Schistosomiasis (also called bilharzia) is a major intravascular soil-transmitted parasitic infection which remains of significant public health importance worldwide, with an estimated 207 million people infected. The disease is caused by six species of blood flukes: Schistosoma haematobium, S. mansoni, S. japonicum, S. intercalatum, S. guineensis and S. mekongi. S. haematobium causes urogenital schistosomiasis while the other species cause intestinal disease.
Schistosomiasis is a debilitating disease. Urogenital schistosomiasis, caused by S. haematobium, can include acute illness such as blood in urine (hematuria) and anemia in children, muscle pain and rashes, while fibrosis of the bladder and ureter, and kidney damage can occur as infections persist. Bladder cancer can be a complication in adults and female genital schistosomiasis may be associated with increased risk of human immunodeficiency virus (HIV) infection. Abdominal pain, diarrhea, rectal bleeding and irreversible liver damage is seen in gastrointestinal form of the disease. Untreated urogenital schistosomiasis may result in various tissue lesions that can be easily infected by other bacteria or viruses.
The schistosome life cycle is maintained in a mammalian definitive host and a freshwater snail intermediate host. Humans acquire the infection following direct contact with water sources containing infectious cercariae (larval forms of the parasite of the Schistosoma). The fork-tailed larvae penetrate human skin and enter the circulation via the capillaries and lymphatics. During penetration, they transform into schistosomula and migrate in the blood circulation. They are then carried around and throughout the body by blood flow for several days before becoming trapped in the vein leading to the liver. During this course of migration, they are found in the lungs in large numbers, as they are temporarily held up in capillaries of the lungs. Here, the male and female worms sexually mature and pair up. The eggs penetrate the vasculature walls and enter either the bladder or intestinal lumen to be shed in urine (urinary schistosomiasis) or stool (intestinal schistosomiasis). The eggs hatch in freshwater sources and release free-swimming miracidia, which then infect a specific freshwater snail intermediate host. Within the snail, the miracidia transform into sporocysts, and after two rounds of asexual reproduction, free-swimming cercariae are released after about 30 days. The cercariae continue the life cycle by penetrating the skin of the definitive mammalian host.
The complex life cycle of schistosomiasis continues when an infected person defecates or urinates in or near freshwater, releasing a proportion of the eggs into the water, which then hatch and infect their intermediate snail host. Infection typically occurs while wading, bathing or washing in contaminated water.
The current laboratory diagnosis can be grouped into the following four main categories: (i) direct parasitological diagnosis; (ii) immunological diagnosis; (iii) DNA and RNA detection; and (iv) use of cytokines, metabolites, and other schistosome molecules as biomarkers.
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