Modeling the Dynamics and Control of Transmission of Schistosoma japonicum and S. mekongi in Southeast Asia

A mathematical model for transmission of schistosomes is useful to predict effects of various control measures on suppression of these parasites. This review focuses on epidemiological and environmental factors in Schistosoma japonicum and Schistosoma mekongi infections and recent advances in mathematical models of Schistosoma transmission.

Schistosome/mollusk: genetic compatibility

Schistosomiasis remains one of the most prevalent parasitic infections and has significant economic and public health consequences in many developing countries. Economic development and improvement in standard of living in these countries are dependent on the elimination of this odious disease. For the control of Schistosomiasis, understanding the host/parasite association is important, since the host parasite relationship is often complex and since questions remain concerning the susceptibility of snails to infection by respective trematodes and their specificity and suitability as hosts for continued parasite development. Thus, the long term aim of this research is to learn more about the genetic basis of the snail/parasite relationship with the hope of finding novel ways to disrupt the transmission of this disease. In the current research, genetic variability among susceptible and resistant strains within and between Biomphalaria glabrata and B. tenagophila was investigated using RAPD-PCR. The results indicate great genetic variations within the two snail species using three different primers (intrapopulational variations), while specimens from the same snail species showed few individual differences between the susceptible and resistant strains (interpopulational variation).

Recent studies on Schistosoma intercalatum: taxonomic status, puzzling distribution and transmission foci revisited

Schistosoma intercalatum, which causes human rectal schistosomiasis in Africa, still presents a great interest for its imprecise taxonomic status and its puzzling distribution in Africa. Two geographically isolated strains of S. intercalatum are recognized, the Lower Guinea strain and the Congo strain, which differ from each other in a number of morphological, biological and biochemical characteristics. Recent molecular data using RAPD markers indicate high divergence between the two strains, with values of Nei and Li's similarity indice allowing recognition of two genetically distinct taxa: experiments on pre- and post-isolating mechanisms are in progress in order to re-evaluate the taxonomic status of this polytypic species. With regard to its geographical distribution, S. intercalatum is characterized by the existence of two stable endemic areas (localized in Lower Guinea and North East of Democratic Republic of Congo) which correspond to the historical areas of species discovery, and the emergence during the last 15 years of new foci of the Lower Guinea strain outside previously known endemic areas. The absence of local adaptation of the Lower Guinea strain to its intermediate host, supported by experimental studies, may help to facilitate the spread of this strain. Nevertheless, the present restricted distribution of this species remains puzzling, because its potential snail hosts (bulinids) are widely distributed throughout much of Africa. Recent experimental and epidemiological studies suggest that interspecific sexual interactions between human schistosomes could have a role in limiting the distribution of S. intercalatum: the competitive sexual processes acting among human schistosomes show that S. haematobium and S. mansoni are always competitively dominant over S. intercalatum. These epidemiological observations lead the authors to distinguish three kinds of transmission foci for S. intercalatum

A schistosomiasis transmission model incorporating concomitant immunity

Este trabalho pretende ser uma contribuiçäo ao estudo dos mecanismos de regulamentaçäo da esquistossomose. Propöe-se uma modificaçäo nos modelos básicos desenvolvidos por McDonald e outros que leva em conta imunidade concomitante como mecanismo de regulaçäo. Portanto, enquanto em outros modelos básicos a regulaçäo se dava exclusivamente no hospedeiro intermediário o modelo aqui apresentado permite incluir a regulaçäo no hospideiro definitivo. Conclui-se que a regulaçäo no hospedeiro definitivo permite um ajuste muito melhor aos dados experimentais

Differentation of the fat body cells in the precocius adults of Schistocerca gregaria: regulation of polyploidy by juvenile hormone

Precocious adults from 2nd and 3rd instar larvae of the desert locust Schistocerca gregaria were used to assess the competence of their fat body to synthesize DNA in response to a juvenile hormone analog (JHA), hydoprene. Autoradiographic studies show that JHA stimulates DNA synthesis since a significant proportion of the fat body nuclei are labelled after treatment with 100 or 200 µg of JHA. Maximum DNA synthesis occurs 24 h after treatment with 100 µg of JHA. The nuclear ploidy classes of the precocious adults from 3rd larvae are similar to those of 1-d-old normal adults, but treatemnt of these precociuos adults with µg of JHA doubles the DNA content resulting in enhanced ploidy classes which resemble those of 10-d-old normal females. In the precocious adults that emerged from 2nd instar larvae the ploidy classes are higher than those of 1-d-old normal adults, and treatment of these precocious adults with JHA results in a further increase in the DNA content of the fat body nuclei leading to the formation of high percentages of 16C and 32C nuclei. The results of these studies suggest that any model on the mode of action of JH should recognize this phenomenon of JH-induced polyploidization in the fat body nuclei.

Immunological aspects of host-schistosome relationships

The complex immunological relationships between schistosomes and their vertebrate hosts are considered to be conveniently divisible into four distinct, though interrelated categories: the parasite's vulnerability to, its evasion of, and its exploitation of the host's immune response, and its stimulation of the host's immune response to produce immunopathology. Some significant recent advances in the first three categories are discussed, as well as their relationships to the fourth category of immunopathology.

Cell-matrix interactions in Schistosomal portal fibrosis: a dynamic event

In recent years, one of the most significant progress in the understanding of liver diseases was the demonstration that liver fibrosis is a dynamic process resulting from a balance between synthesis and degradation of several matrix components, collagen in particular. Thus, fibrosis has been found to be a very early event during liver diseases, be it of toxic, viral or parasitic origin, and to be spontaneously reversible, either partially or totally. In liver fibrosis cell matrix interactions are dependent on the existence of the many factors (sometimes acting in combination) which produce the same events at the cellular and molecular levels. These events are: (i) the recruitment of fiber-producing cells, (ii) their proliferation, (iii) the secretion of matrix constituents of the extracellular matrix, and (iv) the remodeling and degradation of the newly formed matrix. All these events represent, at least in principle, a target for a therapeutic intervention aimed at influencing the experimentally induced hepatic fibrosis. In this context, hepatosplenic schistosomiasis is of particular interest, being an immune cell-mediated granulomatous disease and a model of liver fibrosis allowing extensive studies in human and animals as well as providing original in vitro models.

Study on Schistosoma sinensium in Fang district, Chiangmai province, Thailand.

A survey on human infection and possible natural definitive host of Schistosoma sinensium was carried out in Fang District, Chiangmai Province, North Thailand, where Tricula bollingi snails which harbour cercariae of S. sinensium inhabits. Stool examination of the people in the two villages along the stream, where T. bollingi were found, was by formalin-ether concentration technique and by Stoll's method. The stools were found to be negative for S. sinensium eggs. Field rats were also trapped and examined for the presence of S. sinensium. Adult worms and eggs of S. sinensium were found in the mesenteric veins and livers, respectively, of the field rats, Rattus rattus, captured in the rice fields along the stream.

Susceptibility of snail vectors to Oriental anthropophilic Schistosoma.

Laboratory experiments were carried out to study the susceptibility of snail vectors to Oriental anthropophilic Schistosoma. Oncomelania hupensis hupensis was readily infected with the local strain of Schistosoma japonicum (Chinese strain), and also infected with S. japonicum (Philippines strain). O.h. quadrasi was only susceptible to its S. japonicum (Philippines strain). The Oncomelania races were refractory to S. mekongi, S. japonicum-like species (Malaysian strain). Tricula aperta (beta race) was readily infected with S. mekongi, S. sinensium and S. japonicum-like species from Malaysia, but not S. japonicum. T. bollingi was susceptible to S. sinensium and S. mekongi. Robertsiella kaporensis was only susceptible to the local strain, S. japonicum-like species from Malaysia. Geographical isolation may be the cause of these differences in compatibility between the snail vectors and the schistosome parasites.

Schistosoma incognitum and its zoonotic potential role in Phitsanulok and Phichit provinces, northern Thailand.

A study on Schistosoma incognitum, a blood fluke of a variety of mammals, was conducted in different ecological conditions in Phitsanulok and Phichit, northern Thailand. The intermediate host of S. incognitum in permanent water habitats studied, i.e; swamps and ditches is Radix (Lymnaea) auricularia rubiginosa. Of 44,412 mollusks representing 13 different species collected from 24 water habitats studied, 7,186 were R. a. rubiginosa. S. incognitum infection rate in the snails was 2.1%. 483 Rattus rattus, 8 R. argentiventer, 280 Bandicota indica and 65 B. savilei were found infected with S. incognitum with an overall infection rate of 41.7%. R. argentiventer and B. savilei are reported as new mammalian hosts of the parasite. Also, 3.9% of dogs in the study area were found excreting S. incognitum eggs in their stools for the first time. The possibility of S. incognitum as a zoonotic potentiality to humans is discussed and is still an equivocal issue deserving further study.