Monday, June 27, 2022

Case of the Week 687

 This week's case is generously donated by Dr. Ioana Bujila of the Public Health Agency of Sweden. The patient is a 67 year old woman from Gabon. Blood was examined by direct mount and Giemsa-stained blood films, and the following were identified:

These objects are approximately 228 micrometers in length.

What is your diagnosis? Are there any additional laboratory analyses that are recommended in this case? 

Tuesday, June 14, 2022

Case of the Week 686

This week's case was generously donated by Dr. Mike Mitchell and Laura Derderian. The following structure was seen in a stool concentrated wet mount specimen. It measures 27.5 micrometers in length. This was the only structure present in this specimen. Identification? What additional testing might also be useful in this case?

Sunday, June 12, 2022

Answer to Case 686

 Answer to the Parasite Case of the Week 686: Cystoisospora belli sporulated oocyst

This case shows a single - but classic - sporulated oocyst of Cystoisospora (formerly Isospora) belli. Note that two sporoblasts are present in this case, which is unusual to see in freshly passed stool specimens. Most C. belli oocysts are shed in an immature state - either unsporulated or partially sporulated (with only one sporoblast). They mature further in the environment, with the sporoblast dividing in two so that the mature oocyst has 2 sporoblasts. The sporoblasts become sporocysts with a surrounding cyst wall, and the sporocysts divide twice so that they produce 4 sporozoites each. 

We can make the diagnosis based on its characteristic appearance. However, if we wanted to confirm the diagnosis, Florida Fan notes that the oocysts "autofluoresce brightly under UV light excitation and will stain acid fast too." He goes on to say that "our team members prefer the autofluorescent methodology as it is simple and less time consuming." I know of some other labs that also prefer fluorescent microscopy to modified acid fast (MAF) staining since it is more sensitive. 

You can see some great examples of MAF staining HERE, and an autofluorescent oocyst of C. belli  from Florida Fan HERE.

Thanks again to Dr. Mike Mitchell and Laura Derderian for sharing this great case!

Monday, June 6, 2022

Case of the Week 685

 This is week's case is from our long-time contributor, Idzi Potters from the Institute of Tropical Medicine, Antwerp. The following structures were seen in a iodine wet preparation of concentrated stool. Identification? 

Sunday, June 5, 2022

Answer to Case 685

 Answer to the Parasite Case of the Week 685: Raphides, i.e, needle-shaped crystals of calcium oxalate or calcium carbonate used by several plants such as pineapple, kiwi, and rhubarb as a defense against plant-eating animals. According to Idzi, "The weird feeling you get in your mouth when eating too much pineapple is apparently caused by these crystals. You can find them quite often in stools and they should not be confused with Charcot-Leyden (C-L) crystals!" They can be differentiated from C-L crystals by their long needle-like shape and uniform diameter:

As a comparator, Idzi provided 2 nice photographs of C-L crystals, the slender bipyramidal hexagonal crystals of galectin-10. "They were first noticed in 1851 by Friedrich Albert von Zenker, and later on described in detail in 1853 by Jean-Martin Charcot and Charles-Philippe Robin. After that, in 1872, they were further described by Ernst Viktor von Leyden. Hence their name…"

Charcot-Leyden crystals:

Galectin-10 is contained in the granules from eosinophils (and also from basophils) and spontaneously crystallizes into Charcot-Leyden crystals when it is released in the extracellular environment (by exocytosis and/or cellular breakdown). Charcot-Leyden crystals are therefore indicative of eosinophilic inflammation (caused by asthma, allergic reactions, parasitic infections, etc)."

Thanks again to Idzi for this excellent case!

Tuesday, May 31, 2022

Case of the Week 684

 For our last case of May, we have a fun submission from Dr. Megan Shaughnessy. She noted the following in some fresh monkfish she purchased from a small local grocery store. What is the likely identification? Also, what is the risk to humans if ingested?

Monday, May 30, 2022

Answer to Case 684

Answer to the Parasite Case of the Week 684: Anisakid L3 larvae

The following is our third and final discussion from our amazing guest author and 3rd year medical student, Hadel Go. I'm sure you will all agree that her discussions have been among the best we've ever had on this blog. Congratulations on the excellent work, Hadel!


Worm cases are always my favorite because the comments are either “They are so beautiful!!” (@Parasite_Power on Twitter) or “…That’s a wholelottanope” (Valmik in the comments).

This is a nematode in the Anisakidae family, likely Pseudoterranova or Anisakis spp., the two most common cause of human anisakiasis, as many of you suggested. A third genus, Contracaecum, is also a possibility. Mario George Nascimento and Melinh Luong point out that its dark color implicates Pseudoterranova (vs Anasakis which is usually white/pink/red). 

However definitive identification to the genus level requires closer inspection of the intestinal cecum and its positioning as mentioned by Idzi P. in the comments and seen in Case 563 which he donated.

Diagnostic features of Pseudoterranova and Anasakis spp. include a “mucron… and 3 poorly formed anterior lips with a small boring tooth” (see Case 157 and Case 557). The table below shows some varying characteristics between the two more common culprits:


Anisakis simplex

Pseudoterranova decipiens


White, milky



19-36mm long; 0.3-0.6mm wide

25-50mm, 0.3-1.2mm wide

Other features

Blunt tail, long stomach, Y-shaped lateral cords, no cecum

Anteriorly directed cecum


1. Marine mammals defecate into the sea releasing unembryonated eggs
2. Eggs become embryonated in water
3. L2 larvae form in eggs and hatch out
4. Free swimming L2 larvae are ingested by crustaceans and mature into L3 larvae in the hemocoel
5. Fish (any marine fish is susceptible) and cephalopods eat infected crustaceans
6. Larvae migrate to mesentery and muscle tissues of the new host
7. L3 larvae are transferred from fish to fish until ingested by a marine mammal
8. L3 larvae molt twice in the mammal and develop into adults which produce eggs

Anisakiasis occurs when these parasites at the L3 larval stage are ingested in raw or undercooked saltwater seafood. While some larvae will die following ingestion, or make their way up the esophagus into the oral cavity (much to the dismay of the human host!), they may also attach to the wall of the human esophagus, stomach, or intestine causing damage or inflammation. Since humans are incidental hosts, the anisakids do not mature into adults. Symptoms include abdominal pain, nausea, vomiting, distention, diarrhea, blood/mucus in stool, and mild fever. If a patient is allergic to anisakids, rash, itching, angioedema, or anaphylaxis can occur (even when the worms are dead). Complications include intestinal obstruction, peritonitis, and intestinal perforation. Severe immune response following penetration of intestinal tissue can resemble Crohn’s disease.

These roundworms can be visualized and removed with an endoscope, confirming the diagnosis. If not removed, they will eventually die and can cause inflammation. A biopsy can reveal an eosinophilic granuloma containing the dead nematode. Endoscopic or surgical removal of the parasites is curative. Treatment with albendazole has been used successfully to kill the worms, although not FDA approved for anisakiasis. Unembedded worms are usually eliminated by the body within 3 weeks.

Avoid infection by following one of these FDA recommendations which kills the parasite:
- Cook seafood (145°F or above)
- Freeze at -4°F or below for 7 days
- Blast freeze at -31°F or below until solid and store at -31°F or below for 15 hours
- Blast freeze at -31°F or below until solid and store at -4°F for below for 24 hours

I’ve often been asked if I eat sushi and sashimi knowing the prevalence of these worms… and the answer is YES, I love Japanese cuisine and I eat them regularly. All fish intended for raw consumption in the US are required by the FDA to be frozen as stated above so they should be safe. The dead parasites may still be on the tissue, but they won’t be infectious. I would however be more hesitant to eat raw or undercooked fish when traveling to places where there are less stringent regulations. Ask for sushi/sashimi grade fish and use your best judgement!

Kingdom: Animalia
Phylum: Nematoda (roundworms)
Order: Rhabditida
Family: Anisakidae

Read more:
1. CDC - Anisakiasis
2. Anisakidosis: Perils of the Deep | Clinical Infectious Diseases | Oxford Academic (
3. Anisakis, Something Is Moving inside the Fish - PMC (
4. Assessing the risk of an emerging zoonosis of worldwide concern: anisakiasis - PMC (

Thank you @leon_metlay on Twitter for sharing this paper:
Intestinal anisakiasis. Report of a case and recovery of larvae from market fish - PubMed (

This is my third and final post as a guest blogger here and I’d like to thank Dr. Bobbi Pritt for the opportunity to learn more about these creatures and share what I’ve learned with you all!

Monday, May 23, 2022

Case of the Week 683

 This week's case is from a 4 year old boy with sudden onset of 'explosive' watery diarrhea, accompanied by low grade fever, nausea, vomiting, and loss of appetite. A stool specimen was obtained after 1 week of symptoms, and the following objects were noted on a modified acid fast stain. They measure approximately 5 micrometers in diameter. 

Here are some questions to consider:

1. What parasite is shown?

2. Name 3 common ways that infection is acquired.

3. What is the typical recommended treatment?

4. What are the risk factors for severe infection.

As with last week, Hadel Go is serving as a guest author, and will help us answer these questions later this week. Stay tuned!

Sunday, May 22, 2022

Case of the Week 683

Answer to Case 683: Cryptosporidium sp. oocysts

The following OUTSTANDING discussion is from our guest author, Hadel Go, a third year medical student with a strong interest in clinical parasitology.


We received some excellent feedback! Thank you all for leaving comments on the blog, on Twitter, and on LinkedIn.

Yes, these are Cryptosporidium oocysts. These protozoans can be identified by their size and consistent red color on modified acid-fast staining of stool samples. They can also be diagnosed from H&E stained tissue biopsies (see images here: Cryptosporidium,, but nucleic acid amplification tests (NAATs) and immunofluorescence microscopy have the greatest sensitivity and specificity. The two main species that cause infection in humans are C. parvum and C. hominis, although several others are also capable. Species identification requires molecular methods like NAAT and/or sequencing.

Transmission is fecal-oral through food and water contaminated by stool from an infected person or animal. Some examples include swallowing pool water, eating unwashed fruits/vegetables from unsafe farms where produce is contaminated with feces containing oocysts (@StephenTristam on Twitter noted a strong association with farming communities), handling infected farm animals especially calves (@KevKeel brought up infections among vet students!), caring for an infected person, engaging in anal sexual contact (as mentioned by @RA_MLS), and touching your mouth with contaminated hands.

Cryptosporidiosis will present with explosive watery diarrhea, stomach pain, nausea, vomiting and fever 2-10 days after acquiring the parasite as seen in our 4-year-old case patient. These are self-limiting in most immunocompetent individuals. Replenishing fluids and electrolytes to prevent dehydration is usually sufficient. The infection is mainly concentrated in the small intestine, but complications can lead to malnutrition and wasting due to malabsorption or severe dehydration especially in babies.

Immunocompromised patients such as people with HIV/AIDS, inherited immune diseases, or on immunosuppressive drugs can progress to severe chronic infection which may become fatal. The respiratory tract can also be affected (respiratory cryptosporidiosis) resulting in a persistent cough.

Nitazoxanide is the FDA approved treatment for diarrhea caused by Cryptosporidium in immunocompetent patients ≥ 1 year of age. However, as Idzi P. mentioned in the comments, “efficacy in immunocompromised patients is not yet clear… for patients with HIV, anti-retroviral therapy (to improve immune status) could resolve the symptoms.”

Prevention is essential: practice good hygiene, wash hands often, do not swallow water when swimming, only drink filtered or treated water/ice especially when traveling or camping, practice safe sex, do not consume undercooked meat or unpasteurized milk/apple cider, wash your fruits and veggies. To avoid recurrent outbreaks, please do not go swimming if you have diarrhea or within two weeks of resolution of symptoms.

The differential diagnosis is a protozoa in the same family - Cyclospora cayetanensis (discussed in Case 457). It causes cyclosporiasis which presents similarly and can be differentiated from Cryptosporidium species by measuring the oocysts. Their location within enterocytes also varies; see photos from Case 343. The table below compares the two:


Cryptosporidium parvum

Cyclospora cayetanensis

Oocyst size

4-6 μm

7.5-10 μm

Acid-fast staining

Red staining (can be variable)

Often variable staining

Autofluorescence under UV

No (as pointed out by Idzi P.)


Location in enterocyte

Replicates outside the cytoplasm just below the cell membrane

Replicates within the cytoplasm


Oocysts are immediately infectious leading to massive outbreaks

Person-person transmission unlikely because sporulation can take weeks and occurs in the soil.

- Each sporulated oocysts contains 4 sporozoites.
- Cryptosporidium is resistant to chlorine treatments alone due to their protective outer shell.
- Ultraviolet light and boiling are effective ways to inactivate this protozoan.
- Cryptosporidium parvum oocysts are commonly found in young calves 1-4 weeks old.
- Cryptosporidium zoonotic infection can occur from sheep, cows, pigs, rodents and other animals.

LIFE CYCLE (3 developmental stages in bold)

1.      Oocysts (thick-walled spore, infective form) are shed in stool and ingested by host

2.      Excystation releases sporozoites which parasitize epithelial cells of the GI tract

3.      Meronts undergo asexual multiplication within the brush border

4.      Gamonts are produced from sexual multiplication

a.      Microgamonts = male; these rupture releasing microgametes

b.      Macrogamonts = female

5.      Microgametes fertilize macrogamonts forming a zygote

6.      Zygotes give rise to two types of oocysts

a.      Thin-walled oocysts cause autoinfection

b.      Thick-walled oocysts are excreted into the environment

Domain: Eukaryota
Clade: SAR
Infrakingdom: Alveolata
Phylum: Apicomplexa
Order: Eucoccidiorida
Family: Cryptosporidiidae

More information:
Parasites - Cryptosporidium (also known as "Crypto") | Cryptosporidium | Parasites | CDC
CDC - DPDx - Cryptosporidiosis Life Cycle

Thanks to Idzi P. for sharing the following articles about the 1993 outbreak in Milwaukee, WI (the largest known outbreak affecting ~400,000 people):
1. A Massive Outbreak in Milwaukee of Cryptosporidium Infection Transmitted through the Public Water Supply | NEJM
Costs of Illness in the 1993 Waterborne Cryptosporidium Outbreak, Milwaukee, Wisconsin


Wednesday, May 18, 2022

Case of the Week 682

This week we have a special guest author, Hadel Go, a medical student (Class of 2024) who is greatly interested in parasitology. I'm very excited that Hadel will be our first ever guest author! The following is our case that she will be teaching us about later this week.

The patient is a young man from Madagascar who presented with a 1-week history of chest pain, night sweats, and fever. Chest X-ray showed a right upper lobe peripheral cavitary lesion. Work up for tuberculosis including AFB stained smears of sputum and sputum PCR using the Xpert MTB/RIF assay were negative. Therefore, a wedge resection of the abscess was performed and revealed the following (H&E stain, 400x original magnification):

What is your diagnosis?