Variation of snail's abundance in two water bodies harboring strains of Pseudosuccinea columella resistant and susceptible to Fasciola hepatica miracidial infection, in Pinar del Río Province, Cuba

Modeling the effects of water temperature on the population dynamics of Galba viatrix and infection by Fasciola hepatica: a two-year survey in Andean Patagonia, Argentina

Background

The trematode parasite Fasciola hepatica (liver fluke) can infect livestock, wild mammals, and humans, generating serious economic losses worldwide. Aquatic or amphibious snails of the Lymnaeidae family are the intermediate host of this parasite. Both snail population dynamics and parasite development are closely associated with temperature, although most field studies have recorded air temperature rather than water temperature. Our aim was to statistically model the population dynamics of lymnaeid snails and their infection by F. hepatica under natural environmental conditions in Northwest Andean Patagonia.

Methods

For two years, we sampled snails monthly in four bodies of water, while registering water and air temperature hourly, and assessing F. hepatica infection in snails. Hierarchical Bayesian modeling allowed us to estimate the functional relationship between water temperature and population growth, the probability of detecting snails, and infection by F. hepatica.

Results

A total of 1,411 Galba viatrix snails were collected, identified, and analyzed for F. hepatica infection. All sites showed seasonal variation in the number of snails collected and in water temperature as well as sharp variations in snail counts between surveys adjacent in time. The hierarchical model revealed that water temperature acts, at least, at two different time scales: water temperature at the time of sampling determines snail detection probability, whereas the average water temperature between sampling dates affects lymnaeid population growth. We found maximum F. hepatica prevalences in snails of 40% (2/5 and 4/10), followed by 33% (65/197). These are the highest prevalences recorded in G. viatrix populations in Argentina to date. Our modeling evidenced that the positive effects of water temperature on infection probability increases with snail size and prevalence on the previous survey, while previous prevalence strongly enhances the effects of snail size.

Conclusions

Our results underscore the high temporal and spatial variability in the population of snails and the prevalence of F. hepatica, as well as the major impact temperature has on detecting snails. Our models provide quantifications of the effects of water temperature on the population growth of G. viatrix, its detection, and infection under natural field conditions. These are crucial steps towards generating mechanistic models of F. hepatica transmission that would facilitate the design and simulation of potential interventions based on treatments and on environmental and livestock management, taking into account the specific characteristics of each region.

Environmental influence on abundance and infection patterns of snail intermediate hosts of liver and intestinal flukes in North and Central Vietnam

Liver and intestinal flukes (LIF) are important groups of foodborne zoonotic trematodes (FZTs) in Southeast Asia, including Vietnam. Their complex life cycles require specific freshwater snail species as the obligatory first intermediate hosts. In 2019, we conducted a longitudinal study in Yen Bai and Thanh Hoa provinces in North and Central Vietnam, respectively, to investigate the diversity of LIF and their infection prevalence in relation to snail host abundance and environmental factors. Using a combination of morphological and molecular identification techniques, we identified 10 LIF species infecting 11 snail host species. We observed significant seasonal variation in the mean abundance of several snail host species, with the majority of snails collected during the spring. We also detected seasonal changes in LIF species composition, with the highest species richness reported in the spring. Clonorchis sinensis and Fasciola gigantica, two medically important human liver flukes in Asia, were found only in the spring in Yen Bai. Our study revealed that not all snail host species have the same probability of becoming infected, and we recorded seasonal variations in the prevalence of LIF infection in different snail species in relation to water parameters.

Immunological Resistance of Pseudosuccinea columella Snails From Cuba to Fasciola hepatica (Trematoda) Infection: What We Know and Where We Go on Comparative Molecular and Mechanistic Immunobiology, Ecology and Evolution

One of the most interesting biological models is that of snail-trematode interactions, many of which ultimately result in the transmission of several important diseases, particularly in the tropics. Herein, we review the scientific advances on a trematode-snail system in which certain populations of Pseudosuccinea columella (a common host species for trematodes) have been demonstrated naturally-resistant to Fasciola hepatica, in association with an effective encapsulation of the parasite by innate immune cells of the host, the hemocytes. Emphasis is made on the molecular and immunological features characterizing each P. columella phenotype in relation to their anti-parasitic competence, their distinctive ecological patterns and the existence of a significant cost of resistance. An integrative overview of the resistance to F. hepatica through comparative immunobiology, genetics and ecology is presented to hypothesize on the possible origins and evolution of this phenomenon and to postulate significant roles for parasite mediated-selection and environmental factors in shaping and maintaining the resistant phenotype in the field. Lastly, clues into future experimental perspectives to deeply characterize the interplay between P. columella and F. hepatica and the immunobiology of the resistance are also included. The advances revised in the present paper are only beginning to unravel mechanisms of anti-parasite innate defense responses and their evolutionary bases, and can facilitate the development of prospective approaches towards practical applications of P. columella resistance.

Geography and ecology of invasive Pseudosuccinea columella (Gastropoda: Lymnaeidae) and implications in the transmission of Fasciola species (Digenea: Fasciolidae) - a review

Pseudosuccinea columella is considered invasive and has become an important intermediate host of both Fasciola species in many regions of the world. This systematic review assessed the geographical distribution of P. columella, and its implications in the transmission of Fasciola hepatica and Fasciola gigantica, globally. A literature search was conducted on Google Scholar, JSTOR and PubMed databases using Boolean operators in combination with predetermined search terms for thematic analysis. Results show that P. columella has been documented in 22 countries from Europe (3), Africa (8), Oceania (2), North America (3) and South America (6). Furthermore, this snail species has shown to adapt to and inhabit a vast array of freshwater bodies including thermal lakes and ditches with acidic soils. Studies showed that P. columella transmits F. hepatica, with natural and experimental infections documented in sub-Saharan Africa, Europe, South America and North America. Experimental infection studies in Cuba showed the presence of P. columella populations resistant to F. hepatica infection. Furthermore, some populations of this invasive snail collected from F. hepatica endemic locations in Brazil, Venezuela, Australia, South Africa, Colombia and Argentina were found without Fasciola infection. As a result, the role played by this snail in the transmission of Fasciola spp. in these endemic areas is still uncertain. Therefore, further studies to detect natural infections are needed in regions/countries where the snail is deemed invasive to better understand the veterinary and public health importance of this snail species in Fasciola-endemic areas and determine the global dispersion of resistant populations of P. columella.

Natural resistance to Fasciola hepatica (Trematoda) in Pseudosuccinea columella snails: A review from literature and insights from comparative "omic" analyses

The snail Pseudosuccinea columella is one of the main vectors of the medically-important trematode Fasciola hepatica. In Cuba, the existence of natural P. columella populations that are either susceptible or resistant to F. hepatica infection offers a unique snail-parasite for study of parasite-host compatibility and immune function in gastropods. Here, we review all previous literature on this system and present new "omic" data that provide a molecular baseline of both P. columella phenotypes from naïve snails. Comparison of whole snail transcriptomes (RNAseq) and the proteomes of the albumen gland (2D-electrophoresis, MS) revealed that resistant and susceptible strains differed mainly in an enrichment of particular biological processes/functions and a greater abundance of proteins/transcripts associated with immune defense/stress response in resistant snails. These results indicate a differential allocation of molecular resources to self-maintenance and survival in resistant P. columella that may cause enhanced responsiveness to stressors (i.e. F. hepatica infection or tolerance to variations in environmental pH/total water hardness), possibly as trade-off against reproduction and the ecological cost of resistance previously suggested in resistant populations of P. columella.

Patterns of distribution, population genetics and ecological requirements of field-occurring resistant and susceptible Pseudosuccinea columella snails to Fasciola hepatica in Cuba

Pseudosuccinea columella snails transmit the trematode Fasciola hepatica, but in Cuba, six naturally occurring populations successfully resist parasite infection. Here, we present an updated distribution of P. columella in Cuba; 68 positive sites with the earliest records more abundant in west-central Cuba and with east-central populations generally corresponding to the newest samples. No records were found farther east. The IPA site reported 10.5% prevalence of F. hepatica-infected snails. Population genetics, studied through microsatellites, showed low allelic and multilocus genotypic richness (MLGT), mainly in susceptible populations, strong deviations from panmixia and high self-fertilization rates. Susceptible individuals were grouped in one major cluster containing the majority of MLGT, and two independent clusters grouped the MLGT of resistant individuals from western and central populations, respectively. From these, we propose that several introductions of P. columella occurred in Cuba, primarily in the west, with the early arrivals deriving on the resistant populations. A more recent introduction of susceptible P. columella carrying MLGT T and Y may have occurred, where the latter spread quickly through the island and possibly increase the risk of parasite transmission in Cuba since all snails naturally infected with F. hepatica were carriers of the MLGT Y. Interestingly, even though resistant populations are highly diverse and are likely the oldest within Cuba, they are only found in six localities characterized by soft (total hardness, TH = 6.3 ± 1.03°d) and slightly acidic (pH = 6.2 ± 0.12) waters with low richness in snail species (3.2 ± 1.02). This tendency was also observed in a two-year follow-up ecological study that was conducted on a farm where both phenotypes occurred in sympatry; colonization events by resistant over susceptible snails coincided with a reduction in the pH and TH of the water. A comparison of life traits in susceptible and resistant isolates reared at two different pH/TH conditions (5.9/4°d or 7.8/14°d) showed that low pH/TH negatively affects P. columella, irrespective of the phenotype. However, evidence of higher tolerance (higher survival, life expectancy, egg viability) to such conditions was observed in resistant isolates. Finally, we speculate that the limited distribution of resistant populations might be related to a better exploitation of sites that are less suitable to snails (thus, with lower competition), rather than to a differential ecological restriction to specific environmental conditions from susceptible P. columella.

Role of the lymnaeid snailPseudosuccinea columellain the transmission of the liver flukeFasciola hepaticain Egypt

Experimental infections of three EgyptianPseudosuccinea columellapopulations with sympatric miracidia ofFasciolasp., coming from cattle- or sheep-collected eggs, were carried out to determine the capacity of this lymnaeid to support larval development of the parasite. Using microsatellite markers, the isolates of Egyptian miracidia were identified asFasciola hepatica. Apart from being independent of snail origin, prevalences ranging from 60.4 to 75.5% in snails infected with five miracidia ofF. hepaticawere significantly higher than values of 30.4 to 42.2% in snails with bi-miracidial infections. The number of metacercariae ranged from 243 to 472 per cercarial-shedding snail and was independent of snail origin, parasite origin and miracidial dose used for infection. IfP. columellawas subjected to two successive bi-miracidial infections withF. hepatica, prevalence of infection was 63.3%, with a mean of 311 metacercariae per snail. These values were clearly greater than those already reported forRadix natalensisinfected with the same parasite and the same protocol. Successful experimental infection ofP. columellawithF. hepaticasuggests that this lymnaeid snail is an important intermediate host for the transmission of fascioliasis in Egypt.

Fasciola hepaticain Cuba: compatibility of different isolates with two intermediate snail hosts,Galba cubensisandPseudosuccinea columella

In Cuba, only two lymnaeid snails,Galba cubensisandPseudosuccinea columella, with different ecology and distribution patterns, are intermediate hosts forFasciola hepatica. The compatibility of these two species as hosts was analysed through their rates of infection, the production of rediae and survivorship when exposed toF. hepaticamiracidia. Ten populations ofG. cubensis, eight ofP. columellacollected from various habitats and six isolates ofF. hepaticasampled in slaughterhouses from different localities were tested. Our results clearly demonstrate thatG. cubensisis a more compatible host forF. hepaticain Cuba when compared withP. columella. However, the role thatP. columellamay have in fascioliasis transmission under certain conditions should not be disregarded. Variation in infectivity among isolates ofF. hepaticawere also observed and may explain why some regions in Cuba are more commonly subjected to fascioliasis outbreaks.

First report of larval stages ofFasciola hepaticain a wild population ofPseudosuccinea columellafrom Cuba and the Caribbean

A wild population of the lymnaeid snailPseudosuccinea columellainfected by larval stages ofFasciola hepaticawas discovered in the Pinar del Río Province, Cuba. One of 100 snails was infected in a rice culture field. This is the first time this species has been found acting as intermediate host ofF. hepaticaunder natural conditions, not only for Cuba but also for the Caribbean area.