ENTEROBIUS VERMICULARIS: CHARACTERISTICS, MORPHOLOGY, LIFE CYCLE - BIOLOGY - 2025

Enterobius vermicularis is an obligate parasitic nematode of the digestive tract of the human being, commonly called pinworm in Spanish language and pinworm or threadworm in Saxon language.

They are small, cream-white spindle worms, 8-13 mm long for the female and 2.5-5 mm long for the male. The eggs measure 50-54 μm x 20-27 μm, are ovoid, asymmetrically flattened and almost colorless. The larvae are 140-150 μm in length.

Enterobius vermicularis eggs. Author: DPDx, PHIL, via Wikimedia Commons

This species produces the disease known as enterobiasis. The most affected population is school-age children between 5 and 10 years old. This nematode is possibly the oldest known helminth parasite.

Old records

Thanks to its form of direct transmission from one human to another, without the need for an external phase of the cycle, this parasite has accompanied human migrations throughout the world.

According to the investigations carried out, Enterobius vermicularis was acquired by humans in Africa and from there it accompanied the species in its dispersal throughout all continents.

Paleoparasitology data allow us to reconstruct these routes of spread of infection. This nematode is possibly the oldest known helminth parasite.

Their eggs have been detected in coprolytic carbon (originating from faeces) located in Utah, North America, whose age was dated at 10,000 years. In South America eggs have also been found in human coprolites, in this case 4,000 years ago.

Enterobius vermicularis eggs have been detected in mummified human bodies in different parts of the world:

• In a female adolescent mummified 7,000 years ago in Tehran (Iran).
• Eggs were found in China in a 2,100-year-old mummy.
• In Greenland they were found in a mummy dating from 1,400 AD.
• In Korea on a 17th century female mummy.

General characteristics

Gender

Enterobius vermicularis (formerly known as Oxyuris vermicularis) belongs to the kingdom Animalia, phylum Nematoda, class Secernentea, subclass Spiruria, order Oxyurida, family Oxyuridae.

The genus Enterobius includes about 25 species parasitic in primates, only Enterobius vermicularis parasitizes humans. It is distributed from the arctic zones to the tropics.

Infection

Infection occurs from ingesting nematode eggs from contaminated hands, food, or less commonly from water. The most obvious symptom is anal itching, which is generated by the presence of eggs or by the migration of worms to the outside.

To diagnose the disease, the most reliable method is to apply a plastic tape to the perianal region when the patient awakens, before defecating or performing morning hygiene. The eggs or adults are then identified under a microscope.

Lifecycle

Its life cycle starts from the hatching of ingested eggs once they reach the stomach. The larvae migrate to the ileum, the cecum, and the appendix. Adult females settle in the ileum, cecum, appendix, or colon.

When the uteri are laden with eggs, the worms migrate out of the intestinal lumen through the anal cavity and deposit their eggs on the skin of the perianal region.

To eliminate the parasite E. vermicularis, drugs from the benzimidazole group are used, such as mebendazole and albandazole, or compounds such as piperazine, pyrantel or pirvinium. The treatment must be applied to the entire family group. They must be applied under strict medical indication.

Habitat

Enterobius vermicularis is an obligate parasitic nematode that lives from the Arctic to the tropics. It does not discriminate between poor or rich countries: in some areas of Europe and the United States, up to 100% infection has been detected.

Its host is humans, although cases of parasitosis have been reported in captive specimens of related species such as the chimpanzee and the gibbon.

The eggs adhere to the surfaces thanks to the presence of an external albuminous layer. Up to 50,000 eggs per square meter have been detected on the walls of a school bathroom.

Genetics

The Enterobius vermicularis genome is a 14,010 bp circular DNA molecule that encodes 36 genes (12 proteins, 22 tRNA and 2 rRNA). Interestingly, this mtDNA (mitochondrial DNA) genome lacks atp8, unlike almost all other nematode species that have been investigated.

Phylogeny and taxonomy

Enterobius vermicularis (formerly known as Oxyuris vermicularis) belongs to the kingdom Animalia, phylum Nematoda, class Secernentea, subclass Spiruria, order Oxyurida, family Oxyuridae.

The genus Enterobius includes around 25 species parasitic in primates, while only Enterobius vermicularis parasitizes humans.

Enterobius gregorii, hitherto known in Europe, Africa and Asia, also parasitizes humans. Some argue that this last species is invalid and that it is juvenile forms of Enterobius vermicularis. Species of this genus of nematodes have co-evolved with primates.

Species such as Enterobius serratus, Enterobius pygatrichus and Enterobius zakiri have been described as parasitic in small primates, while Enterobius buckleyi is parasite of the orangutan. Enterobius anthropopitheci occurs in chimpanzees. In the gorilla the species Enterobius lerouxi.

Morphology

It is a small white spindle-shaped worm. The adult female is 8-13 mm long and 0.4-0.5 mm wide; males are smaller in size, reaching 2.5-5 mm in length and 0.5-0.6 mm in width.

They have three lips and a pair of lateral cephalic wings that allow them to attach to the mucosa of the intestine. It has a robust esophagus that ends in a highly developed esophageal bulb. The posterior end of the body is attenuated; hence its old name pinworm (pointed tail).

The reproductive system is highly developed and T-shaped. In cross section, the characteristic eggs are seen inside the uterus. Adult males have a ventrally curved tail, with a caudal wing, and a single large copulatory spicule.

The eggs measure 50-54 μm x 20-27 μm, are ovoid, one of their faces flattened and the other convex, almost colorless. The shell has a thick albuminous layer on the outside that allows it to adhere to surfaces. Then a thin hyaline layer and an embryonic membrane. The larvae are 140-150 μm in length.

Lifecycle

Egg intake and larval development

The life cycle takes place within the lumen of the gastrointestinal tract. After ingestion, the eggs hatch in the stomach and upper part of the small intestine.

The larvae migrate to the ileum, the cecum, and the appendix. After moving twice along the way, they become adults. Infected patients harbor few or several hundred adults.

Establishment and copulation

Adult females (moving at approximately 6-7 cm per 30 min) settle on the lower ileum (where copulation occurs), cecum, appendix, or ascending colon. There, they form tiny ulcerations at the site where they are inserted.

Later secondary infections and hemorrhages are generated that produce small ulcers and submucosal abscesses. Females survive for 37-93 days. Males survive about 50 days, die after copulation and are expelled with the feces.

Oviposition and hatching

Oviposition begins at five weeks. When the uteri are loaded with the eggs, the gravid worm migrates from the colon, through the anus.

While passing through the perianal or perineal skin, the eggs are expelled by uterine contractions, death or disintegration of the worm itself, or by its rupture during scratching. This process takes place overnight. Around 11,000 eggs are produced.

At the time of oviposition, the larvae are immature and non-infectious. Atmospheric oxygen accelerates development. These larvae become infectious after six hours inside the human body, thanks to the effect of body temperature.

Egg survival is optimal at low temperature and high humidity; in hot, dry conditions, infectivity decreases after one or two days. The cycle takes between two and four weeks.

Contagion

Infection occurs by ingesting nematode eggs in contaminated hands, food, or less commonly in water. To a lesser extent, the eggs can enter the body by air through the nasal passages.

The eggs are very light and spread around the room when the bedding is aerated, maintaining their infectivity for a period of three weeks.

There are four transmission methods:

• Direct infection: from the anal and perianal regions due to contamination of the nails by scratching (autoinfection).
• Exposure to viable eggs: This can occur through dirty bedding and other contaminated objects.
• Contaminated dust: containing eggs (from bedding, pajamas, toys, furniture, and the fur of cats and dogs).
• Retroinfection: this occurs when after incubating in the anal mucosa, the larvae migrate towards the sigmoid colon and the cecum.

Enterobius vermicularis is one of the most common intestinal parasites in humans. They normally infect the terminal ileum and the large intestine and are generally considered a harmless parasite that can be easily eradicated with proper treatment.

However, extraintestinal migration of worms, although very rare, can lead to serious health problems or even death. There are reported cases in which the nematode has perforated the large intestine and consequently a bacterial infection has been generated, followed by peritonitis.

epidemiology

It is estimated at more than one billion cases of enterobiasis in the world. The most affected population is school-age children between 5 and 10 years old. The disease spreads easily and is difficult to control in schools, boarding schools, or vacation camps. Transmission within families with infected children is quite common.

Symptoms

The most common symptom is anal or perineal itching. Local tingling and sharp pain may also occur. However, most infections are asymptomatic. Other, more intense symptoms occur at night. These include insomnia, restlessness and fatigue.

Children can become anorexic, lose weight, or suffer from impaired concentration, irritability, emotional instability, and enuresis (involuntary bed-wetting).

It can also cause bruxism or snapping of teeth, nausea, vomiting, diarrhea and sialorrhea (excessive salivation), abdominal pain and even cramps.

As secondary symptoms, scratch wounds and subsequent infections can occur. In girls there are cases of vulvovaginitis.

Some studies associate the action of Enterobius vermicularis with the decrease of trace elements such as copper, zinc and magnesium, relevant for child development.

Enterobius vermicularis is reported to generate toxic metabolites that act on the central nervous system (day and night nervousness, behavior disorders, decreased attention at the school level).

Enterobius vermicularis produces internal ulcerations and small hemorrhages. Although there is controversy in this regard, it is pointed out that severe infections with this nematode can cause appendicitis.

Diagnosis

To diagnose the disease, it is necessary to accurately identify the presence of adult Enterobius vermicularis worms or their eggs.

In identification, it is helpful to apply an adhesive plastic tape (known as the Graham method) to the perianal area at night or shortly after awakening, before defecation. The worms and eggs become attached to it and can be observed under a microscope.

Routine stool tests only give positive results in 5-15% of cases.

Treatment

To prevent infection, hygiene is essential, both personal, such as lingerie and the bathroom. Once the infection occurs, chemical therapy is necessary.

Different drugs from the benzimidazole group are used, such as mebendazole and albandazole, or compounds such as piperazine, pyrantel, or pirvinium. The treatment must be applied to the entire family group. It must be done under strict medical indication.

On the other hand, various natural remedies can be mentioned to eliminate pinworms:

• The juice of papaya or green milky with honey.
• Consume pumpkin seeds or grapefruit seed extract.
• A mixture of milk, garlic, mint, wormwood and honey.

Herbs like yarrow and thyme have also been found to be effective in fighting the parasite. Apple cider vinegar is also mentioned as an effective remedy.

References

1. Araújo A and Ferreira LF. (nineteen ninety five). Oxiuríase and prehistoric migraҫões. History Ciências Saúde-Manguinhos Vol II (1): 99-109.
2. Brooks, DR and Glen, DR. (1982). Pinworms and Primates: A Case Study in Coevolution. Proc. Helminthol. Soc. Wash. 49 (1): 76-85.
3. Cazorla DJ, ME Acosta, A Zárraga and P Morales. (2006). Clinical-epidemiological study of enterobiasis in preschool children and schoolchildren from Taratara, Falcón State, Venezuela. Parasitol Latinoam 61: 43-53.
4. Cook GC. (1994). Enterobius vermicularis infection. Leading article -Tropical infection of the gastrointestinal tract and liver series. Gut. 35: 1159-1162. Hospital for Tropical Diseases, St Pancras Way, London NWI OPE.
5. Kang S, T Sultana, K Eom, YC Park, N Soonthornpong, SA Nadler and JK Park. (2009). The mitochondrial genome sequence of Enterobius vermicularis (Nematoda: Oxyurida) - An idiosyncratic gene order and phylogenetic information for chromadorean nematodes. Gene. 429 (1-2): 87-97.
6. Serpytis M and D Seinin (2012) Fatal case of ectopic enterobiasis: Enterobius vermicularis in the kidneys. Scandinavian Journal of Urology and Nephrology. 46 (1): 70-72.
7. Symmers WSTC. (1950). Pathology of Oxyuriasis with special reference to Granulomas due to the presence of Oxyuris vermicularis (Enterobius vermicularis) and its Ova in the Tissues. Arch. Pathol. 50 (4): 475-516.
8. Teixeira A, Lara C, Leão FG, Almeida A and de Oliveira F. (2013). Enterobius vermicularis eggs in waiting rooms and banheiros of basic health units (UBS) of the municipality of Nova Serrana-MG: contributions for or control. Rev Patol Trop Vol. 42 (4): 425-433.