Schistosomiasis (bilharzias)

 

Schistosomiasis (bilharzias)

Infection occurs when skin comes in contact with contaminated freshwater in which certain types of snails that carry the parasite are living. Freshwater becomes contaminated by schistosome eggs when infected people urinate or defecate in the water. The eggs hatch, and if the appropriate species of snails are present in the water, the parasites infect, develop and multiply inside the snails. The parasite leaves the snail and enters the water where it can survive for about 48 hours. Larval schistosomes (cercariae) can penetrate the skin of persons who come in contact with contaminated freshwater, typically when wading, swimming, bathing, or washing. Over several weeks, the parasites migrate through host tissue and develop into adult worms inside the blood vessels of the body. Once mature, the worms mate and females produce eggs. Some of these eggs travel to the bladder or intestine and are passed into the urine or stool.

 

Schistosomiasis (Bilharziasis) is caused by some species of blood trematodes (flukes) in the genus Schistosoma. The three main species infecting humans are Schistosoma haematobium, S. japonicum, and S. mansoni. Three other species, more localized geographically, are S. mekongi, S. intercalatum, and S. guineensis (previously considered synonymous with S. intercalatum). There have also been a few reports of hybrid schistosomes of cattle origin (S. haematobium, x S. bovis, x S. curassoni, x S. mattheei) infecting humans. Unlike other trematodes, which are hermaphroditic, Schistosoma spp. are dioecous (individuals of separate sexes).

 

Schistosoma eggs are eliminated with feces or urine, depending on species

1.     . Under appropriate conditions the eggs hatch and

2.      release miracidia

3.     , which swim and penetrate specific snail intermediate hosts

4.     . The stages in the snail include two generations of sporocysts 

5.     and the production of cercariae

6.     . Upon release from the snail, the infective cercariae swim, penetrate the skin of the human host

7.      , and shed their forked tails, becoming schistosomulae

8.      . The schistosomulae migrate via venous circulation to lungs, then to the heart, and then develop in the liver, exiting the liver via the portal vein system when mature,  

9.      . Male and female adult worms copulate and reside in the mesenteric venules, the location of which varies by species (with some exceptions)

10.  . For instance, S. japonicumis more frequently found in the superior mesenteric veins draining the small intestine  and S. mansoni occurs more often in the inferior mesenteric veins draining the large intestine 

 

·         . However, both species can occupy either location and are capable of moving between sites. S. intercalatum and S. guineensis also inhabit the inferior mesenteric plexus but lower in the bowel than S. mansoni. S. haematobium most often inhabitsin the vesicular and pelvic venous plexus of the bladder

·        but it can also be found in the rectal venules. The females (size ranges from 7–28 mm, depending on species) deposit eggs in the small venules of the portal and perivesical systems. The eggs are moved progressively toward the lumen of the intestine (S. mansoni,S. japonicum, S. mekongi, S. intercalatum/guineensis) and of the bladder and ureters (S. haematobium), and are eliminated with feces or urine, respectively 

·        .Hosts Various animals such as cattle, dogs, cats, rodents, pigs, horses, and goats, serve as reservoirs for S. japonicum, and dogs for S. mekongi. S. mansoni is also frequently recovered from wild primates in endemic areas but is considered primarily a human parasite and not a zoonosis.

Intermediate hosts are snails of the genera Biomphalaria, (S. mansoni), Oncomelania (S. japonicum), Bulinus (S. haematobium, S. intercalatum, S. guineensis). The only known intermediate host for S. mekongi is Neotricula aperta.

 

Symptoms of schistosomiasis are caused not by the worms themselves but by the body’s reaction to the eggs. Eggs shed by the adult worms that do not pass out of the body can become lodged in the intestine or bladder, causing inflammation or scarring. Children who are repeatedly infected can develop anemia, malnutrition, and learning difficulties. After years of infection, the parasite can also damage the liver, intestine, spleen, lungs, and bladder.

Common symptoms

 

Most people have no symptoms when they are first infected. However, within days after becoming infected, they may develop a rash or itchy skin. Within 1-2 months of infection, symptoms may develop including fever, chills, cough, and muscle aches.

 

Chronic schistosomiasis

Without treatment, schistosomiasis can persist for years. Signs and symptoms of chronic schistosomiasis include: abdominal pain, enlarged liver, blood in the stool or blood in the urine, and problems passing urine. Chronic infection can also lead to increased risk of liver fibrosis or bladder cancer.

Rarely, eggs are found in the brain or spinal cord and can cause seizures, paralysis, or spinal cord inflammation.

 

 

Diagnosis

 

Stool or urine samples can be examined microscopically for parasite eggs (stool for S. mansoni or S. japonicum eggs and urine for S. haematobium eggs). The eggs tend to be passed intermittently and in small amounts and may not be detected, so it may be necessary to perform a blood (serologic) test.

 Schistosomiasis Treatment

Praziquantel is the only drug available to treat schistosomiasis and has been used for decades in mass administration programs targeting entire populations. Treatment does not prevent later re-infections with larval worms if re-exposure occurs. The lack of alternatives to praziquantel is a concern because the parasites could develop resistance to the drug, resulting in no effective treatment for schistosomiasis.

Though safe and moderately effective, praziquantel’s molecular mechanism of action has not been characterized. An NIAID-supported research project is using planarian flatworms that develop two heads and an integrated nervous system under lab conditions to tease out how praziquantel interacts with flatworm nervous system protein receptors. Information gained could be used to develop new and improved treatments that disrupt similar protein receptors in schistosome flatworms.

Other projects are aimed at discovering compounds capable of inhibiting essential parasite enzymes. One research team is screening 100,000 compounds to identify any that inhibit a specific enzyme the parasite needs to establish infection in mammals. Additional screens will be used to separate out those compounds that are the most potent, specific and safe, and that could serve as the basis for new drug therapies. 

Another effort is focused on a protein complex called a proteasome, which is used by several kinds of parasites, including the ones that cause malaria. In studies of malaria parasites, NIAID-supported researchers designed a potent and selective inhibitor of the malarial proteasome. They are now applying the technique to synthesizing proteasome inhibitors that act against S. mansoni parasites. 

 

Safe and effective medication is available for treatment of both urinary and intestinal schistosomiasis. Praziquantel, a prescription medication, is taken for 1-2 days to treat infections caused by all schistosome species.

Prevention

No vaccine is available.

The best way to prevent schistosomiasis is to take the following steps if you are visiting or live in an area where schistosomiasis is transmitted:

• Avoid swimming or wading in freshwater when you are in countries in which schistosomiasis occurs. Swimming in the ocean and in chlorinated swimming pools is safe.
• Drink safe water. Although schistosomiasis is not transmitted by swallowing contaminated water, if your mouth or lips come in contact with water containing the parasites, you could become infected. Because water coming directly from canals, lakes, rivers, streams, or springs may be contaminated with a variety of infectious organisms, you should either bring your water to a rolling boil for 1 minute or filter water before drinking it.

 

•  Bring your water to a rolling boil for at least 1 minute will kill any harmful parasites, bacteria, or viruses present. Iodine treatment alone WILL NOT GUARANTEE that water is safe and free of all parasites.
• Water used for bathing should be brought to a rolling boil for 1 minute to kill any cercariae, and then cooled before bathing to avoid scalding. Water held in a storage tank for at least 1 – 2 days should be safe for bathing.
• Vigorous towel drying after an accidental, very brief water exposure may help to prevent parasites from penetrating the skin. However, do not rely on vigorous towel drying alone to prevent schistosomiasis.