Bilharzia. Schistosomiasis, commonly known as snail fever, bilharzia and bilharziasis, is a disease caused by parasitic worms.
Cheryl Chan 9YC/9F Science/Mrs. Smith
How can I avoid disease? - Bilharzia
Schistosomiasis, commonly known as snail fever, bilharzia and bilharziasis, is a disease caused by parasitic worms. It is the mostly deadly NTD – neglected tropical disease – affecting millions of people each year around the world. Even with a low mortality rate, schistosomiasis is only second to malaria in terms of having a great social, economic, and health impact in tropical regions of the world, and also as the most common parasitic disease among humans.
Schistosomiasis is caused by a pathogen, which is basically a disease-producing agent. The term ‘pathogen’ is generally used to directly refer to infectious organisms; for example, a virus, a bacterium, or fungi. There are several ways where pathogens can enter a human body; mainly by air respiration, water contact or ingestion, soil contact and animal contact or bite. Diseases, illnesses characterized by a patient’s symptoms and physical findings, are sometimes confused with pathogens and the difference is that diseases are what pathogen causes. Our bodies often destroy pathogens before it really causes anything.
The main forms of human schistosomiasis are caused by five different species of flatworms, known as schistosomes: Schistosoma mansoni (causes intestinal schistosomiasis), Schistosoma intercalatum, Schistosoma japonicum (causes Asian forms of intestinal schistosomiasis), Schistosoma mekongi (causes Asian forms of intestinal schistosomiasis) and Schistosoma haematobium (causes urinary schistosomiasis). People get infected when they come into contact with contaminated ponds, streams and rivers inhabited by snails carrying the parasite. It is also common in local waterways polluted with human waste. Normal daily activities for domestic, personal and professional can lead to infection, such as swimming, bathing, fishing, irrigation, rice cultivation etc. Human schistosomiasis is confined to freshwater, and therefore cannot be acquired from saltwater. All ages are at risk for infection, whether travelling or living at endemic areas and freshwater exposure. Individuals at increased hazard include children under the age of 14, laborers with domestic chores centered around freshwater areas, adventure travelers, Peace Corps volunteers, missionaries, soldiers, and ecotourists.
Schistosome parasites have a very complex life cycle, involving many steps. Like an average parasitic worm, they require at least 2 hosts to complete their life cycle. Once entering the snail (the 1st host) through the head or foot, the larvae undergoes a development in the digestive gland of the snail, producing thousands of new parasites. This stage inside the snail host takes between 4-6 weeks. A single snail can release up to 3000 cercariae every day, the resulting parasites, into the freshwater, all capable of infecting human. They remain alive for up to 48 hours and in order to continue, they must penetrate human skin vulnerable in the water not beyond this period of time. Once finding an exposed human body’s skin (the 2nd host), the larvae burrow into the skin in only a few seconds and migrate in the body system from the skin capillaries to the lungs, heart and then finally the liver in 7 days or so. By the time the parasites reach the liver, they mature and form into pairs of adult worms (12 to 16 millimeters in length), a male protecting the female inside his ventral groove. The entire process takes around 45 days and the worms then live together in the blood vessels. The average life expectancy of an adult worm inside a body is 5 years, however can also be up to 20 years. The worms fuels on red blood cells and nutrients, for example sugars and amino acids. This can cause anemia and reduced protection towards other diseases. Female worms will release thousands of eggs which pass out of the body depending on where they are located. If the worms are situated near the bladder, the eggs will invade the bladder wall and are disposed in urine. If in the intestinal blood supply, they will attack the intestine walls and removed in the faeces. However, about 50% of the eggs become trapped in the body and thus, harmful towards many vital organs, including the bladder and the intestines. In the case when people urinate or defecate in freshwater, the eggs then migrate to snails where they eventually hatch and begin the cycle all over again. It is the eggs and not the adult worms responsible for the symptoms of schistosomiasis.
