Schistosoma mansoni

Schistosoma mansoni is a species of parasitic worm, classified within the Platyhelminthes phylum, which affects an estimated 83.31 million throughout the world. This species is the most populous of all members of its genus, occurring in fifty-four countries including areas like Africa, the Middle East, South America, and the Caribbean. This species causes intestinal schistosomiasis in humans. The mapped genome of Schistosoma mansoni and S. japonicum were published in 2009. The associated research from the mapping of these species has shown promising results in new treatments. However, this species’ genome has not yet been completed. The most recent updates were published in 2012.

Schistosoma mansoni differs from other trematodes because it is long in shape, with males reaching an average body length of up to .4 inches and females reaching a length of up to .6 inches. Males differ from females in color, with females appearing to be gray. This is due to the ingestion of blood, which gives the worm the pigment known as hemozoin. Males and females have different reproductive organs, but both sexes have an oral sucker and a second sucker that sits close to it.

The life cycle of Schistosoma mansoni begins when the eggs, found in the feces of a human host, are released into water. Shortly after introduction to water, the eggs hatch into miracidium, which swim around until they find a suitable snail host to infect. These intermediate hosts include Biomphalaria tenagophila, Biomphalaria straminea, Biomphalaria glabrata, and Biomphalaria sudanica. Once the miracidium enters the snail, they develop into “mother” sporocycts than produce “daughter” sporocysts. This process allows thousands of larvae to develop within one snail.

Once the cercaria larvae have developed, they will penetrate through the flesh of the snail once more, swimming into the water during the day light hours. These larvae are able to seek out a human host, often entering the human by means of a hair follicle located on or near the foot. When the larva has successfully penetrated the skin of a human, its head develops into an endoparasitic larva, known as the schistosomule. After spending a few days within the skin of the new host, the larvae will travel through the lymphatics and venules located in the dermal layer of the skin, feeding on blood as they move. The schistosomule reaches the lungs after five to seven days and moves into the circulatory system by way of the heart. This is where males and females may encounter each other and mature into adult worms. Once a monogamous pair comes together, they will travel together to the mesenteric veins to breed.

The full maturation of the male schistosomule is not dependent upon a female, but females must be in the presence of a male to fully mature. If an infection consists of all female parasites, they are unable to reproduce. Despite this relationship to males, it is thought that the development of the females is independent from a male. Once a pair has come together, the female rests within gynaecophoric canal of the male, and the breeding pair moves into the mesenteric circulation system. Females will lay around three hundred eggs each day. These eggs move through the endothelial lining along the venous capillary walls. The eggs will travel through the body to the lumen in the small and large intestines of the human host and are then deposited into the water within the feces.

Schistosoma mansoni is able to avoid the immune system reactions caused by their travel through the blood stream. By using the tegument, a sticky substance with which it covers itself, the worm is able to fight off the antibodies within its host’s blood. It is also able to combat the superoxides that the human host produces, which can severely harm the worm, by producing counteracting antioxidant proteins. The superoxides create a blockage in the vein that a worm is traveling through, and experts have taken advantage of this natural protection by researching it and applying it to treatments. It has been found that by using the inhibitor Thioredoxin Glutathione Reductase (TGR), the worms have a difficult time surviving.

In many cases of Schistosoma mansoni infection, hosts do not show symptoms. In other cases, symptoms appear four to six weeks after initial infection. These include general feelings of un-wellness, fever, and a rash known as swimmer’s itch. This rash can grow over time and appear similar to other rashes or scabies. Between two and ten weeks after infection other symptoms can occur, including cough, aching, gland enlargement, and diarrhea. Infections can cause Katayama fever, an acute form of schistosomiasis that produces symptoms such as lethargy, fever, swelling of the spleen and liver, and bronchospasm.

The biggest threat to a human infected with Schistosoma mansoni is called intestinal schistosomiasis. This occurs when the eggs laid by adult worms become wedged in the intestinal wall, creating a blockage that causes the immune reaction known as a granulomatous reaction. Hosts with this disease may experience blood loss and blockages of the colon, with some individuals appearing to have a “pot belly.” Hosts may also have eggs wedged in the liver, which can lead to elevated pressure within the liver, abdominal fluid buildup, an enlarged spleen, and even deadly amounts of swelling in the gastrointestinal organs and esophagus, which can lead to rupturing that causes death by loss of blood. Despite the danger of these painful symptoms, many infected individuals do not claim to be experiencing them.

Diagnosis of a Schistosoma mansoni infection can be conducted by examining a stool sample and is enhanced when using the Kato-Katz technique. Other testing methods can be used, including the ELISA test, alkaline phosphatase immunoassay (APIA) test, or a circumoval precipitation test (COPT). There are two drugs that commonly used to treat schistosomiasis, known as oxamniquine and praziquantel. Both are equal in efficiency, but praziquantel is more affordable, so it is most often the drug of choice. These medicines treat the symptoms of the disease, but also attempt to prevent the spread of the worm.

Image Caption: Scanning electron micrograph of a pair of Schistosoma mansoni. Credit: Waisberg/Wikipedia