Enterobius Vermicularis: Pinworm Morphology & Life Cycle

Enterobius Vermicularis

• Common names include pinworm, seatworm, and threadworm
• Considered the world's most common parasite, especially affecting children
• Adult worms inhabit the caecum, appendix, and adjacent ascending colon

Morphology

• Adults are short, white, fusiform worms resembling bits of white thread
• The mouth has 3 wing-like, transversely striated cuticular expansions known as cervical alae
• The esophagus features a unique double-bulb structure

Female Worm

• The female worm is 8–13 mm long and 0.3–0.5 mm thick
• Females are oviparous
• Females can survive for 5-12 weeks.

Male Worm

• The male worm measures 2–5 mm in length with a thickness of 0.1–0.2 mm
• Males live for approximately 7–8 weeks

Egg

• The egg is colorless and not bile-stained
• Floats in saturated salt solution
• Has a characteristic elongated ovoid shape, flattened on one side and convex on the other

Life Cycle

• E. vermicularis is monoxenous, completing its entire life cycle within a single host (humans)
• No intermediate host or systemic migration involved
• The natural host is humans
• The infective form is embryonated eggs
• Infection occurs through ingestion of embryonated eggs containing larvae, often via contaminated fingers or autoinfection

Life Cycle Details

• Eggs containing infective larvae, laid on perianal skin, are swallowed and hatch in the intestine
• Larvae molt in the ileum, then migrate to the caecum to mature into adults
• The process from egg ingestion to gravid female development takes 2 weeks to 2 months
• Gravid females migrate to the rectum, exiting the anus at night to lay sticky eggs on perianal and perineal skin and may retreat into the anal canal to lay more eggs
• Female worms may wander into the vulva, vagina, uterus, and fallopian tubes, potentially reaching the peritoneum
• Males are rarely seen due to their lack of migration, they typically die after mating and are passed in the feces

Autoinfection & Retroinfection

• Autoinfection occurs when eggs are ingested due to scratching the perianal area, leading to eggs under the nails, this mode of infection is common in children
• Retroinfection happens when eggs laid on perianal skin hatch immediately into infective larvae and migrate through the anus into the colon

Pathogenicity and Clinical Features

• Enterobiasis is more common in children and occurs more often in females than males
• About one-third of infections are asymptomatic
• Worms cause intense irritation and pruritus (pruritis ani) in the perianal and perineal area when laying eggs, resulting in scratching and skin excoriation
• Nocturnal migration of worms disturbs sleep, and nocturnal enuresis may be observed
• Worms crawling into the vulva and vagina cause irritation and discharge and can ascend to the uterus, fallopian tubes, and peritoneum, potentially causing salpingitis, cervicitis, peritonitis, and urinary tract infections
• Worms have been found in surgically removed appendices, suggesting a role in appendicitis

Laboratory Diagnosis

• Suspect pinworm infestation based on a history of perianal pruritus
• Diagnosis relies on detecting eggs or adult worms
• Feces examination is not useful due to the low presence of eggs
• Eggs are deposited in high numbers on perianal and perineal skin at night; diagnose using swabs collected early morning, pre-bathing
• Swabs from perianal folds are most often positive
• Eggs occasionally found in dirt from under infected children's fingernails

NIH Swab Method

• A widely used method for specimen collection
• Glass rod has transparent cellophane attached to one end with a rubber band
• The cellophane is used for swabbing the perianal area
• Cellophane placed on glass slide and examined microscopically

Scotch Tape Method

• Scotch tape is held sticky side out on a wooden tongue depressor
• Mounted tape is firmly pressed against the anal margin, covering all sides
• A drop of toluene cleans the tape, transfer to a glass slide, tape sticky side down for examination under the microscope

Demonstration of Adult Worm

• Adult worms may be visible on the surface of stools
• They can be found crawling out of the anus of sleeping children
• Detection possible in stools collected after an enema or in the appendix during appendicetomy

Treatment

• Pyrantel pamoate (11 mg/kg once, maximum 1 g), Albendazole (400 mg once), or mebendazole (100 mg once) are used for single-dose therapy
• Piperazine requires daily administration for one week