Paragonimus
These worms’ reproductive organs stand
side by side. Adult worms usually live in the
lungs
of
man
and
carnivores
causing
paragonimiasis, so they are also called lung
flukes. It is essentially a zoonosis. In china,
there are two species of lung flukes of medical
importance: Paragonimus westermani(卫氏并殖
吸虫）, Pagumogonimus skrjabini(斯氏狸殖吸虫).
Paragonimus westermani
This worm was first found in a tiger’s lung in
India in 1877. It is the main kind of lung flukes
infecting man. In our country it is prevalent in 22
provinces, city and autonomic region except
Xinjiang, Xizang, Qinghai, Gansu, Ningxia, Inner
Mongolia and Shandong.
I. morphology
1. Adult:
The body is elliptic, dorsally it is convex,
ventrally it is flattened, reddish-brown in color, about 7.512×4-6×1-3 mm (length:width=2:1).
It looks like half a
peanut. The sizes of two suckers are sub-equal. The ventral
sucker is located in the central position of the body. Two
testes, the ovary and the uterus are situated side by side.
The vitellaria is well developed. The excretory bladder
extends forward to the level of pharynx. The digestive tract
is divided into two ceca.
A adult of Paragonimus westermani
A adult of P. westermani
2. Egg is irregularly elliptic, golden yellow in
color,
80-110×48-60µ
in
size.
The
thick
and
asymmetric shell with a distinct operculum encloses
an ovum surrounded by more than 10 yolk cells.
3. Metacercaria is spherical, about 300400µ with two layers of transparent walls, in crab
and crayfish.
.
The oval ova
have an operculum
and are 80-110 by
48-80 µm. It is
golden
yellow in
color. The shell is
uneven in thickness.
The content is an
ovum and more than
10 yolk cells.
Egg of P. westermani
II. Life cycle
1. Infective stage: metacercaria
2. Infective mode: eating raw fresh water crabs and
crayfish with metacercariae
3. Infective route: by mouth
4. Site of inhabitation: lungs
5. Intermediate hosts: 1st int. host is melania snail(川
卷螺). 2nd int. hosts are crab and crayfish.
6. Reservoir hosts: carnivores such as tiger, lion,
wolf, fox, dog, leopard, cat and etc
7. Life span: 5-6 years
penetrate the diaphragm
adolescents
adults in lungs
eggs
Migrate to the lungs
excystation
In Human Body
duodenum
Discharged
in sputum
man eats raw crab or
or in feces
crayfish with metacercaria
────────────────────────────
Metacercariae
invade crab
In Water
and crayfish
Eggs get into water
25-30℃
3-4weeks
Miracidia hatch out
invades
Cercariae
Escape
Cercariae
Melania snail
asexual reproduction
Daughter rediae
Mother rediae
Sporocysts
III. Pathology and Symptomatology
Adults inhabit lungs, although other organs
are also involved.
1. Pathological lesions may be classified into 4
stages: (1)Invading and migrating stage: After
excystation the adolescents penetrate the intestinal wall and
migrate to the lungs. (2) Suppurative stage. The bleeding
and infiltration of neutrophils and eosinophils surrounding
worms form a capsule, abscess.
(3) Cystic stage, the cyst wall is formed due to the progressive
fibrosis of the surrounding tissue. The cystic contents are
chocolate or rusty thick fluid with eggs and CharcotLeyden crystals( 夏 科 - 雷 登 氏 晶 体 ） , which looks like
sesame paste. The shadow of the cyst can be seen on X-ray.
Patients
cough
out
the
rusty
sputum
when
the
cyst
communicates with the bronchioles. (4) Fibrous-scar stage,
the worms are dead or escape from the cyst. The exudate and
pus are expelled or absorbed and replaced by fibrous-scar
tissue.
2. Clinical manifestation:
Paragonimiasis may be classified into 4 types : (1)Pulmonary
type: the symptoms resemble pulmonary tuberculosis(结核)with low
fever, loss of appetite, night sweating, chest pain, loss of weight and
rusty sputum.
(2) Brain type: manifests epilepsy( 癫 痫 ), hemiplegia( 偏 瘫 ),
monoplegia( 单 瘫 ),aphasia( 失 语 ), visual disturbence and resembles
cerebral cystcercosis(脑囊虫病).
(3)Abdominal type: abdominal pain ， diarrhea or dysentery with
blood, mucus and ova in feces.
(4)Subcutaneous type: the wandering and painless subcutaneous
nodules.
IV. Diagnosis
1. Sputum examination: (1) Alkali digestive method
(10%NaOH), (2) Direct sputum smear
2. Stool examination: (1) Alkali digestion , (2) Water
sedimentation method, (3) Direct fecal smear
3. Biopsy for Subcutaneous type
4. CT for brain type
5. Immunological tests for reference.
V. Treatment and Prevention
1.Treatment: Drug of choice is praziqantel. Other
effective drugs include hexachloroparaxylol, bithionol
(bitin).
2. Prevention:
(1) Health education,
(2) Avoid eating raw fresh water crabs and crayfishes.
(3) Avoid sputum and stool getting into water.
VI. Epidemiology
This disease is prevalent in Far East,
Africa and South America. There are
endemic foci in 22 provinces in Northeast,
East
and
Southwest
of
China.
Its
prevalence is related to eating raw crabs
and crayfishes and the natural foci.