British cyclist Chris Froome, four-time winner of the Tour de France, participated last year in the mythical competition while suffering from an outbreak of schistosomiasis, a parasitic disease caused by a worm called bilharzia. This circumstance, it seems, could explain his poor performance on that occasion.

Bilharzia is the name formerly given to a genus of flatworms that causes the most important infection by this type of organism in humans, schistosomiasis.

It comprises five different species, present in tropical countries throughout the world, and which determine whether the disease is intestinal (Schistosoma mansoni and Schistosoma intercalatum), urinary (Schistosoma haematobium) or intestinal and hepatic (Schistosoma japonicum and Schistosoma mekongi). In addition, three of them (japonicum, mansoni and haematobium) can occasionally cause lesions in the central nervous system by laying eggs in it.

The most common form of infection is bathing in infested waters, in which some snail species that serve as hosts deposit larval forms or may be contaminated by feces of infected people or animals. In this context, the worms penetrate the skin thanks to proteolytic enzymes (capable of breaking down skin proteins).

The larvae mature in the body of the person and lodge in the blood vessels, where they deposit their eggs. Some of these leave the body through urine and feces and others may lodge in body tissues, where they provoke an immune reaction and progressive damage with characteristic calcifications.

In the digestive and hepatic varieties, the typical picture includes abdominal pain, diarrhea and blood in the stool. More secondarily (especially as the disease progresses) there may be hepatomegaly (swelling of the liver), ascites (accumulation of fluid in the peritoneal cavity), hypertension in the abdominal vessels and sometimes splenomegaly (enlargement of the spleen).

On the other hand, in its urogenital manifestation there is hematuria (blood in the urine), fibrosis of the bladder and ureters and sometimes renal lesions. This can lead to complications such as bladder cancer later in life. In women, genital lesions, bleeding, dyspareunia and vulvar nodules may also occur; in men, disorders of the seminal vesicle, prostate and other genital organs. In both cases, infertility may result.

Early treatment of schistosomiasis is essential to reduce damage and avoid more severe symptoms and possible complications. It is known that corticosteroids can be effective for this purpose, and treatment with praziquantel is usually successful in eliminating the parasite. If eggs are detected again in the stool or urine, repeat treatment may be necessary.

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