This week's case was donated by Drs. Michael Mitchell and Theresa Smith. The following structures were seen on a Papanicolaou-stained anal Pap smear who had a history of squamous dysplasia. They measure approximately 55 micrometers long. What is your diagnosis?
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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This week's case is a nice straight-forward one for a change! The following objects were found in a urine specimen and measure ~150 micrometers in length. What is your identification? Photos courtesy of Felicity Norrie.
Answer to the Parasite Case of the Week 766: Schistosoma haematobium eggs
The following outstanding discussion is written by our guest author, Dr. Azra Hasan.
These are Schistosoma haematobium eggs in a urine sample as many of you correctly identified in the comments. The key giveaway was the prominent “terminal spine”, that helps differentiate S. haematobium from other Schistosoma species. On a closer look we can see that the miracidium (larval stage) fills up the egg. (Be sure to check out case 467 which has incredible videos from Idzi Potters, showing a beautiful motile miracidium!). Note that observing motile miracidia within eggs or in urine serves as a direct indication of an active S. haematobium infection and should be reported.
As noted by Florida Fan, typically, Schistosoma haematobium eggs are found in urine, which is a good way to differentiate them from the similar-appearing, but larger, S. intercalatum, which are found in stool. However, in heavy infections, and especially from residents of endemic areas, one may encounter S. haematobium eggs in stool as well. Rarely, they can also be seen in semen and in biopsy tissues from the bladder, as well as the genitalia and rectum. In endemic regions, nearly 75% women with urinary schistosomiasis have S. haematobium eggs in the genitalia. Female genital schistosomiasis (FGS) is often associated with eggs in the cervix, vagina, and/or vulva, with sandy patch lesions and mucosal bleeding sometimes without hematuria. (2)
Satoshi rightly pointed out the RBCs in the background indicating hematuria - During acute infections, hematuria is an important finding along with history that often includes dysuria and frequent urination. Advanced infections may lead to fibrosis, calcification, obstructive uropathy, polyps, or squamous cell carcinoma of the bladder. Imaging often reveals calcifications described as the "eggshell bladder”.
Praziquantel remains the gold standard drug for treatment, but reinfections are common. Many vaccine candidates show promise and would be the answer in endemic regions. (4)
Here are the primary components of the Schistosoma lifecycle:
The following are some helpful diagnostic approaches:
Did you know?
Some interesting case reports and reviews:
More information
This week's case was generously donated by Drs. Cameron Truarn and Damien Bradford from PathWest Laboratory Medicine, and Drs. Huan Zhao and Richard Bradbury from James Cook University, Australia.
The following objects were noted in a bronchoalveolar lavage from a middle-aged immigrant from Asia, now living in Australia. They measure approximately 1000 micrometers long. What is seen here? Please be as specific as possible.
Answer to the Parasite Case of the Week 765: Strongyloides stercoralis L4 larvae
As noted by Idzi Potters, "The presence of a relatively large genital primordium confirms Strongyloides. The apparently tapered tail (not notched) together with the size of the larva point towards the fourth-stage autoinfective larva (L4)."
For those of you who were thinking these were autoinfective filariform (L3) larvae, you were very close! L4 larvae are the next stage in development after autoinfective L3 larvae and may be potentially detected in human respiratory specimens. Richard Bradbury and his colleagues just published a very interesting article in the Journal of Clinical Microbiology on the presence of L4 S. stercoralis larvae in two separate cases, and we received permission from JCM and the Editor-in-Chief to reproduce some of the images here.
The major differences between S. stercoralis L3 and L4 larvae are as follows:
