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Intestinal Trematodes 185

organism that causes a disease known as heterophyiasis,
is found primarily in both the Near and Far East as well as MICROSCOPIC DIAGNOSTIC
parts of Africa. M. yokogawai is found in Asia and Siberia FEATURE
and is known as the causative agent for the disease meta-
gonimiasis. The two organisms are predominantly found
as cat and dog parasites, as well as other fish-eating mam- General Classification—Heterophyid Eggs
mals, so heterophyiasis and metagonimiasis are known as Organism(s) Heterophyes
zoonoses (animal related) when they infect humans. Birds heterophyes and
may also be reservoirs for these related organisms. Metagonimus
yokogawai

Morphology Specimen Required Feces
Stage Egg
The eggs of H. heterophyes and M. yokogawai are Size 30 3 90 μm
indistinguishable from each other. They are small flukes Shape Oval and contains a
known jointly as heterophyids (from the genus compris- formed miracidium
ing these two species) and are approximately 30 μm Shell Thin and smooth
by 15 μm. The eggshells of M. yokogawai appear to be (shell of M. yokogawai
thinner than those of H. heterophyes, although this can thinner)
only be determined by close microscopic attention. The Other Features Eggs are operculated
mature flukes of both species are approximately 1 to and have shoulders
2 mm in length. near the operculum;
sometimes there is a
small boss (protuber-
Symptoms ance) opposite the
operculum
Individuals with light infections are generally asympto- Ova
matic, whereas those with heavier infections with either H. heterophyes M. yokogawai
species may experience diarrhea, abdominal pain, and
eosinophilia. In severe cases, granulomas may develop.
A granuloma is a structure that forms during the process
of inflammation, when large numbers of macrophages
are drawn to an infected area, which then enclose the
area. Granulomas also occur in tuberculosis, leprosy, and
some fungal infections.

Life Cycle

The eggs of the species are ingested by a freshwater C. sinensis
snail. The eggs contain a fully developed miracidium
and, upon ingestion, the miracidium is released from the
egg, upon which it penetrates the tissues of the snail and
develops into the cercarial stage. Mature cercariae are re-
leased into the fresh water, and find fish that become sec-
ond intermediate hosts when they are encysted. When
cercariae encyst into the flesh of the fish, they form merta-
cercariae. The definitive host then ingests the fish contain-
ing metacercariae, at which time they excyst and develop Delmar/Cengage Learning
into adult worms in the small intestine. The adult worms
are then able to produce eggs to continue the life cycle.