Enterobius vermicularis: Introduction, Morphology, Life Cycle, Mode Of

Enterobius vermicularis: Introduction, Morphology, Life Cycle, Mode Of Infection, Pathogenecity, Lab Diagnosis And Treatment

Introduction of Enterobius vermicularis

Enterobius vermicularis is a nematode commonly referred to as pinworm or threadworm or seatworm causes enterobiasis. Prevalence is maximum among the school-going, children. The temperate climate, overcrowding, impaired hygiene, poor personal care are factors promoting infection.

Scientific Classification

Kingdom: Animalia

Phylum: Nematoda

Class: Chromadorea

Order: Rhabditida

Family: Oxyuridae

Genus: Enterobius

Species: E. vermicularis

Binomial name

Enterobius vermicularis

Geographical Distribution of Enterobius vermicularis

The pinworm has a worldwide distribution and is the most common helminth infection in the United States, western Europe, and Oceania.

Habitat

The adult worm remains attached to the large intestine ( caecum, appendix, adjacent portion of colon) by their mouth end.

Morphology of Enterobius vermicularis

Adult worm

Appearance: small, white, thread-like.
Size : male – 2-5mm x 0.1-0.2mm female – 8-13mm x 0.3-0.5mm.
Cervical alae: a wing-like expansion of the cuticle near the anterior end.
Double bulb esophagus: posterior dilated end of the esophagus which forms the globular bulb.
Male: the posterior end is tightly curved, bearing a copulatory bursa with spicules at the posterior end.
Female: posterior one-third is tapering, straight, thin, and pointed.

Eggs

Shape: oval, planoconvex.

Size : 50-60μm x 20-30μm.

Surrounded by double-layered eggshell

Embryonated when passed fresh; contains a tadpole larva inside.

Survives some weeks in fairly high humidity and moderate temperature, few days in dry dust.

Life -cycle of Enterobius vermicularis

The entire life cycle completes in the human gastrointestinal tract o from about 2-8 weeks.

The life cycle begins with eggs being ingested. The eggs hatch in the duodenum. The emerging pinworm larvae grow rapidly to a size of 140 to 150 μm, and migrate through the small intestine towards the colon. During this migration, they molt twice and become adults. Females survive for 5 to 13 weeks and males about 7 weeks. The male and female pinworms mate in the ileum where after the male pinworms usually die, and are passed out with stool. The gravid female pinworms settle in the ileum, caecum, appendix, and ascending colon, where they attach themselves to the mucosa and ingest colonic contents.

Almost the entire body of a gravid female becomes filled with eggs. The estimations of the number of eggs in a gravid female pinworm range from about 11,000 to 16,000. The egg-laying process begins about five weeks after the initial ingestion of pinworm eggs by the human host. The gravid female pinworms migrate through the colon towards the rectum at a rate of 12 to 14 cm per hour. They emerge from the anus, and while moving on the skin near the anus, the female pinworms deposit eggs either through contracting and expelling the eggs, dying and then disintegrating, or bodily rupture due to the host scratching the worm. After depositing the eggs, the female becomes opaque and dies. The female emerges from the anus to obtain the oxygen necessary for the maturation of the eggs.

Pathogenicity of Enterobius vermicularis

Host: only human.

Infective form: embryonated eggs

Mode of transmission :

Ingestion of eggs by contaminated hands (nail-biting, inadequate handwashing).
Exogenous autoinfection: the scrapping of the perianal region due to intense itching, thus contaminating the finger.
Endogenous auto-infection/ retro infection: rétrogrademigration of larvae that have hatched at the perianal region into the rectum and large intestine.

Pathogenesis

Nocturnal migration of gravid females, fully filled with eggs, from the large intestine to the perianal region and start laying eggs.

The human with hypersensitivity to the secretion and excretion of the worms. Rectal pruritus at night

Continuous scratching of the skin

Excoriation of perianal skin

Migration of the worm

Invade female genital tract – vulvovaginitis.

Entrance into the peritoneal cavity – formation of granuloma around eggs or worms which may lead to chronic pelvic peritonitis.

Invade appendix – appendicitis

Others: liver, lungs.

Laboratory diagnosis of Enterobius vermicularis

Sample collection

The adult worms may be detected in the perineal region or on the surface of the stool. These may also be detected in the appendix during an appendectomy. Since eggs are not discharged by the worm into feces, therefore, the fecal examination is not useful in the laboratory diagnosis of threadworm infection. However, in a small proportion of patients stool examination may show the presence of eggs of E. vermicularis. Eggs that are deposited in large numbers on the perianal and perineal skin at night can be demonstrated by scrapping these with NIH swab similar to the adhesive cellophane tape method.
NIH SWAB

Consists of a glass rod at one end of which a piece of transparent cellophane (with sticky surface out) is wrapped and held in place with a rubber band. The other end of the glass rod is fixed in a rubber stopper and kept in a test tube. The cellophane part is used for swabbing by rolling over the perianal area which is then detached, spread over a glass slide, and examined microscopically. Swabs should be taken from three successive days to be considered negative.

Cellophane tape preparation

Microscopy examination

Detect eggs ( planoconvex, 50-60 μm x 20-30μm), with a translucent shell of moderate thickness, containing a larva inside.

Treatment of pinworm

Several drugs are effective against infection with E. vermicularis.

Pyrantel pamoate- Single dose 11 mg per kg of body weight ( max. 1 gm). Mebendazole – single dose of 100mg

Albendazole – single dose of 400 mg
Piperazine citrate in a 7- day course of 65 mg per kg of body weight

Prevention of Enterobius vermicularis

By improving personal hygiene
Proper washing of bed cloths.
Hand washing.

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