Introduction to helminth

Helminth means worm. Their motile activity is accoplished by
wriggling movement. The helminth of medical importance belong
to 3 classes: Nematoda, Trematoda and Cestoda.

Geohelminths refer to the helminths which complete their life
cycles not requiring the processes of the development in
intermediate hosts. They have only one host and a simple life cycle,
such as ascarid, hookworm, pinworm and etc.
 Biohelminths refer to those that have to undergo the
development in intermediate hosts to complete their life cycle ,
such as filaria, liver fluke, pork tapeworm and so on.
Characteristics of Class Nematoda
1. Body is non-segmented and cylindrical in shape.
2. Two sexes are separated. ♀is larger with a straight
tail and a double set of reproductive organs. ♂is smaller
with a curled tail and a single set of reproductive organs.
3. With a complete digestive tract.
4. The body cavity is a protocoele.
5. Life cycle: infective stage is embryonated egg or larva3.
Egg
larva (several stages)
adolescent
adult
molt1
Larva1
molt2
L2
molt3
L3
molt4
L4
Adult
Ascaris Lumbricoides
Ascaris lumbricoides , common saying
“round worm of man”, is the largest of the
intestinal nematodes parasitizing humans. It is
the most common worm found in human. It is
worldwide in distribution and most prevalent
through out the tropics, sub-tropics and more
prevalent in the countryside than in the city
I. Morphology

Adult: The adults are cylindrical in shape, creamywhite or pinkish in color. The female averages 2035cm in length, the largest 49cm. The male is smaller,
averaging 15-31cm in length and distinctly more
slender than the female. The typical curled tail with a
pair sickle like copulatory spines. On the tip of the
head there are three lips, arranged as a Chinese word
“ 品 ” . They have a complete digestive tract.
Reproductive organs are tubular. male has a single
reproductive tubule. The female has two reproductive
tubules and the vulva is ventrally located at the
posterior part of the anterior 1/3 of the body.
Adult worm of A.
lumbricoides
The lips of Ascaris lumbricoides
The three lips
are seen at the
anterior end. The
margin of each lip
is lined with minute
teeth which are not
visible
at
this
magnification.
Ascaris lumbricoides – Lips
Copulatory spines of male

Egg: There are three kinds of the eggs. They are fertilized eggs,
unfertilized eggs and decorticated eggs. We usually describe an
egg in 5 aspects: size, color, shape, shell and content.
1. Fertilized eggs: broad oval in shape, brown in color, an
average size 60× 45µm. The shell is thicker and consists of
ascaroside, chitinous layer, fertilizing membrane and
mammillated albuminous coat stained brown by bile. The content
is a fertilized ovum. There is a new-moon(crescent) shaped clear
space at the each end inside the shell.
2. Unfertilized egg: Longer and slender than a fertilized egg.
The chitinous layer and albuminous coat are thinner than those of
the fertilized eggs without ascaroside and fertilizing membrane.
The content is made of many refractable granules various in size.
3. Decorticated eggs: Both fertilized and unfertilized eggs
sometimes may lack their outer albuminous coats and are
colorless.
Fertilized egg of A. lumbricoides
Fertilized Ascaris Egg
A fertilized Ascaris egg,
still at the unicellular
stage, as they are
when
passed
in
stool.
Eggs are this
stage when passed in
thewhen passed in
stool.
Eggs
are
normally at this stage
when passed in the
stool
Fertilized Ascaris Egg
The ova begin fission
Embryonated Eggs of Ascaris
Freshly Passed Ascaris Eggs From faeces
 The eggs may
appear from light
to dark brown in
color.
 Egg containing a
larva, which will
be infective if
ingested.
Unfertilized egg
The chitinous layer and
albuminous coat are
thinner than those of the
fertilized eggs without
ascaroside and fertilizing
membrane. The content
is
made
of
many
refractable
granules
various in size.
Unfertilized egg
Unfertilized and Fertilized Eggs
II Life Cycle

1. Site of inhabitation: small intestine
2. Infetive stage: embryonated eggs
3. Route of infection: by mouth
4. No intermediate and reservoir hosts
5. Life span of the adult: about 1 year
This worm lives in the lumen of small intestine,
feeding on the intestinal contents, where the fertilized
female lays eggs. An adult female can produce
approximately 240,000 eggs per day, which are passed in
feces. When passed, the eggs are unsegmented and
require outside development of about three weeks until a
motile embryo is formed within the egg.
After the ingestion of embryonated eggs in
contaminated food or drink or from contaminated fingers,
host digestive juices acts on the egg shell and liberate the
larva into the small intestine. These larvae penetrate the
intestinal mucosa and enter lymphatics and mesenteric
vessels. They are carried by circulation to the liver, right
heart and finally to the lungs where they penetrate the
capillaries into the alveoli in which they molt twice and
stay for 10-14days and then they are carried, or migrate, up
the bronchioles, bronchi, and trachea to the epiglottis.
When swallowed, the larvae pass down into the small
intestine where they develop into adults. The time from the
ingestion of embryonated eggs to oviposition by the
females is about 60-75 days. The adult worms live for
about one year. The ascarid life cycle is as the following
diagram.
swallowed
digestive
Infective eggs
small intestine
Larvae hatch out
juice act
penetrate
intestine mucosa
Right heart
Lymphatic or venules
lungs
Bronchiles
penetrate the
alveolar wall
Bronchi
discharged with sputum
Pharynx
return to
Small intestine
discharged
Fertilized eggs
in feces
Out side of
the body
Embryonated eggs(infective eggs)
Liver
molt2,3
Alveoli
Trachea
die
molt 4
Adults
25-30℃,moisture
2-3weeks, molt1
III. Pathogenesis
There are two phase in ascariasis:
1. The blood-lung migration phase of the larvae:
During the migration through the lungs, the larvae
may cause a pneumonia. The symptoms of the
pneumonia are low fever, cough, blood-tinged
sputum, asthma. Large numbers of worms may
give rise to allergic symptoms. Eosionophilia is
generally present. These clinical manifestation is
also called Loeffler’s syndrome.

2. The intestinal phase of the adults. The presence
of a few adult worms in the lumen of the small
intestine usually produces no symptoms, but may give
rise to vague abdominal pains or intermittent colic,
especially in children. A heavy worm burden can
result in malnutrition. More serious manifestations
have been observed. Wandering adults may block the
appendical lumen or the common bile duct and even
perforate the intestinal wall. Thus complications of
ascariasis, such as intestinal obstruction, appendicitis,
biliary ascariasis, perforation of the intestine,
cholecystitis, pancreatitis and peritonitis, etc., may
biliary ascariasis is the
most common complication.
occur, in which
A large mass of Ascaris
biliary ascariasis
Intestinal obstruction
III. Diagnosis
The symptoms and signs are for reference only. The
confirmative diagnosis depends on the recovery and
identification of the worm or its egg.
1. Ascaris pneumonitis: Examination of sputum for
Ascaris larvae is sometimes successful.
2. Intestinal ascariasis: Feces are examined for the ascaris
eggs.
(1) Direct fecal film: It is simple and effective. The
eggs are easily found using this way due to a large number
of the female oviposition, approximately 240,000 eggs per
worm per day. So this method is the first choice.
(2) Brine-floatation method:
(3) Recovery of adult worms: When adults or
adolescents are found in feces or vomit and tissues and
organs from the human infected with ascarids , the
diagnosis may be defined.
V. Epidemiology

World wide distribution, very common in
China, especially in the countryside.
Factors favoring the spread of the transmission:
1. Simple life cycle.
2. Enormous egg production ( 240,000 eggs/ day/
female ).
3. These eggs are highly resistant to ordinary
disinfectants( due to the ascroside). The eggs may
remain viable for several years.
4. Social customs and living habits.
5. Disposal of feces is unsuitable.
VI. Prevention and Treatment
 1.Treatment to ascariasis:
Mebendazole
（甲苯咪唑）, Albendazole（阿苯哒唑或
丙硫咪唑 ）and Levamizole are effective.
 2. Sanitary disposal of feces.
 3. Hygienic habits such as cleaning of hands
before meals.
 4. Health education.
Trichuris trichiura ( Whipworm)
Morphology:
 Adult: The worm looks like a buggy whip, the
anterior 3/5 is slender and the posterior 2/5 is
thick. It is pinkish gray in color. The female worm
is 3-5 cm in length and has a long slender
esophageal region. The male is smaller than the
female and has a curved tail. The reproductive
organs of male and female are all double tubule.
 Egg: It is barrel or spindle in shape and 50 x
20µm in size. It is brownish and has a translucent
polar plug at either ends. The content of the egg is
an undeveloped cell
I.
Adults of T. trichiura
Eggs
of
T.
trichiura under
the high power
II.
Life Cycle:
1. Site of inhabitation: cecum
2. Infective stage: embryonic egg
3. Infectve mode and route: passively swallowed by the
mouth
4. Without intermediate host and reservoir host
5. The life span of the adult is about 3-5years.
Adults
deposit
Eggs
3weeks
Infective eggs
ingested by man
larvae hatch out
in small intestine
invade the intestinal wall
return to the intestinal lumen
develop 3-10 days
Adults
III. Pathogenesis:
1. Light infection: Asymptomatic
2. Middle infection: Clinical manifestations are usually
abdominal pain, anorexia, diarrhea, constipation .
3.
Heavy infection: Bloody diarrhea, emaciation,
prolapse of the anus may occur.
IV. Diagnosis:
Discover the eggs in feces by saturated brine flotation
method or direct fecal smear.
V. Treatment and prevention:
Same as those
of ascariasis
Take Mebendazole 3 days for a treatment course
and repeat next week