Cryptic microsporidian parasites differentially affect invasive and native Artemia spp.

We investigated the host specificity of two cryptic microsporidian species (Anostracospora rigaudi and Enterocytospora artemiae) infecting invasive (Artemia franciscana) and native (Artemia parthenogenetica) hosts in sympatry. Anostracospora rigaudi was on average four times more prevalent in the native host, whereas E. artemiae was three times more prevalent in the invasive host. Infection with An. rigaudi strongly reduced female reproduction in both host species, whereas infection with E. artemiae had weaker effects on female reproduction. We contrasted microsporidian prevalence in native A. franciscana populations (New World) and in both invaded and non-invaded Artemia populations (Old World). At a community level, microsporidian prevalence was twice as high in native compared with invasive hosts, due to the contrasting host-specificity of An. rigaudi and E. artemiae. At a higher biogeographical level, microsporidian prevalence in A. franciscana did not differ between the invaded populations and the native populations used for the introduction. Although E. artemiae was the only species found both in New and Old World populations, no evidence of its co-introduction with the invasive host was found in our experimental and phylogeographic tests. These results suggest that the success of A. franciscana invasion is probably due to a lower susceptibility to virulent microsporidian parasites rather than to decreased microsporidian prevalence compared with A. parthenogenetica or to lower microsporidian virulence in introduced areas.     

Why join groups? Lessons from parasite‐manipulated Artemia

Grouping behaviours (e.g. schooling, shoaling and swarming) are commonly explicated through adaptive hypotheses such as protection against predation, access to mates or improved foraging. However, the hypothesis that aggregation can result from manipulation by parasites to increase their transmission has never been demonstrated. We investigated this hypothesis using natural populations of two crustacean hosts (Artemia franciscana and Artemia parthenogenetica) infected with one cestode and two microsporidian parasites. We found that swarming propensity increased in cestode‐infected hosts and that red colour intensity was higher in swarming compared with non‐swarming infected hosts. These effects likely result in increased cestode transmission to its final avian host. Furthermore, we found that microsporidian‐infected hosts had both increased swarming propensity and surfacing behaviour. Finally, we demonstrated using experimental infections that these concurrent manipulations result in increased spore transmission to new hosts. Hence, this study suggests that parasites can play a prominent role in host grouping behaviours.     

Shared geographic histories and dispersal contribute to congruent phylogenies between amphipods and their microsporidian parasites at regional and global scales

In parasites that strongly rely on a host for dispersal, geographic barriers that act on the host will simultaneously influence parasite distribution as well. If their association persists over macroevolutionary time it may result in congruent phylogenetic and phylogeographic patterns due to shared geographic histories. Here, we investigated the level of congruent evolutionary history at a regional and global scale in a highly specialised parasite taxon infecting hosts with limited dispersal abilities: the microsporidians Dictyocoela spp. and their amphipod hosts. Dictyocoela can be transmitted both vertically and horizontally and is the most common microsporidian genus occurring in amphipods in Eurasia. However, little is known about its distribution elsewhere. We started by conducting molecular screening to detect microsporidian parasites in endemic amphipod species in New Zealand; based on phylogenetic analyses, we identified nine species‐level microsporidian taxa including six belonging to Dictyocoela. With a distance‐based cophylogenetic analysis at the regional scale, we identified overall congruent phylogenies between Paracalliope, the most common New Zealand freshwater amphipod taxon, and their Dictyocoela parasites. Also, hosts and parasites showed similar phylogeographic patterns suggesting shared biogeographic histories. Similarly, at a global scale, phylogenies of amphipod hosts and their Dictyocoela parasites showed broadly congruent phylogenies. The observed patterns may have resulted from covicariance and/or codispersal, suggesting that the intimate association between amphipods and Dictyocoela may have persisted over macroevolutionary time. We highlight that shared biogeographic histories could play a role in the codiversification of hosts and parasites at a macroevolutionary scale.     

Multi-host parasite species in cophylogenetic studies

Cophylogenetic studies examine the relationship between host and parasite evolution. One aspect of cophylogenetic studies that has had little modern discussion is parasites with multiple definitive hosts. Parasite species with multiple host species are anomalous as, under a codivergence paradigm, speciation by the hosts should cause speciation of their parasites. We discuss situations such as cryptic parasite species, recent host switching or failure to speciate that may generate multi-host parasites. We suggest methods to identify which of the mechanisms have led to multi-host parasitism. Applying the suggested methods may allow multi-host parasites to be integrated more fully into cophylogenetic studies.     

Hidden Population Structure and Cross-species Transmission of Whipworms (Trichuris sp.) in Humans and Non-human Primates in Uganda

Background

Whipworms (Trichuris sp.) are a globally distributed genus of parasitic helminths that infect a diversity of mammalian hosts. Molecular methods have successfully resolved porcine whipworm, Trichuris suis, from primate whipworm, T. trichiura. However, it remains unclear whether T. trichiura is a multi-host parasite capable of infecting a wide taxonomic breadth of primate hosts or a complex of host specific parasites that infect one or two closely related hosts.

Methods and Findings

We examined the phylogenetic structure of whipworms in a multi-species community of non-human primates and humans in Western Uganda, using both traditional microscopy and molecular methods. A newly developed nested polymerase chain reaction (PCR) method applied to non-invasively collected fecal samples detected Trichuris with 100% sensitivity and 97% specificity relative to microscopy. Infection rates varied significantly among host species, from 13.3% in chimpanzees (Pan troglodytes) to 88.9% in olive baboons (Papio anubis). Phylogenetic analyses based on nucleotide sequences of the Trichuris internal transcribed spacer regions 1 and 2 of ribosomal DNA revealed three co-circulating Trichuris groups. Notably, one group was detected only in humans, while another infected all screened host species, indicating that whipworms from this group are transmitted among wild primates and humans.

Conclusions and Significance

Our results suggest that the host range of Trichuris varies by taxonomic group, with some groups showing host specificity, and others showing host generality. In particular, one Trichuris taxon should be considered a multi-host pathogen that is capable of infecting wild primates and humans. This challenges past assumptions about the host specificity of this and similar helminth parasites and raises concerns about animal and human health.     

Accumulation and exchange of parasites during adaptive radiation in an ancient lake

In the ancient Lake Baikal, Russia, amphipod crustaceans have undergone a spectacular adaptive radiation, resulting in a diverse community of species. A survey of microsporidian parasites inhabiting endemic and non-endemic amphipod host species at the margins of Lake Baikal indicates that the endemic amphipods harbour many microsporidian parasite groups associated with amphipods elsewhere in Eurasia. While these parasites may have undergone a degree of adaptive radiation within the lake, there is little evidence of host specificity. Furthermore, a lack of reciprocal monophyly indicates that exchanges of microsporidia between Baikalian and non-Baikalian hosts have occurred frequently in the past and may be ongoing. Conversely, limitations to parasite exchange between Baikalian and non-Baikalian host populations at the margins of the lake are implied by differences in parasite prevalence and lack of shared microsporidian haplotypes between the two host communities. While amphipod hosts have speciated sympatrically within Lake Baikal, the parasites appear instead to have accumulated, moving into the lake from external amphipod populations on multiple occasions to exploit the large and diverse community of endemic amphipods in Lake Baikal.     

Ecology,not distance,explains community composition in parasites of sky-island Audubon’s Warblers

Haemosporidian parasites of birds are ubiquitous in terrestrial ecosystems, but their coevolutionary dynamics remain poorly understood. If species turnover in parasites occurs at a finer scale than turnover in hosts, widespread hosts would encounter diverse parasites, potentially diversifying as a result. Previous studies have shown that some wide-ranging hosts encounter varied haemosporidian communities throughout their range, and vice-versa. More surveys are needed to elucidate mechanisms that underpin spatial patterns of diversity in this complex multi-host multi-parasite system. We sought to understand how and why a community of avian haemosporidian parasites varies in abundance and composition across elevational transects in eight sky islands in southwestern North America. We tested whether bird community composition, environment, or geographic distance explain haemosporidian parasite species turnover in a widespread host that harbors a diverse haemosporidian community, the Audubon’s Warbler (Setophaga auduboni). We tested predictors of infection using generalized linear models, and predictors of bird and parasite community dissimilarity using generalized dissimilarity modeling. Predictors of infection differed by parasite genus: Parahaemoproteus was predicted by elevation and climate, Leucocytozoon varied idiosyncratically among mountains, and Plasmodium was unpredictable, but rare. Parasite turnover was nearly three-fold higher than bird turnover and was predicted by elevation, climate, and bird community composition, but not geographic distance. Haemosporidian communities vary strikingly at fine spatial scales (hundreds of kilometers), across which the bird community varies only subtly. The finer scale of turnover among parasites implies that their ranges may be smaller than those of their hosts. Avian host species should encounter different parasite species in different parts of their ranges, resulting in spatially varying selection on host immune systems. The fact that parasite turnover was predicted by bird turnover, even when considering environmental characteristics, implies that host species or their phylogenetic history plays a role in determining which parasite species will be present in a community.     

Prevalence of microsporidian infection in commercially caught pink shrimp,Pandalus jordani

Pink shrimp (Pandalus jordani), a commercially harvested species in Oreegon, Washington, and California, hosts four species of microsporidian parasites: Thelohania butleri, Pleistophora crangoni, and two undescribed species placed in the collective group, Microsporidium. All parasitize the skeletal musculature giving shrimp a whitish, opaque appearance. The prevalence of microsporidian infectionin P. jordani was studied over a 6-year period (1975–1980) and was found to be very low. Of the 207, 942 shrimp exmined, 0.19% were infected with one of the four species. No substantial differences in prevalence were observed between different sampling areas in Oregon and Washington or during the years sampled. The four species occurred in each sampling area in about the same portion. T. butleri was predominant and accounted for 78.7% of all infections. Heavy exploitation of host shrimp is suggested as a possible explanation for the low microsporidian infection rates in the P. jordani population.     

Comparing the potential for dispersal via waterbirds of a native and an invasive brine shrimp

1. Migratory waterbirds are likely to have a major role in the spread of many exotic aquatic invertebrates by passive dispersal. However, in the field, this has so far only been confirmed in the case of the American brine shrimp Artemia franciscana, which is spreading quickly around the Mediterranean region. 2. We compared experimentally the capacity of A. franciscana and the native brine shrimp Artemia parthenogenetica to disperse via migratory shorebirds. After Artemia resting eggs (cysts) were fed to Redshank Tringa totanus and Dunlin Calidris alpina, we compared the proportion that survived gut passage, their hatchability and their retention time within the gut. We also tested the ability of cysts to stick to the feathers of Black‐tailed Godwit Limosa limosa. 3. The proportion of ingested cysts retrieved from faeces was the same for each Artemia species (8%), and there were no significant differences in retention time (mean 1.2 h and maximum 10 h for A. parthenogenetica, 1.4 and 12 h for A. franciscana) or hatchability (11% versus 14%). The two shorebird species showed similar retention times and retrieval rates, but cysts recovered from Dunlin had a significantly higher hatchability. Only one of the 1000 A. parthenogenetica cysts and three of the 1000 A. franciscana cysts stuck to feathers. 4. These results indicate that both non‐native and native brine shrimps have a similar high capacity for endozoochory via birds, and that the invasiveness of A. franciscana is probably explained by its competitive superiority owing to high fecundity and release from cestode parasitism. Owing to their different migratory behaviour, Redshank and Dunlin are likely to have different roles as brine shrimp vectors. Brine shrimps provide a suitable model for understanding the role of birds in the dispersal of exotic aquatic invertebrates.     

Introduced species: domestic mammals are more significant transmitters of parasites to native mammals than are feral mammals

The study of parasitism related to biological invasion has focused on attributes and impacts of parasites as invaders and the impact of introduced hosts on endemic parasitism. Thus, there is currently no study of the attributes of hosts which influence the invasiveness of parasites. We aimed to determine whether the degree of domestication of introduced mammalian species – feral introduced mammals, livestock or pets, hereafter ‘D’ – is important in the spillover of introduced parasites. The literature on introduced parasites of mammals in Chile was reviewed. We designed an index for estimating the relevance of the introduced host species to parasite spillover and determined whether the D of introduced mammals predicted this index. A total of 223 introduced parasite species were found. Our results indicate that domestic mammals have a higher number of introduced parasites and spillover parasites, and the index indicates that these mammals, particularly pets, are more relevant introducers than introduced feral mammals. Further analyses indicated that the higher impact is due to higher parasite richness, a longer time since introduction and wider dispersal, as well as how these mammals are maintained. The greater relevance of domestic mammals is important given that they are basically the same species distributed worldwide and can become the main transmitters of parasites to native mammals elsewhere. This finding also underlines the feasibility of management in order to reduce the transmission of parasites to native fauna through anti-parasitic treatment of domestic mammals, animal-ownership education and the prevention of importing new parasite species.     

Contrasting consequences of different defence strategies in a natural multihost–parasite system

Hosts counteract infections using two distinct defence strategies, resistance (reduction in pathogen fitness) and tolerance (limitation of infection damage). These strategies have been minimally investigated in multi-host systems, where they may vary across host species, entailing consequences both for hosts (virulence) and parasites (transmission). Comprehending the interplay among resistance, tolerance, virulence and parasite success is highly relevant for our understanding of the ecology and evolution of infectious and parasitic diseases. Our work investigated the interaction between an insect parasite and its most common bird host species, focusing on two relevant questions: (i) are defence strategies different between main and alternative hosts and, (ii) what are the consequences (virulence and parasite success) of different defence strategies? We conducted a matched field experiment and longitudinal studies at the host and the parasite levels under natural conditions, using a system comprising Philornis torquans flies and three bird hosts – the main host and two of the most frequently used alternative hosts. We found that main and alternative hosts have contrasting defence strategies, which gave rise in turn to contrasting virulence and parasite success. In the main bird host, minor loss of fitness, no detectable immune response, and high parasite success suggest a strategy of high tolerance and negligible resistance. Alternative hosts, on the contrary, resisted by mounting inflammatory responses, although with very different efficiency, which resulted in highly dissimilar parasite success and virulence. These results show clearly distinct defence strategies between main and alternative hosts in a natural multi-host system. They also highlight the importance of defence strategies in determining virulence and infection dynamics, and hint that defence efficiency is a crucial intervening element in these processes.     

Effect of a nuclear polyhedrosis virus on the relationship between Trichoplusia ni (Lepidoptera: Noctuidae) and the parasite, Hyposoter exiguae (hymenoptera: Ichneumonidae)

The effect of a nuclear polyhedrosis virus on the relationship between Trichoplusia ni and the parasite, Hyposoter exiguae, was investigated to determine if the virus could invade and multiply in the tissues of the parasites, if parasites which emerged from virus-infected T. ni larvae had normal emergence, fecundity, and longevity, and if the parasite could serve as a vector for the virus. Light microscopy revealed particles which appeared to be polyhedra within the lumen of the midgut of parasite larvae from virus-infected hosts. Transmission electron microscopy confirmed the presence of polyhedra and free virions within the midgut of the larvae. Polyhedra or free virions were never found within any parasite tissues. Parasite larvae within hosts exposed to virus before parasitization perished when their hosts died of virus infection. Parasite larvae in hosts exposed to virus after parasitization completed their development before their hosts died of virus infection. The proportion of parasites which survived increased as the time between host parasitization and host virus exposure increased. Parasite larvae which developed in hosts exposed to the virus soon after parasitization spent significantly less time in their hosts than did parasites which developed in noninfected hosts. There was no significant difference in time spent in the pupal stage, percent adult emergence, adult longevity with and without food and water, and fecundity of parasites which developed in virus-infected hosts and those which developed in noninfected hosts. Female parasites laid as many eggs in virus-infected hosts as they did in noninfected hosts. Sixty percent of the female parasites which oviposited in virus-infected hosts vectored infective doses of virus to an average of 6% of the healthy hosts subsequently exposed to them. None of the healthy host larvae exposed to male parasites which had been exposed to virus-infected host larvae became infected with the virus. Forty percent of the female parasites which developed in virus-infected hosts transmitted infective doses of the virus to an average of 65% of the healthy host larvae exposed to them. Ninety percent of the male parasites which developed in virus-infected hosts transferred infective doses of the virus to an average of 21% of the healthy host larvae exposed to them.     

Multi-host Model-Based Identification of Armillifer agkistrodontis (Pentastomida), a New Zoonotic Parasite from China

Background

Pentastomiasis is a rare parasitic infection of humans. Pentastomids are dioecious obligate parasites requiring multiple hosts to complete their lifecycle. Despite their worm-like appearance, they are commonly placed into a separate sub-class of the subphylum Crustacea, phylum Arthropoda. However, their systematic position is not uncontested and historically, they have been considered as a separate phylum.

Methodology/Principal Findings

An appraisal of Armillifer agkistrodontis was performed in terms of morphology and genetic identification after its lifecycle had been established in a multi-host model, i.e., mice and rats as intermediate hosts, and snakes (Agkistrodon acutus and Python molurus) as definitive hosts. Different stages of the parasite, including eggs, larvae and adults, were isolated and examined morphologically using light and electron microscopes. Phylogenetic and cluster analysis were also undertaken, focusing on the 18S rRNA and the Cox1 gene. The time for lifecycle completion was about 14 months, including 4 months for the development of eggs to infectious larvae in the intermediate host and 10 months for infectious larvae to mature in the final host. The main morphological difference between A. armillatus and Linguatula serrata is the number of abdominal annuli. Based on the 18S rRNA sequence, the shortest hereditary distance was found between A. agkistrodontis and Raillietiella spp. The highest degree of homology in the Cox 1 nucleic acid sequences and predicted amino acid sequences was found between A. agkistrodontis and A. armillatus.

Conclusion

This is the first time that a multi-host model of the entire lifecycle of A. agkistrodontis has been established. Morphologic and genetic analyses supported the notion that pentastomids should be placed into the phylum Arthropoda.     

Closely related parasitic plants have similar host requirements and related effects on hosts

The performance of root hemiparasites depends strongly on host species identity, but it remains unknown whether there exist general patterns in the quality of species as hosts for hemiparasites and in their sensitivity to parasitism. In a comparative approach, the model root hemiparasites Rhinanthus minor and R. alectorolophus were grown with 25 host species (grasses, forbs, and legumes) at two nutrient levels. Hosts grown without parasites served as a control. Host species identity strongly influenced parasite biomass and other traits, and both parasites grew better with legumes and grasses than with forbs. The biomass of R. alectorolophus was much higher than that of R. minor with all host plants and R. alectorolophus responded much more strongly to higher nutrient availability than R. minor. The performance of the two species of Rhinanthus with individual hosts was strongly correlated, and it was also correlated with that of R. alectorolophus and the related Odontites vulgaris in previous experiments with many of the same hosts, but only weakly with that of the less closely related Melampyrum arvense. The negative effect of R. minor on host biomass was less strong than that of R. alectorolophus, but stronger relative to its own biomass, suggesting that it is more parasitic. The impact of the two parasites on individual hosts did not depend on nutrient level and was correlated. Several legumes and grasses were tolerant of parasitism. While R. minor slightly reduced mean overall productivity, R. alectorolophus increased it with several species, indicating that the loss of host biomass was more than compensated by that of the parasite. The results show that closely related parasites have similar host requirements and correlated negative effects on individual hosts, but that there are also specific interactions between pairs of parasitic plants and their hosts.     

Host sex and parasite genetic diversity

Is the genetic diversity of parasites infecting male and female hosts equal or different? This is the question we address in this paper by studying the neutral genetic variability of the plathyhelminth trematode Schistosoma mansoni within males and females of its natural murine host Rattus rattus in the marshy forest focus of Guadeloupe (French West Indies). Using seven microsatellite markers, we demonstrate that parasites from male hosts are genetically more diversified than parasites from female hosts. Three hypotheses are discussed that could explain this pattern: 1) a host sex-specific duration of cercariae recruitment; 2) a difference in the behaviour of male and female hosts that would lead to the exposure of males to a greater diversity of parasites; and 3) a host sex-biased immunocompetence that would lead to the selection of more genetically diversified individuals in male than in female rats. This finding is the first empirical evidence that each host sex may play different roles in the maintenance of parasite genetic diversity and so in their evolutionary dynamics and epidemiology.     

Host exposure history and priority effects impact the development and reproduction of a dominant parasite

Within a single organism, numerous parasites often compete for space and resources. This competition, together with a parasite’s ability to locate and successfully establish in a host, can contribute to the distribution and prevalence of parasites. Coinfection with trematodes in snail intermediate hosts is rarely observed in nature, partly due to varying competitive abilities among parasite taxa. Using a freshwater snail host (Biomphalaria glabrata), we studied the ability of a competitively dominant trematode, Echinostoma caproni, to establish and reproduce in a host previously infected with a less competitive trematode species, Schistosoma mansoni. Snails were exposed to S. mansoni and co-exposed to E. caproni either simultaneously or 1 week, 4 weeks, or 6 weeks post S. mansoni exposure. Over the course of infection, we monitored the competitive success of the dominant trematode through infection prevalence, parasite development time, and parasite reproductive output. Infection prevalence of E. caproni did not differ among co-exposed groups or between co-exposed and single exposed groups. However, E. caproni infections in co-exposed hosts took longer to reach maturity when the timing between co-exposures increased. All co-exposed groups had higher E. caproni reproductive output than single exposures. We show that although timing of co-exposure affects the development time of parasite transmission stages, it is not important for successful establishment. Additionally, co-exposure, but not priority effects, increases the reproductive output of the dominant parasite.     

Female biased sex-ratio in Schistosoma mansoni after exposure to an allopatric intermediate host strain of Biomphalaria glabrata

For parasites that require multiple hosts to complete their development, the interaction with the intermediate host may have an impact on parasite transmission and development in the definitive host. The human parasite Schistosoma mansoni needs two different hosts to complete its life cycle: the freshwater snail Biomphalaria glabrata (in South America) as intermediate host and a human or rodents as final host. To investigate the influence of the host environment on life history traits in the absence of selection, we performed experimental infections of two B. glabrata strains of different geographic origin with the same clonal population of S. mansoni. One B. glabrata strain is the sympatric host and the other one the allopatric host. We measured prevalence in the snail, the cercarial infectivity, sex-ratio, immunopathology in the final host and microsatellite frequencies of individual larvae in three successive generations.     

Arabidopsis thaliana and the Robin Hood parasite: a chivalrous oomycete that steals fitness from fecund hosts and benefits the poorest one?

Are parasites always harmful to their hosts? By definition, indeed, but in a few cases and particular environments, hosts experience higher fitness in the presence than in the absence of their parasites. Symbiotic associations form a continuum of interactions, from deleterious to beneficial effects on hosts. In this paper, we investigate the outcome of parasite infection of Arabidopsis thaliana by its natural pathogen Hyaloperonospora arabidopsis. This system exhibits a wide range of parasite impact on host fitness with, surprisingly, deleterious effects on high fecundity hosts and, at the opposite extreme, seemingly beneficial effects on the least fecund one. This phenomenon might result from varying levels of tolerance among host lines and even overcompensation for parasite damage analogous to what can be observed in plant–herbivore systems.     

Host age modulates within-host parasite competition

In many host populations, one of the most striking differences among hosts is their age. While parasite prevalence differences in relation to host age are well known, little is known on how host age impacts ecological and evolutionary dynamics of diseases. Using two clones of the water flea Daphnia magna and two clones of its bacterial parasite Pasteuria ramosa, we examined how host age at exposure influences within-host parasite competition and virulence. We found that multiply-exposed hosts were more susceptible to infection and suffered higher mortality than singly-exposed hosts. Hosts oldest at exposure were least often infected and vice versa. Furthermore, we found that in young multiply-exposed hosts competition was weak, allowing coexistence and transmission of both parasite clones, whereas in older multiply-exposed hosts competitive exclusion was observed. Thus, age-dependent parasite exposure and host demography (age structure) could together play an important role in mediating parasite evolution. At the individual level, our results demonstrate a previously unnoticed interaction of the host''s immune system with host age, suggesting that the specificity of immune function changes as hosts mature. Therefore, evolutionary models of parasite virulence might benefit from incorporating age-dependent epidemiological parameters.     

Do Parasitic Trematode Cercariae Demonstrate a Preference for Susceptible Host Species?

Many parasites are motile and exhibit behavioural preferences for certain host species. Because hosts can vary in their susceptibility to infections, parasites might benefit from preferentially detecting and infecting the most susceptible host, but this mechanistic hypothesis for host-choice has rarely been tested. We evaluated whether cercariae (larval trematode parasites) prefer the most susceptible host species by simultaneously presenting cercariae with four species of tadpole hosts. Cercariae consistently preferred hosts in the following order: Anaxyrus ( = Bufo) terrestris (southern toad), Hyla squirella (squirrel tree frog), Lithobates ( = Rana) sphenocephala (southern leopard frog), and Osteopilus septentrionalis (Cuban tree frog). These host species varied in susceptibility to cercariae in an order similar to their attractiveness with a correlation that approached significance. Host attractiveness to parasites also varied consistently and significantly among individuals within a host species. If heritable, this individual-level host variation would represent the raw material upon which selection could act, which could promote a Red Queen “arms race” between host cues and parasite detection of those cues. If, in general, motile parasites prefer to infect the most susceptible host species, this phenomenon could explain aggregated distributions of parasites among hosts and contribute to parasite transmission rates and the evolution of virulence. Parasite preferences for hosts belie the common assumption of disease models that parasites seek and infect hosts at random.