Infection Rates of Fasciola Intermediate Host Snail Species and Their Distribution in Africa: A Systematic Review and Meta-Analysis.

This systematic review and meta-analysis aimed to collate the infection rates of Fasciola spp. in intermediate host snails and their distribution in Africa. The overall infectivity prevalences of Galba truncatula, Radix natalensis, and Pseudosuccinea columella are 52%, 8%, and 3%, respectively. The intermediate host snails native to Africa (R. natalensis and G. truncatula) have been examined more than the invasive P. columella. The studies included in the review ranged from 1999 to 2022. North Africa has the highest prevalence of G. truncatula, with an infection rate of 52%. The review reveals that naturally infected intermediate host snails (G. truncatula, R. natalensis, and P. columella) are found in various regions of Africa. G. truncatula accounts for 22% (from three countries) of the studies included in the review and it was only found in the North African region with the highest overall infection rate of 52%. More studies on infection rate and distribution are needed to effectively control and prevent future transmissions.

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.

Spread of the fascioliasis endemic area assessed by seasonal follow-up of rDNA ITS-2 sequenced lymnaeid populations in Cajamarca, Peru.

Fascioliasis is a worldwide emerging snail-borne zoonotic trematodiasis with a great spreading capacity linked to animal and human movements, climate change, and anthropogenic modifications of freshwater environments. South America is the continent with more human endemic areas caused by Fasciola hepatica, mainly in high altitude areas of Andean regions. The Peruvian Cajamarca area presents the highest human prevalences reported, only lower than those in the Bolivian Altiplano. Sequencing of the complete rDNA ITS-2 allowed for the specific and haplotype classification of lymnaeid snails collected in seasonal field surveys along a transect including 2007-3473 m altitudes. The species Galba truncatula (one haplotype preferentially in higher altitudes) and Pseudosuccinea columella (one haplotype in an isolated population), and the non-transmitting species Lymnaea schirazensis (two haplotypes mainly in lower altitudes) were found. Climatic seasonality proved to influence G. truncatula populations in temporarily dried habitats, whereas L. schirazensis appeared to be more climatologically independent due to its extreme amphibious ecology. Along the southeastern transect from Cajamarca city, G. truncatula and L. schirazensis shared the same site in 7 localities (46.7% of the water collections studied). The detection of G. truncatula in 11 new foci (73.3%), predominantly in northern localities closer to the city, demonstrate that the Cajamarca transmission risk area is markedly wider than previously considered. Lymnaea schirazensis progressively increases its presence when moving away from the city. Results highlight the usefulness of lymnaeid surveys to assess borders of the endemic area and inner distribution of transmission foci. Similar lymnaeid surveys are still in need to be performed in the wide northern and western zones of the Cajamarca city. The coexistence of more than one lymnaeid transmitting species, together with a morphologically indistinguishable non-transmitting species and livestock movements inside the area, conform a complex scenario which poses difficulties for the needed One Health control intervention.

Lymnaeidae from Santander and bordering departments of Colombia: Morphological characterization, molecular identification and natural infection with Fasciola hepatica.

The Lymnaeidae constitute a family of freshwater gastropod molluscs whose diversity and ecology have been infrequently studied throughout Colombia. Some lymnaeid species act as intermediate hosts of trematode parasites, which are of great importance in both the veterinary and medical fields. Among trematode parasites, Fasciola hepatica is best known for being an important parasite of sheep and cattle for decades and causes significant economic losses in these livestock species. The main objective of this work is to identify the various species of lymnaeids that occupy different geographical regions of Santander and its bordering departments within Colombia. This will expand the knowledge of lymnaeid diversity in Colombia and provide further insight into their role in the transmission of F. hepatica. A total of 118 georeferenced sites between 126 m.a.s.l. and 3870 m.a.s.l. were sampled in Santander, Boyacá, Norte de Santander and Cundinamarca, respectively. Lymnaeid snails were identified according to the morphology of their shells and by several characteristics of their reproductive systems. Species identification was confirmed using DNA barcoding. Four lymnaeid species are reported in the study area: the native Galba cousini and three exotic species, Pseudosuccinea columella, G. truncatula and G. schirazensis. The four species were examined for natural infection with F. hepatica. Infected variants of the main snail host, G. cousini, were found in the Onzaga, Encino and Vetas municipalities of Santander, as well as in the Belén municipality of Boyacá. A second species, G. truncatula was also found naturally infected in Mutiscua municipality of Norte de Santander. The two other species, P. columella and G. schirazensis were found free of infection.

A cascade of biological invasions and parasite spillback in man-made Lake Kariba.

Parasite spillback, the infection of a non-indigenous organism by a native parasite, is a highly important although understudied component of ecological invasion dynamics. Here, through the first analysis of the parasite fauna of lymnaeid gastropods of Lake Kariba (Zimbabwe). We illustrate how the creation of an artificial lake may lead to a cascade of biological invasions in which an invasive aquatic plant promotes the proliferation of invasive gastropods, which in turn alters the epidemiology of trematodiases of potential medical and veterinary importance. Using a new multiplex Rapid Diagnostic PCR assay, we assessed the prevalence of Fasciola sp. infections in the gastropod populations. Both gastropod hosts and trematode parasites were identified using DNA barcoding. We provide the first record of the invasive North-American gastropod Pseudosuccinea columella in Lake Kariba. This species was found at 14 out of 16 sampled sites and its abundance was strongly positively correlated with the abundance of the invasive South-American water hyacinth (Eichhornia crassipes). About 65% of the P. columella specimens analysed were infected with a hitherto unknown Fasciola species. Phylogenetic analyses indicate close affinity to Fasciola hepatica and F. gigantica, which cause fasciolosis, an important liver disease affecting both ruminants and humans. In addition, another non-native Lymnaeid species was found: a Radix sp. that clustered closely with a Vietnamese Radix species. Radix sp. hosted both amphistome and Fasciola trematodes. By linking an invasion cascade and parasite spillback, this study shows how both processes can act in combination to lead to potentially important epidemiological changes.

Molecular detection of natural infection of Lymnaea (Pseudosuccinea) columella (Gastropoda: Lymnaeidae) with Fasciola gigantica (Digenea: Fasciolidae) from two provinces of South Africa.

The main intermediate host of Fasciola gigantica in sub-Saharan Africa is Lymnaea (Radix) natalensis. Lymnaea (Pseudosuccinea) columella is capable of transmitting both F. gigantica and F. hepatica and has been reported to be present in South Africa. To date, no natural infection with F. gigantica has been reported despite the wide distribution of the snail. The aim of this study was to confirm whether L. (P.) columella was transmitting F. gigantica and/or F. hepatica in selected locations of KwaZulu-Natal and Eastern Cape provinces of South Africa. Lymnaea (Pseudosuccinea) columella snails were collected from two locations in two provinces of South Africa and screened for cercariae shedding. This was followed by humanely sacrificing the screened snails, and whole tissue of each individual snail was homogenized and amplified using primers designed to amplify the ITS-1 region of Fasciola spp. No cercariae were shed from the screened snails and molecular analysis showed that snails from the two locations were infected with F. gigantica. This study confirms natural infection of L. (P.) columella with F. gigantica in South Africa, where F. gigantica and F. hepatica have already been reported to coexist. Although L. (P.) columella is able to transmit the two species, surprisingly no infection with F. hepatica was detected from the screened snails. The natural intermediate host of F. gigantica in southern Africa, including South Africa, is Lymnaea (Radix) natalensis and comparative studies are needed to determine the competence of the two snail species in the transmission of F. gigantica.

Dynamics of haemocytes from Pseudosuccinea columella circulating infected by Fasciola hepatica / Dinâmica de hemócitos circulantes de Pseudosuccinea columella infectados por Fasciola hepatica

Abstract The lymnaeids are important in the epidemiology of Fasciola hepatica, a neglected and endemic zoonosis. The interaction between the internal defense system of Pseudosuccinea columella and F. hepatica has been little studied. In the present study the effect of infection by F. hepatica on P. columella circulating haemocytes was investigated. Changes in the average number of total circulating haemocytes have been observed at 30 minutes post-infection and 1, 7, 10, 14, 21, 28 and 50 days post-infection (dpi). Miracidia were observed head-foot and mantle at 30 minutes post-infection. Miracidia/Sporocysts in the mantle skirt 1 dpi, and fully formed sporocysts were observed in the head-foot at 7 dpi. Rediae became evident at 10 dpi and were located between the haemocoel and the muscles from 14 dpi; 50 dpi, the rediae in the digestive gland contained cercariae. The statistical analysis of the total haemocytes of P. columella infected by F. hepatica showed significant differences on the 30 minutes post-infection and 1, 14, 21, and 28 dpi in comparison to uninfected molluscs (0 dpi). Therefore, the interference observed on the internal defence system of P. columella may have direct association with the development of F. hepatica.

Resumo Os limnaeideos são importantes na epidemiologia de Fasciola hepatica, uma zoonose negligenciada e endêmica. A interação entre o sistema interno de defesa de Pseudosuccinea columella e F. hepatica tem sido pouco estudada. No presente estudo, investigou-se o efeito da infecção por F. hepatica nos hemócitos circulantes de P. columella. Alterações no número médio de hemócitos circulantes foram observadas aos 30 minutos e 1, 7, 10, 14, 21, 28 e 50 dias após a infecção (dpi). Miracídios foram observados na região cefalopodal e manto aos 30 minutos após a infecção. Miracídio/esporocistos foram observados no colar do manto ao 1 dpi, e esporocistos totalmente formados na região cefalopodal aos 7 dpi. Rédias tornam-se evidentes aos 10 dpi entre a hemocele e músculos a partir de 14 dpi; e rédias com cercárias próximas a glândula digestiva aos 50 dpi. A análise estatística dos hemócitos totais de P. columella infectados por F. hepatica demonstrou diferenças significativas nos 30 minutos pós-infecção e 1, 14, 21 e 28 dpi em comparação aos moluscos não infectados (0 dpi). Portanto, a interferência observada no sistema de defesa interna de P. columella pode ter associação direta com o desenvolvimento de F. hepatica.

Self-fertilization, long-distance flash invasion and biogeography shape the population structure of Pseudosuccinea columella at the worldwide scale.

Population genetic studies are efficient for inferring the invasion history based on a comparison of native and invasive populations, especially when conducted at species scale. An expected outcome in invasive populations is variability loss, and this is especially true in self-fertilizing species. We here focus on the self-fertilizing Pseudosuccinea columella, an invasive hermaphroditic freshwater snail that has greatly expanded its geographic distribution and that acts as intermediate host of Fasciola hepatica, the causative agent of human and veterinary fasciolosis. We evaluated the distribution of genetic diversity at the largest geographic scale analysed to date in this species by surveying 80 populations collected during 16 years from 14 countries, using eight nuclear microsatellites and two mitochondrial genes. As expected, populations from North America, the putative origin area, were strongly structured by selfing and history and harboured much more genetic variability than invasive populations. We found high selfing rates (when it was possible to infer it), none-to-low genetic variability and strong population structure in most invasive populations. Strikingly, we found a unique genotype/haplotype in populations from eight invaded regions sampled all over the world. Moreover, snail populations resistant to infection by the parasite are genetically distinct from susceptible populations. Our results are compatible with repeated introductions in South America and flash worldwide invasion by this unique genotype/haplotype. Our study illustrates the population genetic consequences of biological invasion in a highly selfing species at very large geographic scale. We discuss how such a large-scale flash invasion may affect the spread of fasciolosis.

Effect of Fascioloides magna (Digenea) on fecundity, shell height, and survival rate of Pseudosuccinea columella (Lymnaeidae).

Infection with Fascioloides magna (Digenea) causes serious damage to liver tissue in definitive hosts represented by ruminants, especially cervids. The distribution of F. magna includes the indigenous areas in North America, and the areas to which F. magna was introduced-Central Europe, Southeast Europe, and Italy. The North American intermediate host of F. magna, the freshwater snail Pseudosuccinea columella (Lymnaeidae), is an invasive species recorded in South America, the Caribbean, Africa, Australia, and west and Southeast Europe. In Europe, Galba truncatula is the snail serving for transmission, but P. columella has potential to become here a new intermediate host of F. magna. Little is known about interactions between F. magna and P. columella. In this study, the susceptibility of P. columella (Oregon, USA) to the infection by a single miracidium of the Czech strain of F. magna and the influence of F. magna on snail fecundity, shell height, and survival were evaluated. The data show that the Oregon strain of P. columella is a highly suitable host for the Czech strain of F. magna, with the infection rate of 74 %. In addition, a negative effect on survival rate of infected snails was recorded only in the late phase of infection. The infection was accompanied by a major reduction in egg mass production and by a decrease in the number of eggs per egg mass. The shell height of infected snails did not significantly differ from that in unexposed controls.

The effect of spirulina (Arthrospira platensis) (Oscillatoriales: Cyanobacteria) on the experimental breeding of Pseudosuccinea columella (Basommatophora: Lymnaeidae) / Efecto de la espirulina (Arthrospira platensis) (Oscillatoriales: Cyanobacteria) en la cría experimental de Pseudosuccinea columella (Basommatophora: Lymnaeidae)

Snails of the family Lymnaeidae, as Pseudosuccinea columella, are the intermediate hosts of Fasciola hepatica, the causative agent of fasciolosis in human and livestock all over the world. A thorough knowledge of snail biology is essential for describing the transmission dynamics and for controlling this disease. Since food quality has had a significant effect on snail growth, fecundity and fertility, in this study we evaluated the use of spirulina (Arthrospira platensis) as a food resource for the artificial breeding of P. columella, an invasive snail and the main intermediate host of F. hepatica in Northeastern Argentina. The main purpose was to measure the effect of spirulina on fitness parameters such as survival rate, growth rate, size at first reproduction, lifetime fecundity and viable offspring. A total of 20 676 newly-laid F2 eggs were used; half of them were fed with lettuce (treatment L) and the other half with lettuce plus spirulina (treatment L+S). In comparison with P. columella snails fed only with lettuce, we found that P. columella fed with lettuce plus spirulina: 1) showed higher survival rates, 2) grew faster and showed higher growth increments, 3) attained sexual maturity earlier in time (L+S:60 days vs. L:120 days) and at a smaller size (L+S:4.8mm vs. L:8.2mm), 4) had a longer reproductive period (L+S:150 days vs. L:90 days), 5) produced a higher number of eggs/snail (L+S:29.6 vs. L:13.3), and 6) showed a higher offspring hatching rate (L+S:70% vs. L:40%). The supplementation of P. columella diet with commercial spirulina enhances it fitness and improved the artificial breeding of this species. Spirulina may have a direct positive effect on P. columella development by consuming it, along with an indirect positive effect by improving the water quality. This rearing technique provided large number of reproducing adults and a continuous production of offspring, which are essential for developing future experimental studies in order to improve our knowledge on P. columella biology.

Los caracoles de la familia Lymneidae, como Pseudosuccinea columella, actúan como hospedadores intermediarios de Fasciola hepatica, el agente etiológico de la fasciolosis, zoonosis que afecta al ganado y al hombre en todo el mundo. Conocer profundamente las características biológicas de estos caracoles resulta esencial para describir la dinámica de transmisión y controlar esta parasitosis. La calidad del alimento afecta significativamente el crecimiento, la fecundidad y la fertilidad de estos caracoles. En este estudio, evaluamos la utilización de la espirulina (Arthrospira platensis) como fuente de alimento para la cría artificial de P. columella, una especie invasora que actúa como el principal hospedero intermediario de F. hepatica en el Noreste Argentino. El objetivo principal de este trabajo fue medir el efecto de la espirulina en parámetros del fitness tales como: tasa de supervivencia, tasa de crecimiento, tamaño que alcanzan a la madurez sexual, duración del período fértil, fecundidad y viabilidad de la descendencia. Se utilizaron 20 767 huevos F2 recién puestos; la mitad de ellos fue alimentado con lechuga (tratamiento L) y la otra mitad con lechuga mas espirulina (tratamiento L+S). En comparación con las P. columella alimentadas solamente con lechuga, las P. columella alimentadas con lechuga mas espirulina: 1) presentaron mayores tasas de supervivencia, 2) alcanzaron mayores tamaños y en menor tiempo, 3) alcanzaron la madurez sexual antes de tiempo (L+S:60 días vs. L:120 días) y a menor tamaño (L+S:4.8mm vs. L:8.2mm), 4) tuvieron un período reproductivo más largo (L+S:150 días vs. L:90 días), 5) produjeron mayor cantidad de huevos/caracol (L+S:29.6 vs. L:13.3) y 6) su descendencia tuvo una mayor tasa de eclosión (L+S:70% vs. L:40%). La incorporación de espirulina como suplemento alimenticio mejoró significativamente los parámetros biológicos de P. columella y maximizó la cría artificial de esta especie en laboratorio. La espirulina podría tener un efecto positivo directo sobre el desarrollo de P. columella mediante su consumo, pero también podría tener un efecto positivo indirecto al mejorar la calidad del agua. Con la técnica de cría desarrollada en este trabajo se obtiene una gran cantidad de caracoles adultos reproductores y una continua producción de huevos que son esenciales para desarrollar futuros estudios que permitan aumentar nuestro conocimiento sobre la biología de P. columella.

Distribución y preferencia de hábitats de moluscos hospederos intermediarios de Fasciola hepatica en Cuba / Distribution and preferred habitats of intermediary host molluscs of Fasciola hepatica in Cuba

INTRODUCCIÓN: se realizó un estudio sobre la ecología de las 2 especies de moluscos lymnaeidos de Cuba. Ambas especies son capaces de servir como hospederos intermediarios de Fasciola hepatica y existen en la mayoría de los ecosistemas de humedales. OBJETIVO: determinar la distribución y la preferencias de hábitat de Fossaria cubensis y Pseudosuccinea columella. MÉTODOS: se analizaron 370 localidades donde existen poblaciones de moluscos fluviales y se asoció la abundancia relativa de cada especie con la diversidad y el grado de antropización en cada sitio. RESULTADOS: se observó que F. cubensis es la especie mejor representada con poblaciones distribuidas por todo el Archipiélago, mientras P. columella solo se extiende desde Pinar del Río hasta Camagüey. Los tipos de hábitat preferidos por ambas especies variaron significativamente. F. cubensis predominó en hábitats antropizados (p= 0,024, M-W U test) y P. columella existe de manera equitativa en sitios naturales y transformados (p= 0,08, M-W U test). Los resultados alcanzados con respecto a la diversidad muestran que F. cubensis es más común en sitios de baja diversidad correspondientes a grados de antropización elevados, todo lo contrario a lo que ocurre con P. columella, quien existe en sitios más diversos. CONCLUSIONES: los datos sobre la distribución y ecología de estas especies refuerzan el hecho de que F. cubensis se comporte como hospedero principal de F. hepatica en Cuba (mayor distribución y hábitats antropizados). Se discuten estrategias de control sobre estas poblaciones(AU)

INTRODUCTION: an ecological study was carried out on the lymnaeids species of Cuba. Both species are capable of serving as intermediary hosts of Fasciola hepatica and occur in most of the wetland ecosystems. OBJECTIVE: to determine distribution and habitat preferences of Fossaria cubensis and Pseudosuccinea columella. METHODS: a total of 370 locations where freshwater snails exist were analysed. Relative species abundance was associated with species diversity and antropic degree for each site. RESULTS: F. cubensis was the best distributed species with populations throughout the island whereas P. columella populations extend from Pinar del Río to Camagüey. The preferred habitats by these species substantially varied, with F. cubensis predominating in transformed habitats (p= 0.024, M-W U test) and P. columella existing in natural and transformed habitats evenly (p= 0.08, M-W U test). The results regarding diversity showed that F. cubensis is more common in sites of low diversity related to higher levels of human activity. Oppositely, P. columella is found in much more diverse ecosystems. CONCLUSIONS: data over the distribution and ecology of these species supports the fact that F. cubensis acts as the main intermediary host of F. hepatica in Cuba (extensive distribution and transformed habitats). Control techniques against these populations were discussed(AU)

Distribución y preferencia de hábitats de moluscos hospederos intermediarios de Fasciola hepatica en Cuba / Distribution and preferred habitats of intermediary host molluscs of Fasciola hepatica in Cuba

INTRODUCCIÓN: se realizó un estudio sobre la ecología de las 2 especies de moluscos lymnaeidos de Cuba. Ambas especies son capaces de servir como hospederos intermediarios de Fasciola hepatica y existen en la mayoría de los ecosistemas de humedales. OBJETIVO: determinar la distribución y la preferencias de hábitat de Fossaria cubensis y Pseudosuccinea columella. MÉTODOS: se analizaron 370 localidades donde existen poblaciones de moluscos fluviales y se asoció la abundancia relativa de cada especie con la diversidad y el grado de antropización en cada sitio. RESULTADOS: se observó que F. cubensis es la especie mejor representada con poblaciones distribuidas por todo el Archipiélago, mientras P. columella solo se extiende desde Pinar del Río hasta Camagüey. Los tipos de hábitat preferidos por ambas especies variaron significativamente. F. cubensis predominó en hábitats antropizados (p= 0,024, M-W U test) y P. columella existe de manera equitativa en sitios naturales y transformados (p= 0,08, M-W U test). Los resultados alcanzados con respecto a la diversidad muestran que F. cubensis es más común en sitios de baja diversidad correspondientes a grados de antropización elevados, todo lo contrario a lo que ocurre con P. columella, quien existe en sitios más diversos. CONCLUSIONES: los datos sobre la distribución y ecología de estas especies refuerzan el hecho de que F. cubensis se comporte como hospedero principal de F. hepatica en Cuba (mayor distribución y hábitats antropizados). Se discuten estrategias de control sobre estas poblaciones.

INTRODUCTION: an ecological study was carried out on the lymnaeids species of Cuba. Both species are capable of serving as intermediary hosts of Fasciola hepatica and occur in most of the wetland ecosystems. OBJECTIVE: to determine distribution and habitat preferences of Fossaria cubensis and Pseudosuccinea columella. METHODS: a total of 370 locations where freshwater snails exist were analysed. Relative species abundance was associated with species diversity and antropic degree for each site. RESULTS: F. cubensis was the best distributed species with populations throughout the island whereas P. columella populations extend from Pinar del Río to Camagüey. The preferred habitats by these species substantially varied, with F. cubensis predominating in transformed habitats (p= 0.024, M-W U test) and P. columella existing in natural and transformed habitats evenly (p= 0.08, M-W U test). The results regarding diversity showed that F. cubensis is more common in sites of low diversity related to higher levels of human activity. Oppositely, P. columella is found in much more diverse ecosystems. CONCLUSIONS: data over the distribution and ecology of these species supports the fact that F. cubensis acts as the main intermediary host of F. hepatica in Cuba (extensive distribution and transformed habitats). Control techniques against these populations were discussed.

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 / s. t

The abundance of freshwater snails in two rural sites of Pinar del Río, Cuba, which harbor Pseudosuccinea columella susceptible and resistant to miracidia of Fasciola hepatica was followed for one year. Susceptible snails were found in the most anthropic site (IPA) whereas the resistant population inhabited the most preserved one (El Azufre). Only two snail species coexisted with P columella at IPA site (Physa cubensis and Tarebia granifera) while five species were found at El Azufre, including an endemic from that province (Hemisinus cubanianus). Populations of both resistant and susceptible snails showed stable densities throughout the year, although the susceptible strain attained higher abundance. The highest densities were observed in April-May 2004 for the susceptible population whereas the resistant strain attained its highest abundance in January 2004. No record of Fossaria cubensis was made and the thiarid T granifera occurred only at low densities. One of the sampled sites (IPA) meets all the conditions for the first report of P. columella naturally infected with larvae of F hepatica(AU)