What is enterobiasis?

Many of us are familiar with the term ‘worm infection’. These include parasitic worms known as pinworms, threadworms or seatworms, all of which are forms of the parasite Enterobius vermicularis. This parasite is responsible for the infection known as enterobiasis.

Enterobius vermicularis (E. vermicularis) is a roundworm (nematode) that commonly infects the gastrointestinal tract of humans. It is naturally found in the colon/caecum, however, migration can occur in when the E. vermicularis moves into the perianal region to reproduce and lay infectious eggs. E. vermicularis can only cause infection by laying new eggs - the mere presence of the worm itself is not enough to cause symptoms.¹

Epidemiological insights of enterobius vermicularis

Enterobius vermicularis infections are a global public health issue, with an estimated 1 billion cases of infection worldwide. Children are most susceptible, largely because they spend time in shared environments such as daycares and schools, where close contact and crowding increase the risk of contracting enterobiasis. In addition, children are developing personal hygiene habits and may have a limited understanding of health, disease prevention. Not washing your hands before meals and after using the toilet can facilitate infection. Nail-biting, thumbsucking or chewing on shared toys can increase the transmission of eggs.²

Enterobius vermicularis is an increasingly common problem in tropical and subtropical regions, where warm temperatures and high humidity promote the development, survival and transmission of E. vermicularis eggs. Regions and neighbourhoods with lower socioeconomic status or poor sanitation are at greater risk, as crowded living allows eggs to transfer easily through shared bedding and clothing. Additionally, limited access to education about hygiene and disease prevention contributes to the disease persisting and spreading.²

Life cycle and transmission

Humans acquire an E. vermicularis infection through the consumption of its eggs. This transmission can occur through ingesting via food or water contaminated with these eggs or through scratching/touching our face and mouth with our own hands that are contaminated with eggs - this is the most common mode of transmission.

The ingested eggs hatch in the small intestine. Larvae are released and migrate into the large intestine. Larvae then mature into adult worms in the colon, caecum and appendix regions. These adult worms live freely in the large intestine, where females then go on to lay eggs with the males aiding in fertilisation.³ After mating, the male worms die and the females become gravid (housing thousands of eggs). 

Typically at night, the gravid females migrate from the colon to the perianal areas of the body in order to release their egg (this can be up to 10,000 eggs). After 4-6 days, the eggs become infective in the perianal skin as they are coated in a sticky substance that causes intense itching.³ These eggs are highly transmissible and can spread to other areas of the body such as in the vulva, ovaries, urethra and appendix.² E. vermicularis eggs can also be spread environmentally onto clothing, bedding and even through dust.

The itching caused by can cause reinfection. When the infected person scratches the perianal area. This causes the eggs to adhere to the fingers and nails, and when touching the mouth they re-enter the body. 

Host-parasite interactions - how does Enterobius vermicularis cause infection?

E. vermicularis is equipped with structural and chemical features that help to cause infection and remain within the body. This allows for successful reproduction within the body and an increased chance of causing infection. 

Eggs in enterobiasis

The E. vermicularis eggs are highly adapted to cause infection. They are encased in an eggshell which protects the parasitic embryo from dehydration and other stresses that are caused by the acidic gastrointestinal environment. This protection allows the eggs to remain viable and promotes hatching. Eggs are also coated in a sticky protein substance, enhancing their adherence to the perianal skin, clothing and bedding - this increases the chances of transmission.³

Rapid reproduction rates

E. vermicularis also has high reproductive rates. After mating, female worms can lay 10,000+ eggs, which allows for high levels of persistence in the body regardless of immune control and hygiene.Laying eggs usually occurs during the night, when the host is less active, and this reduces the chances of worms being cleared by the body.⁴

Intense itching and inflammation

One of the biggest symptoms of enterobiasis is intense itching in the perianal region. The sticky protein layer that coats the eggs causes itching, causing the host to scratch and further transmit eggs causing reinfection. In order to avoid attacks by the immune system, E. vermicularis worms attach loosely to the mucosal tissue of the colon. They are large enough to cause irritation and mild inflammation but, small enough to be undetected by the immune system. 

The Immune response

The innate immune system is the first line of defence against E. vermicularis. This includes the mucosal barriers, eosinophils and mast cells.

The mucosa is the innermost layer of the gastrointestinal tract. The mucosa surrounds the open space within the gastrointestinal tract (the lumen) which comes into contact with digested food. Once adult worms are ingested and detected by the mucosa, the mucosa can produce mucus in order to prevent worms sticking to the surface and causing irritation. Additionally, the mucosa can undergo peristalsis. This is a rhythmic contraction of muscles in the mucosa which directs food through the digestive tract, helping to remove worms.⁵

Eosinophils are a type of white blood cell that are recruited at the sites of irritation caused by parasitic worms. Eosinophils are a key effector cell which coats E. vermicularis worms with antibodies. Eosinophils can also release granules directly onto the worm, these granules consist of chemicals toxic to worms, such as major basic protein (damages the worm's cuticle) and eosinophil cationic protein (disrupts worm membranes). Further, Eosinophils can aid in peristalsis by releasing cytokines (IL-4, IL-5 and TGF-beta) to induce mucus secretion and muscle contraction in order to ‘flush out’ worms through diarrhea and mucus production.⁶

Symptoms and clinical manifestations of enterobiasis

Diagnosis 

E. vermicularis eggs are not typically found in the stool; therefore, the first step of diagnosis is identifying the most common perianal itching symptom and whether this occurs at night. In children, some parents may identify small, white threadlike worms (female E. vermicularis worms) in their child's perianal region or in their stool.

Adult worms may also be seen directly crawling on the perianal skin at night. They can be observed on bedding, underwear and stool, with the help of a torchlight. 

If someone is asymptomatic, the scotch tape/cellophane tape test is used. This involves applying a piece of clear sticky tape against the perianal region before defecation. This should be carried out in the morning before washing. The tape can then be placed on a glass slide in the lab and examined under a microscope. If oval-shaped eggs are seen, this is a clear sign of enterobiasis.³

Treatment of enterobiasis

Treating enterobiasis is simple, but it needs to target both E. vermicularis and the risk of it causing reinfection.

The first line of treatment are anti-worm drugs such as albendazole, mebendazole and pyrantel pamoate. These drugs work to kill the adult worm.³ Although they do not kill the eggs, killing the adult worm helps to prevent reproduction and laying further eggs. Repeat use of this medication ensures that newly hatched worms are killed before they can sexually mature. 

Antihistamines and soothing creams can also be used to relieve severe perianal itching and inflammation.

Prevention

Preventive measures are just as important as treatments. It is crucial to prevent the transmission of eggs, especially in children. Methods of prevention include:

• Regular handwashing, especially before meals, after changing nappies and when using the toilet
• Avoid touching your mouth or wash hands beforehand
• Daily showering, in the morning, to remove eggs laid at night
• Trimming fingernails to reduce egg transmission
• Changing underwear, clothes and bedding daily
• Frequent disinfecting of surfaces to reduce eggs in the environment

Summary

Enterobius vermicularis (E. vermicularis) is a thread-like parasite which causes enterobiasis. Enterobiasis is characterised by the intense itching of the perianal area and can cause gastrointestinal complications such as diarrhea, abdominal pain and nausea. Enterobiasis can be transmitted through the ingestion of food or water contaminated with E. vermicularis eggs, however, the most common route of transmission is through touching/scratching the anal skin and then touching your mouth, transmitting eggs. 

Once ingested, E. vermicularis eggs hatch in the small intestine, the larvae then migrate down into the large intestine in order to mature in adult worms. Adult worms live freely within the large intestine and often attach to the mucosal walls of intestine, causing mild irritation and inflammation. After successful reproduction, female eggs migrate into the perianal region to lay their eggs. This occurs during the night, when the host is less active, in order to prevent immune control. 

The immune system responds to eggs by producing mucus in the large intestine to clear out attached worms, rhythmic muscle contractions (peristalsis) also occur in order to induce bowel movements and assist the clear out of any worms. The presence of the adult worms also stimulates white blood cells known as eosinophils. Once recruited, eosinophils coat the worms in antibodies and then release granules containing toxic chemicals in order to damage the structural integrity of the worms, killing them.

Enterobiasis is treatable with drugs including albendazole and mebendazole which work to kill worms, preventing the chances of reproduction and females laying eggs. These drugs also ensure newly hatched worms are killed before they mature. Enterobiasis can be prevented easily by ensuring proper handwashing, daily showering in the morning to remove eggs and regular laundry of underwear, clothing and bedding.