Medicine for Africa - Medical Information Service

CHISTOSOMIASIS

Definition:

Schistosomiasis is also called bilharzia or bilharziosis, after Dr. Theodor Bilharz, a German pathologist who first described the cause of schistosomiasis in Egypt in 1851; in colloquial terms it is also known as ‘swimmer’s itch’.  Schistosomiasis is a disease caused by a parasitic flatworm, called trematode of the genus Schistosoma, of which there are two major categories of parasites – urinary schistosomisasis is caused by Schistosoma (S.) haematobium, while intestinal schistosomiasis is caused by any organism of the genus S. mansoni and S. japonicum, and less often by S. intercalatum and S. mekongi.

Worldwide, more than 650 million people live in endemic areas, of which about 200 million are affected by the disease.

The larvae of the schistosoma parasites are released by certain susceptible freshwater snails in endemic areas.  The released free swimming larvae are called cercariae, which are no more than 1 mm in length, and live in fresh water for only about 48 hours, during which time they have to penetrate the bare skin of people swimming, bathing or wading through water, or the skin of other susceptible mammals, in order to survive.  Once inside the body, the cercariae transform into the schistosomulum stage, in which they invade blood veins and travel to the pulmonary vasculature, where they enter the general blood circulation via the pulmonary capillaries.  The worms mature and grow up to 10 mm in length in the liver before moving to the mesenteric veins of the intestinal tract (S. mansoni, S. japonicum), or the vesicular tissue of the urinary tract (S. haematobium).  After a few weeks, the female worms begin to produce eggs which can penetrate the walls of the bladder or intestinal tract, and are shed via urine or feces.  Eggs that are not shed can induce intense granulomatous reactions due to their highly antigenic characteristics.  Eggs that are shed, transform again into free-swimming miracidia which will live in fresh water for about two to three weeks, during which time they need to infect a susceptible snail, in order to complete their life cycle and the continuous cycle of infection to humans.  Adult worms can produce between 300 (S. mansoni) and 3000 eggs (S. japonicum) per day, and remain in the human body for an average of 4 ½ years, although some can persist for up to 20 years.

Schistosomiasis is wide spread in developing countries, especially in:

• Africa – all freshwater rivers and lakes in southern and sub-Saharan Africa, including the great lakes, and the Nile River Valley in Egypt;
• Asia – Cambodia, southern China, central Indonesia, Laos, Mekong delta, Philippines, Suriname;
• South America – Brazil, Venezuela;
• Caribbean – Antigua, Dominican Republic, Guadeloupe, Martinique, Montserrat;
• Middle East – Iran, Iraq, Saudi Arabian Peninsula, including Yemen.

Symptoms:

In acute schistosomiasis, a mild maculopapular rash (flat discoloured areas and small raised lumps) may develop in some patients within just hours of exposure to cercarial infection. Usually, the acute stage of the disease is asymptomatic (without showing any symptoms), but can present itself after two to eight weeks following cercarial infestation.  Acute schistosomiasis is now also called ‘Katayama fever’, named after the Katayama valley in Japan, that was hyperendemic for S. japonicum, and where the Japanese Dr. Yoshinao Fujii first described the disease syndrome in 1848. 
                          
Characteristic features of Katayama fever include prolonged fever, marked eosinopilia, unproductive cough, hematuria (blood in urine), loss of appetite with associated weight loss, weakness, headaches, abdominal pain, as well as joint and muscle pain, diarrhea and nausea.  These symptoms often disappear after a few weeks and the patient’s general health status gradually improves, but the acute stage of the disease also bears a mortality rate of up to 25%.  

Chronic schistosomiasis is more common than the acute form and may occur many months to years following exposure, resulting from the body’s immune response against the antigens of the schistosoma eggs.  This immune response causes granuloma formation with associated fibrotic changes. As the worms traveled from the lungs to the liver, hepatosplenomegaly (enlarged liver and spleen) may occur, as well as pulmonary granulomatosis and fibrosis with resulting pulmonary hypertension, hematemesis (vomiting of blood) and the development of cor pulmonale.

The development of egg containing granulomas in the intestinal wall (with S. mansoni or S. japonicum) can cause blood-tinged diarrhea, cramps, inflammatory colonic polyposis and, of course, the excretion of numerous schistosoma eggs, which will be diagnostic of the disease.  

Egg reactive granuloma formation in the urinary bladder (S. haematobium) can cause hematuria, dysuria (pain during urination), with progressive damage to the bladder (polyps and ulcers), ureters and kidneys.  Infection is also associated with an increased risk of developing bladder cancer (squamous cell cancer).

Involvement of the central nervous system (CNS) is rare, but can result in spinal cord involvement leading to paralysis (S. haematobium and S. mansoni) and/or cerebral involvement (most common with S. japonicum) causing seizures and changes in mental status.

Ulcerated or fistulous lesions in the vulvar and perianal area can often be misdiagnosed as lesions derived from sexually transmitted diseases.

 
Diagnosis:

Microscopic identification of schistosoma eggs in urine for S. haematobium, or for S. mansoni or S. japonicum in stool specimens is diagnostic of the disease, and absolutely reliable.  At times, microscopic identification in surgically removed tissue specimens from granulomas can also reveal the schistosoma eggs with their characteristic spine location – S. mansoni with a lateral spine, S. haematobium with a terminal spine, and S. japonicum with a small knob:

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Specific blood tests usually turn positive only after six to eight weeks. A complete blood count (CBC) may reveal extensive eosinophilia, especially during the acute stage of the disease, and/or a more or less pronounced anemia; however, neither result is diagnostic of schistosomiasis.  The same is valid for chemistry tests – increased values in alkaline phosphatase or gamma-glutamyltransferase (GGT) point to liver involvement, and decreased values for renal function may all be present, but do not identify the cause of those changes as being due to schistosomiasis/bilharziosis. 

Imaging studies, such as ultrasonography can reveal hepatosplenomegaly, periportal liver fibrosis and adenepathy, as well as ureteral obstruction and obstructive nephropathy – while these results are non-specific, they can support the diagnosis of schistosomiasis, derived via history, physical examination and laboratory findings such as blood values as described above and microscopic evaluation of urine and/or stool samples.

Chest radiology can reveal pulmonary involvement by way of pulmonary hypertension and cor pulmonale.

Computer tomography (CT-scans) and Magnetic Resonance Imaging (MRI) may show CNS involvement if present, but are expensive and extensive tests which also are non-specific for the diagnosis of the disease itself; they are, however, useful in exploring the extent of an infection.

Serology tests such as the expensive Enzyme-Linked ImmunoSorbent Assays (ELISA) can confirm past exposure. While antibody tests cannot distinguish between an active and an inactive infection, specific antigen testing can help at times to distinguish active from inactive infections – their titers correlate with the extent of the infection, as indicated by individual egg counts.

Surgical procedures, such as a colonic biopsy or a cystoscopy of the urinary bladder, may help to evaluate the extent of the disease progression, and help to diagnose secondary ulcers, polyps, or the development of local malignant tissue changes – but are elaborate and costly.


 
Treatment:

The treatment of choice consists of the anthelmintic drug praziquantel (biltricide), in a dose of 40 mg per kg body weight, given by mouth once, or in two divided doses within 24 hours – this regimen is effective against S. haematobium and S. mansoni.  For the treatment of S. japonicum and S. mekongi, 60 mg per kg of bodyweight, given in three equal doses over 24 hours, is recommended.  The regimens have a success rate of 60 to 98%, primarily relying on the point of time of starting the therapy.  However, treatment may at times exacerbate acute infections due to the increased release of schistosoma antigens, and thus usually requires the addition of corticosteroids.

Praziquantel is considered to be a safe drug that is highly effective in treating an infected patient; however, it does not prevent the re-infection with cercariae, and thus has no value in preventing the disease.

A second drug, oxamniquine (vansil) is well absorbed and an alternative drug for the treatment of S. mansoni, however, it is not effective against S. haematobium and S. japonicum.  Recommended dosage is 12-15 mg per kg of body weight given orally for two days.  Caveat – the drug’s safety for use in pregnant women has not yet been established.


 
Prevention:

At this point in time, there is no vaccine available, nor is there any prophylactic chemotherapeutic drug regimen available.

The only effective prevention against contracting the disease is by staying away from potentially snail and schistosoma infected fresh water sources, such as rivers, lakes and ponds, especially in known endemic areas.

Swimming in the ocean, i.e. salt water, or in chlorinate swimming pools is generally considered safe – at least in regard to schistosoma infections.

Drinking water derived from uncertain sources should be boiled for at least one minute prior to consumption.  Bath water held in a storage tank for at least 48 hours should be safe for showering, otherwise it should be heated up to 125º F (approximately 50º C).

Vigorous drying with a towel after accidental exposure to potentially infected water has been said to provide some degree of help to prevent the parasite from penetrating the skin.  However, this trick is not recommended to rely on as a safe preventive measurement against schistosomiasis infection.

The topical application of the chemical insect repellent DEET (N,N-diethyl-m-toluamide) also can block the skin penetration of cercariae, however, its effect is short lived and not a reliable way of prevention.

It has been reported that the ingestion of Castor Oil may provide some level of prevention against infection, but it has not been proven to be a consistently successful and/or reliable way of preventing schistosomiasis.

Improved sanitation measurements could, over time, decrease the prevalence of the disease considerably.  This approach would require the coordinated efforts of local governments and communities to build sewage plants, which unfortunately often extends beyond the financial possibilities of most communities in many endemic areas of the world.

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