This week's case was generously donated by Florida Fan and is a fun finding to close the year.
The following objects were found in vaginal secretions (Giemsa stain, 1000x original magnification).
What is your diagnosis? Is there anything special about the image shown here?
Answer to the Parasite Case of the Week 797: Trichomonas vaginalis trophozoites.
Note the diagnostic features, including the small nucleus, 4 anterior flagella, an undulating membrane, and posterior axostyle.
Also, as Idzi noted, we caught one in the process of binary fission! You can see two sets of flagella and two nuclei.
Thanks again to Florida Fan for donating this classic case. Happy 2026 everyone!
Wishing you all a very Happy Holiday Season!
On the 12th day of Christmas my true love gave to me...
Twelve merozoites
Eleven festoons
Ten primary uterine branches
Nine gravid proglottids
Eight eggs-in-packet
Seven coelymyarian muscles
Six raptorial claws
Five antennal segments
Four nuclei
Three intermediate hosts
Two paired alae
And a single species of Ascaris 😎
This week's fascinating (and somewhat gruesome) case is from Dr. Richard Bradbury. The following were seen on a bacterial culture (Chocolate/New York agar) plate after 24 hours of incubation in CO2. The specimen was pus from a perforated eardrum from a child in the Gambia. What are the possible explanations for this finding?
Answer to the Parasite Case of the Week 796: Fly larvae; clinical correlation is recommended.
As many of you noted, there are two main possibilities for this finding:
1. This is a case of aural myiasis, in which a fly laid eggs onto the patient's ear - likely in the setting of a pre-existing skin/soft tissue infection.
2. This represents contamination of the culture plate, in which the fly laid eggs on the specimen after it was collected.
In this case, examination of the patient revealed no evidence of aural myiasis, and therefore #2 is the most likely explanation.
We had a similar case of "agarmyiasis" back in 2018. I encourage you to check out the interesting comments and discussion.
Thanks again to Richard Bradbury for donating this great case!
It's the first Monday of the month and time for our case from Idzi Potters and the Institute of Tropical Medicine, Antwerp. Loranne Lambregts is a new colleague of Idzi's and contributed to this case, identifying the object of interest below that was submitted for parasite identification in their lab. It measured approximately 1.5 cm in greatest dimension and was still moving when received! Here it is pressed between two glass slides:
Answer to the Parasite Case of the Week 795: Taenia saginata/suihominis gravid proglottid.
As noted by Anonymous, Patrik, Antoine A, Florida Fan, Menzler, and others, we have a proglottid of Taenia saginata/Taenia suihominis (formerly known as T. asiatica) characterized by >12-13 primary uterine branches on each side of the central uterine stem and a Taenia spp. egg characterized by an internal hooked oncoschere and a thick radially striated outer shell. Assuming the image of the egg was acquired using a 40x objective, we can estimate a size of 35 micrometers in greatest dimension which is consistent with Taenia species. We don't have a patient travel history, so I would sign this case out as "Taenia saginata/Taenia suihominis".
This beautiful preparation shows a fresh proglottid pressed between glass and backlit to show the characteristic branching pattern. I particularly like this technique since it requires only minimal manipulation of the proglottid. It can easily be done with a fresh proglottid, but preserved proglottids need to be cleared and softened through exposure to lactophenol or other preparation.
Injecting India ink into the lateral genital pore of Taenia proglottids can also be used to highlight the uterine branches, but we don't perform this in my lab given the potential infectious risk associated with T. solium.
Regarding motility as a diagnostic feature - I dug into this in Case of the Week 789 by scouring the old literature and found only anecdotal evidence. However, it does seem that Taenia saginata proglottids are more likely to be motile than T. solium. I can't speak to T. suihominis, so if anyone has information to share on this species, I would love to hear it.
Thanks again to Idzi and Loranne for donating this great case!
This week's case is from Dr. Steven Ruhoy and Tracie Rose. The patient is an elderly woman with a very itchy rash which is worse at night. The family practitioner noted excoriations on the plantar aspect, but did not see any definitive burrows. She performed a skin scraping which revealed the following on H&E-stained histologic section. What is your identification?
Answer to the Parasite Case of the Week 794: Winged insect, likely contaminant.
Many of you asked if this could be Pediculus humanus - a thought that I also initially shared. However, Blaine Mathison nicely pointed out that this insect has a wing, which rules out lice, as they are wingless. Here is his annotated image:
Based on this finding, it is likely that this is just a contaminant or inconsequential finding. It doesn't explain the patient's symptoms, and therefore further investigation is warranted.
Thanks again to Dr. Steven Ruhoy and Tracie Rose for donating this challenging case!
These week's case was generously donated by Dr. Linoj Samuel. The following objects were seen in the sputum from a renal transplant recipient. He had presented with pneumonia and Klebsiella sp. bacteremia.
What is your diagnosis?
Answer to the Parasite Case of the Week 793: Strongyloides stercoralis L3 (filariform) larvae
This case shows the classic features of the L3 larvae with a blunt-ended (and subtly-notched) tail.
Some of you noted the shorter larva as well, which is reminiscent of an L1 larva. However, this would be unlikely given the lifecycle of S. stercoralis.
I also considered if this could be an L4 larva, given Dr. Richard Bradbury's excellent 2024 JCM publication. However, these rarely-seen L4 larvae have a pointed or cone-shaped tail rather than a blunt, notched tail.
The number of larvae and the host's immunocompromised state are concerning for hyperinfection syndrome. When we see one of these cases in my lab, we immediately call the patient's primary care team to alert them.
Thanks again to Dr. Linoj Samuel for donating this great case!
This week's impressive case was donated by Drs. Ashley Zeoli, Nico Herrera, Luke Pryke, and Paul Blair. The patient is a young man with no significant medical history who presented with right upper quadrant abdominal pain. The pain had been present for several years, but recently worsened to the point that he sought medical care. He is originally from India.
Here was the CT from admission which shows a large, complex cyst in the liver:
Answer to the Parasite Case of the Week 792: Echinococcal cyst, most likely due to E. granulosus.
This case shows several classic findings:
This week's case is from Drs. Matt Pettengill and Sean Moss. Approximately 10 of the following object were recovered from the hair of patient. She had applied calendula to her scalp earlier in the day. What is your identification?
Answer to the Parasite Case of the Week 791: Not a parasite; most closely resembles calendula seeds.
This was a fun case to try to figure out! Almost everyone correctly recognized that this object was not a parasite, and the history of calendula application quickly pointed many of you toward the true source. I consulted our botanist colleague, Dr. Mary Parker, who agreed that this object closely matches a calendula seed.
Here’s a comparison image from the USDA website (top) and our current case (bottom):
Calendula, a genus in the daisy family (Asteraceae), is commonly known as marigold. Calendula-based products, especially calendula oil, are often promoted for hair growth and scalp soothing, though I wasn’t able to find supporting evidence for these claims in PubMed.
As you can see from the photo, Calendula seeds are small, curved, and ridged, making them look surprisingly similar to certain larval insect forms or even trematodes. A great reminder that context and collaboration are key in solving these “parasite mimicker” mysteries!
Thanks again to Drs. Moss and Pettengill for sharing this great case.
This week's case was generously donated by Dr. Tudo Rares Olariu, the head of the clinical laboratory at the Municipal Clinical Emergency Hospital in Timisoara, Romania. The following object was seen in a stool specimen from a child with multiple mucosanguineous stools. It measures approximately 45 micrometers in greatest dimension. What is your diagnosis?
Answer to the Parasite Case of the Week 790: Rodentolepis (formerly Hymenolepis) nana egg.
This is a classic example of an R. nana egg showing the inner and outer membranes, oncosphere with large splayed hooklets, and polar thickenings from which filaments extend into the space between the oncosphere and outer membrane. It's hard to make out the polar filaments in this particular example, but the smaller size allows us to differentiate it from the larger Hymenolepis diminuta egg.
This week's case was donated by Dr. Blake Cirks. The following object was passed per rectum by an otherwise asymptomatic individual living in Laos. Based on the image and video, what is the most likely diagnosis?
Happy Autumn, everyone! It's the first Monday of the month and time for our monthly case from Idzi Potters and the Institute of Tropical Medicine, Antwerp. The following were seen in a direct wet mount specimen of stool from a patient with chronic constipation. What is your identification?
Answer to Parasite Case of the Week 788: Pentatrichomonas hominis trophozoites
Note the classic 'jerky' motility of P. hominis trophozoites. I've never seen so many P. hominis in a specimen before!
Like Trichomonas vaginalis, there is no known cyst stage of this protozoan parasite. P. hominis trophozoites move using 5 flagella: 4 are directed anteriorly, while the 5th is directed posteriorly, forming the outer edge of an undulating membrane. This results in characteristic motility that Dr. Richard Bradbury likes to describe as "a man trapped inside a plastic bag"! You can see a very nice video and still images of this phenomenon in Case of the Week 737. For those of you who have been following this blog since the beginning (2007!), you may recall that P. hominis was the parasite highlighted in Case of the Week 5. Check out the fun video that Alex Ball made for me back then in the Answer to the Case of the Week 5.
Thanks again to Idzi Potters and the Institute of Tropical Medicine Antwerp for donating this great case!
This week's case is from Dr. Beth Adams who encountered a patient in the Moskitia region of Honduras who had coughed up the following object. What is your presumptive identification?
Answer to the Parasite Case of the Week 787: Adult Ascaris sp.
As Florida Fan and Idzi noted, the curved posterior end suggests that this is a male adult.
Ascaris is the largest nematode to reside in the human intestinal tract. Adults live in the small bowel and must move against peristalsis to avoid being expelled in the stool. The fact that they are freely moving and not attached to the intestinal wall means that they can occasionally end up in ectopic locations such as the biliary tree or appendix. In this case, the worm migrated up the intestine, stomach, and esophagus, and was expelled through the mouth (!) Given its large size, heavy infections can lead to the potential deadly complication of small bowel obstruction.
Some readers made the interesting point that this could be either Ascaris lumbricoides or the zoonotic Ascaris suum, which raises the controversial topic of Ascaris taxonomy!.Although they were long thought to be separate species, Ascaris suum was found to be genetically similar to the human species A. lumbricoides in a 2014 comparative analysis of microRNA profiles, arguing against separation into Ascaris two species. However, a 2020 analysis using whole genome sequencing found significant genetic differentiation between A. lumbricoides and A. suum populations. Therefore, it looks like the two populations are genetically distinct and likely deserve separate classifications. We can hopefully look to future analyses to more fully understand their taxonomic status.
Thanks again to Dr. Beth Adams who donated this case, and to Drs. Mike Adams and Bill Stauffer who shared the image and video with me initially.
This week's case was generously donated by Dr. Richard Bradbury from James Cook University in Australia. The following object was seen in the stool from a patient with advanced HIV infection living in Tanzania. Shown are preparations using differential interference contrast (DIC), darkfield, and fluorescent microscopy (wavelengths unknown). The objects measure approximately 25-30 micrometers in length. What is your identification?
Answer to the Parasite Case of the Week 786: Cystoisospora belli
Idzi nicely described this finding as an "Immature oocyst of Cystoisospora belli (formerly known as Isospora belli) containing only one sporoblast, which will evolve to two sporoblasts and later on will sporulate to become infective."
While we don't know the definitive details of the images that Dr. Bradbury provided, Idzi suggested that "the last one could be autofluorescence at 450-490 nm." and noted that "All coccidia (including Cystoisospora belli) will demonstrate fluorescence (without prior staining) when placed under UV-light. They will show up as bright blue structures when using an excitation filter of 330-365 nm, or green at 450-490 nm. Of note: Cryptosporidium is not placed among the coccidians anymore (moved to the Gregarines), and does not demonstrate autofluorescence either!"
If you are interested about how autofluorescence can be used for diagnosis, you should check out Blaine's and Dr. Marc Couturier's recent paper "Shedding new light on Cyclospora: how the use of ultraviolet fluorescence microscopy can improve diagnosis of cyclosporiasis".
It's the first of the month and time for a case from Idzi Potters and the Institute of Tropical Medicine, Antwerp!
The following images show an object that dropped out of a patient's nose 😮. What is your identification?
Wishing all of my American readers a very happy and restful Labor Day.
Answer to the Parasite Case of the Week 785: Oestrus ovis larva.
This is a fascinating case of a probable 2nd instar stage of Oestrus ovis, commonly known as the sheep nasal botfly. O. ovis, can occasionally cause infection of the eye (ophthalmomyiasis) or, less commonly, the nose and sinuses (rhinomyiasis) in humans. Human infection is an accidental zoonosis and results from deposition of first-instar larvae by adult flies, typically in the ocular or nasal mucosa. Human cases are most prevalent in Mediterranean and other subtropical regions, with seasonal peaks in summer and spring.
Most infestations are self-limited as larvae rarely progress beyond the first instar in humans. Therefore, this is a very interesting presentation of what appears to be a 2nd instar larva involving the nose and/or sinuses.
Diagnosis is based on clinical suspicion and examination of the larvae. First-stage larvae are small (approximately 1–2 mm) and mostly translucent As noted above, this is the most common form seen in humans.
Second-stage larvae are larger (up to 7 mm), more robust, and display increased segmentation, with the body becoming more opaque and the cuticle developing small spines. The oral hooks are more prominent, and the posterior spiracles begin to show more complex structure. This is what I believe this specimen to be.
Third-stage larvae are the largest (up to 21 mm), cylindrical, and have a thick, heavily pigmented cuticle with pronounced transverse bands of spines and well-developed oral hooks in their mature form; the posterior spiracles are fully formed and more sunken into the body. Also, the body is distinctly segmented, and takes on a brown color in the mature form.
Check out these two publications for some great photos of the different stages:
Thanks to all who wrote in on this interesting case, and to Idzi for donating it! Special thanks to Blaine Mathison for his input on larval stage.
Answer to the Parasite Case of the Week 784: Myiasis causing fly larva, most likely Cuterebra species.
Thankfully, Rebecca Black and her colleagues were able to remove the larva from this poor kitty!
As noted by Florida Fan and others, we don't have the posterior spiracles to make a definitive identification. However, based on the presentation, geographic location, dark color, and overall appearance, we can still make an identification of Cuterebra sp. third instar stage larva.
If you are interested, you can read more about this infection in animals at the following excellent sites:
Cuterebra Infestation in Small Animals - Integumentary System - Merck Veterinary Manual
Companion Animal Parasite Council | Cuterebriasis
Genus Cuterebra - Rodent and Lagomorph Bot Flies - BugGuide.Net
The usual hosts of Cuterebra are rodents and lagamorphs. Flies lay eggs on vegetation or woody ground debris, often near the opening of rodent burrows. The eggs hatch to release a first instar stage larva when there is exposure to increased temperature, indicating the presence of a nearby host. The larva then enters host through any natural body opening such as the mouth or existing wounds. It will then migrate to the subcutaneous tissues and enlarges rapidly. It creates a pore from which it can breathe and eventually exit the host, usually in 3-6 weeks.
As you all know, my interest is in human infections, and therefore I posted this case as both a pet lover and a human medical parasitologist. Cuterebra infections (cuterebriasis) can rarely occur in humans, and as with other forms of myiasis, removal of the larva is curative.
Thanks again to student veterinarian, Rebecca Black, for donating this case!
This week's case was generously donated by Dr. Richard Bradbury. The following were seen in a stool specimen from a middle-aged man with diarrhea. Preparations are a concentrated wet prep and trichrome stained permanent mount. Object measure 10-15 micrometers long. What is your identification?
Answer to the Parasite Case of the Week 783: Chilomastix mesnili cysts and trophozoites. Note the classic morphology:
C. mesnili is a non-pathogenic flagellate and therefore not the cause of this patient's diarrhea.
Thanks again to Dr. Bradbury for donating this great case!
This week's case was generously donated by Dr. Adrienne Showler. The following image is from a video capsule endoscopy performed on an immunocompromised patient with diarrhea. The patient has not travelled outside of the US and has no other risk factors for parasitic infection. Numerous of these objects were seen - none were moving. Identification?
Answer to the Parasite Case of the Week 782: Not a parasite.
Most closely resembles banana "seeds" (tannin bodies).
While we will never know for sure, the beaded/fragmented appearance and dark color of these objects allow us to identify this as a non-parasitic object that closely resembles banana tannin bodies. You can read more about banana tannin bodies in my previous cases 139 and 468. In particular, I recommend checking out Case 468 which shows my experiment to recreate a partially-digested banana.
Thanks again to Dr. Adrienne Showler for donating this interesting case!
This week's case features a beautiful video from Dr. Rasool Jafari. The specimen is skin scrapings. What is your identification?
Answer to Parasite Case of the Week 781: Demodex sp. The legs and gnathosoma (with mouthparts) are nicely demonstrated:
