Schistosoma mansoni

Schistosoma mansoni is a trematode parasite (blood fluke) that infects humans specifically in the mesenteric venules of the intestine.

Disease / Pathology

Schistosoma mansoni is the cause of schistosomiasis or Katayama fever.

Katayama fever is the acute form of the disease and symptoms are due to the onset of egg laying in the host. These symptoms are initial immune reactions by the host to the presence of these eggs. A rash or itchy skin may develop as well as cough and musculature aching. Again, all part of the initial immune response. Some individuals may have very mild symptoms or none at all.

Most importantly is the chronic form of the disease. In the chronic form of the disease the adult worms deposit eggs that become lodged in the intestinal wall or pass through to be excreted in the feces. Eggs that become lodged in the intestinal wall result in the formation of granulomas. Antigens secreted through the porous egg walls by the developing miracidium initiate the formation of these granulomas.

Eggs can travel through the venous system to other areas of the body and be deposited with the formation of granulomas. Over time these granulomas produce damage that is irreversible. These chronic infections will last for years with symptoms being vague to almost asymptomatic until the damage to the host is extensive enough to produce symptoms. Damage is especially significant when granuloma formations occur in the brain, lungs, liver, and bladder.

Location in the Host

Schistosoma mansoni adults are located in the mesenteric venules of the intestine.

Geographic Distribution

Schistosoma mansoni can be found in the Caribbean, parts of Africa, Suriname and parts of South America such as Brazil and Venezuela.

Life Cycle

Schistosoma mansoni

Morphology & Diagnosis

Schistosoma mansoni eggs can be detected in a ova and parasite examination, specifically a formalin concentrate of the specimen stained with Lugol’s iodine. Eggs and worms can be also detected via biopsies of the intestinal wall.

Eggs can be difficult to detect in patients simply because the egg shedding in intermittent and is low in numbers. The eggs have a characteristic prominent lateral spine as illustrated in the first image below. With respect to other intestinal parasitic eggs, they are rather large in size ( 114-189um by 45-70um). Eggs can also be detected in biopsies and an excellent example is seen in the photo below.

Adult worms are not seen unless via surgical biopsies.

Diagnosis can be difficult as noted previously since eggs are intermittently shed. The CDC provides a serologic blood test that will detect the disease in patients strongly suspected of being infected but have negative ova and parasite examination results. However, blood testing is only done on blood drawn 6-8 weeks after initial infections. Therefore the complete clinical history should help to determine when and if the test should be performed.


Schistosoma mansoni cercariae – Photo by W. Vientos

Schistosoma mansoni in wet-prep under high power and under oil immersion. Note the prominent lateral spine clearly visible.

Schistosoma mansoni egg in the colon biopsy of a male complaining of vague intestinal discomfort over a period of 3 years. Note the prominent lateral spine at about 7 o-clock – Photo by W. Vientos