Encyclopedia
Schistosomiasis or
bilharzia is a disease affecting many people in
developing countries. In the form of 'acute' schistosomiasis, it is sometimes referred to as
snail fever and cutaneous schistosomiasis may sometimes be commonly called swimmer's itch. In certain African communities the process of overcoming schistosomiasis is an important
rite of passage. Although it has a low
mortality rate, schistosomiasis can be very debilitating.
Types
There are five species of
flatworms that cause schistosomiasis. Each causes a different clinical presentation of the disease. Schistosomiasis may localize in different parts of the body, and its localization determines its particular clinical profile.
- Schistosoma mansoni is an important human parasite [i]. ...
and Schistosoma intercalatum cause intestinal schistosomiasis - Schistosoma haematobium is an important parasite [i]. ...
causes urinary schistosomiasis - Schistosoma japonicum is an important parasite [i] and one of the major infectious agents of schistosomiasis [i] ...
and Schistosoma mekongi cause Asian intestinal schistosomiasis
Geographical distribution and epidemiology
The disease is found in tropical countries in
Africa,
Caribbean, eastern
South America, east
Asia and in the
Middle East.
Schistosoma mansoni is an important human parasite [i]. ...
is found in parts of South America and the Caribbean, Africa, and the Middle East;
S. haematobium in Africa and the Middle East; and
S. japonicum in the
Far East.
S. mekongi and
S. intercalatum are found focally in
Southeast Asia and central
West Africa, respectively.
An estimated 207 million people have the disease, 120 million symptomatic. A few countries have eradicated the disease, and many more are working towards it. The
World Health Organization is promoting efforts working towards this goal. In some cases, urbanization, pollution, and/or consequent destruction of snail habitat has reduced exposure, with a subsequent decrease in new infections. The most common way of getting schistosomiasis in developing countries is by wading or swimming in lakes, ponds and other bodies of water which are infested with the
snails that are the natural reservoirs of the
Schistosoma pathogen.
Life cycle
Schistosomes have a typical
trematode vertebrate-invertebrate lifecycle, with humans being the definitive host. The life cycles of all five human schistosomes are broadly similar: parasite eggs are released into the environment from infected individuals, hatching on contact with fresh water to release the free-swimming miracidium. Miracidia infect fresh-water
snails by penetrating the snail's foot. After infection, close to the site of penetration, the miracidium transforms into a primary sporocyst. Germ cells within the primary sporocyst will then begin dividing to produce secondary sporocysts, which migrate to the snail's hepatopancreas. Once at the hepatopancreas, germ cells within the secondary sporocyst begin to divide again, this time producing thousands of new parasites, known as cercariae, which are the larvae capable of infecting mammals.
Cercariae emerge daily from the snail host in a circadian rhythm, dependent on ambient temperature and light. Young cercariae are highly motile, alternating between vigorous upward movement and sinking to maintain their position in the water. Cercarial activity is particularly stimulated by water turbulence, shadows and human skin chemicals. Penetration of the human skin occurs after the cercaria have attached to and explored the skin. The parasite secretes enzymes that break down the skin's protein to enable penetration of the cercarial head through the skin. As the cercaria penetrates the skin it transforms into a migrating schistosomulum stage.
The newly transformed schistosomulum may remain in the skin for 1-2 days before locating a post-capillary
venule; from here the schistosomulum travels to the lungs where it undergoes further developmental changes necessary for subsequent migration to the liver. Eight to ten days after penetration of the skin, the parasite migrates to the liver sinusoids.
S. japonicum migrates more quickly than S. mansoni, and usually reaches the liver within 6-8 days of penetration. Juvenile
S. mansoni and
S. japonicum worms develop an oral sucker after arriving at the liver, and it is during this period that the parasite begins to feed on red blood cells. The nearly-mature worms pair, with the longer female worm residing in the gynaecophoric channel of the male. Adult worms are about 10 mm long. Worm pairs of S. mansoni and S. japonicum relocate to the mesenteric or rectal veins.
S. haematobium schistosomula ultimately migrate from the liver to the perivesical venous plexus of the bladder, ureters and kidneys through the hemorrhoidal plexus.
Parasites reach maturity in 6-8 weeks, at which time they begin to produce eggs. Adult
S. mansoni pairs residing in the mesenteric vessels may produce up to 300 eggs per day during their reproductive lives.
S. japonicum may produce up to 3000 eggs per day. Many of the eggs pass through the walls of the blood vessels, and through the intestinal wall, to be passed out of the body in faeces.
S. haematobium eggs pass through the ureteral or bladder wall and into the urine. Only mature eggs are capable of crossing into the digestive tract, possibly through the release of proteolytic enzymes, but also as a function of host immune response, which fosters local tissue ulceration. Up to half the eggs released by the worm pairs become trapped in the mesenteric veins, or will be washed back into the liver, where they will become lodged. Worm pairs can live in the body for an average of four to five years, but may persist up to 20 years.
Trapped eggs mature normally, secreting antigens that elicit a vigorous
immune response. The eggs themselves do not damage the body. Rather, it is the cellular infiltration resultant from the immune response that causes the pathology classically associated with schistosomiasis.
Pathology
Above all, schistosomiasis is a chronic disease. Pathology of
S. mansoni and
S. japonicum schistosomiasis includes: Katayama fever, hepatic perisinusoidal egg
granulomas, Symmers’ pipe stem periportal fibrosis, portal hypertension, and occasional embolic egg granulomas in
brain or
spinal cord. Pathology of
S. haematobium schistosomiasis includes: hematuria,
scarring, calcification,
squamous cell carcinoma, and occasional embolic egg granulomas in brain or spinal cord.
Bladder Cancer diagnosis and mortality are generally elevated in affected areas.
Clinical features
Many infections are subclinically symptomatic, with mild anemia and malnutrition being common in endemic areas. Acute schistosomiasis may occur weeks after the initial infection, especially by
S. mansoni and
S. japonicum. Manifestations include:
- Abdominal pain
- Cough
- Diarrhea
- Eosinophilia - extremely high white blood cell count.
- Fever
- Fatigue
- Hepatosplenomegaly - the enlargement of both the liver and the spleen.
Occasionally
central nervous system lesions occur: cerebral granulomatous disease may be caused by ectopic
S. japonicum eggs in the
brain, and granulomatous lesions around ectopic eggs in the
spinal cord from
S. mansoni and
S. haematobium infections may result in a transverse myelitis with flaccid paraplegia. Continuing infection may cause granulomatous reactions and fibrosis in the affected organs, which may result in manifestations that include:
- Colonic polyposis with bloody diarrhea ;
- Portal hypertension with hematemesis and splenomegaly * Cystitis and ureteritis with hematuria, which can progress to bladder cancer;
- Pulmonary hypertension ;
- Glomerulonephritis; and central nervous system lesions.
Laboratory diagnosis
Microscopic identification of eggs in
stool or urine is the most practical method for diagnosis. The stool exam is the more common of the two. For the measurement of eggs in the feces of presenting patients the scientific unit used is epg or eggs per gram. Stool examination should be performed when infection with
S. mansoni or
S. japonicum is suspected, and urine examination should be performed if
S. haematobium is suspected.
Eggs can be present in the stool in infections with all
Schistosoma species. The examination can be performed on a simple smear . Since eggs may be passed intermittently or in small amounts, their detection will be enhanced by repeated examinations and/or concentration procedures . In addition, for field surveys and investigational purposes, the egg output can be quantified by using the Kato-Katz technique or the Ritchie technique.
Eggs can be found in the urine in infections with and with
S. japonicum na S. intercalatum. Detection will be enhanced by centrifugation and examination of the sediment. Quantification is possible by using filtration through a Nucleopore® membrane of a standard volume of urine followed by egg counts on the membrane. Investigation of
S. haematobium should also include a pelvic x-ray as bladder wall calcificaition is highly characteristic of chronic infection.
Recently a field evaluation of a novel handheld microscope was undertaken in Uganda for the diagnosos of intestinal schistosomiasis by a team led by Dr. Russell Stothard who heads the Schistosomiasis Control Iniative at the Natural History Museum, London. His report abstract may be found here:
Tissue
biopsy may demonstrate eggs when stool or urine examinations are negative.
The eggs of
S. haematobium are ellipsoidal with a terminal spine,
S. mansoni eggs are also ellipsoidal but with a lateral spine,
S. japonicum eggs are spheroidal with a small knob.
Antibody detection can be useful in both clinical management and for
epidemiologic surveys.
Treatment
Schistosomiasis is readily treated using a single oral dose of the drug Praziquantel. While Praziquantel is safe and highly effective in curing an infected patient, it does not prevent re-infection by cercariae and is thus not an optimum treatment for people living in endemic areas. As with other major parasitic diseases, there is ongoing and extensive research into developing a vaccine that will prevent the parasite from completing its life cycle in humans.
Antimony has been used in the past to treat the disease. In low doses this
toxic metalloid bonds to
sulfur atoms in
enzymes used by the parasite and kills it without harming the host. This treatment is not referred to in present-day
peer-review scholarship; Praziquantel is universally used. Outside of the US, there is a second drug available for treating Schistosoma mansoni called Oxamniquine.
Prevention and hygiene
The main focus of prevention is eliminating the water-borne snails which are natural reservoirs for the disease. This is usually done by identifying bodies of water, such as lakes, ponds, etc., which are infested, forbidding or warning against swimming and adding niclosamide,
acrolein,
copper sulfate, etc., to the water in order to kill the snails.
In 1989, Aklilu Lemma and Legesse Wolde-Yohannes received the
Right Livelihood Award for their research on the
sapindus-Plant , as a preventative measure for the disease.
Recent studies have indicated the possibility of biocontrol of the parasite. Introducing or adding to existing populations of crayfish in the ponds and rivers where the parasite is prevalent would keep host snail populations down, thus significantly reducing the parasite population.
Another important consideration is the impact of man made irrigations schemes in spreading the disease. These can be designed to make it hard for the snails to colonise the water, and to reduce the contact with the local population. UN guidelines were published in the 1950s, but sadly were still being ignored during the 1980 despite the efforts of some clinicians to get civil engineers to take it into account in their designs.
See e.g. 10. Charnock, Anne Taking Bilharziasis out of the irrigation equation. New Civil Engineer, 7 August. Bilharzia caused by poor civil engineering design due to ingnorance of cause and prevention.
Failure for engineers to take this into account is an interesting example of the Relevance Paradox
See also
References
External links
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- 10. Charnock, Anne Taking Bilharziasis out of the irrigation equation. New Civil Engineer, 7 August. Bilharzia caused by poor civil engineering design due to ingnorance of cause and prevention.
