Chronobiology of trematode cercarial emergence: from data recovery to epidemiological, ecological and evolutionary implications

One major challenge for parasites with complex cycles consists to succeed in the transmission from one host to the next host. To maximize the probability of encountering the right host, numerous trematode species have selected various emergence rhythms occurring during the escape of the short-lived cercariae from the mollusc host. Cercarial shedding patterns are beautiful examples of adaptation of the parasite for a successful rendezvous with its subsequent host. In this review, after an analysis of the technical and statistical aspects specific to such studies, we compile the knowledge and unresolved issues we have about the synchronization of these rhythms, their genetic support and the role of the host physiology or activity. We are also interested on how cercarial rhythmicity influences cercarial densities in waters of transmission sites and then the risk of host infection in case of schistosomiasis. Ecological significance of the inter- and intra-specific diversity of these rhythms is emphasized as well as the evolutionary implication of new chronotypes resulting from the capture of new host species and promoting reproductive isolation and alloxenic speciation. Currently, genome sequence data now available for some trematodes such as the schistosomes provide an unprecedented resource for new research approaches that should contribute identification of the genes and mechanisms involved in determining the cercarial shedding rhythms observed.

Biological control of the snail hosts of Schistosoma mansoni in the Caribbean area using Thiara spp

Field observations and experiments using thiarid snails as competitors of Biomphalaria spp., potential intermediate hosts of Schistosoma mansoni in the Caribbean area, are reviewed. The parthenogenetic snails, Thiara granifera and T. (= Melanoides) tuberculata, were introduced to the Neotropical area in recent decades. In numerous islands and countries, these oriental species have demonstrated their capacity to colonize rapidly and densely many types of habitats while at the same time reducing and even eliminating populations of Biomphalaria spp. The results of field experiments, carried out in several Caribbean islands, have shown the efficiency as well as the limitations of T. tuberculata as a competitor of B. glabrata and B. straminea. In St. Lucia, B. glabrata was apparently eliminated from marshes and streams, 6 to 22 months after the introduction of the competitor. In Martinique, T. tuberculata was introduced into two groups of water-cress beds which constituted the last transmission sites of schistosomiasis on the island. In just less than three years after the introduction of the competitor, both B. glabrata and B. straminea have been eliminated from the transmission sites. In Guadeloupe, several introductions have been carried out in different types of habitat such as permanent ponds, canals, streams and temporary marshes. The findings of all field experiments have indicated that thiarid snails as competitors of pulmonates are favoured by the presence of permanent and stable habitats, preferably shallow, with emergent plants and well oxygenated. On the other hand, the competitor snails are at a disadvantage in waterbodies which are temporary, extremely deep, poorly oxygenated or with a dense mat of floating aquatic vegetation.(ABSTRACT TRUNCATED AT 250 WORDS)

Decline of a sylvatic focus of Schistosoma mansoni in Guadeloupe (French West Indies) following the competitive displacement of the snail host Biomphalaria glabrata by Ampullaria glauca

Grand Etang lake in Guadeloupe is a focus of intestinal schistosomiasis characterized by the virtual absence of human contact and the presence of a heavily infested rat population. The lake, with a surface area of 0.03 km², is situated in rain forest at an altitude of 450 m. Originally it contained an extensive belt of the floating plant Pistia stratiotes which provided a favourable habitat for Biomphalaria glabrata, the intermediate host snail of Schistosoma mansoni. In 1974 and 1975, a low prevalence (about 3%) of S. mansoni was observed in B. glabrata while a high prevalence (about 60%) was recorded in rats. The rat population was infected with another trematode, Ribeiroia guadeloupensis, which requires two intermediate hosts, B. glabrata and the fish Tilapia mossambica. In 1976, the prosobranch snail, Ampullaria glauca, was accidentally introduced into the lake. By 1980, this snail had spread over half of the lake and had caused marked regression of the P. stratiotes layer. Concomitantly, the population of B. glabrata declined to such a point that in 1985 only a small colony remained in the northern section of the lake. The consequences of these changes are reflected in a rapid decline of S. mansoni in the rat population from 61.3% in 1980 to 34.8% in 1981, 6.5% in 1982, 2.8% in 1983, 11.6% in 1984, and 0% in 1985 and 1986.

Estratégias e métodos de controle da esquistossomose

Após considerar algumas situações epidemiológicas e as possibilidades de controle,diferentes programas e métodos foram passados em revista e discutidos. O saneamento do meio, o controle de moluscos, o tratamento da população humana e a educação sanitária foram analisados como partes de projetos integrados, em áreas infectadas por Schistosoma mansoni ou haematobium. De acordo com os objetivos a serem adotados pelas autoridades do País, são descritas quatro alternativas estratégicas, relacionadas com as condições ecológicas, que utilizam em essência: 1) controle malacológico, seguido de tratamento das pessoas parasitadas, e repetido sempre que necessário; 2) controle malacológico e tratamento de massa, se a prevalência for muito elevada, seguido de busca periódica e tratamento dos casos; 3) tratamentos periódicos, nas épocas de baixa ou nula transmissão; 4) erradicação dos vetores (onde for possível) seguida de tratamento dos casos e vigilância epidemiológica dos focos. Dá-se grande ênfase à decisão e ao empenho governamental no controle, à participação da comunidade e à avaliação periódica dos projetos.

Water contact behavior and schistosomiasis in an upper Egyptian village

Preliminary findings of a study of the role of microspatial behavior and local ecological circumstances in the persistence of Schistosoma haematobium infection in El Ayaisha village, Upper Egypt, are presented. Twelve types of water contact activities were studied in three cohorts of 274,324 and 392 male students aged 5-16 during a 2-year period. Swimming and playing resulted in more frequent and intensive contact with potentially infective water and in more pollution of snail habitats with schistosome eggs than any other type of activity. Irrigation was probably not a major cause of S. haematobium infection in schoolboys and drinking water and fishing carried the smallest risk for this age group. Mapping and discriminate analysis of infection and transmission indicators revealed spatial associations between water contact, schistosome transmission and infection among the various age groups. The highest egg excretion rates in individuals and the highest prevalence rates were noted in the neighborhoods nearest to the canals. Water contact and contamination of water by the heavily infected school age boys is seasonally concentrated during the summer, when intermediate host snails and infective cercariae are also most common in the water. Frequency, type and duration of water contact change with age together with types of water bodies used, causing corresponding changes in exposure risk. Nevertheless, other factors, including acquired immunity, preexisting infections, chemotherapy and the effect of the study on the normal water contact behavior of the study population must also be evaluated. Contact with the Nile and the canals by the general nonschool population of El Ayaisha was studied to verify the observations among the schoolboys and to study socially linked activities and the transmission potential of the various transmission sites. Results are evaluated in regard to the feasibility of various schistosomiasis control measures in El Ayaisha.