This week's case was generously donated by Dr. Emily Snaverly. The following images show an incidental finding from screening colonoscopy, measuring approximately 1cm long. The patient is an asymptomatic, middle-aged male with no known travel history whose previous colonoscopy did not show any parasites. What is your identification?
Answer to the Parasite Case of the Week 771: Rodentolepis (Hymenolepis) nana, the dwarf tapeworm
As nicely described by Florida Fan, Idzi Potters, and Menzler, the craspedote positioning of the proglottids, armed rostellum, and characteristic (although immature) eggs with prominently splayed, large hooklets all point to R. nana. If you look very closely, you can make out the polar filaments:
This week's case is from Idzi Potters and the Institute of Tropical Medicine, Antwerp. The following structures were seen in a stool specimen from a child living in a rural area of Laos. They measure approximately 60 micrometers in greatest dimension. What is your identification?
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Answer to the Parasite Case of the Week 770: Schistosoma mekongi
The following excellent discussion is by our guest author, Dr. Asra Hasan:
This week’s answer is Schistosoma mekongi! With that, we’ve shown a Schistosoma trio already for the new year—S. haematobium, S. mansoni, and now S. mekongi. Great job, Florida fan! You correctly spotted the inconspicuous lateral spine and the internal miracidium of S. mekongi. And a shoutout to our anonymous writer—your answer was pretty close, as S. mekongi looks a lot like S. japonicum, just smaller, and with a slightly different geographic range.
The endemic area for S. mekongi is along the lower Mekong River—Laos, Thailand, and Cambodia—hence, the population at risk is comparatively smaller than for other Schistosoma species.
Have a read of the clinical vignette below:
“A 42-year-old male rice farmer from a rural village in Cambodia presents to a local clinic with complaints of chronic diarrhea, abdominal pain, and progressive weight loss over the past six months. He also reports intermittent fever and generalized weakness. On examination, he has hepatosplenomegaly and mild ascites. Laboratory results reveal eosinophilia, and stool microscopy shows small, subspherical eggs with a minute lateral spine at one end. The patient has frequent exposure to river water while working in the fields.” And that would qualify as a typical case for this week—S. mekongi!
The life cycle is similar to other Schistosoma species discussed on this blog (for example, Case 766), except that the definitive hosts are both humans and dogs, and the intermediate host is the snail Neotricula aperta. S. mekongi primarily affects the intestines, liver, and spleen, and rarely, disseminated sites such as the brain.
Diagnosis
Microscopic examination of stool after sedimentation concentration is used to detect eggs.
Antigen tests: Point-of-care lateral flow assays detecting CCA are used for field screening. In the lab, ELISA has been used to detect circulating schistosome antigens in serum and urine and may be the preferred method for confirming diagnosis. Since stool examination can detect schistosome eggs weeks after cure, detection of circulating antigens (CAA & CCA) in blood or urine provides evidence of an ongoing active infection, as both antigens are rapidly cleared from circulation.
Antibody tests can detect evidence of infection, but cannot distinguish current from past infection, and are not particularly useful due to cross-reactions with other helminth infections.
Treatment
Praziquantel is the drug of choice.
Prevention of schistosomiasis involves universal treatment campaigns and that has shown dramatic decrease in disease burden although it has not been helpful in eliminating the disease completely requiring repeated campaigns. Travelers can prevent schistosomiasis by avoiding bathing, swimming, wading, or other contact with freshwater in disease-endemic countries.
And finally, a mention about the World Health Organization. The World Health Organization's roadmap for eliminating neglected tropical diseases recommends targeting Schistosoma mekongi for elimination. Current strategies in affected communities include: Preventive chemotherapy targeting at-risk populations (e.g., entire villages along the Mekong) and distribution of information and education, improvements in water, sanitation, and hygiene.
Some more reading:
CDC resources: https://www.cdc.gov/schistosomiasis/resources/
Answer to the Parasite Case of the Week 769: Live Schistosoma mansoni ova. Note the motile miracidium with its characteristic 'flame cell' - a specialized cell that is part of its excretory system, characterized by a flickering, flame-like movement of cilia which helps to expel waste products from the egg (see image below, and be sure to check out the video HERE.
The following excellent discussion is written by our guest author, Dr. Azra Hasan:
Everyone rightly recognized the oval egg with a” lateral spine”, the hallmark feature of Schistosoma mansoni.
Sometimes, it may be necessary to tap the coverslip to move the eggs; the lateral spine may not be visible if the egg is turned on its side. In very light or chronic infections, eggs may be tough to detect in stool; therefore, multiple stool examinations may be required and a biopsy and/or immunologic tests for antigen or antibodies help diagnose infection in these patients.
Schistosomiasis (also known as bilharziasis) spreads when people come in contact with freshwater contaminated by microscopic swimming cercariae released from infected snails. These cercariae penetrate intact skin and mature into adult worms as shown in the life cycle below. Schistosoma mansoni is endemic in Africa, South America, and the Caribbean. Humans are the definitive host and snails (Biomphalaria spp.). are the intermediate hosts. Adult worms reside in blood vessels of the intestines, laying eggs that trigger inflammation, fibrosis, and organ damage.
Clinical features
Fun Facts:
Treatment:
Praziquantel is the gold standard. Oxamniquine is effective only against S. mansoni. Resistance to both these drugs has been reported. Early treatment prevents portal hypertension and related complications
Prevention with improved sanitation and avoiding wading in insanitary water, is better than cure! (Praziquantel kills adult worms but not immature larvae- It is important to note that If treatment starts early, always follow up with retreatment).
Salmonella-schistosome syndrome: When a secondary bacterial infection (usually Salmonella) occurs, anti-schistosome and antibacterial therapy should be given together; the disease remains if the schistosome is not treated simultaneously.
Some interesting case reports:
- Paste the PMID number in PubMed for full-text articles
- Type ‘Schistosoma’ in the search bar of this blog and walk through the pages for more images and reads.
This week's case was generously donated by Dr. Mike Mitchell. The following organism was found in the agar of a bacterial urine culture. It is about 1.5 mm in greatest dimension. Would you report this finding, and if so, how?
Answer to the Parasite Case of the Week 768: Non-parasitic psocid (booklouse/barklouse). In case you were wondering, Psocid is pronounced SO-sid.
Many thanks to everyone on social media and the blog comments, including Florida Fan, Idzi, and Blaine.
Idzi noted " Definitely a book louse (psocid). Possibly Liposcelis sp. (Order Psocodea - suborder Troctomorpha - Family Liposcelididae). Although these insects resemble lice, these critters are not true lice, not even parasites! They live in moist places and feed primarily on molds, fungi, starch, etc. They can be easily recognized by that funny weird "nose" (or "clypeus" in scientific terms) they have."
Idzi further suggested "...not to report this, as it concerns in incidental find or a contaminant. If found on a more regular basis, try to eliminate the source of contamination as quick as possible." It's likely that it was introduced into the specimen or the agar plate from the environment.
I laughed out loud at some of the clever comments I received:
Jefferson Vaughan: I love those little guys. So humble. So innocuous..except to librarians"
James Richardson: "Risk factor-reading on the toilet?"
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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