ENTEROBIASIS (caused by Enterobius vermicularis)

Enterobiasis is a parasitic infection caused by an intestinal nematode known as E. vermicularis. The disease which is commonly known as threadworm or pinworm infection is mostly found in children than in adults due to poor personal hygiene in the former. Pinworm infection is one of the oldest of all the helminthic infections, and the disease is widespread in nature. Enterobiasis, a worldwide parasitic infection is characterized by intense irritation around the anal or perianal region of affected people; and this causes occasional scratching of the anus by the infected persons. This can cause feacal contamination of the hands or fingers by infective eggs of the parasite especially in children, and infected fingers can introduce the parasite into the intestinal tract via the mouth. Though commonly found in young people and children, enterobiasis can also occur in people of all ages.

Enterobius vermicularis inhabits the colon and the anal skin or perianal region of infected human hosts. E. vermicularis or pinworm is also known to colonize the caecum and appendix, and this can cause appendicitis.

Type and morphology of Enterobius vermicularis infection

Enterobius vermicularis exist as adult male and female worms. The worm’s produce resistant infective eggs that is oval in shape. Infective eggs of E. vermicularis hatch into larva which eventually develops into adult worms in the colon or large intestine of infected human hosts. Infective eggs are highly resistant to desiccation, and they remain viable for long period in beddings and other materials on which they are shed.

Vector, reservoir and habitat of Enterobius vermicularis

Enterobius vermicularis or pinworm has no insect or animal vector. Humans are the definitive or final host of the parasite. No intermediate host exists for E. vermicularis; and the adult worms of the parasite are usually found in the caecum, colon, perineum or perianal region.

Clinical signs and symptoms of Enterobius vermicularis infection

Enterobiasis is usually a subclinical parasitic infection with mild or no severe clinical symptoms. Itching or scraping at the perianal region or anus is often the most noticeable sign of the disease even though E. vermicularis infection produces no severe disease in humans. Scratching of the perianal region which normally occurs in the nighttime can cause secondary bacterial infection; and the infection of the genital and urinary tract by E. vermicularis can occasionally occur in female hosts due to the close proximity of the genitalia or vagina to the anus in females. Serious complications associated with pinworm or threadworm infection occur when adult worms enter the appendix to cause appendicitis.

Pathogenesis of Enterobius vermicularis infection

Enterobiasis occurs following the ingestion of infective eggs of E. vermicularis either through self contamination or via the anal-oral route (Figure 1). Human hosts harbouring pinworm in their colon or large intestine may occasionally experience re-infection. After ingestion, infective eggs hatch in the colon to larval forms which develop into adult worms in the large intestine. The appendix or caecum may occasionally be colonized by adult worms; and this may incite appendicitis. Mature female worms produce infective eggs few weeks after infection. The eggs which are resistant to desiccation are usually deposited at the perianal skin and beddings of infected individuals. Scratching of the anus during an infection can re-introduce infective pinworm eggs into the body via contaminated fingers i.e. through the anal-oral route. And the larva hatched from infective eggs at the perineum or perianal skin can migrate back into the colon where they develop into adult male and female worms. Shed eggs of E. vermicularis are infective; and they can be accidentally ingested via contaminated hands or fingers especially after scratching the anus.

Figure 1: Life cycle of Enterobius vermicularis. 1. Eggs of E. vermicularis are deposited on perianal folds after ingestion by a human host. 2. Self-infection occurs by transferring infective eggs to the mouth with hands that have scratched the perianal area. 3. Person-to-person transmission can also occur through handling of contaminated clothes or bed linens of infected persons. Enterobiasis may also be acquired through surfaces in the environment that are contaminated with pinworm eggs (e.g., curtains and carpeting). Some small number of eggs may become airborne and inhaled. These would be swallowed and follow the same development as ingested eggs. Following ingestion of infective eggs, the larvae hatch in the small intestine of the human host. 4. After hatching, the adult worms formed establish themselves in the colon (large intestine). The time interval from ingestion of infective eggs of E. vermicularis to oviposition by the adult females is about one month, and the life span of the adult worms is about two months.  5. Gravid females migrate nocturnally outside the anus and oviposit (i.e. it produces ova or egg) while crawling on the skin of the perianal area of the human host. The larvae contained inside the eggs develop (the eggs become infective) in 4 to 6 hours under optimal conditions, and retro-infection (i.e. the migration of newly hatched larvae from the anal skin back into the rectum) may occur but the frequency with which this happens is still unknown. CDC

The diagnosis is made by identifying pinworms or their eggs. Worms can sometimes be seen on the skin around the anus 2–3 hours after falling asleep. Pinworm infection is diagnosed in the laboratory by the identification of E. vermicularis worms from perianal specimens or eggs recovered from the anus of infected persons during physical clinical examination of the perineum or anus. E. vermicularis worms are small worms with pointing ends especially at the tail region (Figure 2); and they resemble a piece of thread, thus the name threadworm or pinworm. Eggs of E. vermicularis (Figure 3) can also be identified from faecal samples or perianal swabs. Enterobiasis can be treated with mebendazole, pyrantel pamoate and albendazole. It is important to treat family members of infected persons because they may have acquired or ingested the infective egg of the parasite by sharing beds or clothing. Infected persons should be properly treated, and the beddings and anal region should also be washed to avoid disease transmission in the family since the resistant infective eggs of E. vermicularis are very infectious when shed by gravid female worms.

Laboratory diagnosis, treatment, control and prevention of Enterobius                         vermicularis infection

The diagnosis is made by identifying pinworms or their eggs. Worms can sometimes be seen on the skin around the anus 2–3 hours after falling asleep. Pinworm infection is diagnosed in the laboratory by the identification of E. vermicularis worms from perianal specimens or eggs recovered from the anus of infected persons during physical clinical examination of the perineum or anus. E. vermicularis worms are small worms with pointing ends especially at the tail region (Figure 2); and they resemble a piece of thread, thus the name threadworm or pinworm. Eggs of E. vermicularis (Figure 3) can also be identified from feacal samples or perianal swabs. Enterobiasis can be treated with mebendazole, pyrantel pamoate and albendazole. It is important to treat family members of infected persons because they may have acquired or ingested the infective egg of the parasite by sharing beds or clothing. Infected persons should be properly treated, and the beddings and anal region should also be washed to avoid disease transmission in the family since the resistant infective eggs of E. vermicularis are very infectious when shed by gravid female worms.

Figure 2: A wet preparation showing the mature worm of E. vermicularis. Note the cervical alae at the anterior end of the worm, a feature that is unique to pinworms. The human pinworm, Enterobius vermicularis are easily transmitted from human to human and they are particularly common in children. They cause anal itching in infected human hosts.

Figure 3: Eggs of E. vermicularis. Notice the pointed tail and characteristic anterior end of the pinworm. These features coupled with the small-slender morphology of the worm are why they are often called pinworm or threadworm. The nematode (roundworm) Enterobius vermicularis (previously Oxyuris vermicularis) is also called human pinworm. The adult females measure about 8 to 13 mm while the adult male measures about 2 to 5 mm. Human hosts are considered to be the only hosts of E. vermicularis.

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