Schistosomiasis, also known as snail fever, bilharzia, and Katayama fever is a tropical disease, specifically a helminthiasis caused by the parasitic flatworm of the genus Schistosoma. Schistosomiasis affects humans as well as other animals. It affects the urinary tract or the intestines. Symptoms include abdominal pain, diarrhea, bloody stool, or blood in the urine. Those who have had a schistosomiasis infection for a long time may experience liver damage, kidney failure, infertility, or bladder cancer. In children, schistosomiasis may cause failure to thrive, poor growth, and learning difficulties. Schistosomiasis is classified as a helminth infection.
Schistosomiasis is spread by contact with fresh water contaminated with parasites released from infected freshwater snails. Medical diagnosis is made by finding the parasite's eggs in a person's urine or stool. Diagnosis can also be confirmed by finding antibodies against the disease in the blood.
Methods of preventing the disease include improving access to clean water and reducing the number of snails. In areas where the disease is common, the medication praziquantel may be given once a year to the entire at-risk group. This is done to decrease the number of people infected, and consequently, the spread of the disease. Praziquantel is also the treatment recommended by the World Health Organization (WHO) for those who are known to be infected.
The disease is especially common among children in developing countries, as children there are more likely to play in contaminated water. Schistosomiasis is also common among women, as they generally have greater exposure through daily chores that involve water, such as washing clothes and fetching water. Other high-risk groups include farmers, fishermen, and people using unclean water during daily living.
In 2019, schistosomiasis impacted approximately 236.6 million individuals across the globe. Each year, it is estimated that between 4,400 and 200,000 individuals succumb to it. The illness predominantly occurs in regions of Africa, Asia, and South America, where approximately 700 million individuals in over 70 nations reside in regions where the disease is prevalent. In tropical regions, schistosomiasis ranks as the second most economically significant parasitic disease, second only to malaria. Schistosomiasis is classified as a neglected tropical disease.
Signs and symptoms
Many individuals do not experience symptoms. If symptoms do appear, they usually have an incubation period of about 4–6 weeks. The first symptom of the disease may be a general feeling of illness. Within 12 hours of infection, an individual may develop cercarial dermatitis due to irritation at the point of entrance, commonly referred to as "swimmer's itch". The rash that may develop can mimic scabies and other rashes.
The manifestation of a schistosomal infection varies over time as cercariae, (the larval form of the parasite) and later the adult worms and their eggs, migrate through the body. If eggs migrate to the brain or spinal cord, seizures, paralysis, or spinal cord inflammation are possible.
Acute infection Manifestation of acute infection from schistosomiasis includes cercarial dermatitis (hours to days) and acute systemic schistosomiasis (2–8 weeks) which can include symptoms of fever, myalgia, a cough, bloody diarrhea, chills, or lymph node enlargement. Some patients may also experience dyspnea and hypoxia associated with the development of pulmonary infiltrates.
Cercarial dermatitis The first potential reaction is an itchy, maculopapular rash that, within the first 12 hours to days of penetration, results from cercariae penetrating the skin. The first time a non-sensitized person is exposed, the rashes are usually mild with an associated prickling sensation that quickly disappears on its own since this is a type of hypersensitivity reaction. In sensitized people who have previously been infected, the rash can develop into itchy, red, raised lesions (papules) with some turning into fluid-filled lesions (vesicles). Previous infections with cercariae causes a faster developing and worse presentation of dermatitis due to the stronger immune response. The round bumps are usually one to three centimeters across.
Because people living in affected areas have often been repeatedly exposed, acute reactions are more common in tourists and migrants. The rash can occur between the first few hours and a week after exposure, and they normally resolve on their own in around 7–10 days. For human schistosomiasis, a similar type of dermatitis called "swimmer's itch" can also be caused by cercariae from animal trematodes that often infect birds. Cercarial dermatitis is not contagious and can not be transmitted from person-to-person.
Symptoms may include: Flat, red rash Small red, raised pimples Small red blisters Prickling or tingling sensation, burning, itching of the skin
Scratching the rash can lead to secondary bacterial infection of the skin, thus it is important to refrain from scratching. Common treatments for itching include corticosteroid cream, anti-itch lotion, application of cool compresses to rash, bathing in Epsom salts or baking soda, and in severe cases, prescription strength cream and lotions. Oral antihistamines can also help relieve the itching.
Acute schistosomiasis (Katayama fever) Acute schistosomiasis (Katayama fever) may occur weeks or months (around 2–8 weeks) after the initial infection as a systemic reaction against migrating schistosomulae as they pass through the bloodstream, through the lungs, and to the liver; and also against the antigens of eggs. Similarly to swimmer's itch, Katayama fever is more commonly seen in people with their first infection such as migrants and tourists, and it is associated with heavy infection. However, it is also seen in native residents of China infected with S. japonicum. S. japonicum can cause acute schistosomiasis in a chronically infected population, and can lead to a more severe form of acute schistosomiasis.
Symptoms may include: Dry cough with changes on chest X-ray Fever Fatigue Muscle aches Headache Malaise Abdominal pain Diarrhea Enlargement of the liver (hepatomegaly), the spleen (splenomegaly), or both (hepatosplenomegaly) Hives Acute schistosomiasis self-resolves within 2–8 weeks in most cases, but a small proportion of people have persistent weight loss, diarrhea, diffuse abdominal pain, and rash.
Neurological complications may include: Spinal cord inflammation (transverse myelitis) may occur if worms or eggs travel to the spinal cord during this acute phase of infection. Headaches Disturbances of sensorium Hemiplegia Tetraplegia Visual impairment Ataxia/ speech impairment Motor paralysis Cardiac complications may include:
Myocarditis Pericarditis Asymptomatic myocardial ischemia
Treatment may include: Corticosteroid such as prednisone is used to alleviate the hypersensitivity reaction and reduce inflammation. Praziquantel can be administered to kill adult schistosomes and prevent chronic infection in addition to corticosteroid therapy. It is ineffective for recent infections as it only targets adult worms rather than premature schistosomulae. Therefore, a repeat praziquantel treatment several weeks after initial infection may be warranted. It is recommended to treat with praziquantel 4–6 weeks after initial exposure since it targets adult worms. For acute schistosomiasis (AS), praziquantel is ineffective on schistosomulae after 7 days and does not prevent the chronic phase of the disease. Too early treatment can worsen symptoms of AS. In some cases, this worsening of symptoms can be life-threatening by causing encephalitis related to vasculitis, myocarditis, or pulmonary events. Oxamniquine (50 mg/kg once) can be administered in the early phase of schistosomiasis. It is more effective against schistsomulae than praziquantel, but only with S. mansoni. This prevents the chronic S. mansoni infection and egg-laying stages. Artemether is an artemisin derivative efficient against schistosomulae aged 7–21 days, but only reduces S. mansoni infection by 50% in exposed children.
Chronic infection In long-established schistosomiasis, adult worms lay eggs that can cause inflammatory reactions. The eggs secrete proteolytic enzymes that help them migrate to the bladder and intestines to be shed. The enzymes also cause an eosinophilic inflammatory reaction when eggs get trapped in tissues or embolize to the liver, spleen, lungs, or brain. The long-term manifestations are dependent on the species of schistosome, as the adult worms of different species migrate to different areas. Many infections are mildly symptomatic, with anemia and malnutrition being common in endemic areas.
Intestinal schistosomiasis The worms of S. mansoni and S. japonicum migrate to the gastrointestinal tract and liver veins. Eggs in the gut wall can lead to pain, bloody stool, and diarrhea (especially in children). Severe disease can lead to narrowing of the colon or rectum.
In intestinal schistosomiasis, eggs become lodged in the intestinal wall during their migration from the mesenteric venules to the intestinal lumen, and the trapped eggs cause an immune system reaction called a granulomatous reaction. They mostly affect the large bowel and rectum, and involvement of the small bowel is rare. This immune response can lead to colonic obstruction and blood loss. The infected individual may have what appears to be a potbelly. There is a strong correlation between the morbidity of intestinal schistosomiasis and the intensity of infection. In light infections, symptoms may be mild and can go unrecognized. The most common species to cause intestinal schistosomiasis are S. mansoni and S. japonicum, however, S. mekongi and S. intercalatum can also cause this disease.
Symptoms may include: Abdominal pain and discomfort Loss of appetite Mucous diarrhea with or without gross blood Blood in feces that is not visibly present (fecal occult blood) Abdominal distention
Complications may include: Intestinal polyps Intestinal ulcers Iron-deficient anemia Fistula Bowel strictures (narrowing of colon or rectum) Protein-losing enteropathy Partial or complete bowel obstruction Appendicitis (rare) Approximately 10–50% of people living in endemic regions of S. mansoni and S. japonicum develop intestinal schistosomiasis. S. mansoni infection epidemiologically overlaps with high HIV prevalence in Sub-Saharan Africa, where gastrointestinal schistosomiasis has been linked to increased HIV transmission.
Hepatosplenic schistosomiasis Eggs also migrate to the liver, leading to fibrosis in 4 to 8% of people with chronic infection, mainly those with long-term heavy infection.
Eggs can become lodged in the liver, leading to portal hypertension, splenomegaly, the buildup of fluid in the abdomen, and potentially life-threatening dilations or swollen areas in the esophagus or gastrointestinal tract that can tear and bleed profusely (esophageal varices). This condition can be separated into two phases: inflammatory hepatic schistosomiasis (early inflammatory reaction) and chronic hepatic schistosomiasis. Most common species to cause this condition are S. mansoni, S. japonicum, and S. mekongi.
Inflammatory hepatic schistosomiasis occurs mainly in children and adolescents due to early immune reaction to eggs trapped within the periportal and presinusoidal spaces of the liver creating numerous granulomas. Liver function is unaffected, and the severity of liver and spleen enlargement is correlated to the intensity of the infection. It is characterized by an enlarged left lobe of the liver with a sharp edge and an enlarged spleen with nodules. The liver and spleen enlargement is usually mild, but in severe cases, they can enlarge to the level of the belly button and even into the pelvis.