Schistosoma Mansoni – Morphology, Life Cycle and Pathogenicity
Schistosoma mansoni is geographical located in Africa, especially Egypt and the wet tropical regions, the West Indies, and the eastern coast of tropical South America.
Habitat of Schistosoma Mansoni
The Adults are found in the haemorrnoidal plexuses of man and of certain species of rodents and baboons.
(b) Eggs are found in feces, rarely in urine and infective only to snail.
(c) All larval stages occur in fresh-water snails, viz. species of Biomphalaria in Africa, and species of Australorbis and Tropocorbis in the American continent.
(d) Infective cercariae escape from the mollusc and have a short free-swimming life in water; they can penetrate the unbroken skin of the final host.
Morphological Characters
The alimentary canal resembles that of S. haematobium except that, in the male. After primary bifurcation, the two limbs of the intestine re-unite in the anterior half of the body. This common caecum usually bifurcates again, the branches re-uniting in a trunk which pursues a wavy course to end blindly near the posterior extremity.
Male: The adult male of schistosoma mansoni measures about 1cm in length and has a breadth of about 1.3mm. Behind the ventral sucker the body is covered with in-numerable tubercles which are distinctly larger than those on the male of S. haematobium. The genital organs resemble those of S. haematobium except that there are eight spherical testes.
Female: The female measures about 1.1cm in length and about 170cm in breadth. The skin is smooth. The ovary is situated in the anterior half of the body, just in front of the junction of the intestinal caeca. In consequence of the anterior position of the ovary, the uterus is short and rarely contains more than three eggs. The vitelline glands occupy the posterior half to two-thirds of the body.
The eggs are bluntly oval, have a lateral spine; and measure about 140cm in length by about 60cm in breadth. The miracidium, sporocyst, and cercaria, resemble those of S. haemotobium very closely.
Life History of Schisitosoma Mansoni
The eggs are passed in feces, rarely urine. The succeeding stages are like those of S. haematobium except that development takes place in various species of planorbid snails. As in the case of S. haematobium, the cercariae, after escaping from the snail, die unless they penetrate the final host within some thirty hours. The incubation period in man is about two months. Immunity has been demonstrated in chronic infections, and this is passed to infants before birth and protects them for about six months.
Pathogenicity
Infection is often subclinical, but in the earlier stages of infection there may be a pronounced eosinophilia, a definite leucocytosis, symptoms of toxaemia such as urticaria, and pulmonary signs. Later, there may be abdominal pain, a pronounced dysentery, blood and mucus being passed in the feces, as a result of the eggs ulcerating through the tissue of the intestine. In very chronic infection there may be prolapse of the rectum. The liver and spleen are enlarged, the former showing cirrhosis.
Schistosoma mansoni infections are endemic in parts of the world with poor sanitation. The parasite is spread by fecal-oral contact with contaminated water. Young children who spend much time playing or bathing in ponds and rivers are especially at risk.
In a study of hepatosplenic schistosoma patients, interferon-gamma protected against periportal fibrosis and TNF-alpha aggravated it. These results highlight the opposing costs and benefits of host defenses against Schistosoma.
Infection of Schistosoma Mansoni
The trematode Schistosoma mansoni causes the tropical disease schistosomiasis, where the cercariae penetrate skin and develop into adult worms that live in the bloodstream. The worms cause debilitating disease in many parts of the world, mainly in areas with freshwater. The infection can remain dormant for several years, and some people don’t know they have it – though sometimes you may get itchy red bumps (called urticarial) or watery diarrhea on the surface of your skin where the worms burrow in. If left untreated, schistosomiasis can damage organs such as the bladder and kidneys.
The infection is transmitted by contact with freshwater from lakes or rivers where the snails that complete the schistosomal life cycle are found. It can also be spread by swimming in contaminated waters or showering in untreated river water. People who have not been in these freshwater environments are unlikely to be infected, but people who regularly take showers from freshwater sources where schistosoma cercariae hatch and grow into adult worms may develop the disease.
Adult worms are released into the bloodstream where they can be ingested by other snails or by humans. The infection can be detected by a simple test of urine called POC-CCA (Point of Care CCA). The test detects Circulating Cathodic Antigen, one of the parasitic gut associated glycoproteins regurgitated by living adult worms in the human host. The presence of the antigen in urine is indicative of active S. mansoni infection. The test has a high sensitivity of up to 100% and is particularly useful in endemic populations where a single positive result closely correlates with prevalence using KK microscopy.
The worms are a source of debilitating and chronic disease in many parts of the world. The disease is exacerbated by poverty, malnutrition and poor sanitation, but the main factor is prolonged contact with contaminated freshwater from rivers or lakes where the worms are endemic. The infection can be treated with a course of tablets. Despite the widespread distribution of the disease, it is possible to control the outbreaks by reducing the use of untreated freshwater, removing snail bait and using effective medicines.
Symptoms of Schistosoma Mansoni
Schistosoma parasites are free-swimming larvae called cercariae that are shed by freshwater snails of the genus Biomphalaria. These snails are considered intermediate hosts and serve as a reservoir for the schistosomes in endemic areas. The cercariae can then enter the human body through contaminated drinking water and become free-swimming in the bloodstream. After entering the human body, the cercariae migrate through blood vessels to reach the hepatic (liver) or vesicular (bladder) blood systems. During this process, they change from larvae to male and female adult worms.
Schistosomiasis is primarily an infection of the liver and bladder, although it can also affect other organs such as the intestines, kidneys, lungs, and pancreas. Most people don’t have any symptoms when they first get infected, but within a few months of becoming infected, some people begin to have early symptoms such as itching and a skin rash. Later, if the infection is chronic (long-lasting), it can cause complications such as inflammation or scarring of the liver, spleen, and intestines, or a buildup of fluid in the abdomen (ascites).
Acute schistosomiasis, known as bilharzia or snail fever, is associated with intense itching, a skin rash, abdominal tenderness or pain and diarrhea. This form of the disease is mainly caused by the release of immuno-complexes by the immune system in response to antigens released by the Schistosoma cercariae. This results in the formation of hepatic granulomas that result in hepatocellular necrosis and portal hypertension, with subsequent muscular wasting and low serum albumin in some patients.
Some patients may develop a decompensated liver disease associated with abdominal distension, hepatomegaly, and cirrhosis, which is usually the result of years of chronic schistosomiasis and alcoholism. Portal hypertension can also lead to varices in the oesophagus, which are at risk of rupturing and causing severe vomiting (hematemesis). This disease is sometimes mistaken for alcoholic liver disease. In this case, treatment of the underlying alcoholism may improve the patient’s condition. A number of tests, including a complete blood count, hepatitis B and C viral antibody testing, a chest X-ray and an ultrasound, and a liver biopsy, are used to confirm the diagnosis.
Diagnosis of Schistosoma Mansoni
The trematode Schistosoma mansoni is the causative agent of the debilitating disease schistosomiasis. It is transmitted through freshwater snails of the genus Biomphalaria acting as intermediate hosts. Infection is prevented by enhancing sanitation and controlling the snail population.
Blood tests and, more recently, polymerase chain reaction (PCR) tests can detect antibodies against schistosomal egg proteins in the body, but these methods do not indicate whether or how many worms are present or their location within the body. Infection stages are determined by how many eggs are excreted in the stool or urine and by clinical signs and symptoms.
Intestinal schistosomiasis is associated with abdominal pain, loss of appetite and bloody diarrhoea. The intensity of these symptoms depends on the number of infecting worms and varies according to the species; Schistosoma intercalatum, for example, has milder intestinal symptoms than Schistosoma mekongi and Schistosoma japonicum. During a chronic infection, liver and kidney function may be impaired. Blood tests (complete blood count, bile acid profiles, serum creatinine) and ultrasound examination can be helpful in assessing the severity of the organ damage. In Katayama fever, eosinophils (a type of white blood cell) are often elevated in the blood.
If not treated, schistosomiasis can cause serious complications in the liver, intestines and bladder. Women can develop urogenital schistosomiasis, which increases the risk of acquiring HIV because the urethra becomes damaged. It also can lead to anemia.
Treatment is by drugs that kill the adult worms. Praziquantel, delivered orally, is the drug of choice. It is effective for people living in endemic areas and for travellers or migrants from endemic areas.
Despite the effectiveness of treatment, schistosomiasis remains a significant problem in many countries. The burden is particularly heavy in sub-Saharan Africa and in tropical regions with inadequate water supplies. Infected people should avoid contact with freshwater bodies of water and, if this is not possible, drink bottled or treated drinking water. There is no vaccine against schistosomiasis but, in addition to treatment with praziquantel, molluscicides such as sodium pentachlorophenate, dinitro-o-cyclohexylphenol and copper sulfate should be used to control snail populations.
Treatment of Schistosoma Mansoni
The treatment for schistosomiasis involves medication that eliminates the worms or at least reduces their numbers. It also includes medications to treat complications of the infection.
Treatment with praziquantel (PZQ) is the most effective method for eliminating or reducing the number of schistosoma eggs. It works by inhibiting the schistosomal enzymes, hexokinase and adenyl phosphatase. The medication is taken orally and can be given to adults, children and infants. It is also effective against many other trematodes, including the malaria parasite, Plasmodium falciparum.
Most people continuously exposed to endemic schistosomiasis develop partial immunity after early teenage years, and the number of established worms within them is reduced by natural worm death over time. This decrease in worms and the resolution of granulomatous reactions to eggs within the host, contribute to a reduction in the intensity of symptoms.
Acute schistosomiasis is a common problem in travellers to endemic areas, and presents with abdominal pain in the right upper quadrant, diarrhoea (with or without blood) and malaise. Chronic hepatosplenic schistosomiasis affects mostly people from poor rural areas with long-standing infections, and the progression to disease is slower than in acute schistosomiasis.
Symptoms in this phase include hepatosplenomegaly, fever, fatigue and weakness. Blood tests can reveal hepatic dysfunction and anaemia, while ultrasounds, chest X-rays, CT scans and magnetic resonance imaging (MRI) may be used to assess the presence and extent of liver damage.
The primary methods for preventing schistosomiasis are avoiding contact with bodies of water where the intermediate snail species completes its life cycle, and ensuring that human waste is collected, transported and disposed of properly. These measures are particularly important for young children, who are more likely to swim and bathe in contaminated water than adults. Mass community and school-based treatment programs with praziquantel, education campaigns and the use of molluscicides to reduce snail populations are also employed to control schistosomiasis in endemic countries. These programs have had some success, but are expensive and require sustained efforts.
