Making the right choice: testing the drivers of asymmetric infections within hosts and their consequences for pathology

Into the danger zone: How the within-host distribution of parasites controls virulence

Despite the importance of virulence in epidemiological theory, the relative contributions of host and parasite to virulence outcomes remain poorly understood. Here, we use reciprocal cross experiments to disentangle the influence of host and parasite on core virulence components-infection and pathology-and understand dramatic differences in parasite-induced malformations in California amphibians. Surveys across 319 populations revealed that amphibians' malformation risk was 2.7× greater in low-elevation ponds, even while controlling for trematode infection load. Factorial experiments revealed that parasites from low-elevation sites induced higher per-parasite pathology (reduced host survival and growth), whereas there were no effects of host source on resistance or tolerance. Parasite populations also exhibited marked differences in within-host distribution: ~90% of low-elevation cysts aggregated around the hind limbs, relative to <60% from high-elevation. This offers a novel, mechanistic basis for regional variation in parasite-induced malformations while promoting a framework for partitioning host and parasite contributions to virulence.

Putting infection on the map: Using heatmaps to characterise within- and between-host distributions of trematode metacercariae

The location of parasites within individual hosts is often treated as a static trait, yet many parasite species can occur in multiple locations or organs within their hosts. Here, we apply distributional heat maps to study the within- and between-host infection patterns for four trematodes (Alaria marcianae, Cephalogonimus americanus, Echinostoma spp. and Ribeiroia ondatrae) within the amphibian hosts Pseudacris regilla and two species of Taricha. We developed heatmaps from 71 individual hosts from six locations in California, which illustrate stark differences among parasites both in their primary locations within amphibian hosts as well as their degree of location specificity. While metacercariae (i.e., cysts) of two parasites (C. americanus and A. marcianae) were relative generalists in habitat selection and often occurred throughout the host, two others (R. ondatrae and Echinostoma spp.) were highly localised to a specific organ or organ system. Comparing parasite distributions among these parasite taxa highlighted locations of overlap showing potential areas of interactions, such as the mandibular inner dermis region, chest and throat inner dermis and the tail reabsorption outer epidermis. Additionally, the within-host distribution of R. ondatrae differed between host species, with metacercariae aggregating in the anterior dermis areas of newts, compared with the posterior dermis area in frogs. The ability to measure fine-scale changes or alterations in parasite distributions has the potential to provide further insight about ecological questions concerning habitat preference, resource selection, host pathology and disease control.

Can predators stabilize host-parasite interactions? Changes in aquatic predator identity alter amphibian responses and parasite abundance across life stages

The role of parasites can change depending on the food web community. Predators, for instance, can amplify or dilute parasite effects on their hosts. Likewise, exposure to parasites or predators at one life stage can have long-term consequences on individual performance and survival, which can influence population and disease dynamics. To understand how predators affect amphibian parasite infections across life stages, we manipulated exposure of northern leopard frog (Rana pipiens) tadpoles to three predators (crayfish [Orconectes rusticus], bluegill [Lepomis macrochirus], or mosquitofish [Gambusia affinis]) and to trematode parasites (Echinostoma spp.) in mesocosms and followed juveniles in outdoor terrestrial enclosures through overwintering. Parasites and predators both had strong impacts on metamorphosis with bluegill and parasites individually reducing metamorph survival. However, when fish were present, the negative effects of parasites on survival was not apparent, likely because fish altered community composition via increased algal food resources. Bluegill also reduced snail abundance, which could explain reduced abundance of parasites in surviving metamorphs. Bluegill and parasite exposure increased mass at metamorphosis, which increased metamorph jumping, swimming, and feeding performance, suggesting that larger frogs would experience better terrestrial survival. Effects on size at metamorphosis persisted in the terrestrial environment but did not influence overwintering survival. Based on our results, we constructed stage-structured population models to evaluate the lethal and sublethal effects of bluegill and parasites on population dynamics. Our models suggested that positive effects of bluegill and parasites on body size may have greater effects on population growth than the direct effects of mortality. This study illustrates how predators can alter the outcome of parasitic infections and highlights the need for long-term experiments that investigate how changes in host-parasite systems alter population dynamics. We show that some predators reduce parasite effects and have indirect positive effects on surviving individuals potentially increasing host population persistence.

QUANTIFYING BILATERAL INFECTION PATTERNS IN THE TREMATODE ALLOGLOSSIDIUM RENALE

Within-host distributions of parasites can have relevance to parasite competition, parasite mating, transmission, and host health. We examined the within-host distribution of the adult trematode Alloglossidium renale infecting the paired antennal glands of grass shrimp. There are 4 possible parasite distributions for infections of paired organs: random, uniform, biased aggregation to 1 particular organ (e.g., left vs. right), or inconsistently biased (aggregated, but does not favor 1 side). Previous work has shown that morphological asymmetries in hosts can lead to biased infections of paired organs. Apparent symmetry between the antennal glands of grass shrimp leads to the prediction that there would be no bias for 1 particular organ. However, an alternative prediction stems from the fact that A. renale is hermaphroditic: aggregation between glands would increase outcrossing opportunities and thus, avoid inbreeding via self-mating. Existing methods to test for an overall pattern did not apply to the A. renale system because of low-intensity infections as well as many 0 values for abundance per unit of the antennal gland. Hence, we used Monte Carlo simulations to determine if the observed overall patterns differed from those expected by randomly allocating parasites into groups of 2. We found that in 3 of 4 data sets, A. renale infections did not deviate from random distributions. The fourth data set had a more uniform pattern than expected by chance. As there was no aggregation between glands and the proportion of worms in single gland infections did not differ from that expected by chance alone, we found no evidence of inbreeding avoidance as might be manifested via a within-host distribution. Given the large proportion of worms in single infections, we predict as a major evolutionary outcome that populations of A. renale will be largely inbred.

Timing and order of exposure to two echinostome species affect patterns of infection in larval amphibians

The study of priority effects with respect to coinfections is still in its infancy. Moreover, existing coinfection studies typically focus on infection outcomes associated with exposure to distinct sets of parasite species, despite that functionally and morphologically similar parasite species commonly coexist in nature. Therefore, it is important to understand how interactions between similar parasites influence infection outcomes. Surveys at seven ponds in northwest Pennsylvania found that multiple species of echinostomes commonly co-occur. Using a larval anuran host (Rana pipiens) and the two most commonly identified echinostome species from our field surveys (Echinostoma trivolvis and Echinoparyphium lineage 3), we examined how species composition and timing of exposure affect patterns of infection. When tadpoles were exposed to both parasites simultaneously, infection loads were higher than when exposed to Echinoparyphium alone but similar to being exposed to Echinostoma alone. When tadpoles were sequentially exposed to the parasite species, tadpoles first exposed to Echinoparyphium had 23% lower infection loads than tadpoles first exposed to Echinostoma. These findings demonstrate that exposure timing and order, even with similar parasites, can influence coinfection outcomes, and emphasize the importance of using molecular methods to identify parasites for ecological studies.

Influence of an extreme climatic event on bilateral asymmetry and occurrence of eye flukes in perch (Perca fluviatilis) and roach (Rutilus rutilus) from a lake in southeast England

Eye flukes are common infections of freshwater fish and their occurrence as metacercarial stages may occur non-randomly resulting in an asymmetrical distribution within the host eyes. However, from previous studies the presentation of bias by these trematodes lacks consistency suggesting that congenital asymmetrical effects are unlikely to be the cause and exogenous factors, such as environmental stress, may be more influential. The present study, undertaken over a 4-year period, investigates the impact of an extreme heatwave and drought on the annual bilateral asymmetry and occurrence of two eye fluke species (Diplostomum sp. in the lens and Tylodelphys sp. in the vitreous humour) from perch (Perca fluviatilis) and roach (Rutilus rutilus). The onset of the extreme climatic event resulted in a significant negative effect on the occurrence of the eye flukes. Bilateral asymmetry, which was present within both trematode species and hosts, appeared to be highly variable between eye fluke and fish species and also year of study. However, during the pre-drought period both host species demonstrated significant asymmetry for Tylodelphys sp. but not for Diplostomum sp. while during the drought this bias was reversed. The potential role of fluctuating asymmetry of fish hosts in structuring the bilateral asymmetry of eye flukes is discussed.

Influence of Ligula intestinalis plerocercoids (Cestoda: Diphyllobothriidea) on the occurrence of eyeflukes in roach (Rutilus rutilus) from a lake in south-east England

Vertebrate hosts commonly harbour concurrent infections of different helminth species which may interact with each other in a synergistic, antagonistic or negligible manner. Direct interactions between helminths that share a common site in the host have been regularly reported, but indirect interactions between species that occur in different sites are rarely described, especially in fish hosts. Plerocercoids of Ligula intestinalis are common infections of the peritoneal (body) cavity of roach (Rutilus rutilus) in freshwater habitats. These larval cestodes can cause extensive systemic pathologies to the fish host, which in turn may alter its susceptibility as a target host for other helminth species. The present study, using an existing dataset, investigates the influence of L. intestinalis (ligulosis) on frequently occurring eyefluke infections in roach sampled from a lake in south-east England. The occurrence of two species of eyefluke (Diplostomum sp. and Tylodelphys sp.) in the roach population demonstrated no significant levels of interaction with each other. The prevalence but not mean intensity or abundance of Diplostomum sp. was significantly increased in ligulosed roach, while the incidence of Tylodelphys sp. remained unchanged. Analyses of bilateral asymmetry in the occurrence of eyeflukes in left and right eyes of infected fish demonstrate that Tylodelphys sp. shows significant asymmetry in non-ligulosed roach, which is not replicated in ligulosed individuals. In contrast, Diplostomum sp. shows no evidence of asymmetry in either ligulosed or non-ligulosed fish.

Exposure to a cyanobacterial toxin increases larval amphibian susceptibility to parasitism

Anthropogenic activities are promoting the proliferation of aquatic primary producers in freshwater habitats, including cyanobacteria. Among various problems stemming from eutrophication, cyanobacterial blooms can be toxic due to the production of secondary compounds, including microcystins such as microcystin-LR (MC-LR); however, it is unknown whether cyanotoxins can affect the susceptibility of aquatic vertebrates such as fish and larval amphibians to parasites or pathogens even though infectious diseases can significantly affect natural populations. Here, we examined how exposure to environmentally relevant concentrations of MC-LRs affected the resistance of larval amphibians (northern leopard frog, Rana pipiens) to infection by a helminth parasite (the trematode Echinostoma sp.), and whether this was manifested by reductions in host anti-parasite behavior. Exposure to a relatively high (82 μg L^-1) concentration of MC-LR caused over 70% mortality, and tadpoles that survived exposure to the low MC-LR (11 μg L^-1) treatment had significantly higher infection intensities than those in the control; however, anti-parasite behavior was not affected by treatment. Our results indicate that MC-LR can have both direct and indirect negative effects on larval amphibians by increasing their mortality and susceptibility to parasitism, which may have implications for other aquatic vertebrates in eutrophic habitats dominated by cyanobacteria as well.

Effects of Echinostoma trivolvis metacercariae infection during development and metamorphosis of the wood frog (Lithobates sylvaticus)

Many organisms face energetic trade-offs between defense against parasites and other host processes that may determine overall consequences of infection. These trade-offs may be particularly evident during unfavorable environmental conditions or energetically demanding life history stages. Amphibian metamorphosis, an ecologically important developmental period, is associated with drastic morphological and physiological changes and substantial energetic costs. Effects of the trematode parasite Echinostoma trivolvis have been documented during early amphibian development, but effects during later development and metamorphosis are largely unknown. Using a laboratory experiment, we examined the energetic costs of late development and metamorphosis coupled with E. trivolvis infection in wood frogs, Lithobates [=Rana] sylvaticus. Echinostoma infection intensity did not differ between tadpoles examined prior to and after completing metamorphosis, suggesting that metacercariae were retained through metamorphosis. Infection with E. trivolvis contributed to a slower growth rate and longer development period prior to the initiation of metamorphosis. In contrast, E. trivolvis infection did not affect energy expenditure during late development or metamorphosis. Possible explanations for these results include the presence of parasites not interfering with pronephros degradation during metamorphosis or the mesonephros compensating for any parasite damage. Overall, the energetic costs of metamorphosis for wood frogs were comparable to other species with similar life history traits, but differed from a species with a much shorter duration of metamorphic climax. Our findings contribute to understanding the possible role of energetic trade-offs between parasite defense and host processes by considering parasite infection with simultaneous energetic demands during a sensitive period of development.