Dana M. Calhoun

Endohelminths in Bird Hosts from Northern California and an Analysis of the Role of Life History Traits on Parasite Richness.

Abstract The life history characteristics of hosts often influence patterns of parasite infection either by affecting the likelihood of parasite exposure or the probability of infection after exposure. In birds, migratory behavior has been suggested to affect both the composition and abundance of parasites within a host, although whether migratory birds have more or fewer parasites is unclear. To help address these knowledge gaps, we collaborated with airports, animal rescue/rehabilitation centers, and hunter check stations in the San Francisco Bay Area of California to collect 57 raptors, egrets, herons, ducks, and other waterfowl for parasitological analysis. After dissections of the gastrointestinal tract of each host, we identified 64 taxa of parasites: 5 acanthocephalans, 24 nematodes, 8 cestodes, and 27 trematodes. We then used a generalized linear mixed model to determine how life history traits influenced parasite richness among bird hosts, while controlling for host phylogeny. Parasite richness was greater in birds that were migratory with larger clutch sizes and lower in birds that were herbivorous. The effects of clutch size and diet are consistent with previous studies and have been linked to immune function and parasite exposure, respectively, whereas the effect of migration supports the hypothesis of “migratory exposure” rather than that of “migratory escape.”

Experimental Infections of Bluegill with the Trematode Ribeiroia ondatrae (Digenea: Cathaemasiidae): Histopathology and Hematological Response

Infections by the digenetic trematode, Ribeiroia ondatrae, cause severe limb malformations in many North American amphibians. Ribeiroia ondatrae also infects fishes as second intermediate hosts, but less is known about the pathology and immune responses initiated in infected fish, even though reports of infected fish date back to early 1900s. To this end, we experimentally exposed juvenile Bluegills Lepomis macrochirus to three doses of R. ondatrae cercariae and monitored the pathology, parasite infection success, and humoral responses over 648 h. All exposed fish became infected with metacercariae, and the average infection load increased with exposure dose. Histologically, infection was associated with acute hemorrhages in the lateral line and local dermis at 36 h, followed by progressive granulomatous inflammation that led to the destruction of encysted metacercariae. Correspondingly, over the course of 648 h we observed an 85% decline in average infection load among hosts, reflecting the hosts clearance of the parasite. Infection was not associated with changes in fish growth or survival, but did correlate with leukocytosis and neutrophilia in circulating host blood. Understanding the physiological responses of R. ondatrae in Bluegill will help to clarify the ecological effects of this parasite and provide a foundation for subsequent comparisons into its effects on behavior, individual health, and population dynamics of Bluegill.

Role of Antimicrobial Peptides in Amphibian Defense Against Trematode Infection

Antimicrobial peptides (AMPs) contribute to the immune defenses of many vertebrates, including amphibians. As larvae, amphibians are often exposed to the infectious stages of trematode parasites, many of which must penetrate the host’s skin, potentially interacting with host AMPs. We tested the effects of the natural AMPs repertoires on both the survival of trematode infectious stages as well as their ability to infect larval amphibians. All five trematode species exhibited decreased survival of cercariae in response to higher concentrations of adult bullfrog AMPs, but no effect when exposed to AMPs from larval bullfrogs. Similarly, the use of norepinephrine to remove AMPs from larval bullfrogs, Pacific chorus frogs, and gray treefrogs had only weak (gray treefrogs) or non-significant (other tested species) effects on infection success by Ribeiroia ondatrae. We nonetheless observed strong differences in parasite infection as a function of both host stage (first- versus second-year bullfrogs) and host species (Pacific chorus frogs versus gray treefrogs) that were apparently unrelated to AMPs. Taken together, our results suggest that AMPs do not play a significant role in defending larval amphibians against trematode cercariae, but that they could be one mechanism helping to prevent infection of post-metamorphic amphibians, particularly for highly aquatic species.

Noxious newts and their natural enemies: experimental effects of tetrodotoxin exposure on trematode parasites and aquatic macroinvertebrates

ABSTRACT The dermal glands of many amphibian species secrete toxins or other noxious substances as a defense strategy against natural enemies. Newts in particular possess the potent neurotoxin tetrodotoxin (TTX), for which the highest concentrations are found in species within the genus Taricha. Adult Taricha are hypothesized to use TTX as a chemical defense against vertebrate predators such as garter snakes (Thamnophis spp.). However, less is known about how TTX functions to defend aquatic‐developing newt larvae against natural enemies, including trematode parasites and aquatic macroinvertebrates. Here we experimentally investigated the effects of exogenous TTX exposure on survivorship of the infectious stages (cercariae) of five species of trematode parasites that infect larval amphibians. Specifically, we used dose‐response curves to test the sensitivity of trematode cercariae to progressively increasing concentrations of TTX (0.0 [control], 0.63, 3.13, 6.26, 31.32, and 62.64 nmol L−1) and how this differed among parasite species. We further compared these results to the effects of TTX exposure (0 and 1000 nmolL−1) over 24 h on seven macroinvertebrate taxa commonly found in aquatic habitats with newt larvae. TTX significantly reduced the survivorship of trematode cercariae for all species, but the magnitude of such effects varied among species. Ribeiroia ondatrae – which causes mortality and limb malformations in amphibians – was the least sensitive to TTX, whereas the kidney‐encysting Echinostoma trivolvis was the most sensitive. Among the macroinvertebrate taxa, only mayflies (Ephemeroptera) showed a significant increase in mortality following exogenous TTX exposure, despite the use of a concentration 16x higher than the maximum used for trematodes. Our results suggest that maternal investment of TTX into larval newts may provide protection against certain trematode infections and highlight the importance of future work assessing the effects of newt toxicity on both parasite infection success and the palatability of larval newts to invertebrate predators. HIGHLIGHTSTrematode cercariae exposed to tetrodotoxin all exhibited reduced survivorship.Trematodes survivorship varied, Ribeiroia ondatrae was the least sensitive to TTX.Host toxicity may decrease infection in naturally occurring populations.Of macroinvertebrates, only mayflies showed a decrease in survivorship to TTX.Maternal investment of TTX into newts may provide protection against trematodes.

Parasites of invasive freshwater fishes and the factors affecting their richness

Freshwater environments are threatened by nonnative species introductions, often involving fishes. Parasites and pathogens introduced with fishes have the potential to infect native taxa and should be investigated. We examined 726 fishes representing 6 invasive species from 27 ponds in California to evaluate how parasite richness, infection prevalence, and parasite abundance varied with host body size, host species, pond identity, and environmental variables. We identified 14 parasitic taxa based on morphological and molecular techniques. Overall, 60% of fish were infected with ≥1 parasite taxa: 6 trematodes, 3 monogeneans, 3 nematodes, 1 larval cestode (unidentified), and 1 acanthocephalan. Micropterus salmoides had the highest average infection prevalence (85% ± 0.08 SE, n = 17 site–year combinations), the highest average infection abundance (292.51 ± 119.10 SE parasites/fish), and the 2nd- highest γ diversity (10 taxa; Lepomis macrochirus had 11 total parasite taxa). In contrast, Gambusia affinis consistently supported the lowest parasite abundance, infection prevalence, and richness. Across all fish species, larger body size was associated with a broader diversity of parasites, but host sex had no effect. Increased pond area, pH, and aquatic community diversity also were correlated positively with parasite richness in fish populations, consistent with potential influences of colonization opportunities and the ‘diversity begets diversity’ hypothesis. A shortage of historical information on the native ranges of observed parasites precluded an assessment of whether these infections were introduced with their fish hosts. Nonetheless, they have the potential to directly or indirectly affect interactions between introduced fishes and native fauna, particularly given the near-absence of native lentic fishes in this region and the fact that several infections are shared with sensitive endemic taxa, such as amphibians.

The influence of landscape and environmental factors on ranavirus epidemiology in a California amphibian assemblage

A fundamental goal of disease ecology is to determine the landscape and environmental processes that drive disease dynamics at different biological levels to guide management and conservation. Although ranaviruses (family Iridoviridae) are emerging amphibian pathogens, few studies have conducted comprehensive field surveys to assess potential drivers of ranavirus disease dynamics.We examined the factors underlying patterns in site-level ranavirus presence and individual-level ranavirus infection in 76 ponds and 1,088 individuals representing 5 amphibian species within the East Bay region of California.Based on a competing-model approach followed by variance partitioning, landscape and biotic variables explained the most variation in site-level presence. However, biotic and individual-level variables explained the most variation in individual-level infection.Distance to nearest ranavirus-infected pond (the landscape factor) was more important than biotic factors at the site-level; however, biotic factors were most influential at the individual-level. At the site level, the probability of ranavirus presence correlated negatively with distance to nearest ranavirus-positive pond, suggesting that the movement of water or mobile taxa (e.g., adult amphibians, birds, reptiles) may facilitate the movement of ranavirus between ponds and across the landscape.Taxonomic richness associated positively with ranavirus presence at the site-level, but vertebrate richness associated negatively with infection prevalence in the host population. This might reflect the contrasting influences of diversity on pathogen colonization versus transmission among hosts.Amphibian host species differed in their likelihood of ranavirus infection: American bullfrogs (Rana catesbeiana) had the weakest association with infection while rough-skinned newts (Taricha granulosa) had the strongest. After accounting for host species effects, hosts with greater snout-vent length had a lower probability of infection.Our study demonstrates the array of landscape, environmental, and individual-level factors associated with ranavirus epidemiology. Moreover, our study helps illustrate that the importance of these factors varies with biological level.

Parasite richness and abundance within aquatic macroinvertebrates: testing the roles of host‐ and habitat‐level factors

The importance of parasites as both members of biological communities and as structuring agents of host communities has been increasingly emphasized. Yet parasites of aquatic macroinvertebrates and the environmental factors regulating their richness and abundance remain poorly studied. Here we quantified parasite richness and abundance within 12 genera of odonate naiads and opportunistically sampled four additional orders of aquatic macroinvertebrates from 35 freshwater ponds in the San Francisco Bay Area of California, USA. We also tested the relative contributions of host- and habitat-level factors in driving patterns of infection abundance for the most commonly encountered parasite (the trematode Haematoloechus sp.) in nymphal damselflies and dragonflies using hierarchical generalized linear mixed models. Over the course of two years, we quantified the presence and intensity of parasites from 1,612 individuals. We identified six parasite taxa: two digenetic trematodes, one larval nematode, one larval acanthocephalan, one gregarine, and a mite, for which the highest infection prevalence (39%) occurred in the damselfly genus, Ishnura sp. Based on the hierarchical analysis of Haematoloechus sp. occurrence, infection prevalence and abundance were associated predominantly with site-level factors, including definitive host (frog) presence, nymphal odonate density, water pH and conductivity. In addition, host suborder interacted with the presence of fishes, such that damselflies had higher infection rates in sites with fish relative to those without, whereas the opposite was true for dragonfly nymphs. These findings offer insights into the potential interaction between host- and site-level factors in shaping parasite populations within macroinvertebrate taxa.

The influence of landscape and environmental factors on ranavirus epidemiology in amphibian assemblages

Aim To quantify the influence of a suite of landscape, abiotic, biotic, and host-level variables on ranavirus disease dynamics in amphibian assemblages at two biological levels (site and host-level). Location Wetlands within the East Bay region of California, USA. Methods We used competing models, multimodel inference, and variance partitioning to examine the influence of 16 landscape and environmental factors on patterns in site-level ranavirus presence and host-level ranavirus infection in 76 wetlands and 1,377 amphibian hosts representing five species. Results The landscape factor explained more variation than any other factors in site-level ranavirus presence, but biotic and host-level factors explained more variation in host-level ranavirus infection. At both the site- and host-level, the probability of ranavirus presence correlated negatively with distance to nearest ranavirus-positive wetland. At the site-level, ranavirus presence was associated positively with taxonomic richness. However, infection prevalence within the amphibian population correlated negatively with vertebrate richness. Finally, amphibian host species differed in their likelihood of ranavirus infection: American Bullfrogs had the weakest association with infection while Western Toads had the strongest. After accounting for host species effects, hosts with greater snout-vent length had a lower probability of infection. Main conclusions Strong spatial influences at both biological levels suggest that mobile taxa (e.g., adult amphibians, birds, reptiles) may facilitate the movement of ranavirus among hosts and across the landscape. Higher taxonomic richness at sites may provide more opportunities for colonization or the presence of reservoir hosts that may influence ranavirus presence. Higher host richness correlating with higher ranavirus infection is suggestive of a dilution effect that has been observed for other amphibian disease systems and warrants further investigation. Our study demonstrates that an array of landscape, environmental, and host-level factors were associated with ranavirus epidemiology and illustrates that their importance vary with biological level.

Circadian rhythms of trematode parasites: applying mixed models to test underlying patterns

Circadian rhythms of parasites and their hosts can influence processes such as transmission, pathology and life cycle evolution. For trematode parasites that depend on free-living infectious stages (i.e. cercariae) to move among host species, the timing of parasite release is hypothesized to increase the likelihood of contacting a host. Yet, a persistent challenge in studying such biorhythms involves selection of appropriate analytical techniques. Here, we extend a generalized linear mixed modelling (GLMM) framework to cosinor analyses, thereby allowing flexibility in the statistical distribution of the response variable, incorporation of multiple covariates and inclusion of hierarchical grouping effects. By applying this approach to 93 snails infected with trematode parasites from freshwater pond ecosystems, we detected non-random rhythms in six of eight species, with variation in both the timing of peak cercariae release (between 5:10 and 21:46 h) and its magnitude (between 13 and 386). The use of GLMM yielded more accurate and precise estimates of the cosinor parameters compared with classical least-squares (LS) based on a simulation-based sensitivity analysis. The sensitivity analysis revealed that the amplitude and rhythm-adjusted mean values from the LS models diverged from the true values at some limits. We highlight the importance of novel analytical approaches for evaluating parasite circadian rhythms and investigating their underlying mechanisms.