Schistosomiasis (Snail Fever) Causes, Symptoms, Treatment, Diagnosis, Prevention (original) (raw)

What is schistosomiasis (snail fever)?

Life cycle of schistosomiasis

Life cycle of schistosomiasis

Schistosomiasis is a disease that is caused by parasites (genus Schistosoma) that enter humans by attaching to the skin, penetrating it, and then migrating through the venous system to the portal veins where the parasites produce eggs and eventually, the symptoms of acute or chronic disease (for example, fever, abdominal discomfort, blood in stools). Health officials consider the disease to be a worm infection or helminthiasis.

Bilharziasis, bilharzia, bilharziosis, and snail fever or, in the acute form, Katayama fever are alternate names for schistosomiasis. Theodore Bilharz identified the parasite Schistosoma haematobium in Egypt in 1851. Schistosomiasis is the second most prevalent tropical disease in the world; malaria is the first. The disease is found mainly in developing countries in Africa, Asia, South America, the Middle East, and the Caribbean and is considered one of many tropical diseases that can be soil-transmitted (or water-transmitted). In the U.S., it is diagnosed in tourists who have visited these developing countries and in visitors from these countries, or lab accidents. More than 200,000 people die each year in Sub-Saharan Africa from this infection. The type of snail that is part of the parasite's life cycle (see below) is not endemic (non-endemic) to U.S. freshwater sources in the U.S.

In 2014, an outbreak occurred in Corsica, France, in people swimming in the Cavu River. This was the first reported locally acquired Schistosoma infection in France.

What causes schistosomiasis?

Parasites of the genus Schistosoma (S. mansoni, S. mekongi, S. intercalatum, S. haematobium, and S. japonicum) cause the disease. The disease in humans is part of the complicated life cycle of the parasites that are illustrated in the figure below. Humans enter freshwater areas that contain snails that grow Schistosoma sporocysts that develop into free-swimming cercariae shed by freshwater snails (Biomphalaria and Bulinus genus), considered to be an intermediate host. The cercariae can attach to and penetrate the human skin, migrate to blood vessels, and through lung blood capillaries reach the portal blood or vesicular (bladder) blood systems. During this migration, the cercariae change and develop from schistosomula into male and female adult parasitic worms. The worms incorporate human proteins into their surface structures, so most humans produce little or no immune response to the parasites. After parasite mating occurs in the portal or vesicular blood system, egg production occurs. In contrast to the adult parasites, the parasite's eggs stimulate a strong immune response in most humans. Some eggs migrate through the bowel or bladder tissue and are shed in feces or urine to soil or water, while other eggs are swept into the portal blood and lodge in other tissue sites. Eggs shed into urine or feces may reach maturity in freshwater (a hatched egg develops into a miracidium) and complete their life cycle by infecting susceptible snails. In addition, some adult worms may migrate to other organs (for example, the eyes or liver). This life cycle is further complicated by S. japonicum species that may also infect domesticated and wild animals, which can then serve as another host system. S. haematobium is the species that usually infects the human bladder tissue, while the other species usually infect the bowel tissue.

The acute and chronic symptoms of schistosomiasis are thought to be mainly due to the egg migration through tissue and the human immune response to the eggs. Chronic symptoms are mainly due to eggs that are not shed from the body. Complications (for example, hepatomegaly or enlarged liver and bladder cancer) related to the disease are thought to occur due to long-term exposure to the highly antigenic eggs.

IMAGES Schistosomiasis See pictures of Bacterial Skin Conditions See Images

What are the symptoms of schistosomiasis?

Although a few patients may have minor skin irritation when the cercariae enter the skin, most people do not develop symptoms until the eggs develop (about one to two months after initial skin penetration). Then, fever, chills, cough, and muscle aches can begin within one to two months of infection. However, most people have no symptoms at this early phase of infection. Unfortunately, a few patients develop acute schistosomiasis (Katayama fever) during this one- to two-month period, and their symptoms resemble those of serum sickness and are as follows:

The majority of people who develop chronic schistosomiasis have symptoms developed months or years after the initial exposure to the parasites. The following is a list of most symptoms associated with chronic schistosomiasis. Patients usually have a few of these symptoms.

People associated with freshwater sources in areas where Schistosoma are endemic should seek medical care if they develop symptoms of acute schistosomiasis (see above, especially for abdominal pain, blood in stools or urine, and fever). Those with diagnosed chronic schistosomiasis should seek medical care if their chronic symptoms increase (especially abdominal pain, shortness of breath, bloody diarrhea or bloody urine, seizures, or mental status changes). Anyone with undiagnosed schistosomiasis who develops the symptoms listed above should seek medical care and inform the caregivers that they have been exposed to freshwater sources in areas where the disease is endemic either as residents of the areas or as a tourist.

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Diagnosis of schistosomiasis

The presumptive diagnosis of schistosomiasis is based on the medical caregiver's history and physical examination of the patient. It is important to know that a person has inhabited or visited areas of the world where the disease is endemic, especially if the person has had skin exposure to freshwater lakes and streams. If the patient has that history and has symptoms that are described above, a presumptive diagnosis may be made. However, because symptoms of schistosomiasis resemble those of serum sickness and other diseases, definitive diagnostic tests are usually required. Thick fecal smears and urine concentration tests (for example, the Kato-Katz test) are used to determine if any Schistosoma eggs are present. If eggs are found, the patient is definitively diagnosed with schistosomiasis. In addition, most eggs from each species are shaped differently so it is possible to determine which Schistosoma is infecting the patient. Sometimes the definitive diagnosis is made by examination of biopsy samples of tissue when the eggs are visualized in the infected tissue.

Blood tests and, more recently, polymerase chain reaction (PCR) tests can help confirm the diagnosis, but positive results may only indicate past exposure. However, these tests are not usually positive until the patient has been infected for about six to eight weeks because it takes time for the eggs to develop and stimulate the human immune response. Each Schistosoma species has different egg proteins that can be detected by antibody tests. The PCR test is available from the U.S. Centers for Disease Control and Prevention.

Many other tests and procedures may be necessary to establish the diagnosis, especially if no eggs are found in the feces or urine, which is often the situation in chronic schistosomiasis. Colonoscopy, cystoscopy, endoscopy, and liver biopsy are all methods that can be used to obtain tissue biopsy material. In addition, ultrasound, chest X-rays, CT scan, MRI, and echocardiograms may be used to determine the extent of the infection in various organ systems. Most physicians will run additional blood tests (complete blood count [CBC], liver function tests, renal function tests) to determine if organs have been damaged by the parasites.

What is the treatment for schistosomiasis?

Currently, the drug used in most people is praziquantel (Biltricide); however, it only is effective against adult worms and does not affect eggs or immature worms. Treatment with this drug is simple and its dose is based on the patient's weight with two doses given in one day. However, the drug causes rapid disintegration of the worm which, in turn, allows the human immune system to attack the parasite. This immune response can cause localized reactions, which may increase the patient's symptoms. Corticosteroids are often used to reduce the symptoms of this reaction. Unfortunately, this response limits the use of praziquantel. Praziquantel and oxaminquine or artemether are used by some clinicians early in infections, or to treat individuals infected with both malaria and schistosomes, respectively.

Ocular schistosomiasis should not be treated with this praziquantel. Other organs with heavy parasite infections may not function well and require supportive care until the hyperimmune response abates after drug administration. Other drugs (oxamniquine, metrifonate, artemisinins, and trioxolanes) have been used in some patients but have limited effectiveness. New drugs are in development. Infectious disease specialists, ophthalmologists, and surgeons may treat someone with a schistosomiasis infection.

Surgical care may include removal of tumor masses, ligation of esophageal varices, shunt surgeries, and granuloma removal.

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What are the complications of schistosomiasis?

The complications that may develop with schistosomiasis usually occur in individuals harboring many parasites and eggs, especially when the eggs and parasites have migrated to other organs. In general, complications usually involve the cardiopulmonary, gastrointestinal, and central nervous systems (CNS), the liver and spleen, and urinary tracts along with the liver and spleen. Some of the major complications are high blood pressure (hypertension), seizures, bacterial infections, urinary obstruction, organ damage or destruction, and death.

What is the prognosis for schistosomiasis?

Early antiparasitic treatment, especially with acute schistosomiasis, may allow people to recover completely without developing chronic disease. A few people get the disease but recover completely. Even patients with early chronic disease can improve with drug treatment. However, the prognosis is worse for people who have other health problems (for example, with a suppressed immune system, HIV, or chronic infections such as malaria) and subsequently get infected with Schistosoma. People with chronic disease may improve with careful antiparasitic drug treatments and symptomatic treatment of the complications associated with schistosomiasis.

Is it possible to prevent schistosomiasis?

Theoretically, the disease can be prevented by avoiding all human skin contact with freshwater sources where schistosomiasis and the snails that complete their life cycle are endemic. However, this is unlikely to occur in most developing countries. Disease control officials' reports of attempts to decrease or eliminate snails from some freshwater sources using molluscicides (snail bait) have cited a decrease in the number of people infected, but this often requires repeat treatments of contaminated environments and some efforts have been stopped because of limited success.

Unfortunately, people who are treated and have no symptoms of the disease can easily become reinfected if exposed to the cercariae; as the human immune response to this disease often is not able to prevent reinfection. There is no commercially available vaccine against Schistosoma, but research is ongoing and perhaps a vaccine may be available in a few years.

Children of school age are at risk or at high risk for the disease because they often have skin and bare feet exposed to contaminated water and soil.

Medically Reviewed on 8/2/2023

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