The schistosomes offer a classic example of the biological cycle of trematodes, worms a few inches long with typically flattened or cylindrical bodies:
The distribution of schistosomiasis in the World is conditioned both by the presence of the specific mollusks that are its intermediate hosts and by the hygienic habits of the populations, here is an overview:
The Schistosoma haematobium on which the disease depends is a cylinder-shaped, unsegmented-bodied trematode with an oral and ventral sucker. The male and female live mated in the veins of the bladder plexus of the human being, which is their usual host.
The eggs deposited by the females penetrate by enzymatic action into the bladder and are excreted along with urine. When they come in contact with fresh water, they hatch and release larvae, or miracids. These reach small freshwater snails, into which they penetrate, develop and transform into cercariae, which return again to water. Humans become infected when they come in contact with water infested with cercariae, which are equipped with a kind of tail and are able to cross the skin barrier.
When cercariae penetrate through the skin, it results in an itchy dermatitis that lasts 2-3 days. After four or more weeks, irregular fever, malaise, and generalized pain appears. Specific symptomatology occurs after three or more months, which is the time it takes for the cercariae, which have reached the liver, to develop into mature worms.
These then move to the venous plexuses of the bladder, where fertilized females lay myriads of eggs that attempt to pass into the bladder cavity.
It is in connection with this passage that the first episodes of terminal hematuria (blood in the urine) occur, followed by pollakiuria (the need to urinate frequently throughout the day) and pain during urination. The disease tends to become chronic and present complications that progressively worsen the patient's condition.
The most frequent complication is phosphate stones of which schistosoma eggs come to represent the central core. Such stones can be found not only in the bladder cavity but also in the renal pelvis and ureters, giving the possibility of urine stagnation and the secondary onset of hydronephrosis (accumulation of urine inside the kidneys).
The irritant action of the schistosome is also a complication of the urinary tract.
The irritating action of eggs and stones can lead to the formation of polypoid growths and adenopapillomas of the bladder mucosa. Hyperplasia of the prostate is not uncommon. In the advanced stages of the disease, patients present with progressive loss of strength, a state of hypochromic anemia, and recurrent fevers.
Since schistosomiasis cannot occur without body contact with water infested with cercariae, prevention is to avoid full or partial bathing in pools, ponds or rivers with contaminated or suspected water.
Intestinal schistosomiasis is widespread in the Nile Valley, parts of Central, East and West Africa as well as Central and South America. It is due to Schistosoma mansoni, a trematode worm that lives in the branches of the lower mesenteric veins of the colon wall. The eggs penetrate the intestinal lumen and are excreted with the feces.
Also in this form, symptoms are closely related to the penetration of cercariae into the body. The specific localization of S. mansoni in the intestine initially causes colic and diarrhea associated with tenesmus (spasmodic contractions at the anal sphincter). There is anorexia and emaciation, while the colon becomes enlarged and spastic and painful.
As the disease progresses, cirrhosis of the liver caused by metastasized eggs arises with a picture of portal hypertension and enlarged spleen and then ascites (pathologic collection of fluid in the abdominal cavity). Because splenomegaly tends to assume considerable proportions, it is accompanied by hypochromic anemia and irregular fever.This picture, known as Egyptian splenomegaly, is often mistakenly referred to chronic malaria.
Asian schistosomiasis is widespread in the Far East, particularly in China, Japan, and the Philippines. It is due to Schistosoma japonicum, a trematode worm that lives in the branches of the lower and upper mesenteric veins. The eggs penetrate the intestinal lumen and behave like those of Schistosoma mansoni.
Symptomatology is similar to that of intestinal schistosomiasis, but certainly of greater severity. There is often a continuous fever accompanied by considerable anemia.
Parasitology and Tropical Medicine, Chieffi G.