Lymnaea cousinni (Gastropoda: Lymnaeidae) as transmitter of fascioliasis.

In the article entitled “First report of Lymnaea cousini Jousseaume, 1887 naturally infected with Fasciola hepatica (Linnaeus, 1758) (Trematoda: Digenea) in Machachi, Ecuador” recently published by Angel Villavicencio A and Mauricio Carvalho de Vasconcellos in Memorias do Instituto Oswaldo Cruz (vol. 100, issue 7, pages 735-737, November 2005), it is stated that their finding in Ecuador represents the first report of specimens of this lymnaeid species naturally infected by the liver fluke. However, it is well known from long ago that this species acts as intermediate host of fascioliasis in Andean countries. In the first half of the last century, Brumpt et al. (1939-1940) already demonstrated that L. bogotensis Pilsbry, 1935, a synonym of L. cousini proposed and established by Hubendick (1951) and always accepted (see Pointier et al. 2004), even also recognized by Villavicencio and Carvalho de Vasconcellos (2005) in their paper here in question, is the intermediate host of Fasciola hepatica in the surroundings of Santa Fé de Bogotá, Colombia. Materials studied by Villavicencio and Carvalho de Vasconcellos (2005) were from Machachi, in the Andean region of Ecuador. These authors further note that fascioliasis prevalence in humans in the Andean region of Ecuador ranges from 24 to 53% and add the reference of the Servicio Ecuatoriano de Sanidad Animal (SESA 2003) concerning this information. However, these very high human prevalences do not fit with results obtained in surveys carried out up to the present in Andean communities of Ecuador, in which human prevalences always appear to be low or very low: 6% by serology (Trueba et al. 2000); 0.5% by coprology (Gozalbo et al. 2004). Authors additionaly note that, according to WHO (1995), almost 200,000 people in Ecuador are infected, but it shall be clarified that these WHO data were only estimations available at the beginning of the 90s decade. Villavicencio and Carvalho de Vasconcellos (2005) emphasize the high prevalence of infection of 31.43% by F. hepatica they detected in L. cousini snails as the highest value ever reported for lymnaeid snails naturally infected with this parasite. However, although this prevalence is evidently high when compared to current prevalences by fasciolids in lymnaeid snails, which are usually less than 5%, F. hepatica prevalences in snails similar and even higher than that reported by Villavicencio and Carvalho de Vasconcellos (2005) have been found elsewhere. In fascioliasis human endemic areas of Andean countries, for instance, a confirmed 31.6% prevalence by F. hepatica in Galba truncatula was detected in the center of the Northern Bolivian Altiplano village of Tambillo (Bargues et al. 1995), that is, even in a human settlemen instead of in a field place inhabited by livestock. The fasciolid prevalence reported by Villavicencio and Carvalho de Vasconcellos (2005) was found in 70 L. cousini snail specimens collected in a 4.5-m2 area they covered in a private farm of Machachi. Unfortunately, nothing is mentioned about the crowding of the cattle nor about the seasonal managament of cows and fascioliasis prevalences of cattle in that farm. Information on all these aspects is crucial to explain the probable concentration of higly infected cattle for water drinking in the water body these authors studied, as to be able to deduce whether this high prevalence is only the result of a peculiar private farm situation in a given moment of the year or it may be more largely extrapolated, as to evaluate the real epidemiological significance of L. cousini in fascioliasis transmission in Ecuador. There is finally also concern about the acceptance of this prevalence as really and only belonging to F. hepatica. The doubts appear after analyzing the results of Brumpt et al. (1939-1940), who stated that the lymnaeid species here in question appears to be a mollusc of low parasitic efficacy for F. hepatica, owing to its very low fasciolid infection percentages, and that this lymnaeid additionally shows to develop the role of intermediate host for other trematode species. Villavicencio and Carvalho de Vasconcellos (2005) do not mention how they microscopically classified the fluke larval stages and the only colour photograph included in the paper showing a redia and a cercaria does not allow to ascertain their specific ascription. It is well known that F. hepatica larval stages are very similar to those of other digenean species (i.e., bird trematodes). That is why the classification of fasciolid larval stages found in nature always needs a verification, usually made by experimental infection of a definitive host (whether a laboratory model or a domestic herbivore livestock species) and subsequent species classification by microscopical study of mounted trematode adult specimens. Another wellknown way is by studying the cercarial chaetotaxy (Richard 1971), although nowadays molecular tools as rDNA ITS-2 and ITS-1 sequences are also available for direct application to the larval stages (Mas-Coma et al. 2001).