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schistosomiasis

Tuesday 17 March 2009

Definition: Schistosomiasis infects approximately 200 million persons and kills approximately 280,000 annually. Most of the mortality comes from hepatic granulomas and fibrosis, caused by Schistosoma mansoni in Latin America, Africa, and the Middle East and Schistosoma japonicum and Schistosoma mekongi in East Asia.

Images

- digestive schistosomiasis

In addition, Schistosoma haematobium, found in Africa, causes hematuria and granulomatous disease of the bladder, resulting in chronic obstructive uropathy.

Schistosoma sp.

Schistosoma sp. are infectious agents of schistosomiasis.

There are 5 principal species that infect man which can be classified by the appearance of the egg:

- Eggs w/ terminal spine: Schistosoma haemotobium, Schistosoma intercalatum
- Eggs w/ lateral spine: Schistosoma mansoni
- Round/ovoid, minutely spined: Schistosoma japonicum, Schistosoma mekongi

They are members of the group of worms called flukes. However, the schistosomes have the following differences:

- Sexes are separate
- Cylindrical in shape
- Male: short and stumpy with copulatory groove.
- Female: long and slender
- Only one intermediate host

Pathology

Schistosoma sp. are infectious agents of schistosomiasis. Schistosomiasis is a complex of acute and chronic diseases caused by worms belonging to the genus Schistosoma, or blood flukes, which live in the blood vessels of humans and other animals.

It is the most important helminth infection on the globe infecting 200 million in approximately 75 countries, second only to malaria in terms of socioeconomic & public health importance

It is frequently a “man-made” disease as it is associated with the development of irrigation systems and hydroelectric dams in third world countries.

People become infected by exposure to contaminated fresh water. This water gets contaminated by infected people working in the rice field, fisherman in the lake or children playing who indiscriminately defecate or urinate in the water.

Pathogenesis

Schistosomiasis is transmitted by freshwater snails that live in the slow-moving water of tropical rivers, lakes, and irrigation ditches, ironically linking agricultural development with spread of the disease.

Infectious schistosome larvae (cercariae) swim through fresh water and penetrate human skin with the aid of powerful proteolytic enzymes that degrade the keratinized layer.

Within the skin, schistosome larvae shed a surface glycocalyx that protects the organisms from osmotic shock; this glycocalyx, however, activates complement by the alternative pathway and is recognized by many human antischistosome antibodies. Schistosomes migrate into the peripheral vasculature, traverse to the lung, and settle in the portal or pelvic venous system, where they develop into adult male and female schistosomes.

Females produce hundreds of eggs per day, around which granulomas and fibrosis form. Some schistosome eggs are passed from the portal veins through the intestinal wall into the colonic lumen, are shed with the feces, and release into freshwater miracidia that infect the snails to complete the life cycle.

The immune response to S. mansoni and S. japonicum eggs in the liver causes the severe pathology of schistosomiasis.167 While the immune response does provide some protection in animal models, the price of this response is granuloma formation and hepatic fibrosis. Acute schistosomiasis in humans can be a severe febrile illness that peaks about 2 months after infection.

The T-helper response in this early stage is dominated by TH1 cells, and the IFN-γ produced by T cells may stimulate macrophages to produce high levels of the pyrogens TNF, IL-1, and IL-6. Chronic schistosomiasis is associated with a dominant TH2 response, although TH1 cells persist. Stimulation of TH2 cells may be due to proteins in the parasite egg that cause mast cells to produce IL-4, which induces further TH2 differentiation and amplifies the response.

Both types of T-helper cells appear to stimulate formation of granulomas in the liver. Eggs become surrounded by granulomatous lesions composed of macrophages, lymphocytes, eosinophils, and connective tissue. Severe hepatic fibrosis is a serious manifestation of chronic schistosomiasis. In animal models, IL-13, produced by TH2 cells, increases fibrosis by increasing synthesis of proline, an important amino acid in collagen.

Morphology

In mild S. mansoni or S. japonicum infections, white, pinhead-sized granulomas are scattered throughout the gut and liver. At the center of the granuloma is the schistosome egg, which contains a miracidium; this degenerates over time and calcifies.

The granulomas are composed of macrophages, lymphocytes, neutrophils, and eosinophils; the last-mentioned are distinctive for helminth infections. The liver is darkened by regurgitated heme-derived pigments from the schistosome gut, which, like malaria pigments, are iron-negative and accumulate in Kupffer cells and splenic macrophages.

In severe S. mansoni or S. japonicum infections, inflammatory patches or pseudopolyps may form in the colon. The surface of the liver is bumpy, whereas cut surfaces reveal granulomas and a widespread fibrous portal enlargement without distortion of the intervening parenchyma by regenerative nodules. Because these fibrous triads resemble the stem of a clay pipe, the lesion is named pipe-stem fibrosis.

Many of these portal triads lack a vein lumen, causing presinusoidal portal hypertension and severe congestive splenomegaly, esophageal varices, and ascites.

Schistosome eggs, diverted to the lung through portal collaterals, may produce granulomatous pulmonary arteritis with intimal hyperplasia, progressive arterial obstruction, and ultimately heart failure (cor pulmonale).

On histologic examination, arteries in the lungs show disruption of the elastica layer by granulomas and scars, luminal organizing thrombi, and angiomatoid lesions similar to those of idiopathic pulmonary hypertension.

Patients with hepatosplenic schistosomiasis also have an increased frequency of mesangioproliferative or membranous glomerulopathy (Chapter 20), in which glomeruli contain deposits of immunoglobulin and complement but rarely schistosome antigen.

In S. haematobium infection, bladder inflammatory patches due to massive egg deposition and granulomas appear early, and when they erode, they cause hematuria.

Later, the granulomas calcify and develop a "sandy" appearance, which, if severe, may line the wall of the bladder and cause a dense concentric rim (calcified bladder) on radiographic films.

The most frequent complication of S. haematobium infection is inflammation and fibrosis of the ureteral walls, leading to obstruction, hydronephrosis, and chronic pyelonephritis. There is also an association between urinary schistosomiasis and squamous cell carcinoma of the bladder.

Prevention

There is not a vaccine or any other prophylaxis available for prevention of schistosomiasis. Prevention is by avoiding fresh water. According to the CDC, because there is no practical way for the traveler to distinguish infested from noninfested water, travelers should be advised to avoid wading, swimming or other contact with freshwater in disease-endemic countries.

Links

- examiner.com