Answer to the Parasite Case of the Week 767: Enterobius vermicularis egg
The following discussion is written by our outstanding guest author, Dr. Azra Hasan:
The image shows the classic "D-shaped" (plano-convex) egg of E. vermicularis, round on one side and flattened along the other, measuring 50-60 µm × 20–30 µm, containing a folded larva.
Followers correctly identified the characteristic plano-convex-shaped egg of Enterobius vermicularis, commonly known as the pinworm, named for the pointed, pin-like tail of the female adult worm. Finding parasitic ova on pap smear is not uncommon and in this case the location (anal pap test), shape, and size, strongly point to Enterobius vermicularis.
The following amazing images from another case show a larva hatching from the egg! Many thanks to Dr. Kristin Galan for donating this image, and to Dr. Nicole Brammer Hubbard for coordinating its inclusion in this blog.
The parasite thrives due to its easy mode of transmission. It is transmitted through anus-finger-mouth contact, nail-biting, unsupervised body hygiene, poor compliance with basic hand hygiene, soiled nightwear, rarely through airborne eggs, and often by fomites such as contaminated furniture, toys, etc.
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Life Cycle of Enterobius vermicularis
The cycle begins when a human host ingests embryonated eggs, often contaminated fingers or fomites. The eggs hatch in the small intestine, where larvae undergo two molts to become adult worms. Adult worms then migrate to the large intestine, primarily making the cecum, appendix, and ascending colon their home.
However, the males mostly die here and pass out in the stool. The gravid female leaves this safe residence every night to travel to the perianal folds to lay her eggs (oviposition).
(If you’d like to see a pretty gravid female pinworm, have a look at the video uploaded on Case 732.)
The cycle completes when scratching transfers eggs from the anus to the mouth, (autoinfection).
Retroinfection may also occur, with larvae re-entering the rectum.
(Don’t miss the poem by Blaine Mathison, Case 526, on Mrs. Pinworm’s travel and mission.)
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Clinical manifestations and a story
The typical symptom of enterobiasis is intense pruritus in the perianal region, especially at night. This occurs due to the nocturnal migration and egg deposition by the female worm.
Severe infections can lead to irritability, disturbed sleep, and secondary bacterial infections from persistent scratching. While the adult female typically resides in the large intestine and moves sporadically to the anus, she may sometimes migrate into ectopic sites such as the female genitourinary tract, causing vulvovaginitis. Several case reports describe various ectopic locations, such as fallopian tubes, omentum, peritoneum, liver, lungs, spleen, and kidneys. Granulomas often form around degenerating worms and eggs in these locations and appear as nodules.
Speaking of ectopic sites and pinworms, I’m instantly reminded of a case I had encountered during residency. A child had come to the ophthalmology clinic with a red, inflamed eye, blurred vision, and intermittent throbbing pain. A live, wriggling worm was found in the conjunctival fold during examination. This was sent to the lab, and, the culprit turned out to be an adult female Enterobius vermicularis. That memory has stayed with me ever since! The ectopic location was caused by the transfer of worms and eggs to the eye through contaminated fingers. Dr. Pritt published a similar case with her fellow, Dr. Esther Babady, back in 2011.
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In the Laboratory:
1. Cellulose acetate tape test (Scotch tape test in the US, Sellotape test in the UK, named after the brand) is the diagnostic method of choice. The paddle test is very simple and user-friendly. It is a neat commercially prepared diagnostic tool that uses a plastic paddle coated with adhesive to collect eggs from the perianal region for the microscopic examination.
2. As the female worm’s migration is sporadic, more than one test is recommended to rule out infection. Tests are best performed early in the morning before the patient washes or bathes.
3. While adult worms or eggs may occasionally appear in stool, this is incidental; stool samples are not the specimen of choice.
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Let the Parents Know:
• Pets at home, such as dogs or cats, do not carry the pinworm.
• Swimming pool chlorination will not eliminate pinworm eggs.
• Cockroaches can serve as mechanical vectors of the eggs—get rid of any in your home.
• Emphasize proper hand hygiene, including techniques, duration, and the
role of fingernails in the spread of the infection.
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Prevention and Treatment:
Prevention focuses on personal hygiene: frequent handwashing, trimming nails, and cleaning and disinfecting contaminated surfaces such as bedding, clothing, and toys.
Medications such as albendazole and mebendazole are effective for treatment. However, reinfection is common due to environmental contamination. Family members and close contacts are often treated as well. For stubborn cases, a prolonged treatment regimen may be needed.
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Some Interesting Reads on Pinworm:
1. The Diagnosis and Treatment of Pinworm Infection, Dtsch Arztebl Int, 2019.
2. Enterobius vermicularis in the Nose: A Rare Entity, Indian J Otolaryngol
Head Neck Surg, 2005.
3. Large Bowel Obstruction Secondary to Enterobius vermicularis Pseudotumor, BMJ
Case Rep. 2022 Nov 29;15(11): e252676.
4. Enterobiasis as a Neglected Worldwide Disease: A Call to Action, Rev Soc Bras Med Trop.
2024 Oct 28;57: e011022024.
5. The Cockroach as a Host for Trichinella and Enterobius vermicularis: Implications for
Public Health, Hawaii Med J. 2004 Mar;63(3):74-7.
6. CDC: http://www.cdc.gov/parasites/pinworm/
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