Schistosomiasis Dr. Mohammad Shakeeb, MD Specialist in clinical pathology/Microbiology and immunology

Schistosomiasis is a major parasitic infection of tropical areas, with some 200 million infections worldwide. blood flukes: dorsoventrally flattened helminths The three schistosomes most frequently associated with human disease are : Schistosoma mansoni. Schistosoma japonicum. Schistosoma haematobium.

schistosomiasisbilharziasis snail fever They collectively produce the disease called schistosomiasis, also known as bilharziasis or snail fever. The schistosomes differ from other flukes: They are male and female rather than hermaphroditic. Their eggs do not have an operculum.

They also are obligate intravascular parasites and are not found in cavities, ducts, and other tissues. The infective forms are skin-penetrating cercariae liberated from snails. These differ from other flukes in that they are not eaten on vegetation, in fish, or in crustaceans.

life cycle Infection is initiated by ciliated, free swimming, freshwater cercariae that penetrate intact skin. Enter the circulation Develop in the intrahepatic portal circulation (S. mansoni and S. japonicum) In the vesical, prostatic, rectal, and uterine plexuses and veins (S. haematobium).

The female has a long, cylindric body, whereas the shorter male, which appears cylindric, is actually flat. The cylindric appearance derives from folding the sides of the body to produce a groove, the gynecophoral canal, in which the female resides for fertilization. Both sexes have oral and ventral suckers and an incomplete digestive system, which is typical of a fluke.

As the worms develop in the portal circulation, they elaborate a remarkable defense against host resistance. They coat themselves with substances that the host recognizes as itself; consequently, there is little host response directed against their presence in blood vessels. This protective mechanism accounts for chronic infections that may last 20 to 30 years or longer.

After developing in the portal vein, the male and female adult worms pair up and migrate to their final locations, where fertilization and egg production begin. intestinal schistosomiasis. S. mansoni and S. japonicum are found in mesenteric veins and produce intestinal schistosomiasis. vesicular schistosomiasis. S. haematobium occurs in veins around the urinary bladder and causes vesicular schistosomiasis.

On reaching the submucosal venules of their respective locations, the worms initiate oviposition, which may continue at the rate of 300 to 3000 eggs daily for 4 to 35 years. The eggs elicit an intense inflammatory reaction, with mononuclear and polymorphonuclear cellular infiltrates and the formation of microabscesses.

In addition, the larvae inside the eggs produce enzymes that aid in tissue destruction and allow the eggs to pass through the mucosa and into the lumen of the bowel and bladder They are passed to the external environment in the feces and urine, respectively.

miracidia The eggs hatch quickly on reaching fresh water to release motile miracidia. The miracidia then invade the appropriate snail host. Where they develop into thousands of infectious cercariae. The free-swimming cercariae are released into the water, where they are immediately infectious for humans and other mammals.

Pthogenesis The infection is similar in all three species of human schistosomes in that disease results primarily from the host’s immune response to the eggs. The very earliest signs and symptoms are due to the penetration of the cercariae through the skin. Immediate and delayed hypersensitivity to parasite antigens result in an intensely pruritic papular skin rash.

Katayama syndrome The onset of oviposition results in a symptom complex known as Katayama syndrome. marked by fever, chills, cough, urticaria, arthralgias, lymphadenopathy, splenomegaly, and abdominal pain. This syndrome is typically seen 1 to 2 months after primary exposure and may persist for 3 months or more.

It is thought to result from the massive release of parasite antigens, with subsequent immune complex formation. Associated laboratory abnormalities include leukocytosis, eosinophilia, and polyclonal gammopathy. granulomasfibrosis. The more chronic and significant phase of schistosomiasis is due to the presence of eggs in various tissues and the resulting formation of granulomas and fibrosis.

The retained eggs induce extensive inflammation and scarring. The clinical significance of which is directly related to the location and number of eggs.

Schistosoma mansoni Schistosoma mansoni occurs in Africa, especially in the tropical areas and the Nile delta, southern Africa, and Madagascar, Brazil, Venezuela, Surinam, and certain Caribbean islands, including Puerto Rico. Adult S. mansoni live primarily in the portal vein and in the distribution of the inferior mesenteric vein.

Initial deposition of eggs in the large intestine may produce abdominal pain and dysentery, with abundant blood and mucus in the stool. Eggs may be detected in feces at this time. Chronic infection may result in liver fibrosis and portal hypertension, depending on the number of worms present. Eggs may be more difficult to find in feces during this stage.

Eggs, which measure 116–180 μm by 45–58 μm, are oval, with a large distinctive lateral spine that protrudes from the side of the egg near one end.

Schistosoma japonicum occurs in China, southeast Asia, and the Philippines, causes disease that is clinically similar to that of S. mansoni but often more serious because many more (up to 10 times as many) eggs are produced by S. japonicum. The disease has been essentially eliminated from Japan, although animal reservoirs still exist.

Adult worms live primarily in the distribution of the superior mesenteric vein, and eggs readily reach the liver, inducing fibrosis and portal hypertension as a common complication of chronic infection. The smaller size of the eggs predisposes them to dissemination, especially to the brain and spinal cord. The eggs are broadly oval, measuring 75–90 μm by 60–68 μm, and have an inconspicuous lateral spine, which may be difficult to demonstrate.

Schistosoma haematobium Urinary schistosomiasis occurs in many parts of Africa, the Middle East, and Madagascar. Parasites migrate via the hemorrhoidal veins to the venous plexuses of the urinary bladder, prostate, uterus, and vagina. One of the earliest and most common symptoms of infection is hematuria, especially at the end of micturition.

Chronic infection may cause pelvic pain and bladder colic, with an increased desire to urinate. Accumulation of eggs in the tissues may result in hypertrophy of the urothelium, squamous metaplasia, and marked fibrosis, which may progress to obstruction and, ultimately, renal failure.

Urinary schistosomiasis also has been associated with squamous cell carcinoma of the bladder. Eggs are recovered from the urine by examination of spun sediment. They are elongate, measuring 112–180 μm by 40–70 μm, and have a characteristic terminal spine.

DIAGNOSIS Definitive diagnosis of schistosomiasis requires the recovery of the characteristic eggs in urine, stool, or biopsy specimens. In S haematobium infections, eggs are most numerous in urine samples obtained at midday, especially the last drops voided. Cystoscopy with biopsy of the bladder mucosa may be required for the diagnosis of mild infection.

Eggs of S mansoni and S japonicum are passed in the stool. Results of rectal biopsy may be positive when those of repeated stool examinations are negative. Conventional serologic tests detect circulating antibodies with sensitivities exceeding 90%, but cannot distinguish active from inactive infection

Enzyme immunoassay (EIA)- based reagent strip (dipstick) tests, capable of detecting circulating, genus-specific, adult worm antigens in blood and urine, are rapid, simple, and sensitive. They are particularly helpful in the diagnosis of Katayama syndrome in those returning from endemic areas.

because antigen levels drop rapidly after successful therapy, these tests may prove helpful in distinguishing active from inactive disease.

TREATMENT Praziquantel Praziquantel, which is active against all three species of schistosomes, is the agent of choice.

prevention Prevention involves proper disposal of human waste and eradication of snail host when possible. Swimming in areas of endemic infection should be avoided.