Schistosomiasis
There are five species of the genus Schistosoma which commonly cause
disease in humans: S. haematobium, S. mansoni, S. japonicum, S.
mekongi and S. intercalatum.
S. haematobium was discovered by Theodor Bilharz in Cairo in 1861 and
the genus is sometimes called bilharzia and the disease is bilharziasis.
The ovum is passed in the urine or faeces of infected individuals and gains
access to fresh water, where the ciliated miracidium inside it is liberated and
enters its intermediate host, the freshwater snail in which it multiplies, Large
numbers of fork-tailed cercariae are then liberated into the water, where they
may survive for 2–3 days. Cercariae can penetrate the skin or the mucous
membrane of the mouth of their definitive host (humans). They transform
into schistosomulae and moult as they pass through the lungs; and are
carried by the blood stream to the liver, and so to the portal vein, where they
mature.
The male worm is up to 20 mm in length and the more cylindrical female,
usually enfolded longitudinally by the male, is rather longer. Within 4–6
weeks of infection, they migrate to the venules draining the pelvic viscera,
where the females deposit ova.
Pathology
The pathological changes and symptoms depend on the species and the stage
of infection, Most disease is due to the passage of eggs through mucosa and
to the granulomatous reaction to eggs deposited in tissues.
The eggs of S. haematobium pass mainly through the wall of the bladder, but
may also involve rectum, seminal vesicles, vagina, cervix and uterine tubes.
S. mansoni and S. japonicum eggs pass mainly through the wall of the lower
bowel or are carried to the liver. The most serious, but rare, consequence of
ectopic deposition of eggs is transverse myelitis
and paraplegia.
Granulomas are composed of macrophages, eosinophils, and epithelioid and
giant cells around an ovum. Later, there is fibrosis and eggs calcify, often in
sufficient numbers to become radiologically visible.
Eggs of S. haematobium and other species (S. mansoni and S. japonicum)
after the development of portal hypertension may reach the lungs, causing
pulmonary hypertension.
Clinical features
During the early stages of infection, there may be itching lasting 1–2 days at
the site of cercarial penetration. After a symptom-free period of 3–5 weeks,
acute schistosomiasis (Katayama syndrome) may present with allergic
manifestations, such as urticaria, fever, muscle aches, abdominal pain,
headaches, cough and sweating. On examination, hepatomegaly,
splenomegaly, lymphadenopathy and pneumonia may be present. There is
eosinophilia and serology is positive, these allergic phenomena (Katayama
syndrome) may be severe in infections with S. mansoni and S. japonicum,
but are rare with S. haematobium. The features subside after 1–2 weeks.
Chronic schistosomiasis is due to egg deposition and occurs months to years
after infection. The symptoms and signs depend upon the intensity of
infection and the type of schistosoma.
Schistosoma haematobium
Humans are the only natural hosts of S. haematobium, which is highly
endemic in Egypt, East Africa and the Middle East.
Painless terminal haematuria is usually the first and most common symptom.
Frequency of micturition follows, due to bladder neck obstruction. Later, the
disease may be complicated by frequent urinary tract infections, bladder or
ureteric stone formation, hydronephrosis, and ultimately renal failure with a
contracted calcified bladder. Pain is often felt in the iliac fossa or in
the loin, and radiates to the groin. There is a strong epidemiological
association of S. haematobium infection with squamous cell carcinoma of
the bladder.
Females may develop schistosomal papillomas of the vulva, and
schistosomal lesions of the cervix may be mistaken for cancer. Intestinal
symptoms may follow involvement of the bowel wall.
Adult worms can live for 20 years or more and lesions may progress, these
patients should always be treated.
Schistosoma mansoni
Characteristic symptoms begin 2 months or more after infection. They may
be slight, like malaise, or consist of abdominal pain and frequent stools
which contain blood-stained mucus. With severe advanced disease,
increased discomfort from rectal polyps. The early hepatomegaly is
reversible, but portal hypertension may cause massive splenomegaly,
fatal haematemesis from oesophageal varices, or progressive ascites.
Liver function is initially preserved because the pathology is fibrotic rather
than cirrhotic.
S. mansoni predispose to the carriage of Salmonella.
Schistosoma japonicum, S. mekongi and
S. intercalatum
In addition to humans, the adult worm of S. japonicum infects the dog, rat,
pig, horse and sheep, it is present in china.
The pathology of S. japonicum is similar to that of S. mansoni, but as this
worm produces more eggs, so the lesions tend to be more extensive and
widespread. The clinical features resemble those of severe infection with
S. mansoni, with added neurological features. The small and large bowel
may be affected, and hepatic fibrosis with splenic enlargement is usual.
Deposition of eggs or worms in the CNS, especially in the brain, causes
symptoms in about 5% of infections, such as epilepsy, blindness, hemiplegia
or paraplegia.
Investigation
The diagnosis depends on finding eggs or serologic evidence to S.
haematobium, dipstick urine testing shows blood and albumin. The terminal
spined eggs can usually be found in the terminal stream urine exam.
Ultrasound exam to assess the urinary tract; bladder wall thickening,
hydronephrosis and bladder calcification can be detected.
In S. mansoni or S. japonicum stool examination of eggs with lateral spine
can usually be found.
When the infection is light, a rectal biopsy can be examined. Serological
tests (ELISA) are useful as screening tests but remain positive after
treatment and cure.
Management
The aim of treatment is to kill the adult worm and so stop egg-laying.
Praziquantel is the drug of choice for all forms of schistosomiasis. It
achieves cure in 80% of patients and over 90% reduction in egg laying in the
remaining individual.
Praziquantel therapy in early infection reverses pathologies such as
hepatomegaly and bladder wall thickening.
Surgery is indicated for residual lesions, stricture may require plastic
procedures, granuloma in the brain or spinal cord may require neurosurgery
if it does not respond to chemotherapy and corticosteroid.
Prevention
The life cycle is terminated if the ova in urine or faeces are not allowed to
contaminate fresh water containing the snail host. The provision of latrines
and of a safe water supply remains a major problem in rural areas, in case of
S. japonicum there are so many hosts besides humans so that a proper use of
latrines would be of little value, mass treatment of the population helps in S.
haematobium and S. mansoni but of little value in S. japonicum, personal
protection is to prevent contact with contaminated water.