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 molt2 molt3 molt4

Larva1 L2 L3 L4 Adult

Ascaris Lumbricoides Ascaris lumbricoides , common saying “rou

nd worm of man”, is the largest of the intestinal

nematodes parasitizing humans. It is the most c

ommon worm found in human. It is worldwide i

n distribution and most prevalent through out t

he tropics, sub-tropics and more prevalent in th

e countryside than in the city

I. Morphology Adult: The adults are cylindrical in shape, creamy-

white or pinkish in color. The female averages 20-35cm 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, sti

ll 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, feed

ing 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 digestiveInfective eggs small intestine Larvae hatch out juice act

penetrate Lymphatic or venules Liverintestine mucosa penetrate the molt2,3Right heart lungs Alveoli alveolar wall

Bronchiles Bronchi Trachea

discharged with sputum die

return to molt 4Pharynx Small intestine Adults

discharged 25-30 ,moisture℃Fertilized eggs Out side of in feces the body 2-3weeks, molt1 Embryonated eggs(infective eggs)

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 occur, in which biliary ascariasis is the most common complication.

A large mass of Ascaris

biliary ascariasis

Intestinal obstruction

III. Diagnosis The symptoms and signs are for reference only. The confir

mative 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 adolesce

nts 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 Chi

na, 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)I.       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

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.

deposit 3weeks ingested by man Adults Eggs Infective eggs larvae hatch out in small intestine invade the intestinal wall return to the intestinal lumen Adults develop 3-10 days

III. Pathogenesis: 1.  Light infection: Asymptomatic 2. Middle infection: Clinical manifestations are usually

abdominal pain, anorexia, diarrhea, constipation . 3. Heavy infection: Bloody diarrhea, emaciation, prola

pse of the anus may occur.

IV. Diagnosis: Discover the eggs in feces by saturated brine flotation m

ethod 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