Metacercarial utilization of a naturally infected single species (Lymnaea peregra) snail community by Echinoparyphium recurvatum

Order and timing of infection with different parasite life stages impacts host and parasite life histories

Co-exposure to multiple parasites can alter parasite success and host life history when compared to single infections. These infection outcomes can be affected by the order of parasite arrival, the host immune response, and the interspecific interactions among co-infecting parasites. In this study, we examined how the arrival order of two trematode parasites, Schistosoma mansoni and Echinostoma caproni, influenced parasite ecology and the life history of their snail host, Biomphalaria glabrata. Snail hosts were exposed to E. caproni cercariae before, with, and after their exposure to S. mansoni miracidia. We then measured the effects of this timing on infection prevalence, infection intensity of E. caproni metacercariae, and cercarial output of S. mansoni, as well as on snail reproduction and survival. Snails infected only with S. mansoni and snails exposed to E. caproni after S. mansoni both shed more cercariae than simultaneously exposed snails. Additionally, S. mansoni prevalence was lower in snails that were first exposed to E. caproni compared to snails that were exposed to E. caproni after S. mansoni. Moreover, snails exposed to E. caproni before S. mansoni did not differ in their survival compared to control snails, whereas simultaneously exposed snails and snails exposed to E. caproni after S. mansoni had lower survival than control snails. Combined, this prevalence and survival data suggest a potential protective role of early E. caproni exposure. The timing of E. caproni exposure impacts S. mansoni establishment and reproduction, but host survival patterns are likely driven by S. mansoni prevalence alone.

Vertebrates as uninfected disseminators of helminth eggs and larvae

The passive dispersal of non-mobile organisms by vertebrates (zoochory) is a common mechanism used to explain their often widespread distribution. Transport occurs either internally via the vertebrate digestive tract (endozoochory), or externally be adhering to skin, feathers or fur (ectozoochory), and its success is due to both physiological and ecological factors associated with the disseminating 'hosting' animal. Helminth eggs and larvae are generally non-mobile stages that are largely dependent on the movement of another animal, typically a host, for geographical dissemination. Studies on the zoochory of helminths by vertebrates are extensive and particularly long-standing, stretching back to the 19th century, although this literature is often overlooked when considering the biogeography of parasites. This review assesses the potential of helminths to be dispersed passively by zoochory examining evidence from both laboratory and field studies. The physiological dynamics of the vertebrate intestines and skin surface as hostile environments, as well as the characteristics of eggs and larvae which may facilitate successful transport are evaluated. The various mechanisms of helminth endo- and ectozoochory are presented and the likelihood of long-distance dispersal determined. It is concluded that zoochory is a potentially important means of disseminating parasites.

Snail-borne zoonotic trematodes in edible viviparid snails obtained from wet markets in Northern Thailand

Edible viviparid snails are suspected to be the source of human echinostomiasis in Thailand, but little is known about the prevalence of viviparid snail parasitization by echinostomes in snails sold for human consumption in wet markets. Here, the prevalence of echinostome metacercariae and the association between parasitic intensity and host size, as well as the relationship to host species, were investigated. In total, 1100 viviparid snails belonging to four species (Cipangopaludina annandalei, Filopaludina martensi martensi, F. sumatrensis polygramma and F. doliaris) were obtained from wet markets in Chiang Rai Province, Northern Thailand. All snail species were found to be infected with echinostome metacercariae with total prevalence and mean intensity values of 16.5% and 16.4, respectively. The metacercariae of the avian trematode, Thapariella anastomusa were found in Filopaludina spp. snails. Interestingly, C. annandalei identified in this study had not previously been considered a food source and this is the first report of echinostome metacercaria in this snail species. Results confirmed the role of C. annandalei as the second intermediate host of the trematode; however, the species identity of these echinostomes remains unclear. Comparative morphology of these metacercariae and their adult stage require further study, and molecular markers are necessary to confirm their identification. Prevalence and intensity of echinostomes metacercariae was significantly associated with snail species.

Relative importance of chemical attractiveness to parasites for susceptibility to trematode infection

While the host immune system is often considered the most important physiological mechanism against parasites, precontact mechanisms determining exposure to parasites may also affect infection dynamics. For instance, chemical cues released by hosts can attract parasite transmission stages. We used the freshwater snail Lymnaea stagnalis and its trematode parasite Echinoparyphium aconiatum to examine the role of host chemical attractiveness, physiological condition, and immune function in determining its susceptibility to infection. We assessed host attractiveness through parasite chemo-orientation behavior; physiological condition through host body size, food consumption, and respiration rate; and immune function through two immune parameters (phenoloxidase-like and antibacterial activity of hemolymph) at an individual level. We found that, although snails showed high variation in chemical attractiveness to E. aconiatum cercariae, this did not determine their overall susceptibility to infection. This was because large body size increased attractiveness, but also increased metabolic activity that reduced overall susceptibility. High metabolic rate indicates fast physiological processes, including immune activity. The examined immune traits, however, showed no association with susceptibility to infection. Our results indicate that postcontact mechanisms were more likely to determine snail susceptibility to infection than variation in attractiveness to parasites. These may include localized immune responses in the target tissue of the parasite. The lack of a relationship between food consumption and attractiveness to parasites contradicts earlier findings that show food deprivation reducing snail attractiveness. This suggests that, although variation in resource level over space and time can alter infection dynamics, variation in chemical attractiveness may not contribute to parasite-induced fitness variation within populations when individuals experience similar environmental conditions.

Snail species diversity impacts the infection patterns of Echinostoma spp.: Examples from field collected data

Rapid losses of biodiversity due to the changing landscape have spurred increased interest in the role of species diversity and disease risk. A leading hypothesis for the importance of biodiversity in disease reduction is the dilution effect, which suggests that increasing species diversity within a system decreases the risk of disease among the organisms inhabiting it. The role of species diversity in trematode infection was investigated using field studies from sites across the U.S. to examine the impact of snail diversity in the infection dynamics of both first and second intermediate larval stages of Echinostoma spp. parasites. The prevalence of Echinostoma spp. sporocysts/rediae infection was not affected by increases in snail diversity, but significant negative correlations in metacercariae prevalence and intensity with snail diversity were observed. Additionally, varying effectiveness of the diluting hosts was found, i.e., snail species that were incompatible first intermediate hosts for Echinostoma spp. were more successful at diluting the echinostome parasites in the focal species, while H. trivolvis, a snail species that can harbor the first intermediate larval stages, amplified infection. These findings have important implications not only on the role of species diversity in reducing disease risk, but the success of the parasites in completing their life cycles and maintaining their abundance within an aquatic system.

Infection of Potamopyrgus antipodarum (Gray, 1843) (Gastropoda: Tateidae) by trematodes in Poland, including the first record of aspidogastrid acquisition

The prosobranch gastropod Potamopyrgus antipodarum (Gray, 1843) is poorly understood as a parasite host outside its native New Zealand, including in Europe. Our aim was to ascertain whether non-native P. antipodarum could acquire aspidogastrids or digeneans in habitats where these parasites are found in native hosts. We examined 2400 P. antipodarum individuals from Sosno Lake (Poland). The majority of snails were adult females. No males were found. We found five P. antipodarum individuals with Aspidogaster conchicola and 39 snails with metacercariae of Echinoparyphium aconiatum Dietz 1909 or E. recurvatum (Linstow, 1873). Snails with metacercariae and unparasitized snails, but not snails with A. conchicola, produced embryos. Ours is the first record of an Aspidogastrea - P. antipodarum association.

Tranmission pattern differences of miracidia and cercariae larval stages of digenetic trematode parasites

Digenetic trematodes have complex life cycles involving multiple hosts and free-living larval stages. Some species have 2 lar-val stages that infect snails, with miracidia and cercariae using these molluscs as first and second intermediate hosts, respec-tively. Although both larval stages may infect the same snail species, this is accomplished using different chemical cues and may be influenced by different biotic and abiotic factors. Significant differences in the infection patterns of these parasitic stages regarding host size and density were observed in 2 separate field studies. The prevalence of sporocysts/rediae and mean abundance of Echinostoma spp. metacercariae infection were positively correlated with host size, while the prevalence of Echinostoma spp. cercariae infection was positively correlated with host density across 5 different pulmonate snail species. Larger snails within a given species tend to be older and the increased exposure time may be responsible for the positive correlations with host size. Additionally, infection by miracidia in more vagile snail hosts was influenced by trematode species richness at a sample site, which may be attributed to increased encounter rate as a result of increased movement by the snail hosts. Echinostoma spp. metacercariae prevalence was influenced by host density, possibly due to high abundances of larval clones and their response to more generalized chemical cues attributed to low host specificity by cercariae. Although they can infect the same gastropod hosts, miracidia and cercariae infection are dependent on different factors at both the individual and population level of their snail hosts.

Differences in natural infections of two mortality-related trematodes in lesser scaup and American coot

Populations of North American waterbirds, particularly lesser scaup, have been declining due to habitat disturbance, changing food resources, contaminants, bad water quality, and competition. However, epizootic diseases, including parasitism, may also play an important role in further decline. Trematode-associated mortality of migrating waterbirds, mainly American coot and lesser scaup, has been occurring in the Upper Mississippi River National Wildlife and Fish Refuge since 2002. We examined the levels of infective stages of Cyathocotyle bushiensis and Sphaeridiotrema globulus in the invasive, intermediate host snail, Bithynia tentaculata, during the fall of 2005 and compared these to infection levels in moribund or dead bird hosts. Our results show different infection levels of these 2 parasites in the 2 bird species; C. bushiensis is found more frequently in coot, and S. globulus is more common in scaup. This result is interesting because both bird species are presumed to forage on the same snail population and thus should be experiencing the same extent of exposure. These differences in infections could be attributed to differences in resources of gastrointestinal tracts of coot and scaup, or host resistance. Alternatively, differences in feeding behaviors of coot and scaup may also contribute to differential infections of the 2 trematodes.

The effects of host size and temperature on the emergence of Echinoparyphium recurvatum cercariae from Lymnaea peregra under natural light conditions

The production of cercariae from their snail host is a fundamental component of transmission success in trematodes. The emergence of Echinoparyphium recurvatum (Trematoda: Echinostomatidae) cercariae from Lymnaea peregra was studied under natural sunlight conditions, using naturally infected snails of different sizes (10-17 mm) within a temperature range of 10-29 degrees C. There was a single photoperiodic circadian cycle of emergence with one peak, which correlated with the maximum diffuse sunlight irradiation. At 21 degrees C the daily number of emerging cercariae increased with increasing host snail size, but variations in cercarial emergence did occur between both individual snails and different days. There was only limited evidence of cyclic emergence patterns over a 3-week period, probably due to extensive snail mortality, particularly those in the larger size classes. Very few cercariae emerged in all snail size classes at the lowest temperature studied (10 degrees C), but at increasingly higher temperatures elevated numbers of cercariae emerged, reaching an optimum between 17 and 25 degrees C. Above this range emergence was reduced. At all temperatures more cercariae emerged from larger snails. Analysis of emergence using the Q10 value, a measure of physiological processes over temperature ranges, showed that between 10 and 21 degrees C (approximately 15 degrees C) Q10 values exceeded 100 for all snail size classes, indicating a substantially greater emergence than would be expected for normal physiological rates. From 14 to 25 degrees C (approximately 20 degrees C) cercarial emergence in most snail size classes showed little change in Q10, although in the smallest size class emergence was still substantially greater than the typical Q10 increase expected over this temperature range. At the highest range of 21-29 degrees C (approximately 25 degrees C), Q10 was much reduced. The importance of these results for cercarial emergence under global climate change is discussed.

The effects of temperature on the distribution and establishment ofEchinoparyphium recurvatummetacercariae inLymnaea peregra

The establishment and distribution ofEchinoparyphium recurvatummetacercariae in the second intermediate host,Lymnaea peregra, were investigated at a temperature range of 5–29°C. Preliminary studies on the survival and infectivity ofE. recurvatumcercariae showed that both parameters were temperature dependent. No cercarial transmission occurred at 5 or 10°C. Nevertheless, the transmission efficiency (1/H0) indicated that transmission was temperature independent in the temperature range 17–25°C and was much lower than in previous studies using this host–parasite system. These differences were attributed to low cercarial densities used in this study. The effect of temperature on encystment site choice (mantle cavity, kidney, pericardium) by metacercariae showed that the mantle cavity was the prime site of encystment, followed by the pericardium and the kidney. Temperatures at the lower and upper ranges (14 and 29°C), however, caused a significant reduction in encystment in the mantle cavity but not in the pericardium or kidney. The importance of cercarial densities, the physiological mechanisms influencing metacercarial distribution and their implications for parasite transmission to the definitive host are discussed.