Schistosoma mansoni 2188496 223684100 2008-07-05T07:02:36Z Kelidimari 5317994 {{Taxobox | name = ''Schistosoma mansoni'' | image = Schistosoma_mansoni2.jpg | image_width = 200px | image_caption = Schistosomes ''encopula'' | regnum = [[Animal]]ia | phylum = [[Platyhelminthes]] | classis = [[Trematoda]] | subclassis = [[Digenea]] | ordo = [[Strigeidida]] | genus = ''[[Schistosoma]]'' | species = '''''S. mansoni''''' | binomial = ''Schistosoma mansoni'' | binomial_authority = [[Luigi Sambon|Sambon]], 1907 }} <gallery> Image:Mansoni.jpg|Schistosoma mansoni cross section. </gallery> '''''Schistosoma mansoni''''' is a significant [[Parasitic worm|parasite]] of humans, one of the major agents of [[schistosomiasis]]. Of the [[trematoda|trematodes]], schistosomes are atypical in that the adult stages have two sexes (dioecious) and are located in blood vessels of the definitive host. Most other trematodes are [[hermaphroditic]] and are found in the intestinal tract or in organs, such as the liver. The lifecycle of schistosomes includes two hosts: a definitive host (i.e., man) where the parasite undergoes sexual reproduction , and a single intermediate snail host where there are a number of asexual reproductive stages. ''S. mansoni'' is named after [[Patrick Manson|Sir Patrick Manson]], who first identified it. == Morphology of adult helminths== Schistosomes, unlike other trematodes, are long and slim worms. The male ''S. mansoni'' is approximately 1 cm long (0.6 to 1.4 cm) and is 0.11 cm wide. It is white, and it has a funnel-shaped oral sucker at its anterior end followed by a second pediculated sucker. The external part of the worm is composed of seven layers and is continually renewed as the outer layer is continually shedding. The tegument bears a large number of small tubercules. The suckers have small thorns in their inner part as well as in the buttons around them. The male genital apparatus is composed of 6 to 9 testicular masses, situated dorsally. There is one deferent canal beginning at each testicle which is connected to a single deferent that dilates into a reservatory, the seminal vesicle, located at the beginning of the gynacophoric canal. The copula happens through the coaptation of the male and female genital orifices.<ref name = "Rey">{{cite book | last = Rey | first = Luíz | title = Parasitologia | year = 1991 | month = | publisher = Editora Guanabara Koogan S.A | location = [[Rio de Janeiro, RJ]] | language = | id = ISBN 85-277-0189-8 | doi = | pages = 351&ndash;62 | chapter = | chapterurl = | quote = }} </ref> The female has a cilindric body, longer and thinner than the male (1.2 to 1.6 cm long by 0.016 cm wide). The female parasite is darker, and it looks gray. The darker color is due to the presence of a pigment (hemozoin) in its digestive tube. This pigment is derived from the digestion of blood. The [[ovary]] is elongated and slightly lobulated and is located on the anterior half of the body. A short oviduct conducts to the ootype which continues with the uterine tube. In this tube it is possible to find 1 to 2 eggs (rarely 3 to 4) but only 1 egg is observed in the ootype at any one time. The genital pore opens ventrally. The posterior two-thirds of the body contain the vittelogenic glands and their winding canal, which unites with the [[oviduct]] a little before it reaches the ootype. The digestive tube begins at the anterior extremity of the worm, at the bottom of the oral sucker. The digestive tube is composed of an [[esophagus]] which divides in two branches (right and left) and that reunite in a single [[cecum]]. There isn't any [[anus]]. ==Epidemiology== <!-- Unsourced image removed: [[Image:schistosoma_mansoni_Map.gif|thumb|350px|left||Geografic distribution of Schistosoma mansoni infection.]] --> Schistosoma mansoni is a parasite that is found in [[Africa]], [[Madagascar]], parts of [[South America]] (such as [[Venezuela]] and [[Brazil]]), [[Puerto Rico]] and the [[West Indies]]. Among human parasitic diseases, schistosomiasis (sometimes called bilharziasis) ranks second behind [[malaria]] in terms of socio-economic and public health importance in tropical and subtropical areas. The disease is endemic in 76 developing countries, infecting more than 200 million people in rural agricultural and peri-urban areas, and placing more than 600 million people at risk.<ref>{{cite journal | last = Oliveira, G. | coauthors = Rodrigues N.B., Romanha, A.J., Bahia, D. | year = 2004 | title = Genome and Genomics of Schistosomes | journal = Can. J. Zool. | volume = 82 | issue = | pages = 375–90 | doi = 10.1139/Z03-220 | id = | url = | format = | accessdate = }}</ref> Of the infected patients, 20 million suffer severe consequences from the disease, and 120 million are symptomatic. Some estimate that there are approximately 20,000 deaths related to schistosomiasis yearly. In many areas, schistosomiasis infects a large proportion of children under 14 years of age. An estimated 500–600 million people worldwide are at risk from the disease. ==Life Cycle== <!-- Unsourced image removed: [[Image:schistosoma_mansoni_lyfe_cycle.gif|thumb|300px|Schistosoma mansoni lyfe cycle.] --> After the eggs of the human-dwelling parasite are emitted in the feces and into the water, the ripe [[miracidium]] hatches out of the egg. The hatching happens in response to temperature, light and dilution of feces with water. The miracidium searches for a suitable freshwater [[snail]] (''[[Biomphalaria]] glabrata'', ''B. straminea'' or ''B. tenagophila'') to act as an intermediate host and penetrates it. Following this, the parasite develops via a so-called mother-sporocyst and daughter-sporocyst generation to the [[cercaria]]. The purpose of the growth in the snail is the numerical multiplication of the parasite. From a single miracidium result a few thousand cercaria, every one of which is capable of infecting man. The cercaria emerge from the snail during daylight and they propel themselves in water with the aid of their bifurcated tail, actively seeking out their final host. When they recognise human [[skin]], they penetrate it within a very short time. This occurs in three stages, an initial attachment to the skin, followed by the cercaria creeping over the skin searching for a suitable penetration site, often a [[hair follicle]], and finally penetration of the skin into the [[Epidermis (skin)|epidermis]] using [[proteolytic]] secretions from the cercarial post-acetabular, then pre-acetabular [[glands]]. On penetration, the head of the cercaria transforms into an endoparasitic [[larva]], the schistosomule. Each schistosomule spends a few days in the skin and then enters the circulation starting at the dermal [[lymphatics]] and [[venule]]s. The schistosomule migrates to the [[lungs]] (5–7 days post-penetration) and then moves via circulation through the left side of the [[heart]] to the hepatoportal circulation (>15 days) where, if it meets a partner of the opposite sex, it develops into a sexually mature adult and the pair migrate to the mesenteric veins.<ref>{{cite web | url = http://www.dpd.cdc.gov/dpdx/HTML/Schistosomiasis.htm | title = DPDx - Schistosomiasis | accessdate = 2007-14-06 | publisher = [[Centers for Disease Control|CDC]] }} </ref> Male schistosomes undergo normal maturation and morphological development in the presence or absence of a female, although behavioural, physiological and antigenic differences between males from single-sex, as opposed to bisex, infections have been reported. On the other hand, female schistosomes do not mature without a male. Females schistosomes from single-sex infections are underdeveloped and exhibit an immature reproductive system. Although the maturation of the female worm seems to be dependent on the presence of the mature male, the stimuli for female growth and for reproductive development seem to be independent from each other. The adult female worm resides within the adult male worm's gynacophoric canal, which is a modification of the ventral surface of the male forming a groove. The paired worms move against the flow of blood to their final niche in the mesenteric circulation where they begin egg production (>32 days). The ''S. mansoni'' parasites are found predominantly in the small inferior mesenteric blood vessels surrounding the large intestine and caecal region of the host. Each female lays approximately 300 eggs a day (one egg every 4.8 minutes), which are deposited on the [[endothelial]] lining of the venous [[capillary]] walls.<ref>{{cite journal | last = Loverde, P.T. | coauthors = Chen, L. | year = 1991 | title = Schistosome female reproductive development | journal = Parasitol. Today | volume = 7 | issue = 11 | pages = 303–308 | doi = | id = | url = | format = | accessdate = }}</ref> Most of the body mass of female schistosomes is devoted to the reproductive system. The female converts the equivalent of almost her own body dry weight into eggs each day. The eggs move into the [[lumen]] of the host's [[intestines]] and are released into the environment with the feces. ==Genome== Schistosoma mansoni has 8 pairs of [[chromosomes]] (2n = 16)—7 autosomal pairs and 1 sex pair. The female schistosome is heterogametic, or ZW, and the male is homogametic, or ZZ. Sex is determined in the zygote by a chromossomal mechanism. The Schistosoma [[genome]] is approximately 270 MB with a GC content of 34%, 4–8% highly repetitive sequence, 32–36% middle repetitive sequence and 60% single copy sequence. Numerous highly or moderately repetitive elements have been identified, and their frequency in genomic sequence data also suggests at least 30% repetitive [[DNA]]. Chromosomes range in size from 18 to 73 MB and can be distinguished by size, shape and C banding. There are estimated to be 15–20 thousand expressed [[genes]]. <ref>{{cite web | url = http://www.sanger.ac.uk/Projects/S_mansoni/ | title = Schistosoma mansoni Genome Project | accessdate = 2007-14-06 | publisher = [[Wellcome Trust Sanger Institute|Sanger Institute]] }} </ref> In June 2003, a ~5x whole genome [[shotgun sequencing]] project was initiated at the [[Sanger Institute]]. Together with the shotgun data being generated by [[TIGR]], an ~8x coverage of the genome will be obtained, assembled and annotated. <ref>{{cite web | url = http://www.tigr.org/tdb/e2k1/sma1/ | title = Schistosoma mansoni Genome Project | accessdate = 2007-14-06 | publisher = [[TIGR|The Institute for Genomic Research]] }} </ref> Also in 2003, 163,000 ESTs ([[expressed sequence tag]]s) were generated (by a [[consortium]] headed by the [[University of São Paulo]]) from six selected developmental stages of this parasite, resulting in 31,000 assembled sequences and an estimated 92% of the 14,000-gene complement. <ref>{{cite web | url = http://verjo18.iq.usp.br/schisto/ | title = Schistosoma mansoni EST Genome Project | accessdate = 2007-14-06 | publisher = [[University of São Paulo]] }} </ref> ==Pathology== [[Image:schistosoma mansoni.jpg|thumb|300px|left||A ''Schistosoma mansoni'' egg with the characteristic lateral spine]] Schistosome eggs, which may become lodged within the hosts tissues, are the major cause of pathology in schistosomiasis. Some of the deposited eggs reach the outside environment by passing through the wall of the intestine; the rest are swept into the circulation and are filtered out in the periportal tracts of the liver resulting in periportal fibrosis. Onset of egg laying in humans is sometimes associated with an onset of fever (Katayama fever). This "acute schistosomiasis" is not, however, as important as the chronic forms of the disease. For ''S. mansoni'' and ''S. japonicum'' these are "intestinal" and "hepatic schistosomiasis", associated with formation of [[granulomas]] around trapped eggs lodged in the intestinal wall or in the liver, respectively. The hepatic form of the disease is the most important, granulomas here giving rise to [[fibrosis]] of the liver and [[hepatosplenomegaly]] in severe cases. Symptoms and signs depend on the number and location of eggs trapped in the tissues. Initially, the inflammatory reaction is readily reversible. In the latter stages of the disease, the pathology is associated with [[collagen]] deposition and fibrosis resulting in organ damage that may be only partially reversible. Granuloma formation is initiated by antigens secreted by the miracidium through microscopic pores within the rigid egg shell, and there is strong evidence that the vigorous granulomatous response, rather than the direct action of parasite egg antigens, is responsible for the pathologic tissue manifestations in schistosomiasis.<ref>{{cite journal | last = Boros, D.L. | coauthors = | year = 1989 | title = Immunopathology of Schistosoma mansoni infection | journal = Clin Microbiol Rev. | volume = 2 | issue = 3 | pages = 250–69 | doi = | pmid = : 2504481 | url = http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pubmed&pubmedid=2504481 | format = pdf | accessdate = 2007-06-14 }}</ref> The granulomas formed around the eggs impair blood flow in the liver and consequently induce [[portal hypertension]]. With time, [[collateral circulation]] is formed and the eggs disseminate into the lungs, where they cause more granulomas, pulmonary arteritis and, later, [[cor pulmonale]]. A contributory factor to portal hypertension is Symmers' fibrosis, which develops around branches of the portal veins. This fibrosis occur only many years after the infection and apparently is caused in part by soluble egg antigens and various immune cells which react to them. ==Symptoms== Many individuals do not experience symptoms. If symptoms do appear, it usually takes four to six weeks from the time of infection. The first symptom of the disease may be a general ill feeling. Within twelve hours of infection, an individual may complain of a tingling sensation or light [[rash]], commonly referred to as "swimmer's itch", due to irritation at the point of entrance. The rash that may develop can mimic [[scabies]] and other types of rashes. Other symptoms can occur two to ten weeks later and can include [[fever]], aching, [[cough]], [[diarrhea]], or gland enlargement. These symptoms can also be related to avian schistosomiasis which does not cause any further symptoms in humans. ===Katayama fever=== Another primary condition, called Katayama fever, may also develop from infection with these worms, and it can be very difficult to recognize. Symptoms include fever, lethargy, the eruption of pale temporary bumps associated with severe itching (urticarial) rash, liver and spleen enlargement, and bronchospasm. ===Intestinal schistosomiasis=== In intestinal schistosomiasis, eggs become lodged in the intestinal wall and cause an immune system reaction called a [[granuloma|granulomatous reaction]]. This immune response can lead to obstruction of the colon and blood loss. The infected individual may have what appears to be a potbelly. Eggs can also become lodged in the liver, leading to high blood pressure through the liver, enlarged spleen, the buildup of fluid in the abdomen, and potentially life-threatening dilations or swollen areas in the esophagus or gastrointestinal tract that can tear and bleed profusely (esophageal varices). Rarely, the central nervous system is affected. Individuals with chronic active schistosomiasis may not complain of typical symptoms. ==Diagnosis and treatment== Diagnosis of infection is confirmed by the identification of eggs in stools. Eggs of ''Schistosoma mansoni'' are approximately 140 by 60 µm in size and have a lateral spine. The diagnosis is improved by the utilization of the [[Kato technique|Kato-Katz technique]] (a semi-quantitative stool examination technique). Other methods which can be used are enzyme linked immunosorbent assay (ELISA), circumoval precipitation test (COPT) and alkaline phosphatase immunoassay (APIA).<ref>{{cite web | url = http://www2.ttcn.ne.jp/~akky/parasite/clinic.htm | title = Clinical Aspects | accessdate = 2007-14-06 | publisher = [[University of Tsukuba]] School of Medicine }} </ref> Currently there are two drugs available, [[Praziquantel]] and [[Oxamniquine]], for the treatment of <ref>{{cite web | url = http://www.emedicine.com/med/topic2071.htm | title = eMedicine - Schistosomiasis | accessdate = 2007-14-06 | publisher = eMedicine }} </ref> schistosomiasis. They are considered equivalent in relation to efficacy and safety. Due to its lower cost per treatment, Praziquantel is generally considered the first option for treatment. The recommended dose is: Praziquantel, 60mg/kg of body weight for children up to 15 years old, and 50mg/kg of body weight for adults; Oxamniquine, 15mg/kg for adults, and 20mg/kg for children up to 15 years old. Both drugs are administered as a single dose treatment. The treatment objective is to cure the disease and to prevent the evolution of infected patients to the chronic form of the disease. All confirmed cases should be treated. == See also == * [[Parasitology]] * [[Schistosomiasis]] * ''[[Schistosoma haematobium]]'' * ''[[Schistosoma japonicum]]'' * [[Genital schistosomiasis]] ==References== {{Reflist|2}} ==Further reading== * Adel A. F. Mahmoud. ''Schistosomiasis.'' World Scientific Publishing Company, Imperial College Press, London, 2001. (ISBN 978-1860941467) [[Category:Parasites]] [[es:Schistosoma mansoni]] [[fr:Schistosoma mansoni]] [[ja:マンソン住血吸虫]] [[pt:Esquistossomo]]