Schistosomiasis Life Cycle in Humans and Snails

What is Schistosomiasis?

Schistosomiasis is an infectious disease caused by parasitic flatworms belonging to the family Schistosomatidae. Schistosomiasis is caused by contact with water contaminated with snails that carry worms. The symptoms of this disease include swelling of the intestines, liver, bladder and other organs. Schistosomiasis has been an epidemic in countries like Asia, Africa, South America and the Caribbean. This infectious disease is prominent in areas with poor hygienic conditions caused by poverty; lacking availability to health care services and good infrastructure. Also called Bilharzia, the parasite was first discovered as a disease caused in the 1850s by Theodor Bilharz. He was a German pathologist working in Egypt. This disease has affected more than 200 million people annually across the world. 

The three main species of schistosome that cause schistosomiasis in humans are as follows:

1. Schistosoma haematobium

2. Schistosoma mansoni

3. Schistosoma japonicum

Other species of schistosoma can cause disease in other animals. Schistosoma bovis infects domestic cattle, impacting on their health and commercial productivity are the examples of this disease.

Life Cycle of Schistosomiasis

1. The Egg Stage:

Based on the kind of species of worm, female fluke measuring up to 25 mm in length releases approximately 3,500 eggs daily into the bloodstream. These eggs migrate to the intestine or bladder and are released into the environment through feces or urine.

2. The Larval Stages:

1. Miracidium: When the eggs are in water and under favourable conditions, they hatch and release miracidium larvae. Miracidium larva are ciliated larvae that swim to the intermediate host, in this case snails.

2. Cercariae: After further development, the larvae transform into a fork-tailed larva called cercaria. The cercariae larvae emerge from the snails into water. When these larvae get in contact with a mammal, they drop their tail and penetrate the skin tissues where they feed on blood. They become schistosomula, once the cercaria larva drops its tail.

3. Schistosomula: To the heart, and later enters the liver, the schistosomula travels through the lungs, where they undergo development. After the schistosomula becomes mature, they exit the liver through the portal vein system.

3. Adult Stage:

During this stage the male and female worms are sexually mature. The male and female worms mate. The adult worms reside in the intestine and this location varies depending on the kind of species. They do not remain in one place and migrate to different locations inside human beings. The eggs produced by fertilized female worms are moved to the lumen of the intestine, the bladder, the ureters and released via feces and urine. Then the entire cycle begins all over again.

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Conclusion

The symptoms of schistosomiasis include cough, fever, skin irritation, inflammation of liver. In more severe cases, there is blood found in urine and feces. There are different types of schistosomiasis found in different locations. For example S. haematobium are found in Africa and parts of the Middle East, S. japonicum are found in China and the Philippines.

Fun Facts

1. Schistosomiasis is a tropical disease caused by a parasite. Through contact with fresh water contaminated with the parasite’s larvae, the parasite is being transmitted.

2. Particularly poor communities are being affected by schistosomiasis because of the lack of clean water, adequate sanitation or readily available medical treatment. By these factors they are being put at a higher risk of developing schistosomiasis.

3. In sub-Saharan Africa, more than 200,000 deaths per year are due to schistosomiasis.

4. Lack of hygiene and certain play habits of school-aged children such as swimming or fishing in infested water make them especially vulnerable to infection.

5. People are infected during routine agricultural, domestic, occupational, and recreational activities, which expose them to infested water.

6. With the focus on reducing disease through periodic use of a large scale of population, the schistosomiasis treatment can be controlled with praziquantel, a more comprehensive approach of  reducing transmission includes potable water, adequate sanitation, and snail control.

7. At least 290.8 million people estimatedly shows that it was requirement of preventive treatment for schistosomiasis in 2018, out of which more than 97.2 million people were reported to have been treated.

8. The schistosomiasis can be controlled based on a large-scale treatment of a group of populations who are at risk, with access to safe water, improved sanitation, hygiene education, and snail control.