Nematode(线虫 )

Round Worms

Introduction

Among the commonest of all parasites and responsible for diseases of major importance in humans

Non-segmented roundworms belonging to the Phylum Nemathelminthes, Class Nematoda

The sexes are usually separate (Dioecious), the male which is smaller than the female commonly has a curved posterior end

Morphology Cylindrical and slender Bilaterally symmetrical Sex-differentiated Celomic cavity: protocoele The supporting body wall consists of cuticle layer,

syncytial layer (subcutical layer) and longitudinal muscular layer

The alimentary tract is a simple tube extending from the mouth to the anus

Morphology

No circulatory system The reproductive system is in tubular

form with dioecious (sex-differentiation): The male reproductive organs are

situated in the posterior third of the body as a single coiled tube, the various parts of which are differentiated as testis, vas deferens, seminal vesicle, and ejaculatory duct, also cloaca and spicule

The female reproductive system may be either a single or a bifurcated tube, differentiated into ovary, oviduct, seminal receptacle, uterus, ovejector and vagina

Digestive system

female reproductive system

male reproductive system

Physiology The methods of obtaining food may be classified as

Sucking with ingestion of blood (Ancylostoma) Ingestion of lysed tissues and blood (Trichuris) Feeding on the intestinal contents (Ascaris) Ingestion of nourishment from the body fluids (filarial worms)

Metabolism: mainly aerobic metabolism, and most species need free-living periods for larvae, which are capable of withstanding a wide range of environmental condition

During larval development, nematodes pass through several molts or ecdysis, both inside and outside the host

Life cycle Geo-helminths

Only one host: the larvae pass from host to host directly or after a free-living existence

Transmission to a new host depends upon the ingestion of the mature infectious eggs with

larvae (Ascaris, Pinworm) the penetration of the skin or mucous

membranes by the larvae (Hookworm) Bio-helminths

Have an intermediate host Transmission to a new definitive host is

intermediated by the arthropod--- (filarial worm)

Common medical nematodes species

Intestinal lumen residing nematodes: Ascaris, Hookworm, Whip worm, Pinworm----the

adult parasite inhabit in human intestinal tract

Blood and tissue residing nematodes: Filaria, Thichinella----the location of the adult

parasite is blood or tissue

Ascaris Lumbricoides(似蚓蛔线虫 ,蛔虫 )

General Introduction Common saying “round worm of man”

The largest of the intestinal nematodes

parasitizing humans

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

Adult: 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, has a

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 ” 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

Morphology

Adult worm of A. lumbricoides

The lips of A. 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

Egg There are three kinds of the eggs

fertilized eggs unfertilized eggs decorticated eggs

We usually describe an egg in 5 aspects size, color, shape, shell and content

Morphology

Fertilized egg: an average size 60×45µm broad oval in shape brown in color The shell is thick Albuminous coat is thick and stained brown by bile The content is a fertilized ovum There is a new-moon(crescent) shaped clear space at each end inside the shell

Morphology

Unfertilized egg Longer and slender than fertilized egg The shell and albuminous coat are thinner

than those of the fertilized egg The content is made of many refractable

granules various in size

Decorticated egg: Both fertilized and unfertilized eggs

sometimes may lack their outer albuminous coats and are colorless

Morphology

Life Cycle

Site of inhabitation: small intestine Infective stage: embryonated eggs Route of infection: by mouth Blood-lung migration: intestine--- blood intestine--- blood

stream --- right side of the heart --- lung stream --- right side of the heart --- lung --- respiratory tree --- coughed up and --- respiratory tree --- coughed up and swallowed --- small intestineswallowed --- small intestine

Life Cycle

No intermediate and reservoir hosts The time from the ingestion of

embryonated eggs to oviposition by the females is about 60-75 days

Life span of the adult: about 1 year Female may produce approximately

240,000 eggs per day, which are passed in feces

Life Cycle

1. The blood-lung migration phase of the larvae: During the migration through the lungs, the larvae may cause a pneumonia (temporary).

The symptoms of the pneumonia are low fever, cough, blood-tinged sputum, asthma

The clinical manifestation is also called Loeffler’s syndrome

Pathogenesis

2. The intestinal phase of the adults• No symptoms to vague abdominal pains or

intermittent colic, especially in children• A heavy worm burden can result in malnutrition• Wandering adults may block the appendical lumen or

the common bile duct and even perforate the intestinal wall which cause complications of ascariasis:

• intestinal obstruction• Appendicitis• biliary ascariasis (the most common one)• perforation of the intestine• cholecystitis, pancreatitis and peritonitis

Pathogenesis

The symptoms and signs are for reference only

Intestinal ascariasis: feces are examined for the ascaris eggs Direct fecal film: it is simple and effective and

is the first choice brine-floatation method: recovery of adult worms: when adults or

adolescents are found in feces or vomit and tissues

Ascaris pneumonitis: examination of sputum for Ascaris larvae is sometimes

Diagnosis

Worldwide distribution, very common in China, especially in the countryside.

Infection rate:

rural >urban;

children > adults

Epidemiology

Factors favoring the spread of the transmission: Simple life cycle Enormous egg production ( 240,000 eggs/ day/

female ) Eggs are highly resistant to ordinary disinfectants

( due to the ascroside) which may remain viable for several years

Social customs and living habits. Disposal of feces is unsuitable

Treatment to ascariasis: Mebendazole, Albendazole are effective

Sanitary disposal of feces. Hygienic habits such as cleaning of

hands before meals. Health education.

Prevention

Trichuris trichiura (毛首鞭形线虫 ) (whipworm,鞭虫 )

General Introductions A common nematode residing in human colon Human infection with whipworm once be

identified in coprolites of prehistoric man Worldwide distributed in the same areas

where the Ascaris are found Trichuriasis are usually not serious clinically,

but overwhelming infections leading to death have been reported in children

Morphology: adults

The adult female measures about 35-50 mm in length, and the male about 30-45 mm

Morphology: eggs

☆ barrel-shaped or fusiform with bipolar prominences (plugs)

☆ measures 50-55 by 22-24 µm

☆ brown, smooth shell

☆ contains a single-cell ovum

Life cycle No intermediate host Infected through

fecal-oral route by the embryonated egg

Ileocecal portion is the most favorite site of residing

Pathogenesis Symptoms are determined largely by the worm

burden: less than 10 worms are asymptomatic Only heavily infected patients develop clinical

disease: Chronic diarrhea, characterized by mucous stools,

and associated with tenesmus(里急后重 ) If the diarrhea is protracted, the patient may develop

rectal prolapse, more likely to occur in small children Many individuals infected with whipworm tend to be

malnourished and anemic

Pathogenesis

prolapsed rectum

whipworms are often seen attached to the rectal tissue

Laboratory Diagnosis

"Fecal direct smear " or

"sedimentation" are the methods of choice for etiological diagnosis

Epidemiology1. The infection is most common in tropical

areas where prevalence as high as 80% has been documented

2. Commonly coexisting with Ascaris infections

Principles of Control

Chemotherapy Albendazole (400 mg per day for 3 days) Mebendazole (100 mg per dose, twice a day for 2

days)Proper disposal of feces is the primary means of prevention

In areas of the world where untreated human feces are used to fertilize crops, control of this infection is impossible

Hygeian education

Enterobius vermicularis ( 蠕形住肠线虫 ) (Pinworm,蛲虫 )

Introduction

1. A colon residing nematode of smaller size.

2. One of the most widely prevalent parasitic nematodes of human being.

3. Affecting mainly children below the age of 12 years. No host other than man.

Morphology

Adult :Small nematodes of

8~13mm in length by

0.3~0.6 mm in diameter,

with the spindly shape Egg : smaller than that of

ascaris, asymmetrical,

with a thin and colorless

shell female

egg

male

anterior end of adult

Life Cycle

1. Adults inhabit in the cecum. Female lay eggs in the perianal region of the host.2. No intermediate host, the egg takes around 6 hours to be matured and becomes the infective stage.3. Mode of infection may be auto-infection or cross-infection.

Pathogenesis 1.The migratory phase of the pinworm is restricted to the

gastrointestinal tract and thus the host does not experience any systemic reactions.

2. Perianal itching may be complicated. Heavy infections in children may also produce such symptoms as sleeplessness, weight loss, hyperactivity, grinding of teeth, abdominal pain, and vomiting.

3. Ectopic parasitism may happen, which can cause uro-genital inflammations or even the pelvic cavity

be involved occasionally.

Laboratory Diagnosis

Anal swab method:Anal swab method:

Cotton swabCotton swab

Cellophane tapeCellophane tape

Epidemiology

Endemic in children concentrated units. There is no differences on the basis of sex, race, or socio-economic class.

Infection rate:

urban > rural;

children > adults

Principles of Control

1. Chemotherapy

2. Personal Sanitation