Schistosoma mansoni

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.

During its lifetime, this parasite lives in the bodies of two different hosts. The first, the intermediate host, is usually a freshwater snail from the family Planorbidae. The ciliated larvae mature into sporocysts in these snails, making the snail a producer of cercariae for the remainder of its life. Adults mature and reproduce in the mesenteric portal system of a wide variety of hosts such as man, mice and hamsters. They thrive in tropical and equatorial environments because there are more people and a wide range of molluscs to act as hosts, as well as many rivers to transfer and carry the miracidia and cercariae.

Schistosoma mansoni is a parasite and during its lifetime, usually lives in two different hosts. Its life cycle is typical of many blood flukes. After copulation, females release eggs into the water or soil outside of the body of the host through its feces. Once in the water, the eggs hatch and the miracidium (free-swimming ciliated larvae) penetrates the intermediate host (usually a snail from the family Planorbidae). There, it develops into a sporocyst. This sporocyst reproduces asexually and yields more sporocysts. They enter the host by boring into its tissue after secreting hyaluronidase (an enzyme that dissolves hyaluronic acid, which is one of the principle substrates of connective tissue). About 4-6 weeks after the intermediate host is infected, the parasite will produce cercariae, the juvenile fork-tailed larvae stage of development. The cercariae are then released from the intermediate host. They swim around until they find a definitive host. A behavior of the cercariae that is unique is that they swim upwards rapidly and then become motionless with their forked tails extended, and then they sink slowly downwards. They continue this pattern of alternately rising and sinking so that the currents will carry them away from the habitat of their intermediate host. (This host will be a continuous producer of cercariae in the water for the remainder of its life.) The cercariae of both sexes, after a period of swimming and drifting, penetrate into the definitive host (they can enter through the lining of the throat and esophagus; however, they usually infect through contact with skin while it is submerged in contaminated water) and migrate via the blood to the lungs. They then travel to the hepatic portal and mesenteric veins, where they mature (another unique characteristic of this worm is that the females do not mature unless maturing males are present), and the copulation and egg-laying processes begin again.

Shistosomula (the tail-less larvae of the worm) mature and reproduce in the hepatic portal system of their definitive (final) host. The blood in this system is rich in soluble food materials such as amino acids and monosaccharides. The shistosome worms ingest blood from the hepatic and mesenteric veins. Females feed more actively than males because the nutritional requirements of egg production are much greater than the nutritional requirements needed for sperm production.

In countries affected by Schistosoma mansoni, such as Egypt, where the climate is very dry, agricultural development is hindered because they can not irrigate the land. The most detrimental effect of the parasite, however, is schistosomiasis. It occurs in many third world or underdeveloped countries, and it has a serious negative economic impact. The disease spreads rapidly people are ignorant of the danger of the parasite and the disease caused by it. The disease not only affects humans but animals and lifestock as well. Animals are needed for their meat as well as for manual labor for farmers. When the disease afflicts animals, human food sources are affected as well. Another negative effect of the parasite is that the government must put a substantial amount of funding into the purification of water. This can be a time consuming and expensive project if it is possible at all

Research in the area of contolling this parasite is a major source of income. Many governmental research laboratories in the countries affected by this parasite, such as the National Research Center in Dokki, Egypt, focus on schistosomiasis. Because schistosomiasis is so easily spread through contact with contaminated water, the water in the countries affected by the disease is virtually unusable. This provides an industry centered around water purification andthe production of bottled, purified water.

The recovery of eggs from a 5,000 year old Egyptian mummy has shown that the disease, schistosomiasis, caused by this parasite has been present since prehistoric times. The recent report of newly discover strains illustrates the continuing versatality of these parasites.

Schistosomiasis is a progressive disease, gradually affecting different parts of the body. First, it affects the lungs, causing a bronchial cough, and then it developes into a typical allergic response, coinciding with the growth of the worms, with symptoms such as generalized rash, fever and diarrhea. As the worms migrate and destroy small blood vessels, they cause bleeding and mucus formation, and some pain. The eggs cause swelling in the tissues in which they are laid, and the organs eventually lose flexibilty. Blood vessels that drain the organs become blocked. The portal circulation is damaged and the liver shrinks and stops functioning normally, causing the body to become emaciated but the abdomen and spleen to swell. Eventually the disease is fatal because the liver stops functioning altogether and toxic wastes build up within the body.