Key Stages and Transmission in the Schistosomiasis Life Cycle
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:
Schistosoma haematobium
Schistosoma mansoni
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:
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.
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.
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.
[Image will be Uploaded Soon]
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
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.
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.
In sub-Saharan Africa, more than 200,000 deaths per year are due to schistosomiasis.
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.
People are infected during routine agricultural, domestic, occupational, and recreational activities, which expose them to infested water.
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.
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.
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.
FAQs on Schistosomiasis Life Cycle: Step-by-Step Guide
1. What are the main stages in the life cycle of the Schistosoma parasite?
The life cycle of Schistosoma is digenetic, meaning it requires two hosts to complete its cycle. The key stages are:
- Egg Stage: Parasite eggs are released from an infected human (the definitive host) via faeces or urine into freshwater.
- Miracidium Stage: The eggs hatch in the water, releasing free-swimming larvae called miracidia.
- Snail Infection: The miracidia actively seek and penetrate a specific freshwater snail (the intermediate host).
- Asexual Reproduction: Inside the snail, the miracidia transform into sporocysts, which multiply asexually to produce thousands of new larvae.
- Cercaria Stage: These new larvae, called cercariae, are released from the snail back into the water. They are the infective stage for humans.
- Human Infection: Cercariae penetrate the unbroken skin of a human who comes in contact with contaminated water.
- Maturation: Inside the human body, the cercariae develop into adult worms (schistosomulae), which migrate to the blood vessels of the liver and then to the intestines or bladder to mate and lay eggs, starting the cycle again.
2. What is the difference between the definitive and intermediate hosts in the schistosomiasis life cycle?
In the schistosomiasis life cycle, the hosts are distinguished by the type of parasitic development that occurs within them:
- The definitive host is the organism where the parasite reaches sexual maturity and reproduces. For Schistosoma, the definitive host is a human. Adult male and female worms live and mate within the human's blood vessels.
- The intermediate host is the organism where the parasite undergoes asexual reproduction and larval development. For Schistosoma, the intermediate host is a specific species of freshwater snail. Inside the snail, the parasite multiplies from a single miracidium into thousands of infective cercariae.
3. How does the Schistosoma parasite enter the human body?
The Schistosoma parasite enters the human body through direct skin penetration. The infective larval stage, known as the cercaria, is released from infected snails into freshwater. When a person wades, swims, or bathes in this contaminated water, the free-swimming cercariae are attracted to the skin. They attach themselves, shed their tails, and secrete enzymes that allow them to burrow through the skin and enter the bloodstream. This process does not require any cuts or open wounds.
4. What are the different larval forms of Schistosoma and their specific roles?
The Schistosoma life cycle features several distinct larval forms, each with a specific function:
- Miracidium: A ciliated, free-swimming larva that hatches from the egg in freshwater. Its sole purpose is to find and infect the correct snail intermediate host.
- Sporocyst: A non-motile, sac-like stage that develops from the miracidium inside the snail. It functions as a reproductive factory, asexually producing numerous daughter sporocysts or cercariae.
- Cercaria: A free-swimming larva with a forked tail, released from the snail. This is the infective stage for humans, designed to penetrate the skin of the definitive host.
- Schistosomulum: The stage immediately after the cercaria penetrates the human skin and loses its tail. It migrates through the host's circulatory system to the liver, where it matures into an adult worm.
5. How do the eggs of Schistosoma cause the primary symptoms of schistosomiasis?
Interestingly, the main symptoms of schistosomiasis are not caused by the adult worms, but by the body's immune reaction to the parasite's eggs. While some eggs are excreted to continue the life cycle, many become trapped in the host's tissues, particularly the liver and bladder. The immune system recognizes these trapped eggs as foreign bodies and forms intense inflammatory reactions called granulomas around them. Over time, this chronic inflammation leads to tissue damage, fibrosis (scarring), and organ dysfunction, resulting in the severe clinical manifestations of the disease.
6. Why are freshwater snails so crucial for the transmission of schistosomiasis?
Freshwater snails are crucial because they serve as the obligatory intermediate host where the parasite undergoes massive amplification. A single miracidium that infects a snail develops into sporocysts, which asexually produce thousands of infective cercariae. This amplification step dramatically increases the number of infective parasites in the environment, significantly raising the chances of human infection. Without the specific snail host, the parasite's life cycle is broken, and transmission cannot occur. Therefore, snail control is a key strategy in preventing schistosomiasis.
7. How does the Schistosoma life cycle demonstrate biological adaptation?
The Schistosoma life cycle is a masterclass in biological adaptation. It shows adaptations for survival in multiple environments (water, snail, human) and for ensuring transmission. Key adaptations include:
- High Egg Production: Adult worms produce hundreds of eggs per day to increase the chances of some reaching freshwater.
- Host Seeking: Miracidia have chemoreceptors to actively locate their specific snail host.
- Asexual Amplification: The sporocyst stage in the snail multiplies the parasite's numbers exponentially, compensating for high mortality rates in the external environment.
- Skin Penetration: Cercariae are adapted with enzymes and a powerful muscular tail to actively breach the skin barrier of the human host.
- Immune Evasion: Adult worms can coat themselves with host antigens, effectively camouflaging themselves from the human immune system and allowing them to live for years.
8. Which stage of the Schistosoma life cycle is typically targeted by treatment medication like Praziquantel?
The most common treatment for schistosomiasis, the drug Praziquantel, is primarily effective against the adult worm stage of the parasite's life cycle. It is not very effective against the immature schistosomulae or the eggs. This is why treatment is often delayed for a few weeks after a person is exposed to contaminated water. This delay allows the immature parasites to develop into adults, which are vulnerable to the drug. The medication causes severe spasms and paralysis of the adult worms' musculature, leading to their detachment from the blood vessel walls and subsequent destruction by the host's immune system.
