Schistosomes and humans relationship nature

Schistosomiasis - Wikipedia

schistosomes and humans relationship nature

The anti-schistosomal drug praziquantel is safe and efficacious against adult worms of all the six Schistosoma spp. infecting humans; however. Schistosoma is a genus of trematodes, commonly known as blood flukes. They are parasitic flatworms responsible for a highly significant group of infections in humans termed schistosomiasis, . a rare infection in humans and is considered to be a zoonosis. The natural vertebrate host is von Muller's rat (Rattus muelleri). We are researching the impact of human schistosomiasis treatment strategies of the ZEST programme in relation to the lifecycle of schistosomiasis. 1/2.

Interactions between schistosomiasis and human immunodeficiency virus in Western Kenya

The origin of this species is East Africa, where it coexists with its sister species, S. Interactions between these species potentially influence their epidemiology, ecology, and evolutionary biology, because they infect the same species of hosts and can hybridize.

schistosomes and humans relationship nature

Over two years, we examined their distribution in Kenya to determine their degree of overlap geographically, within snail hosts, and in the water column as infective stages. Both species were spatially and temporally patchy, although S.

Both species overlap in the time of day they were present in the water column, which increases the potential for the species to coinfect the same host and interbreed. Peak infective time for S. Three snails were coinfected, which was more common than expected by chance. These findings indicate a lack of obvious isolating mechanisms to prevent hybridization, raising the intriguing question of how the two species retain separate identities.

Introduction One of the world's most prevalent neglected diseases is schistosomiasis, which is caused by flatworms of the genus Schistosoma. It is estimated that million people world wide are infected [1]. Schistosomiasis is notable for its chronic nature, for being difficult to control on a sustained basis, and for the limited options currently available for control [2].

Schistosoma mansoni is the most widespread and best known of the human-infecting schistosomes. It is a genetically diverse parasite with complex epidemiology, particularly in East Africa, which is also its hypothesized place of origin [3]. Epidemiological studies of S.

schistosomes and humans relationship nature

By examining snails, the obligatory hosts for the larval stages of schistosomes, we can gain a much needed perspective, one that allows the determination of where human-infective cercariae are actually being produced, and thus identifies likely sites of active transmission.

Also, during the molluscan phase of the schistosome life cycle, schistosome sporocysts may encounter other individuals of the same or a related schistosome species, or of unrelated species of digenetic trematodes see [5] for an overview of some of the possible interactionspotentially influencing the dynamics of transmission.

Molecular epidemiological investigations have shown that S.

Population genetics of schistosomiasis

In western Kenya, where our studies were undertaken, S. This species is typically considered a parasite of rodents although it has been reported from wild felids, canids, and even humans, although this latter observation has not been confirmed with molecular techniques [9] — [14].

schistosomes and humans relationship nature

Evidence from experimental infections of baboons suggests S. In some locations such as Guadeloupe, rodents are the exclusive definitive host for S.

Overlap of both schistosome species in the same individual rodent host was reported by Schwetz [18] who found eggs of both species in rodents the Democratic Republic of the Congo, although he considered the eggs shaped like those of S.

Both schistosome species infect the same species of Biomphalaria snails and past reports indicate that they can infect the same individual snail host [19] ; therefore they potentially influence each other in terms of infection patterns, development, and cercarial release patterns.

Also, these two species hybridize readily in the laboratory [20] — [22] and a natural hybrid has been found from a snail in the Lake Victoria region [23]. Hybridization is an important epidemiological concern because hybrids could directly infect humans or lead to gene introgression between the species, which both could alter their biology and capacity to cause pathology. However, in the face of possible hybridization and definitive and intermediate host overlap, these two species are apparently able to maintain their identity [23]which unless contact is very recent, suggests the presence of isolating mechanisms including ecological, geographical, or temporal isolation.

Most schistosome cercariae emerge from their snail hosts following a predictable circadian pattern [25] — [27]one that is genetically controlled [28]. Schistosoma mansoni cercariae are diurnal and are typically released during daylight hours, but populations vary concerning their exact time of emergence [17] and references therein.

Previous studies have shown that S. These emergence times correspond to times when their putative hosts are present in the water and available for infection, humans during the day and rodents at night.

However, schistosome cercariae remain active and infective in the water column for up to 9 hours in an experimental setting [30]. This longevity creates the potential for overlap in actual transmission times, even if the cercariae emerge at different times. Using schistosome specimens derived from field collections of snails over a two year period in the Lake Victoria region of Kenya, and applying molecular techniques to these specimens, we addressed several questions concerning the epidemiology of S.

Schistosoma - Wikipedia

Does either species outnumber the other in terms of number of snails infected and number of cercariae produced per snail? Stool examination should be performed when infection with S. Eggs can be present in the stool in infections with all Schistosoma species. Because eggs may be passed intermittently or in small numbers, their detection is enhanced by repeated examinations or concentration procedures, or both.

Eggs can be found in the urine in infections with S. Quantification is possible by using filtration through a nucleopore filter membrane of a standard volume of urine followed by egg counts on the membrane.

  • Schistosomiasis

Tissue biopsy rectal biopsy for all species and biopsy of the bladder for S. Test sensitivity and specificity vary widely among the many tests reported for the serologic diagnosis of schistosomiasis and are dependent on both the type of antigen preparations used crude, purified, adult worm, egg, cercarial and the test procedure.

Centers for Disease Control and Preventiona combination of tests with purified adult worm antigens is used for antibody detection. Immunoblots with adult worm microsomal antigens are species-specific, so a positive reaction indicates the infecting species. The presence of antibody is indicative only of schistosome infection at some time and cannot be correlated with clinical status, worm burden, egg production, or prognosis.

People and livestock are strictly prohibited from entering the water! The World Health Organization is promoting these efforts. In some cases, urbanization, pollution, and the consequent destruction of snail habitat have reduced exposure, with a subsequent decrease in new infections. The drug praziquantel is used for prevention in high-risk populations living in areas where the disease is common.

The detailed specifications laid out in various United Nations documents since the s could have minimized this problem. Irrigation schemes can be designed to make it hard for the snails to colonize the water and to reduce the contact with the local population. After the construction of fourteen large dams, greater increases in schistosomiasis occurred in the historical habitats of native prawns than in other areas.