Case of the Week 562

This week's case was generously donated by Dr. David Hernandez Gonzalo. The patient is a middle-aged male who presented with abdominal discomfort and nausea. An endoscopy was performed which revealed the following object in the gastric antrum:

The worm was sent to pathology and was sectioned, revealing the following (H&E):

Identification?

Answer to Case 562

Answer to Parasite Case of the Week 562: Likely anisakid, given the gross and microscopic appearance and position in the body. This also fits with the clinical history that I held back - this patient was an avid sushi lover!

From the gross image, you can tell that this is a roundworm, and that it appears to be embedded in the gastric mucosa:

You can also see that the caliber of the worm is similar throughout its length. That, as well as the location (stomach), make Trichuris trichiura unlikely. The longer length also allows us to rule out Enterobius vermicularis and the hookworms (which admittedly would also be unlikely in the stomach).

The microscopy is a bit harder to interpret, given that we only have a portion of the worm to examine. What we can make out, however, is the thick eosinophilic cuticle, tall coelomyarian musculature with multiple muscle cells (polymyarian), the intestinal tract that runs the length of the visible portion of the section, and the lack of reproductive structures, which are consistent with an L3 anisakid larva:

I believe that the netting like structure is a section through the muscle cells. We unfortunately can't see the lateral cords (best seen in cross-section) and additional features that would help us identify the anisakid to the genus level (e.g., cecum). This brings us to the conclusion that we always draw from this case, which is to send the worm to the microbiology lab and not the surgical pathology bench!

From Blaine:

Why are pathologists so quick to slice-and-dice

When a diagnosis would be much more precise

If they just left it intact

and the microbiologists have a whack

Alas now a diagnosis is a roll of the dice

Case of the Week 561

This week's case features photos and videos from my fabulous Technical Specialists, Heather Arguello and Emily Fernholz. The following were seen in bronchoalveolar lavage fluid. Identification?

Answer to Case 561

Answer to Parasite Case of the Week 561: Strongyloides stercoralis L3 (filariform) larvae.

As noted by Florida Fan, the notched tail is clearly shown in this case. Indeed, this is the best example I've ever seen. The notched tail can be used to differentiate the L3 larvae of S. stercoralis with those of hookworm (the latter has a pointed tail).

Old One also pointed out that the filariform esophagus reaches to the midpoint of the S. stercoralis L3 larva. In comparison, the esophagus only reaches to ~1/3 of the hookworm L3 larva. (The CDC has a nice schematic of these differences which you can view HERE.) I can't tell if the background cells in the current case are eosinophils, but peripheral and local eosinophilia are common features of strongyloidiasis.

BW in VT noted that the patient may have acquired infection years ago, and maintained infection via ongoing autoinfection. This is an important component of the S. stercoralis life cycle, as it allows for infection to persist for decades. Should the patient become immunocompromised, the ongoing infection can produce a hyperinfection state, leading to severe morbidity and possible death.

From Blaine: Silly stringy Strongyloides stercoralis stealthily slithers in sputum, spied by sensational Specialists.

Case of the Week 560

This week's case is a composite photo I created for my 2019 calendar for the month of September. The accompanying questions are:

1. What is the parasite shown? (measure ~60 micrometers long)
2. Why is this a suitable parasite for September?
3. What is the significance of the other objects in the picture? 

Answer to Case 560

Answer to Parasite Case of the Week 560: "Back to school" pinworm

This fun image had a question in 3 parts:

1. What is the parasite shown? Eggs of Enterobius vermicularis
2. Why is this a suitable parasite for September? September is the time that kids go back to school in America, and thus are more likely to obtain easily-transmitted parasites such as pinworm and head lice. As Nema noted, pinworm is "a parasite that is transmitted efficiently at the start of the school year because the embryonated eggs easily pass from the hands (unwashed) from one child to another and can also pass through the fomites"
3. What is the significance of the other objects in the picture? The backpack full of school supplies gives you the "back-to-school" link, whereas the tape refers to the cellulose tape method in which clear (not frosted!) tape is applied to the perianal skin folds to collect eggs (and occasionally adult females) that may be present. This test is commonly called the "Scotch" tape test in the U.S. after a common brand of tape; in the UK, it may be referred to as the Sellotape method - another common local brand.

In my lab, we prefer to use a plastic paddle with an adhesive surface to more efficiently and safely sample the perianal skin (a.k.a. the 'bum paddle' according to Marc). The paddle is easily transported to the lab inside the accompanying tube, and can then be examined directly under the microscope. The Graham test is another commercial collection method available in Europe.

Case of the Week 559

We're one week late due to my crazy travel schedule, but without further delay, here is our monthly case by Idzi Potters and the Institute of Tropical Medicine, Antwerp. It's short and sweet: The following was found in a stool specimen from a 3 year old child with diarrhea. It measures approximately 80 micrometers in diameter. Identification?

Answer to Case 559

Answer to Parasite Case of the Week 559: Hymenolepis diminuta
This was another great case by Idzi Potters. This egg had all of the characteristic features of H. diminuta: large size (80 microns), and lack of polar filaments between the striated outer membrane and the smooth inner membrane.You can also nicely see some of the hooks of the internal 6-hooked oncosphere:

Here is a side-by-side comparison of H. diminuta eggs, and the eggs of the related cestode, Hymenolepis nana. You can see that they are very similar appearing, but H. nana eggs are smaller and have polar filaments that originate from the inner membrane and extend out into the space between membranes (arrows in image below).

Some pondered why the parasite featured this week was named "diminuta" since its eggs are bigger than those of H. nana. Blaine helped us with that answer to this: "H. diminuta was described earlier (1819) then H. nana (1851). So, at the time of its description, H. diminuta may have been the smallest tapeworm known from humans. "Nana" comes from the Latin 'nanus' meaning dwarf or small." Isn't etymology fun?

A fun saying for today: People who confuse etymology and entomology bug me in a way I can't put into words 

Case of the Week 558

This week's case presents a bit of a conundrum. The patient is a 50 year old woman with recent travel to Kenya. She presents with acute onset of fever and chills and was tested by a rapid malaria antigen test (P. falciparum and Pan-malaria antigens) and was negative. A follow-up Giemsa-stained thin blood smear from the same blood collection shows the following:

Identification based on the blood smear? How might this correlate with the rapid antigen test?

Answer to Case 558

Answer to Parasite Case of the Week 558: Plasmodium falciparum malaria, >10% parasitemia. NEGATIVE rapid antigen.

So why is the rapid antigen test negative???

As noted by our readers, there are many possible reasons for a positive blood smear and negative rapid malaria antigen test (RDT). Here are our options, along with the reasons why each is or isn't a likely explanation in this case:

1. This is babesiosis, and not malaria. This is a very important consideration given the morphologic similarities between Babesia spp. and Plasmodium falciparum. However, the moprhologic features in this case are highly consistent with P. falciparum, including the presence of relatively-homogenous rings, without the size and shape pleomorphism usually seen with Babesia spp. There are also applique forms and headphone forms (arrows, below) which are characteristic, but not definitive, for P. falciparum infection. There may also be a hint of hemazoin (malaria pigment), but it is not obvious. Overall, we can rule out babesiosis based on the microscopic morphology.
   
2. The negative RDT is due to deletion of the P. falciparum histidine rich protein II repeat region in the parasite infecting this patient. This deletion has been reported in some African and South American countries, including Kenya where this patient had recently traveled. While this is a good thought, it would not explain why the pan-malaria antigen band (in this case, aldolase) is also absent, resulting in a completely negative RDT result. Thus, we can also exclude this as the reason for the negative RDT. 
3. This is the well-described prozone (or 'hook') phenomenon, where antigen excess (seen in cases like this with high parasitemia) binds to both the capture and detection antibodies and interferes with the formation of an antibody-antigen-antibody 'sandwich'. (You can refresh your memory on how a lateral flow immunoassay works by reading this fairly well-written Wikipedia article on malaria RDTs HERE). Prozone effect is well-described for malaria RDTs. You can read a nice study on how often this might occur HERE. In my mind, this is the most likely cause of the false negative result seen in this case.
4. The kit is faulty, or the test was not performed correctly. We unfortunately can't rule this out.

What additional testing would help us sort out the cause of the false negative RDT?

As some of you mentioned, confirmatory testing by another method would be useful to prove this was indeed malaria. Fortunately, we were able to perform PCR and it was strongly positive for P. falciparum, thus confirming our microscopic impression. To evaluate for possible RDT prozone phenomenon, we could also test serial dilutions of the fresh blood specimen to see if specimens with diluted antigen become positive. Repeat RDT testing would confirm that the test was performed correctly to begin with. Finally, we could easily confirm that the test kit isn't expired by checking the box that was used, and checking the storage conditions to ensure that they were appropriate for the test kits.

While we don't have a definitive answer for this case, I thought it was an excellent opportunity to discuss how the prozone phenomenon is a real risk in cases of high malaria parasitemia and may result in false negative results. This is one reason why malaria RDT results should always be followed gold standard blood film examination (or PCR). Also, we always need to remember that no lab test is perfect, and must always be interpreted in the context of the individual patient. 

Thank you all for the excellent comments and discussion on this case!