Trematodes (Flukes)

Department of Parasitology,

Xiangya Medical School,

Central South University

Paragonimus westermani

(lung fluke)

More than 50 species of paragonimus have

been reported as parasites.

P. Westermani is widely distributed in East Asia

and is the most important species.

It was first discovered from Bengal tigers that

had died in the zoos in Europe in 1878.

In 1880, human infections were first found in

Taiwan of China

Discovery

Morphology1 、 Adult worm Thick, reddish-brown in color,a

flattened ventral side,a swelled dorsal

side, like half a peanut,(7.5-12.0mmx4-6mm)

Oral sucker & ventral sucker are

similar in size;ventral sucker is at pre-

equator

Two testes, the ovary and the uterus

are situated side by side

Cecum with two winding branches

Vitelline follicles are extensive in lateral

fields

Morphology

1 、 Adult worm

Surface with spines

Morphology

2 、 Egg

Average size: 85μm X 53μm

Golden yellow,

Irregular elliptic,

Thick and asymmetric shell with a

big operculum,

Inside, a egg cell surrounded by

about ten yolk cells

Morphology

3 、 Metacercaria Spherical,

About 300-400µm with

two layers of transparent

walls in crab and crayfish

Life Cycle

Life Cycle

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Definitive host

(Human,cat,dog,tiger)

1st intermidate host

(snails in fresh water)

2nd intermidate host

(crab and crayfish)

Definitive host

(Human,cat,dog,tiger)

melania snail

Crayfish

Crab

Development in human

Life Cycle

Infection

Stage metacercaria

Mode ① eating raw crab or crayfish with

Metacercaria

② eating raw transport host, such as wild pig

③ drinking raw stream water

Intestine

abdominal cavity subcutaneous tissue

abdominal wallLiver,kidneyReproductive sytem

thoracic cavity

(diaphragm)

capsule of heart lung

brain

（ intestinal wall ）

Life Cycle

Development in human

Migration

Development in humanLife Cycle

Residence

Stage

Site

Life span

Adults,Younger worms

Adults: lungYounger worms: liver,kidney,pancreas,brain, subcutaneous

Habitus Wandering

5-6 years, up to 20 years

Development in human

Life Cycle

Discharge

Stage

Mode

Eggs

In the sputum or stool

Pathogenesis

Pathological changes in host are caused by: A. Mechanical injury by migration and inhabitation of

the worm

B. Immunopathological reactions by secretions and

excrements of the worm

Process of the paragonimiasis:

A. Acute stage:

B. Chronic stage:

Pathogenesis

Acute stage:

several days to 1 month after infection mainly caused by invasion and migration of

the young flukes

some people may be asymptomatic

tired, loss of appetite, fever,diarrhea,

abdominal pain,chest pain, cough and

eosinophilia etc.

Chronic stagePathogenesis

Also classified into 3 stages:

Abscess stage:The bleeding and infiltration of

neutrophils and eosinophils surrounding worms form a capsule, abscess.

Cystic stage: the cyst wall is formed due to the progressive

fibrosis of the surrounding tissue. A. The cystic contents are chocolate or rusty thick fluid with eggs and Charcot-Leyden crystals, which looks like sesame paste.

B. The shadow of the cyst can be seen on X-ray. C. Patients cough out the rusty sputum when the cyst communicates with the bronchioles

Pathogenesis

Chronic stage

Fibrous-scar stage: After worm die or move to other

sites, the cyst will be filled with fibrous scar.The

exudate and pus are expelled or absorbed and

replaced by fibrous-scar tissue.

Pathogenesis

Infected lungs Adults worms in lungs

Clinical Manifestation Paragonimiasis may be classified into 4 types :

Pulmonary paragonimiasis: the symptoms resemble pulmonar

y tuberculosis with low fever, loss of appetite, night sweating, loss of weigh

t,chest pain, chronic cough and blooded sputum or rusty sputum

Cerebral paragonimiasis : manifests epilepsy ( 癫痫 ), paraly

sis ( 瘫痪 ), visual disturbance , psychomotor symptoms, etc

Abdominal paragonimiasis: abdominal pain ， diarrhea or dyse

ntery with blood.

Cutaneous paragonimiasis: the wandering and painless subcut

aneous nodules.

Diagnosis History of eating raw crustaceans (crabs)

or inadequately cooked pork

Parasitological examination : Based on microscopic demonstration of eggs in stool or sputum. But the eggs may not be present until 2 to 3 months after infection.

A. Sputum examination:

(1) Alkali digestive method (10%NaOH),

(2) Direct sputum smear

B. Stool examination:

(1) Water sedimentation method,

(2) Direct fecal smear

Biopsy for Subcutaneous type The worms may be discovered during surgery

or biopsy; typical pathological changes can

be found,such as worms cavity or tunnel,

eosinophilic infiltration, Charcot-Leyden crystals, ect.

Immunological tests for reference IDT(intradermal test)

ELISA for special antibodies

dot-ELISA for Cag

X-ray /CT examination for chest or brain

Diagnosis

Epidemiology This disease is prevalent in Far East, Africa and South

America.

In China, it is endemic in 27 provinces except Tibet,

Xinjiang, Inner Mongolia, Qinghai, and Ningxia. Its

prevalence is related to the natural focus.

In some forest and desert the par

asitic zoonoses transmit among

vertebrate, which areas is called

natural endemic focus.

Endemic areas Natural focus

Epidemiology

Epidemiology Its prevalence is also related to eating raw crabs and crayfishes.

In the Far East, crabs are frequently eaten after they have

been slightly salted, pickled,or immersed briefly in wine

(drunken crab),practices that are seldom lethal to the

metacercaria.

Fresh crab juice, which is used for the treatment of infertility in Cameroon and of measles in Korea.

Children living in endemic areas may be infected while

handling or ingesting crabs during the course of play.

Prevention and control

Treatment:

praziqantel, 75mg/(kg.d) for two days

Prevention:

(1) Health education,

(2) Avoid eating raw fresh water crabs and

crayfishes.

(3) Avoid sputum and stool getting into water.

Questions

What are the intermediate and definitive hosts?

How do humans contract this parasite?

How to diagnose this disease?

Pagumogonimus skrjabini

It was first reported by Chen in 1959, mainly caused cutaneous paragonimiasis

Its Life cycle is similar to P. westermani’s

Definitive hosts: Paguma, cats, dogs etc

Human: non-normal host Its larvae cann’t develop into mature worm in human.

Pathological changes are caused by migration

of larvae

Diagnosis is based on Immunological test

and biopsy

Fasciolopsis buski

largest intestinal fluke , also named as “giant

Asian intestinal fluke”,which can cause

Fasciolopsis

In 1873, first cases of Fasciolopsis was found

in Guangzhou

Introduction

Morphology

1 、 Adult worm

long elliptic, very big, 3~8cm,

flesh-colored, like a ginger slice.

The ventral sucker is much larger

than the oral sucker and is

located close to it.

Two coral-liked testes are located

in the posterior half of the body.

The tegument is densely covered

with spines.

Morphology

2 、 Egg

the largest helminth egg

oval, yellowish,

a thin shell,

a small operculum,

Inside an egg cell surrounded

by some yolk cells

Life Cycle

Definitive hosts: human and pigs

Pigs are the most important reservoir hosts

Intermediate host: Planorbis snail

Medium of water plants: water chestnut,

bamboo , caltrop, lotus

Life Cycle

Life Cycle

Me

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Ad

ults

Ju

ven

iles

Ad

ult

s

Eg

gs

Mira

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Definitive host

(Human,pig)

intermidate host

(Planorbis snail)

Aquatic plant medium

(water chestnut, water caltrop)

Definitive host

(Human,pig)

Development in human

Life Cycle

Infection

Stage metacercaria

Modeeating raw water plants with

metacercariae

Development in humanLife Cycle

Residence

Stage

Site

Life span

Adults

Small intestine

1-4 years

Mode Attatch to the intestine wall

Development in human

Life Cycle

Discharge

Stage

Mode

Eggs, Adults

In the feces

Pathogenesis

Mechanical injury due to the attachment of the adults

Spoliation of nourishment, covering the wall of intestine

to affect absorption

Allergy caused by secretions and excrements

Intestinal obstruction by a mass of the worms

Hemorrhage, ulceration, abscess of the intestinal wall

Clinical Manifestation

Most of infection cases are asymptomatic.

Some patients may have abdominal discomfort ,

nausea, vomiting and chronic diarrhea.

Some infected children manifest anemia,

weight loss, edema of leg and face even

ascites.

Diagnosis

Stool examination:

Based on dentification of typical eggs or the

adult flukes in the stool.

Seroimmunological test:

useful in detecting early infection or general

survey.

Epidemiology

Mainly distributed in temperate zone or

subtropics of Asian

In China, it is found in 18 provinces except

north and west regions.

The prevalence of fasciolopiasis is related to

growing water plants and feeding pigs on

water plants

Prevention and controlTreatment:

praziqantel, a single 15mg/kg

bithionol (bitin).

Prevention:

(1) Health education,

(2) Avoid eating raw water-plants or avoid feeding

pigs on raw water plants,

(3) Deal with night soil

P. westermani C. sinesis F. buski

Definitive human human human

Reservoir host tiger, dog etc dog, cat etc pig

Paratenic host boar etc no no Egg sputum/feces feces feces

Miracidium water snail water Sporocyst snail snail snail

Rediae snail mother snail snail

Daughter snail snail

Cercariae snail/water

Metacercariae crab/crayfish fish/shrimp water vegetation

Larvae migration no no

Adult worm lungs/other organs bile ducts intestine

Ectopic parasitism yes no no

Summary

Life Cycle

Transport host: wild pig(boar)

The larva of some parasites can invade a non-normal host,

but can not develop, and only keep the larva stage. If the l

arva enter a normal definitive host, it can continue to devel

op into adult worm. The non-normal host is called a parate

nic host or transport host.