Schistosomiasis is a parasitic disease caused by flukes (trematodes) of the genus Schistosoma. After malaria and intestinal helminthiasis, schistosomiasis is the third most devastating tropical disease in the world, being a major source of morbidity and mortality for developing countries in Africa, South America, the Caribbean, the Middle East, and Asia. (See Epidemiology and Prognosis.) [1]

More than 140 million people, 90% of who live in Africa, are infected with schistosomiasis. [2, 3] An estimated 700 million people are at risk of infection in 76 countries where the disease is considered endemic, as their agricultural work, domestic chores, and recreational activities expose them to infested water. [1, 4] Globally, 200,000 deaths are attributed to schistosomiasis annually. [5] Transmission is interrupted in some countries. [4] (See Etiology and Epidemiology.)

The World Health Organization (WHO) estimates that about 220.8 million people required preventive treatment for schistosomiasis in 2017. An estimated 102.3 million people were treated the same year. [3]

Sometimes referred to as bilharzias, bilharziasis, or snail fever, schistosomiasis was discovered by Theodore Bilharz, a German surgeon working in Cairo, who first identified the etiological agent Schistosoma hematobium in 1851. [6] A Schistosoma egg is seen below.

Egg of Schistosoma hematobium, with its typical terminal spine. View Media Gallery

Most human schistosomiasis is caused by S haematobium, S mansoni, and S japonicum. Less prevalent species, such as S mekongi and S intercalatum, may also cause systemic human disease. Less importantly, other schistosomes with avian or mammalian primary hosts can cause severe dermatitis in humans (eg, swimmer's itch secondary to Trichobilharzia ocellata). (See Etiology.)

Characteristics of schistosomiasis

Schistosomiasis is due to immunologic reactions to Schistosoma eggs trapped in tissues. Antigens released from the egg stimulate a granulomatous reaction involving T cells, macrophages, and eosinophils that results in clinical disease (see the image below). Symptoms and signs depend on the number and location of eggs trapped in the tissues. Initially, the inflammatory reaction is readily reversible. In the latter stages of the disease, the pathology is associated with collagen deposition and fibrosis, resulting in organ damage that may be only partially reversible. (See Pathophysiology, Etiology, and Presentation.)

Granuloma in the liver due to Schistosoma mansoni. The S mansoni egg is at the center of the granuloma. View Media Gallery

Eggs can end up in the skin, brain, muscle, adrenal glands, and eyes. As the eggs penetrate the urinary system, they can find their way to the female genital region and form granulomas in the uterus, fallopian tube, and ovaries. Central nervous system (CNS) involvement occurs because of embolization of eggs from the portal mesenteric system to the brain and spinal cord via the paravertebral venous plexus. [7, 8, 9]

Snail hosts

The different species of Schistosoma have different types of snails serving as their intermediate hosts; these hosts are as follows [10, 11, 12] :

Biomphalaria for S mansoni

Oncomelania for S japonicum

Tricula (Neotricula aperta) for S mekongi

Bulinus for S haematobium and S intercalatum

At-risk populations

Today, 120 million people are symptomatic with schistosomiasis, with 20 million having severe clinical disease. [1] More than 200,000 deaths per year are due to schistosomiasis in sub-Saharan Africa. [13] Women washing clothes in infested water are at risk. [14] Hygiene and playing in mud and water make children vulnerable to infection. Forty million women of childbearing age are infected. [15] Approximately 10 million women in Africa have schistosomiasis during pregnancy. [15] In endemic areas, the infection is usually acquired as a child. [4]

In Brazil and Africa, refugee movements and migration to urban areas are introducing the disease to new locations. Increasing population size and corresponding needs for power and water have led to increased transmission. Infections are not uniformly distributed within communities. It has been estimated that 5-10% of an endemic community may be heavily infected, and the remainder has mild to moderate infections. The risk of infection is highest amongst those who lived near lakes or rivers. [16] In Uganda, almost no transmission was found to have occurred at altitudes greater than 1400 m or where the annual rainfall was less than 900 mm. [16]

With the rise of tourism and travel, an increasing number of tourists are contracting it. Tourists often present with severe acute infection and unusual problems including paralysis.

The intensity and prevalence of infection rises with age and peaks usually between ages 15 and 20 years. In older adults, no significant change is found in the prevalence of disease, but the parasite burden or the intensity decreases. [10, 17, 18] The disease is not endemic in United States.

Complications

Complications of schistosomiasis include the following: