DEFINITION
Schistosomiasis is 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 hematobium 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 from lab accidents. More than 200,000 people die each year in Sub-Saharan Africa from this infection.
Causes
Parasites of the genus Schistosoma (S. mansoni, S. mekongi, S. intercalatum, S. hematobium, and S. japonicum) cause the disease. 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 by 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, 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. hematobium is the species that usually infects the human bladder tissue, while the other species usually infect the bowel tissue.
Signs and symptoms
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 for serum sickness and are as follows:
• Fever
• Abdominal pain (liver/spleen area)
• Bloody diarrhea or blood in the stools
• Cough
• Malaise
• Headache
• Rash
• Body aches
The majority of people who develop chronic schistosomiasis have symptoms develop 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.
• Abdominal pain
• Abdominal swelling (ascites)
• Bloody diarrhea or blood in the stools
• Blood in the urine and painful urination
• Shortness of breath and coughing
• Weakness
• Chest pain and palpitations
• Seizures
• Paralysis
• Mental status changes
• Lesions on the vulva or the perianal area
Treatment
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 on 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.
Prevention
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.