Schistosomiasis

Schistosomiasis is a significant neglected tropical disease caused by parasitic flatworms known as schistosomes.

These parasites belong to a group of infections called helminthiasis. The disease primarily affects the human urinary tract or the intestines. It can also impact many other animal species. Because it is widespread and affects millions, it is a major global health concern. In tropical regions, it is the second most economically significant parasitic disease, following malaria.

The infection begins when a person comes into contact with fresh water. This water contains cercariae, which are the larval forms of the parasite. These larvae are released from infected freshwater snails into the water.

When cercariae penetrate the skin, they can cause cercarial dermatitis. This is often called "swimmer's itch" and appears as an itchy, red, maculopapular rash. This rash can include flat lesions, raised pimples, or small blisters. For people who have been infected before, the immune response is stronger. This leads to a faster and more severe skin reaction.

After entering the skin, the parasites undergo several stages of development. The cercariae transform into schistosomulae. These young worms migrate through the bloodstream and the lungs. Eventually, they reach organs like the liver or the intestines to become adult worms.

The adult worms then lay eggs within the host. These eggs secrete proteolytic enzymes to help them migrate through tissues. This migration is necessary for the eggs to be shed in urine or stool. However, the movement of these eggs can also cause serious damage to the body.

Infections can be categorized into acute and chronic stages. Acute schistosomiasis, also known as Katayama fever, occurs 2 to 8 weeks after infection. This stage is a systemic reaction to the migrating worms and their antigens. Symptoms include fever, fatigue, muscle aches, cough, and diarrhea. It can also cause enlargement of the liver or spleen.

If the eggs migrate to the brain or spinal cord, neurological complications like seizures or paralysis may occur. Chronic infection happens when the disease persists for a long time. In this stage, the body's immune system reacts to eggs trapped in various tissues.

Chronic intestinal schistosomiasis is caused by species like *S. mansoni* and *S. japonicum*. These worms live in the veins of the gastrointestinal tract. When eggs get trapped in the intestinal wall, they cause a granulomatous reaction. This immune response can lead to abdominal pain, bloody stool, and diarrhea. Severe cases can result in intestinal polyps, ulcers, or even a complete bowel obstruction. In children, this can lead to poor growth and learning difficulties.

Chronic hepatic schistosomiasis affects the liver and spleen. This occurs when eggs become lodged in the liver, leading to fibrosis.

This scarring can cause portal hypertension and an enlarged spleen. In severe cases, it can lead to esophageal varices, which are swollen areas in the esophagus. These can tear and cause heavy bleeding. The disease can also cause fluid buildup in the abdomen.

Such complications can be life-threatening for those with long-term, heavy infections.

Schistosomiasis is a massive global health challenge. In 2019, it impacted approximately 236.6 million individuals. It is estimated that between 4,400 and 200,000 people die from the disease each year. About 700 million people live in regions where the disease is prevalent. These regions are mostly located in Africa, Asia, and South America.

Children and women are often at higher risk due to water exposure during play or daily chores.

Medical professionals use several methods to manage and prevent the disease. Diagnosis is made by finding parasite eggs in urine or stool, or by detecting antibodies in the blood. The World Health Organization recommends the medication praziquantel for infected individuals.

In some areas, praziquantel is given to entire groups once a year to reduce the number of infected people. Prevention also involves improving access to clean water and reducing the number of snails in the environment.