Schistosomiasis Life Cycle

Introduction

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.

Let’s look at the life cycle of schistosoma to get a better understanding.

Life Cycle of Schistosomiasis:

1. The Egg Stage –

Based on the 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 faeces or urine.

2. The Larval Stages –

• Miracidium: When the eggs are in water and under favourable conditions, they hatch and release miracidium larvae. Miracidium larva is the ciliated larva that swims to the intermediate host, in this case, snails.
• 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. Once the cercaria larva drops its tail, they become schistosomula.
• Schistosomula: The schistosomula travels through the lungs, to the heart, and later enters the liver 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 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 faeces and urine. Then the entire cycle begins all over again.

Conclusion

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

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