Parasites In Humans
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Schistosoma - Blood Flukes

Schistosomiasis (also known as snail fever or bilharzia) is caused by blood flukes from the genus Schistosoma. More than 200 million people are infected worldwide. Mostly in freshwaters where there are many snails which are the intermediate host. There are five main species infecting humans: Schistosoma mansoni, S. haematobium, S. japonicum and two geographically localized species S. intercalatum and S. mekongi.

Schistosoma requires the use of two hosts to complete its life cycle. Depending on the Schistosoma species their eggs are shed either in the feces or urine of an infected human. Eggs can survive up to a week in dry land. If the feces end up in water, larvae called miracidia hatch and start finding certain species of freshwater snails. When they find a snail they penetrate its foot and transform into sporocysts (another larval form). These primary sporocysts multiply asexually into secondary sporocysts and travel to the snail's hepatopancreas. They multiply asexually producing hundreds of cercariae (another larval form). (The process from sporocyst to cercaria takes a few months.) Cercariae exit the snail and start waiting in the water. They can survive about 48 hours in favourable conditions. When they sense that human skin is near, they quickly swim and attach with suckers. They find a suitable spot (usually a hair follicle) and penetrate the skin using special enzymes. As they enter they transform into schistosomulae (another larval form). Only head parts enter, they leave tails behind. Each schistosomula stays a few days in the skin and then enters the bloodstream through dermal lymphatic vessels or blood venules. They travel in the bloodstream to get to specific blood veins. In humans Schistosoma reaches fertility in 6–8 weeks. The newly developed adult females and males find each other and pair up. Adult blood flukes are 1–2 cm long. Males make a gynaecophoric channel for the longer and thinner females to reside. The worm pair then travel to rectal or mesenteric veins. They attach to the venous wall with oral and ventral suckers and can live for many years. Females lay eggs on the endothelial lining of the venous capillary walls at the rate of 300–3000 eggs per day depending on the Schistosoma species. Some eggs are flushed by circulating blood ending up causing inflammation in organs such as liver or lungs. Most eggs however travel to the lumen of the intestinal tract (S. japonicum and S. mansoni) and of the ureters and bladder (S. haematobium), thus exiting the body in the feces or urine. Mature eggs produce special enzymes and can penetrate many membranes such as rectal veins or intestinal wall. The eggs get out of the body and the cycle starts again.

Schistosoma life cycle

Schistosoma species can migrate around and are not bound to just one location. But each species has a preferred location. For example, S. japonicum resides more frequently in the veins that drain the small intestine. S. mansoni is found more often in the veins that drain the large intestine. S. haematobium occurs usually in the venous plexus of bladder, but can also be found in the rectal venules.

The first symptoms are a rash or itch during the first few days. Within two months chills, cough, diarrhea, fatigue, fever and muscle aches can occur. Usually however during the first few weeks schistosomiasis is asymptomatic. The disease is worse for children who can develop anemia, learning difficulties and malnutrition. After years of infection eggs inflame organs such as the liver, bladder and lungs. If eggs end up in the brain or spinal cord, they can cause paralysis, seizures or inflammation of the spinal cord.

Diagnosis is done from a stool or urine sample by microscopic examination. You need to provide your health care provider with the samples. Eggs can be present in the feces in infections of all Schistosoma species and in the urine in infections of S. haematobium and S. japonicum. Tissue biopsy (bladder or rectal biopsy) can also be used in finding eggs, if stool or urine samples are negative. It takes about two months for the parasite to mature into reproducing adult and only after that time will there be eggs present.

Schistosomiasis is treated with praziquantel according to the advice of your health care provider. For infections caused by S. mansoni oxamniquine can be used in areas where praziquantel is less effective.

Geographic distribution

  • Africa: all freshwater in sub-Saharan and southern Africa, also in the Nile River valley in Egypt
  • Caribbean: Antigua, Dominican Republic, Guadeloupe, Martinique, Montserrat, Saint Lucia (low risk) 
  • South America: Brazil, Suriname, Venezuela
  • Southern China
  • Southeast Asia: Cambodia, central Indonesia, Laos, Mekong delta and Philippines Schistosoma mansoni adult pair
  • The Middle East: Iran, Iraq, Saudi Arabia, Yemen.

Avoid freshwater lakes, rivers or streams in areas where schistosomiasis occurs. Drying your skin with a towel immediately after swimming might sweep some larval blood flukes away. Swimming in the salty ocean and in chlorinated swimming pools is quite safe. Human feces should be prevented of getting into water to prevent snails from getting infected. In addition to humans Schistosoma species infect many animals such as cats, dogs, horses, pigs, rodents and goats.

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