An update and review of arthropod vector sensory systems: Potential targets for behavioural manipulation by parasites and other disease agents.

For over a century, vector ecology has been a mainstay of vector-borne disease control. Much of this research has focused on the sensory ecology of blood-feeding arthropods (black flies, mosquitoes, ticks, etc.) with terrestrial vertebrate hosts. Of particular interest are the cues and sensory systems that drive host seeking and host feeding behaviours as they are critical for a vector to locate and feed from a host. An important yet overlooked component of arthropod vector ecology are the phenotypic changes observed in infected vectors that increase disease transmission. While our fundamental understanding of sensory mechanisms in disease vectors has drastically increased due to recent advances in genome engineering, for example, the advent of CRISPR-Cas9, and high-throughput "big data" approaches (genomics, proteomics, transcriptomics, etc.), we still do not know if and how parasites manipulate vector behaviour. Here, we review the latest research on arthropod vector sensory systems and propose key mechanisms that disease agents may alter to increase transmission.

The proof is in the poo: Non-invasive method to detect endoparasitic infection.

Almost every animal trait is strongly associated with parasitic infection or the potential exposure to parasites. Despite this importance, one of the greatest challenges that researchers still face is to accurately determine the status and severity of the endoparasitic infection without killing and dissecting the host. Thus, the precise detection of infection with minimal handling of the individual will improve experimental designs in live animal research. Here, we quantified extracellular DNA from two species of endoparasitic worm that grow within the host body cavity, hairworms (phylum Nematomorpha) and mermithids (phylum Nematoda), from the frass of their insect host, a cave weta (Orthoptera: Rhaphidophoridae) and an earwig (Dermaptera: Forficulidae), respectively. Frass collection was done at two successive time periods, to test if parasitic growth correlated with relative DNA quantity in the frass. We developed and optimized two highly specific TaqMan assays, one for each parasite-specific DNA amplification. We were able to detect infection prevalence with 100% accuracy in individuals identified as infected through post-study dissections. An additional infection in earwigs was detected with the TaqMan assay alone, probably because some worms were either too small or degraded to observe during dissection. No difference in DNA quantity was detected between sampling periods, although future protocols could be refined to support such a trend. This study demonstrates that a noninvasive and minimally stressful method can be used to detect endoparasitic infection with greater accuracy than dissection alone, helping improve protocols for live animal studies.

Resetting our expectations for parasites and their effects on species interactions: a meta-analysis.

Despite the ubiquitous nature of parasitism, how parasitism alters the outcome of host-species interactions such as competition, mutualism and predation remains unknown. Using a phylogenetically informed meta-analysis of 154 studies, we examined how the mean and variance in the outcomes of species interactions differed between parasitized and non-parasitized hosts. Overall, parasitism did not significantly affect the mean or variance of host-species interaction outcomes, nor did the shared evolutionary histories of hosts and parasites have an effect. Instead, there was considerable variation in outcomes, ranging from strongly detrimental to strongly beneficial for infected hosts. Trophically-transmitted parasites increased the negative effects of predation, parasites increased and decreased the negative effects of interspecific competition for parasitized and non-parasitized heterospecifics, respectively, and parasites had particularly strong negative effects on host species interactions in freshwater and marine habitats, yet were beneficial in terrestrial environments. Our results illuminate the diverse ways in which parasites modify critical linkages in ecological networks, implying that whether the cumulative effects of parasitism are considered detrimental depends not only on the interactions between hosts and their parasites but also on the many other interactions that hosts experience.

Infection patterns and new definitive host records for New Zealand gordiid hairworms (phylum Nematomorpha).

Some parasites modify the phenotype of their host in order to increase transmission to another host or to an environment suitable for reproduction. This phenomenon, known as host manipulation, is found across many parasite taxa. Freshwater hairworms are known for the behavioural changes they cause in their terrestrial arthropod hosts, increasing their likelihood of entering water to exit the host and reproduce. Understanding how infected arthropods move around in the natural environment could help uncover alterations in spatial distribution or movement induced by hairworms in their terrestrial definitive hosts. Moreover, few hairworm-host records exist for New Zealand, so any additional record could help elucidate their true host specificity. Here, we investigated whether infected terrestrial arthropods were more likely to approach streams in two subalpine communities of invertebrates, using a spatial grid of specialised pitfall traps. Although hairworm infection could not explain the movements of arthropod hosts near streams, we found several new host records for hairworms, including the first records for the recently described Gordionus maori. We also found some new host-parasite associations for mermithid nematodes. These records show that the host specificity of hairworms is quite low, suggesting that their diversity and distribution may be greater than what is currently known for New Zealand.

Host specificity and the reproductive strategies of parasites.

Environmental stability can have profound impacts on life history trait evolution in organisms, especially with respect to development and reproduction. In theory, free-living species, when subjected to relatively stable and predictable conditions over many generations, should evolve narrow niche breadths and become more specialized. In parasitic organisms, this level of specialization is reflected by their host specificity. Here, we tested how host specificity impacts the reproductive strategies of parasites, a subject seldomly addressed for this group. Through an extensive review of the literature, we collated a worldwide dataset to predict, through Bayesian multilevel modelling, the effect of host specificity on the reproductive strategies of parasitic copepods of fishes or corals. We found that copepods of fishes with low host specificity (generalists) invest more into reproductive output with larger clutch sizes, whereas generalist copepods of corals invest less into reproductive output with smaller clutch sizes. The differences in host turnover rates through an evolutionary timescale could explain the contrasting strategies across species observed here, which should still favour the odds of parasites encountering and infecting a host. Ultimately, the differences found in this study reflect the unique evolutionary history that parasites share both intrinsically and extrinsically with their hosts.

Building a comprehensive phylogenetic framework in disease ecology.

Disease spillover can have dramatic consequences in multispecies systems, potentially leading to the emergence of zoonoses. To better understand disease emergence patterns, an approach encompassing species relatedness metrics is needed. We show that integrating phylogenetic information in disease ecology is still lagging, and we highlight potential solutions to solve this problem.

The return to land: association between hairworm infection and aquatic insect development.

Host manipulation by parasites can shape host behaviour, community structure, and the flow of energy through food webs. A well-known example of host manipulation comes from hairworms (phylum Nematomorpha), which somehow cause their terrestrial insect definitive hosts to enter water, a phenomenon that has received lots of attention in recent years. However, little focus has been directed towards the interactions between hairworms and their aquatic insect hosts and the return of dormant hairworms from water to land. Here, we ask whether hairworm cyst infections impact, either directly or indirectly, the life history of their aquatic transport hosts. By observing the development of last-instar Olinga jeanae (Trichoptera: Conoesucidae) caddisfly larvae naturally infected with Gordius-type hairworm cysts under controlled conditions, we found that higher numbers of cysts per infected caddisfly correlated with a decrease in time to pupation. These new observations suggest that, apart from the striking host manipulation that brings the parasite from land to water, the presence of dormant hairworms is associated with changes in the development of their aquatic hosts, either through direct or indirect mechanisms, which may accelerate their transition from water to land.

Come with me if you want to live: sympatric parasites follow different transmission routes through aquatic host communities.

Community composition, including the relative density of each host species, plays a vital role in the transmission of parasites or disease in freshwater ecosystems. Whereas some host species can effectively transmit parasites, others can act as dead ends (non-viable transmission routes), accumulating large numbers of parasites throughout their life, thus becoming important sinks for parasite populations. Although population sinks have been identified in certain host-parasite systems, robust field estimates of the proportions of parasites that are lost to these hosts are lacking. Here, we quantified the distribution of encysted larval hairworms (phylum Nematomorpha), common parasites in lotic ecosystems, in two subalpine stream communities of New Zealand. With parasite and host population densities calculated per m2, we identified which host species most likely contributed to the transmission of three sympatric hairworm morphotypes identified in both streams, and which species acted as population sinks. We also tested for seasonal patterns and peaks in the abundance of each morphotype in the two communities over the sampling season. Finally, we tested whether hosts emerging from the streams had comparable abundances of hairworm morphotypes throughout the sampling period. For each morphotype, different key sets of host species harboured more hairworms on average (abundance) than others, depending on the stream. For one morphotype in particular, two species of hosts were found to be important population sinks that inhibited over a third of these parasites from completing their life cycle. We also observed a clear peak in abundance for another hairworm morphotype during summer. Our data suggest that hosts emerging from the streams matched their aquatic counterparts with respect to hairworm abundance, indicating no infection-dependent reduction in emergence success. Our findings suggest that, depending on relative community composition, sympatric parasites follow different host transmission pathways, some of which lead to dead ends that potentially impact overall infection dynamics. In turn, this information can help us understand the spread or emergence of disease in both freshwater and terrestrial environments, since hairworms infect terrestrial arthropods to complete their life cycle.

Host Manipulation, Gene Editing, and Non-Traditional Model Organisms: A New Frontier for Behavioral Research?

Insects and parasites dominate the biosphere, in terms of known biodiversity and mode of life, respectively. Consequently, insects play a part in many host-parasite systems, either as parasite, host, or both. Moreover, a lot of these systems involve adaptive parasite-induced changes of host phenotype (typically behavior or morphology), which is commonly known as host manipulation. While many host manipulation systems have been described within the last few decades, the proximate mechanisms that underpin host phenotypic change are still largely unknown. Given the intimate co-evolutionary history of host-parasite systems, teasing apart the intricate network of biochemical reactions involved in host manipulation requires the integration of various complementary technologies. In this perspective, we stress the importance of multidisciplinary research on host manipulation, such as high-throughput sequencing methods (genomics and transcriptomics) to search for candidate mechanisms that are activated during a manipulation event. Then, we argue that gene editing technologies, specifically the CRISPR-Cas9 system, are a powerful way to test for the functional roles of candidate mechanisms, in both the parasite and the host. Finally, given the sheer diversity of unique host-parasite systems discovered to date, there is indeed a tremendous potential to create novel non-traditional model systems that could greatly expand our capacity to test the fundamental aspects of behavior and behavioral regulation.

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.

The people vs science: can passively crowdsourced internet data shed light on host-parasite interactions?

Every internet search query made out of curiosity by anyone who observed something in nature, as well as every photo uploaded to the internet, constitutes a data point of potential use to scientists. Researchers have now begun to exploit the vast online data accumulated through passive crowdsourcing for studies in ecology and epidemiology. Here, we demonstrate the usefulness of iParasitology, i.e. the use of internet data for tests of parasitological hypotheses, using hairworms (phylum Nematomorpha) as examples. These large worms are easily noticeable by people in general, and thus likely to generate interest on the internet. First, we show that internet search queries (collated with Google Trends) and photos uploaded to the internet (specifically, to the iNaturalist platform) point to parts of North America with many sightings of hairworms by the public, but few to no records in the scientific literature. Second, we demonstrate that internet searches predict seasonal peaks in hairworm abundance that accurately match scientific data. Finally, photos uploaded to the internet by non-scientists can provide reliable data on the host taxa that hairworms most frequently parasitize, and also identify hosts that appear to have been neglected by scientific studies. Our findings suggest that for any parasite group likely to be noticeable by non-scientists, information accumulating through internet search activity, photo uploads, social media or any other format available online, represents a valuable source of data that can complement traditional scientific data sources in parasitology.

iParasitology: Mining the Internet to Test Parasitological Hypotheses.

Digital data (internet queries, page views, social media posts, images) are accumulating online at increasing rates. Tools for compiling these data and extracting their metadata are now readily available. We highlight the possibilities and limitations of internet data to reveal patterns in host-parasite interactions and encourage parasitologists to embrace iParasitology.

The Adaptiveness of Host Behavioural Manipulation Assessed Using Tinbergen's Four Questions.

Host organisms show altered phenotypic reactions when parasitised, some of which result from adaptive host manipulation, a phenomenon that has long been debated. Here, we provide an overview and discuss the rationale in distinguishing adaptive versus nonadaptive host behavioural manipulation. We discuss Poulin's criteria of adaptive host behavioural manipulation within the context of Tinbergen's four questions of ethology, while highlighting the importance of both the proximate and evolutionary explanations of such traits. We also provide guidelines for future studies exploring the adaptiveness of host behavioural manipulation. Through this article, we seek to encourage researchers to consider both the proximate and ultimate causes of host behavioural manipulation to infer on the adaptiveness of such traits.

When fiction becomes fact: exaggerating host manipulation by parasites.

In an era where some find fake news around every corner, the use of sensationalism has inevitably found its way into the scientific literature. This is especially the case for host manipulation by parasites, a phenomenon in which a parasite causes remarkable change in the appearance or behaviour of its host. This concept, which has deservedly garnered popular interest throughout the world in recent years, is nearly 50 years old. In the past two decades, the use of scientific metaphors, including anthropomorphisms and science fiction, to describe host manipulation has become more and more prevalent. It is possible that the repeated use of such catchy, yet misleading words in both the popular media and the scientific literature could unintentionally hamper our understanding of the complexity and extent of host manipulation, ultimately shaping its narrative in part or in full. In this commentary, the impacts of exaggerating host manipulation are brought to light by examining trends in the use of embellishing words. By looking at key examples of exaggerated claims from widely reported host-parasite systems found in the recent scientific literature, it would appear that some of the fiction surrounding host manipulation has since become fact.

A molecular war: convergent and ontogenetic evidence for adaptive host manipulation in related parasites infecting divergent hosts.

Mermithids (phylum Nematoda) and hairworms (phylum Nematomorpha) somehow drive their arthropod hosts into water, which is essential for the worms' survival after egression. The mechanisms behind this behavioural change have been investigated in hairworms, but not in mermithids. Establishing a similar mechanistic basis for host behavioural change between these two distantly related parasitic groups would provide strong convergent evidence for adaptive manipulation and insight into how these parasites modify and/or create behaviour. Here, we search for this convergence, and also contrast changes in physiology between hosts infected with immature and mature mermithids to provide the first ontogenetic evidence for adaptive manipulation by disentangling host response and pathology from the parasite's apparent manipulative effects. We used SWATH-mass spectrometry on brains of Forficula auricularia (earwig) and Bellorchestia quoyana (sandhopper), infected with the mermithids Mermis nigrescens and Thaumamermis zealandica, respectively, at both immature and mature stages of infection, to quantify proteomic changes resulting from mermithid infection. Across both hosts (and hairworm-infected hosts, from earlier studies), the general function of dysregulated proteins was conserved. Proteins involved in energy generation/mobilization were dysregulated, corroborating reports of erratic/hyperactive behaviour in infected hosts. Dysregulated proteins involved in axon/dendrite and synapse modulation were also common to all hosts, suggesting neuronal manipulation is involved in inducing positive hydrotaxis. Furthermore, downregulation of CamKII and associated proteins suggest manipulation of memory also contributes to the behavioural shift.

Varying levels of melanotic encapsulation of gordiid hairworm cysts (Nematomorpha) by aquatic insect larvae: seasonal and host effects.

The defence reactions of insects to parasitic invaders are both varied and complex. Melanisation of pathogens is often an important step in insect immunity and can play a key role in isolating parasites. Within samples collected from a subalpine stream in New Zealand during two consecutive seasons (i.e., winter and spring), we observed and categorised different levels of melanotic encapsulation by aquatic insect larvae to dormant Gordius sp. hairworm (Phylum Nematomorpha) cysts, a relatively obscure group of parasites. Some of these insect species act as intermediate transport hosts in the complex life cycle of hairworms. Based on these new observations, we calculated the melanisation response for an abundant species of caddisfly larvae (Olinga sp.) using the proportion of non-melanised cysts per individual host. We tested the hypothesis that season and total number of cysts in an infected host impact its melanisation response. Also, we explored the effect of host body size on the total number of cysts it carries. We found that the total number of cysts does not affect the melanisation response of the host. Season did have an impact on the melanisation response in Olinga sp., with lower levels observed in the spring. Additionally, larger caddisfly larvae harboured more cysts than smaller ones. Since little is known about the cryptic interactions between hairworms and their intermediate hosts, this new information adds some complexity to this poorly understood group of parasites.

Let's go swimming: mermithid-infected earwigs exhibit positive hydrotaxis.

Certain species of parasites have the apparent ability to alter the behaviour of their host in order to facilitate the completion of their own life cycle. While documented in hairworms (phylum Nematomorpha), the ability for mermithid parasites (from the sister phylum Nematoda) to force hosts to enter water remains more enigmatic. Here, we present the first experimental evidence in a laboratory setting that an insect which normally never enters open water (the European earwig Forficula auricularia) will readily enter the water when infected with a mermithid nematode (Mermis nigrescens). Only adult mermithids appear capable of inducing this polarising shift in behaviour, with mermithid length being a very strong predictor of whether their host enters water. However, mermithid length was only weakly associated with how long it took an earwig to enter water following the beginning of a trial. Considering the evidence presented here and its alignment with a proteomic investigation on the same host-parasite system, this study provides strong evidence for adaptive behavioural manipulation and a foundational system for further behavioural and mechanistic exploration.

Parasitological research in the molecular age.

New technological methods, such as rapidly developing molecular approaches, often provide new tools for scientific advances. However, these new tools are often not utilized equally across different research areas, possibly leading to disparities in progress between these areas. Here, we use empirical evidence from the scientific literature to test for potential discrepancies in the use of genetic tools to study parasitic vs non-parasitic organisms across three distinguishable molecular periods, the allozyme, nucleotide and genomics periods. Publications on parasites constitute only a fraction (<5%) of the total research output across all molecular periods and are dominated by medically relevant parasites (especially protists), particularly during the early phase of each period. Our analysis suggests an increasing complexity of topics and research questions being addressed with the development of more sophisticated molecular tools, with the research focus between the periods shifting from predominantly species discovery to broader theory-focused questions. We conclude that both new and older molecular methods offer powerful tools for research on parasites, including their diverse roles in ecosystems and their relevance as human pathogens. While older methods, such as barcoding approaches, will continue to feature in the molecular toolbox of parasitologists for years to come, we encourage parasitologists to be more responsive to new approaches that provide the tools to address broader questions.