ENTOMOLOGY 156 BIOLOGY OF PARASITISM

LABORATORY 3: TREMATODES

Human Schistosomiasis

It is estimated that over 200 million people in the world suffer from this disease! This parasite, because of its acute and chronic damage and severe suffering, has earned itself a place as one of the major targets of the World Health Organization's Special Programme for Research and Training in Tropical Diseases.

There are four species of schistosomes which normally infect humans: S. mansoni, S. japonicum, S. mekongi, and S. haematobium. We will look at the life cycle of S. mansoni but the others are very similar to it.

Schistosoma mansoni (blood fluke)

Adults, with females snugly enclosed by the males in a gynecophoric canal, are found in the branches of the inferior mesenteric vein of people. These flukes are very slender and elongate, no doubt an adaptation to their habitat. In these trematodes, the sexes are separate; after copulation, the eggs produced will be passed into the blood vessels by the female.

The eggs will be carried to the intestinal wall where they migrate into the lumen and are passed out with the feces.

Examine the eggs of S. mansoni; compare them to those of S. japonicum .

S. mansoni egg (from WWW)
S. japonicum egg (from WWW)
S mansoni egg in liver section (UCD slide)
S. mansoni egg (UCD slide)
Cross section of adult in liver (UCD slide)
S. mansoni egg (UCD slide)
S. mansoni egg (UCD slide)

Upon contact with water the non-operculated, embryonated eggs open and miracidia emerge. In the snail, sporocysts and daughter sporocysts develop asexually (no redia stage), and cercariae migrate out of the snail. When the cercariae contact the skin of humans, they actively penetrate it.

After entering the host, the cercariae lose their characteristic forked "tail" and migrate through the blood stream and eventually to the liver.

Migrating eggs can cause tissue damage. Swelling of the liver and spleen as well as cirrhosis of the liver are common occurrences (note the swollen stomach of the boy in the photo on display). Acute cases of schistosomiasis can and often do lead to death.

There are also schistomsomes of cattle and sheep, as well as birds and other vertebrates. When these cercaria penetrate human skin, an unsightly dermatitis can develop, called "swimmers-itch" or cercarial dermatitis.

Observe the various life cycle stages of S. mansoni on display.

S. mansoni male and female (from WWW)
S. mansoni male and female in copula (from WWW)
S. mansoni miracidium (from WWW)
S. mansoni cercaria (from WWW)
S. mansoni adult male (UCD slide)
S. mansoni adult male (UCD slide)
S. mansoni adult male (UCD slide)
S. mansoni adult male (UCD slide)
S. mansoni adult male (UCD slide)
S. mansoni cercaria (UCD slide)
S. mansoni cercaria (UCD slide)

References

Barbosa, F. S. and D. P. P. Costa. 1981. Incapacitating effects of schistosomiasis mansoni on the productivity of sugarcane cutters in northeastern Brazil. Am. J. Epidemiol. 114: 102-111.

Barbour, A. D. 1982. Schistosomiasis. In R. M. Anderson (Ed.), Population Dynamics of Infectious Diseases: Theory and Applications, pp. 180-208.

Kloos, H. and K. Thompson. 1979. Schistosomiasis in Africa: An ecological perspective. J. trop. Geog. 48: 31-46.

Kloos, H., G. DeSole and A. Lemma. 1981. Intestinal parasitism in seminomadic pastoralists and subsistence farmers in and around irrigation schemes in the Awash Valley, Ethiopia, with special emphasis on ecological and cultural associations. Social Science and Medicine 15B: 457-470.

Miller, F. Dew., et al. 1978. Aspects of environmental health impacts of the Aswan High Dam on rural population in Egypt. Prog. Wat. Tech. 11: 173-180.

Weisbrod, B. A., et al. 1973. Disease and Economic Development: The Impact of Parasitic Diseases in St. Lucia. Madison: University of Wisconsin.

Wilson, R. A., G. Smith and M. R. Thomas. 1982. Fascioliasis. In R. M. Anderson (Ed.), Population Dynamics of Infectious Diseases: Theory and Applications, pp. 262-319.

LAB 3 INDEX

LAB INDEX