A variety of diseases throughout the world are caused by three species of blood
flukes: intestinal blood flukes, urinary blood flukes, and Oriental blood flukes.
a. Intestinal Blood Flukes. The intestinal blood fluke, Schistosoma mansoni,
commonly attacks humans in China, Taiwan, Southeast Asia, India, Africa, and northern
South America. Fluke eggs are shed in stools that reach the water. The eggs hatch to
produce free-swimming larvae that penetrate and develop in the flesh of snails, the
intermediary host. Cercariae, the final free-swimming larval stage of the fluke, escape
from the snails and encyst on various water plants. Humans become infected when
they eat uncooked plants, usually water chestnuts, or other caltrops. The adult flukes
live in a human's small intestine attached to the mucosa or buried in mucous secretions.
These adult flukes mature in about three months.
b. Urinary Blood Flukes. The urinary blood fluke, Schistosoma hematobium, is
a parasite which occurs in Egypt, Africa, and in parts of the Middle East. The general
process of development in the first stages is the same for all blood flukes. The adult
urinary blood fluke enters the body of the final host, a human, and matures in the
venous plexuses of the bladder, prostate, and uterus. Eggs are passed in the urine or
retained in the tissues, particularly the bladder wall and the female genital organs.
Among the problems caused by this parasite are fibrosis, ulceration, and granuloma and
papilloma formation, bladder wall calcification, chronic cystitis, pyelitis, and
pyelonephritis. In Egypt, bladder cancer is common in advanced cases.
Figure 3-10. Adult fluke in a bile duct.
c. Oriental Blood Flukes. Infection by the Oriental blood fluke, Schistosoma
japonicum, is prevalent in China, Japan, Philippines, and Sulawesi. In some areas,
more than half the population is infected. The initial stages of development are the
same as for the intestinal and urinary blood fluke. This adult parasite, however,
develops in the veins of the small intestine. Some eggs are carried upstream to various
organs, causing a variety of symptoms including enlargement of the liver.