Trophic transmission of parasites and host behavior modification

Unhealthy herds and the predator–spreader: Understanding when predation increases disease incidence and prevalence

Disease ecologists now recognize the limitation behind examining host–parasite interactions in isolation: community members—especially predators—dramatically affect host–parasite dynamics. Although the initial paradigm was that predation should reduce disease in prey populations (“healthy herds hypothesis”), researchers have realized that predators sometimes increase disease in their prey. These “predator–spreaders” are now recognized as critical to disease dynamics, but empirical research on the topic remains fragmented. In a narrow sense, a “predator–spreader” would be defined as a predator that mechanically spreads parasites via feeding. However, predators affect their prey and, subsequently, disease transmission in many other ways such as altering prey population structure, behavior, and physiology. We review the existing evidence for these mechanisms and provide heuristics that incorporate features of the host, predator, parasite, and environment to understand whether or not a predator is likely to be a predator–spreader. We also provide guidance for targeted study of each mechanism and quantifying the effects of predators on parasitism in a way that yields more general insights into the factors that promote predator spreading. We aim to offer a better understanding of this important and underappreciated interaction and a path toward being able to predict how changes in predation will influence parasite dynamics.

Two's a crowd? Crowding effect in a parasitic castrator drives differences in reproductive resource allocation in single vs double infections

The 'crowding effect' is a result of competition by parasites within a host for finite resources. Typically, the severity of this effect increases with increasing numbers of parasites within a host and manifests in reduced body size and thus fitness. Evidence for the crowding effect is mixed - while some have found negative effects, others have found a positive effect of increased parasite load on parasite fitness. Parasites are consumers with diverse trophic strategies reflected in their life history traits. These distinctions are useful to predict the effects of crowding. We studied a parasitic castrator, a parasite that usurps host reproductive energy and renders the host sterile. Parasitic castrators typically occur as single infections within hosts. With multiple parasitic castrators, we expect strong competition and evidence of crowding. We directly assess the effect of crowding on reproductive success in a barnacle population infected by a unique parasitic castrator, Hemioniscus balani, an isopod parasite that infects and blocks reproduction of barnacles. We find (1) strong evidence of crowding in double infections, (2) increased frequency of double infections in larger barnacle hosts with more resources and (3) perfect compensation in egg production, supporting strong space limitation. Our results document that the effects of crowding are particularly severe for this parasitic castrator, and may be applicable to other castrators that are also resource or space limited.

Monsters of the sea serpent: parasites of an oarfish, Regalecus russellii

Examination of a small portion of the viscera of an oarfish ( Regalecus russellii ) recovered from Santa Catalina Island, southern California, revealed numerous tetraphyllidean tapeworm plerocercoids, Clistobothrium cf. montaukensis; 2 juvenile nematodes, Contracaecum sp.; and a fragment of an adult acanthocephalan, family Arhythmacanthidae. This suggests that the fish was relatively heavily parasitized. The presence of larval and juvenile worms suggests that oarfish are preyed upon by deep-swimming predators such as the shortfin mako shark, Isurus oxyrinchus , known to be a definitive host for the adult tapeworm, and also by diving mammals such as sperm whales, Physeter catodon L., hosts of Contracaecum spp. nematodes.

Infectious disease agents mediate interaction in food webs and ecosystems

Infectious agents are part of food webs and ecosystems via the relationship with their host species that, in turn, interact with both hosts and non-hosts. Through these interactions, infectious agents influence food webs in terms of structure, functioning and stability. The present literature shows a broad range of impacts of infectious agents on food webs, and by cataloguing that range, we worked towards defining the various mechanisms and their specific effects. To explore the impact, a direct approach is to study changes in food-web properties with infectious agents as separate species in the web, acting as additional nodes, with links to their host species. An indirect approach concentrates not on adding new nodes and links, but on the ways that infectious agents affect the existing links across host and non-host nodes, by influencing the 'quality' of consumer-resource interaction as it depends on the epidemiological state host involved. Both approaches are natural from an ecological point of view, but the indirect approach may connect more straightforwardly to commonly used tools in infectious disease dynamics.

When parasites become prey: ecological and epidemiological significance of eating parasites

Recent efforts to include parasites in food webs have drawn attention to a previously ignored facet of foraging ecology: parasites commonly function as prey within ecosystems. Because of the high productivity of parasites, their unique nutritional composition and their pathogenicity in hosts, their consumption affects both food-web topology and disease risk in humans and wildlife. Here, we evaluate the ecological, evolutionary and epidemiological significance of feeding on parasites, including concomitant predation, grooming, predation on free-living stages and intraguild predation. Combining empirical data and theoretical models, we show that consumption of parasites is neither rare nor accidental, and that it can sharply affect parasite transmission and food web properties. Broader consideration of predation on parasites will enhance our understanding of disease control, food web structure and energy transfer, and the evolution of complex life cycles.

Forming foci of transmission: the effects of resource utilization, species interaction, and parasitism on molluscan movement

Animal aggregation to environmental cues provides opportunities for parasite transmission between individual hosts of the same or different species. Better characterization of host behavioral responses to environmental stimuli in the absence and presence of parasites will improve our understanding of how foci of transmission form. The behavioral response patterns of two co-occurring freshwater snail species ( Lymnaea elodes (Say, 1821) and Helisoma trivolvis (Say, 1817) (=  Planorbella trivolvis (Say, 1817))) were assessed in response to three environmental stimuli (crayfish (genus Orconectes  Cope, 1872) carrion, vegetation, or temperature gradient). Experiments were conducted with single species and species interactions. In addition, parasitized L. elodes were included in a single-species experiment and a species-interaction experiment. Snail species differed in the direction and magnitude of their responses to the environmental stimuli. Species interactions did not affect the responses to two of the stimuli for either species; however, interspecific interactions affected the response to high temperature in both species. Behavioral responses were altered in the presence of parasites for both the infected and uninfected hosts, suggesting parasitism is an important biotic factor in animal movement. This experimental study indicates co-occurring species respond to environmental factors in different ways. Furthermore, species interactions and parasitism within a guild can have strong effects on animal movement and potentially on parasite transmission.

Increased susceptibility to predation and altered anti-predator behaviour in an acanthocephalan-infected amphipod

According to the 'parasitic manipulation hypothesis', phenotypic changes induced by parasites in their intermediate hosts are effective means of increasing trophic transmission to final hosts. One obvious prediction, although seldom tested, is that increased vulnerability of infected prey to an appropriate predator should be achieved by the parasite altering the anti-predator behaviour of its intermediate host. In this study, we tested this prediction using the fish acanthocephalan Pomphorhynchus tereticollis and the freshwater amphipod Gammarus pulex. Firstly, we estimated the relative vulnerability of infected and uninfected gammarids to predation by the bullhead Cottus gobio in the field. Second, we investigated under experimental conditions how two common anti-predator behaviours of aquatic invertebrates, refuge use and short-distance reaction to predator chemical cues, were affected by infection status. We found that the prevalence of infection in the field was 10 times higher among gammarids collected from the stomach contents of bullheads compared with free-ranging individuals collected in the same river. In a microcosm uninfected gammarids, but not infected ones, increased the use of refuge in the presence of a bullhead. Finally, a behavioural experiment using an Y-maze olfactometer showed opposite reactions to predator odour. Whereas uninfected gammarids were significantly repulsed by the chemical cues originating from bullheads, infected ones were significantly attracted to the odour of the predator. Taken together, our results suggest that the alteration of anti-predator behaviour in infected G. pulex might enhance predation by bullheads in the field. Reversing anti-predator behaviour might thus be an efficient device by which parasites with complex life-cycles increase their trophic transmission to final hosts. Further studies should pay more attention to both the increased vulnerability of infected prey to an appropriate predator in the field and the influence of parasitic infection on the anti-predator behaviour of intermediate hosts.

Effects of trematode double infection on the shell size and distribution of snail hosts

Infection with larval trematodes sometimes alters the phenotypes of their snail hosts. While some trematode species have distinct effects on host phenotypes, it is still unclear how snail phenotypes are altered when they are parasitized with multiple trematode species. Here, we report that double infection with trematode species averages the effects of parasitic alteration on host phenotype. We found that snail hosts Batillaria attramentaria (Batillariidae) infected with Cercaria batillariae (Heterophyidae) have abnormally large shells and distribute in lower areas of the intertidal zone. Snails with another dominant trematode species, the renicolid cercaria I (Renicolidae), have slightly larger shells and distribute in upper areas of the intertidal zone. A number of double infections with both trematodes was observed in this study. Snails infected with both trematode species exhibited an intermediate size and inhabited a depth between those of snails solely infected with either trematode species, suggesting that the two trematodes simultaneously affected the snail phenotypes. Because altered host phenotypes are frequently beneficial to parasites, two trematode species may compete for successful transmission through alteration of host phenotypes.

Parasites alter host phenotype and may create a new ecological niche for snail hosts

By modifying the behaviour and morphology of hosts, parasites may strongly impact host individuals, populations and communities. We examined the effects of a common trematode parasite on its snail host, Batillaria cumingi (Batillariidae). This widespread snail is usually the most abundant invertebrate in salt marshes and mudflats of the northeastern coast of Asia. More than half (52.6%, n=1360) of the snails in our study were infected. We found that snails living in the lower intertidal zone were markedly larger and exhibited different shell morphology than those in the upper intertidal zone. The large morphotypes in the lower tidal zone were all infected by the trematode, Cercaria batillariae (Heterophyidae). We used a transplant experiment, a mark-and-recapture experiment and stable carbon isotope ratios to reveal that snails infected by the trematode move to the lower intertidal zone, resume growth after maturation and consume different resources. By simultaneously changing the morphology and behaviour of individual hosts, this parasite alters the demographics and potentially modifies resource use of the snail population. Since trematodes are common and often abundant in marine and freshwater habitats throughout the world, their effects potentially alter food webs in many systems.