What is schistosomiasis?
Schistosomiasis is a disease that is caused by parasites that enter humans by attaching to the skin, penetrating it, and then migrating through the blood system. This disease is also known as bilharziasis, bilharzia, bilharziosis, and snail fever.
Theodore Bilharz identified the parasite Schistosoma hematobium in Egypt in 1851. Schistosomiasis is the second most prevalent tropical disease in the world; malaria being the first. The disease is found mainly in developing countries in Africa, Asia, South America, the Middle East, and the Caribbean. More than 200 million people in at least 74 countries are estimated to have the disease.
What causes schistosomiasis?
Parasites called Schistosoma cause the disease. The disease in humans is part of the complicated life cycle of the parasites.
Eggs are laid in some freshwater areas, from faeces or urine of infected person. These undergo some development in the water into a form that penetrate the snail and develop into the free-swimming form that can penetrate human skin (cercariae). The cercariae can attach to and penetrate the human skin, when man enters those fresh waters either to fish or swim or wash (activities that are common in rural areas).
These cercariae migrate through the blood to the portal blood or bladder blood systems. During this migration, the cercariae change and develop into male and female adult parasitic worms. The worms incorporate some human proteins into their surface structures, so most humans produce little or no immune response to the parasites. Parasite mating occurs in the portal or vesicular blood system producing eggs. Unlike the presence of the adult parasites, the parasite’s eggs stimulate a strong immune response by most humans.
Some eggs migrate through the bowel or bladder tissue and are shed in feces or urine, while other eggs are swept into the portal blood and lodge in other tissue sites. Eggs shed into urine or feces may reach maturity in freshwater and complete their life cycle by infecting yet more susceptible snails. In addition, some adult worms may migrate to other organs (for example, eyes or liver).
Some species can also infect domesticated and wild animals, which can then serve as another host system. S. hematobium is the species that usually infects the human bladder tissue, while the other species usually infect the bowel tissue.
The acute and chronic symptoms of schistosomiasis are thought to be mainly due to the egg migration through tissue and the human immune response to the eggs. Chronic symptoms are mainly due to eggs that are not shed from the body. Complications related to the disease are thought to occur due to long-term exposure to the highly antigenic eggs.
We have tried to keep this simple. We will progress in the next post with further discussion on schistosomiasis.
Dr Adenike Ojumu – Health Editor