) flea assemblages is similar between functionally similar flea and host species, respectively. Analogously to testing for phylogenetic signal, we applied Mantel tests to investigate the correlation between flea or host functional distances calculated from functional dendrograms and dissimilarities in sets of interacting partners. In all realm-specific networks, functionally similar fleas tended to exploit similar hosts often belonging to the same genus, whereas functionally similar hosts tended to harbour similar fleas, again often belonging to the same genus. The strength of realm-specific functional signals and the frequency of detecting a significant functional signal in the regional networks differed between realms. The frequency of detecting a significant functional signal in the regional networks correlated positively with the network size for fleas and with the number of hosts in a network for hosts. A functional signal in the regional networks was more frequently found for hosts than for fleas. We discuss the mechanisms behind the functional signal in both fleas and their hosts, relate geographic functional signal patterns to the historic biogeography of fleas and conclude that functional signals in the species composition of host spectra for fleas and of flea assemblages for hosts result from the interplay of evolutionary and ecological processes.

Environmental parasitology: stressor effects on aquatic parasites

Anthropogenic stressors are causing fundamental changes in aquatic habitats and to the organisms inhabiting these ecosystems. Yet, we are still far from understanding the diverse responses of parasites and their hosts to these environmental stressors and predicting how these stressors will affect host-parasite communities. Here, we provide an overview of the impacts of major stressors affecting aquatic ecosystems in the Anthropocene (habitat alteration, global warming, and pollution) and highlight their consequences for aquatic parasites at multiple levels of organisation, from the individual to the community level. We provide directions and ideas for future research to better understand responses to stressors in aquatic host-parasite systems.

Impact of host sex and age on the diversity of endoparasites and structure of individual-based host-parasite networks in nyalas (Tragelaphus angasii Angas) from three game reserves in KwaZulu-Natal province, South Africa

In recent years, numerous studies have examined the effect of host sex and age on the structure of parasite communities in several host taxa under various environmental conditions and in different geographic regions. However, the influence of such factors on the structure of host-parasite networks has received less attention, and remarkably few studies have been carried out on large terrestrial mammals. In this study, we investigated the effects of host age and sex on the parasite infra- and component communities of nyalas (Tragelaphus angasii) and on the structure of individual-based nyala-endoparasite networks. We also aimed to evaluate to what extent these effects vary spatially and if they are mediated by conservation management. Based on a data set of internal macroparasites of 74 nyalas from three game reserves in KwaZulu-Natal province, we found that host age strongly influenced parasite community structure as well as the structure of parasite-nyala networks, whereas host sex played a minor role. However, the effects of both host sex and age were mediated by environmental conditions and thus led to different patterns at the three localities. Our findings highlight that host-parasite communities from different localities should not be pooled when conducting host-parasite network and community studies as this may bias results and mask patterns that are typical for a given locality.

Life history mediates the association between parasite abundance and geographic features

Though parasites are ubiquitous in marine ecosystems, predicting the abundance of parasites present within marine ecosystems has proven challenging due to the unknown effects of multiple interacting environmental gradients and stressors. Furthermore, parasites often are considered as a uniform group within ecosystems despite their significant diversity. We aim to determine the potential importance of multiple predictors of parasite abundance in coral reef ecosystems, including reef area, island area, human population density, chlorophyll-a, host diversity, coral cover, host abundance, and island isolation. Using a model selection approach within a database of more than 1200 individual fish hosts and their parasites from 11 islands within the Pacific Line Islands archipelago, we reveal that geographic gradients, including island area and island isolation, emerged as the best predictors of parasite abundance. Life history moderated the relationship; parasites with complex life cycles increased in abundance with increasing island isolation, while parasites with direct life cycles decreased with increasing isolation. Direct life cycle parasites increased in abundance with increasing island area, though complex life cycle parasite abundance was not associated with island area. This novel analysis of a unique dataset indicates that parasite abundance in marine systems cannot be predicted precisely without accounting for the independent and interactive effects of each parasite's life history and environmental conditions.

Functional Groups of Metazoan Parasites of the Dusky Flounder (Syacium papillosum) as Bioindicators of Environmental Health of the Yucatan Shelf

We aimed to compare the percentage of explained variance given by the relationship of species and functional groups of metazoan parasites of the dusky flounder Syacium papillosum and environmental variables from water and sediments in the Yucatan shelf (YS). Parasite data were obtained from 127 S. papillosum specimens collected from 17 of 67 stations. At each station, 46 environmental variables were measured, including hydrocarbons, heavy metals, and physicochemical variables from water and sediments. Fifteen functional groups were defined based on biological characteristics of 48 parasite species. Our multivariate statistical analyses showed that species and functional groups produced similar explained variance values (47.3% and 50% respectively). However, using functional groups the time and financial resources were minimal compared with those used for morphological and molecular identification to produce the species composition matrix. Thus, functional groups are the best choice from the point of view of saving time and money.

Trypanosoma spp. Neobats: Insights about those poorly known trypanosomatids

Bats are infected with several trypanosomatid species; however, assessing the diversity of this interaction remains challenging since there are species apparently unable to grow in conventional culture media. Accordingly, the ecology and biology of the Molecular Operational Taxonomic Units (MOTUs) Trypanosoma spp. Neobats are unknown. Therefore, we performed the molecular characterization targeting the 18S small subunit rDNA from the blood clot of 280 bats of three Brazilian regions (Paraíba, Rio de Janeiro and Acre states), bypassing the selective pressure of hemoculture. From 68 (24%) positive blood clot samples, we obtained 49 satisfactory sequences. Of these successfully sequenced results, T. spp. Neobats (1, 3 and 4) represented 67%, with the most abundant T. sp. Neobat 4 (53%). Our results show: (1) high abundance and wide geographic range of T. sp. Neobat 4, restricted to Carollia bats; (2) high infection rate of T. sp. Neobat 4 in Carollia perspicillata populations (mean 26%); (3) infection with the monoxenous Crithidia mellificae; and (4) a new MOTU (T. sp. Neobat 5) in Artibeus cinereus, positioning in the Trypanosoma wauwau clade. These data corroborate the importance of bats as hosts of many Trypanosoma species and C. mellificae. They also show that the diversity of the T. wauwau clade is underestimated and warn about the high magnitude of trypanosomes we overpass with the hemoculture. Our findings combined with previous data show that T. spp. Neobats include host-specific and host-generalist species, probably playing different ecological roles: T. sp. Neobat 1 shows broad host range; T. spp. Neobat 3 and 4 are restricted to Artibeus and Carollia, respectively. Finally, T. Neobat 4 seems to be a well-succeeded parasite, especially within C. perspicillata metapopulations across a wide geographical distribution. This work is a step forward to understand the biology and life history of T. spp. Neobats.

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’.

Life in the margins: host-parasite relationships in ecological edges

Transitional zones, such as edge habitat, are key landscapes for investigating biodiversity. "Soft edges" are permeable corridors that hosts can cross, while "hard edges" are impermeable borders that hosts cannot pass. Although pathogen transmission in the context of edges is vital to species conservation, drivers of host-parasite relationships in ecological edges remain poorly understood. Thus, we defined a framework for testing hypotheses of host-parasite interactions in hard and soft edges by (1) characterizing hard and soft edges from both the host and parasite perspectives, (2) predicting the types of parasites that would be successful in each type of edge, and (3) applying our framework to species invasion fronts as an example of host-parasite relationships in a soft edge. Generally, we posited that parasites in soft edges are more likely to be negatively affected by habitat fragmentation than their hosts because they occupy higher trophic levels but parasite transmission would benefit from increased host connectivity. Parasites along hard edges, however, are at higher risk of local extinction due to host population perturbations with limited opportunity for parasite recolonization. We then used these characteristics to predict functional traits that would lead to parasite success along soft and hard edges. Finally, we applied our framework to invasive species fronts to highlight predictions regarding host connectivity and parasite traits in soft edges. We anticipate that our work will promote a more complete discussion of habitat connectivity using a common framework and stimulate empirical research into host-parasite relationships within ecological edges and transitional zones.

The performance of taxonomic and trait-based approaches in the assessment of dusky flounder parasite communities as indicators of chemical pollution

We assessed the performance of taxonomic and several functional trait-based approaches in the assessment of spatial and temporal patterns of dusky flounder (Syacium papillosum) parasite assemblages along the Yucatan shelf to determine their potential as bioindicators of marine chemical pollution. Fish specimens were collected throughout three research cruises that took place in 2015, 2016 and 2018. In addition to the traditional taxonomic approach, four trait-based approaches were performed including community-weighted means (CWM), functional trait niche (FTN), functional groups (FGs), and Rao's functional diversity (FD). Significant spatial and temporal variations in parasite communities were detected using the taxonomic approach. In general, these variations were also reflected in the four trait-based approaches performed, indicating that changes in taxa composition and abundance also resulted in functional composition shifts. Resemblance matrices of both taxonomic and functional trait approaches were significantly correlated. Variations in taxonomic and trait-based composition using the four approaches were significantly correlated with depth, and at least one chemical pollutant variable. Feeding mode, transmission, life stage and attachment structure displayed spatial variability and significant correlations with predictor variables, which indicates that this set of attributes functions as a good surrogate for assessing variations in the functional composition of flatfish parasite communities in relation to pollution. FTN and CWM were the approaches that best detected spatio-temporal variation. CWM and FD were best suited for detecting pollution gradients. These results reveal the feasibility of using trait-based approaches to assess marine parasite communities as bioindicators of chemical pollution. Functional traits of marine metazoan parasites are as good indicators of the effect of oil pollution as taxonomic diversity. This may be a time-saving and cost-effective approach to performing environmental assessments.

The rise of big data in disease ecology

Big data have become readily available to explore patterns in large-scale disease ecology. However, the rate at which these public databases are exploited remains unknown. We highlight trends in big data usage in disease ecology during the past decade and encourage researchers to integrate big data into their study framework.

Understanding host utilization by mosquitoes: determinants, challenges and future directions

Mosquito host utilization is a key factor in the transmission of vector-borne pathogens given that it greatly influences host-vector contact rates. Blood-feeding patterns of mosquitoes are not random, as some mosquitoes feed on particular species and/or individuals more than expected by chance. Mosquitoes use a number of cues including visual, olfactory, acoustic, and thermal stimuli emitted by vertebrate hosts to locate and identify their blood meal sources. Thus, differences in the quality/intensity of the released cues may drive host selection by mosquitoes at both inter- and intra-specific levels. Such patterns of host selection by mosquitoes in space and time can be structured by factors related to mosquitoes (e.g. innate host preference, behavioural plasticity), to hosts (e.g. emission of host-seeking cues, host availability) or to both (e.g. pathogen infection). In this study, we review current evidence, from phenomena to mechanisms, of how these factors influence host utilization by mosquitoes. We also review the methodologies commonly used in this research field and identify the major challenges for future studies. To bridge the knowledge gaps, we propose improvements to strengthen traditional approaches and the use of a functional trait-based approach to infer mosquito host utilization in natural communities.

Anuran's habitat use drives the functional diversity of nematode parasite communities

Understanding the processes responsible for structuring communities has been a challenge in ecology, and parasite communities are an excellent system to address this issue. The use of different diversity metrics can help us to understand the determinants of the structure of parasite communities, and in this sense, functional diversity indexes make it possible to measure the variability of organism traits in communities. In this study, we investigate how host body size and habitat use influence the functional diversity of nematode parasite infracommunities. We collected and examined 213 individuals of 11 species of anurans in an area of the Brazilian Atlantic Forest, calculated Rao's quadratic entropy as a measure of functional diversity of parasite infracommunities, and tested if this index was related to host body size and habitat use with an analysis of covariance (ANCOVA). Anuran species varied in body size (from 1.80 to 10.35 cm) and habit use (arboreal, terrestrial, and semiaquatic), and in the functional diversity of parasite infracommunities (Rao's quadratic entropy ranged from 0 to 0.196). We observed that anurans with larger body size and terrestrial habit showed significantly greater functional diversity of parasites. We conclude that anuran characteristics drive the functional diversity of nematode parasite communities, and highlight the importance of using different diversity metrics to understand the determinants in the host-parasite interaction.

Assembly rules of helminth parasite communities in grey mullets: combining components of diversity

Organisms aggregate in ecological communities. It has been widely debated whether these associations are explained by deterministic or, in contrast, random processes. The answer may vary, depending on the level of an organisational scale (α, β and γ) and the facet of diversity considered: taxonomic, functional and phylogenetic. Diversity at the level of a sampling unit (i.e. host individual) is the α diversity; β diversity represents the extent of dissimilarity in diversity among sampling units (within a level of an organisational scale, β1; between levels of an organisational scale, β2); and the total diversity of a system is γ diversity. Thus, the combination of facets and levels of a scale may be useful to disentangle the mechanisms driving the composition of a parasite community. Using helminth parasite taxonomic, functional, and a proxy for phylogenetic diversity of three species of grey mullets (Teleostei: Mugilidae) from the Mediterranean Sea, we show that random and deterministic processes of different nature explain the assemblage of parasite communities. The parasite community at a host individual (α) was invariably a random subset of the total diversity in the community for the three facets of diversity. At the β1 level, taxonomic diversity was lower than expected by chance, whereas functional diversity and the proxy for phylogenetic diversity were random. At the β2 level, diversity patterns suggested environmental filtering of the parasite assemblage: species, trait, and phylogenetic compositions of parasite communities seemed to depend primarily on the species of host, but also on the locality and season. Our study shows that parasite communities are not totally understood if any of the components (i.e. facets and levels) of diversity is neglected.