Paragonimus Westermani
By
Leo Pedretti
Elizabeth Hall
Introduction
• Causal Agent: Paragonimus westermani
also known as the oriental lung fluke.
• Disease: Paragonimiasis
• More than 30 species of trematodes
(flukes) of the genus Paragonimus have
been reported to infect animals and
humans. Among them, more than 10
species are reported to infect humans, the
most common is P. westermani.
Geographic Distribution
• P. westermani infections occur in limited areas
where local people eat improperly cooked
crustaceans.
• P. westermani occurs in the Far East specifically
in the countries of Korea, Japan, China, Taiwan,
far-east Russia, Malaysia, India, the Philippines,
and Indonesia.
• Other species of Paragonimus are encountered
in Asia, the Americas, and Africa.
Morphology
• The living adult worms are a pinkish-brown
color and bean shaped (7 to 15 mm in length
to 8 mm in width, and 3 to 5 mm in
thickness). It contains a characteristic ovary
in the middle of the worm.
• The golden brown colored immature eggs are
approximately 45-60 μm by 80-100 μm.
• The metacercariae in the second
intermediate host are spherical in shape
measuring 220-450 μm.
Intermediate Hosts
• The eggs are passed in sputum or feces.
• The eggs flow downstream and have a small chance of survival
but this is offset by the fact that the eggs are produced in large
numbers.
• The miracidium hatches and penetrates its first intermediate host
a snail in the family of Thieridae.
• In the snail, the miracidium forms a sporocyst that produces
rediae, which in turn develop many cercariae.
• The cercariae are spined with knoblike tails and minute oral
stylets. It is capable of creeping over rocks in inchworm fashion.
• It enters its second intermediate host of a crab or crayfish. There
are at least 11 different species it infects. Also, there is some
evidence that the crabs can be infected by eating snails.
• Once in the crab, they encyst in the muscles and viscera.
Definitive Host
• The last stage of the parasites
development is fulfilled when a
mammalian host ingests an infected crab.
• Specifically humans, pigs, dogs, and a
variety of feline species.
• Infections of P. westermani can persist in
humans for up to 20 years!
Pathophysiology
• When humans ingest raw infected crustaceans,
larval flukes develop in the small intestine,
penetrate the intestinal wall into the peritoneal
cavity 30 minutes to 48 hours after excysting. They
then migrate into the abdominal wall or liver, where
they undergo further development. Approximately 1
week later, adult flukes reenter from the abdominal
cavity and penetrate the diaphragm to reach the
pleural space and lungs. Flukes mature, a fibrous
cyst wall develops around them, and then egg
deposition starts 5-6 weeks after infection.
• The symptoms of the early stages of this disease
appear to be few with some people being
asymptomatic.
Pathophysiology
Once the parasite is in the lung or another
organ, the worm stimulates an inflammatory
response that eventually coats tissue.
If worms enter the CSF of the spinal cord, it
can result in partial or total paralysis.
There have also been fatal cases of
Paragonimiasis by infection of the heart.
Cerebral cases result in cerebral cysticercosis
(condition in which fluid-filled cysts
surrounding the worm are present).
Diagnosis
• The adult worms can be discovered during surgery or biopsy.
• However, it is usually through microscopic examination of the
characteristic eggs present in sputum, aspired pleural fluid, feces, and
matter of ulcers caused by the parasite.
• The eggs may not be present in these sources until 2 to 3 months after
infection.
• X-ray examination of a pulmonary infection may be mistaken for
tuberculosis, pneumonia, spirochatosis, etc.
• Cerebral involvement requires differentiation from tumors,
cysticercosis, cysts, encephalitis, and others.
• Since egg detection rates are low, it would be useful to utilize
serological techniques to detect Paragonimus antibodies.
• ELISA serological tests are highly sensitive at >92% detection.
• Intradermal skin tests performed with an extract of adult Paragonimus
is sensitive and has few false positives. The results of the skin test
may remain positive for up to 20 years after the infection has been
cured.
• An assay that detects worm antigens with monoclonal antibodies is
also available and can be used in conjunction with the intradermal skin
test.
Treatment
• The most common and effective drug to treat the lung
infection is praziquantel. Bithionol is an alternative
drug but has more side effects.
• The drug is usually taken for 2 days and has a high
success rate at or above 95% in the acute phase of
infection.
• The drug works by increasing cell membrane
permeability in susceptible worms, resulting in loss of
intracellular calcium, massive contractions, and
paralysis of musculature. Produces vacuolization and
disintegration of schistosome tegument, followed by
attachment of phagocytes to parasite and death.
Treatment
• Extrapulmonary lesions may need to be
surgically excised.
• Intraventricular shunts may also be
needed to manage hydrocephalus.
• Therapy may also be required for seizures
caused by an inflammatory reaction to
dying worms in the brain.
Complications
• Pulmonary complications include
pneumonia, bronchitis, bronchiectasis
(bronchial dilation), lung abscess,
pleural effusion, and empyema (pus in
the plural cavity).
• Cerebral complications include
seizures and coma.
• Skin complications include migratory
allergic skin lesions.
Prognosis
• The prognosis is good, with therapeutic cure
rates between 90 and 100%.
• Symptoms resolve rapidly, and eggs
disappear from the sputum in a few weeks
following treatment. (Pulmonary
paragonimiasis may be self-limited, with
lesions resolving in 5-10 years in light
infections.)
• Resolution of abnormalities on chest
radiographs may take several months,
depending on the chronicity of the disease.
• Cerebral infections may be associated with
persistent seizures.
Prevention
• The spread of the infectious form of this
parasite is controlled by cooking
freshwater crabs and crayfish until the
flesh is pearly and opaque.
• Some species of this parasite carrying
crab do occur in the Americas and Africa.
So, crabs should be cooked everywhere
as a general rule.
Pictures
The egg form leaves the definitive
host and hatches in the miracidium
that penestrates the snail.
The adult fluke is found in
its mammalian host.
Pictures
The metacercaria is the form
ingested by humans.
The cercaria is the form that
penetrates the crab.
Questions
• 1) What are the intermediate hosts?
~ snails and crabs
• 2) What was wrong with the man in the case
study report?
~ He had chronic cerebral paragonimiasis resulting in
multiple calcified cysts in the brain.
• 3) What problems were there with the diagnosis
of the man?
~ He had the parasites so long that serological tests showed low
antibody levels and no eggs or cysts where found in either stool or
sputum.
•4) How do humans contract this disease?
~ By ingesting undercooked or uncooked freshwater crabs or
crayfish
References
• Centers for Disease Control & Prevention (2004, May 5).
Paragonimiasis. [On-Line]. Available:
http://www.dpd.cdc.gov/DPDx/HTML/Paragonimiasis.as
p?body=Frames/MR/Paragonimiasis/body_Paragonimiasis_page1.htm.
• eMedicane (2006, March 30). Paragonimiasis. [On-Line].
Available: http://www.emedicine.com/ped/topic1729.htm.
• Kong, J. (2003). Paragonimus westermani. [On-Line].
Available:
http://www.atlas.or.kr/atlas/alphabet_view.php?my_code
Name=Paragonimus%20westermani.
• Schmidt, G. D., & Roberts, L. S. (2005). Digeneans. In
Roberts, L. S., & Janovy, J. Jr. (Eds.), Foundations of
Parasitology (7th ed, pp. 279-283). Boston : Mc Graw
Hill.