Sunday, May 30, 2021

Answer to Case 641

 Answer to the Parasite Case of Week 641: Strongyloides stercoralis L3 (filariform) larvae.

As mentioned in the comments, other items on the differential diagnosis include larvae of the hookworms and Ascaris lumbricoides. The larvae of these nematodes are only rarely seen during the initial stage of infection, in which larvae migrate to the lung before reaching their permanent destination in the intestinal tract. For that reason, it is very rare to identify them in clinical specimens. In comparison, S. stercoralis has an ongoing autoinfection cycle in humans which results in recurrent migration of L3 larvae through the lung. This normal low-level of autoinfection is often subclinical and allows the infection to persist for decades. However, immunocompromised patients (such as those receiving corticosteroids like this patient) are at risk for hyperinfection, during which large numbers of L3 larvae leave the intestinal tract and migrate to the lungs and other organs. This is the common scenario in which larvae are found in respiratory specimens.

In addition to the clinical indicators of strongyloidiasis (patient from Kentucky, receiving immunosuppressive agents), there are a few morphologic features that can be used for differentiating S. stercoralis larvae from other nematode larvae found in the lung. One of the primary features is the shape of the tail. S. stercoralis L3 larvae have a subtly-notched tail, whereas the hookworm L3 larvae (the stage found in the lung, and rarely in unfixed stool examined several days after passage) have a pointed tail. I believe that A. lumbricoides L3 larvae also have a pointed tail - can anyone tell me for sure?

Monday, May 24, 2021

Case of the Week 640

Here's a fun case for you all - submitted for arthropod identification. Thoughts?

Sunday, May 23, 2021

Answer to Case 640

 Answer to the Parasite Case of the Week 640: Pseudoscorpion

Despite the scary look of this arthropod's front pincers (enlarged pedipalps), it is not a scorpion and does not harm humans. In fact, pseudoscorpions can be helpful; Dwight Ferris pointed out that they feed on other arthropods pests that you don't want in your house such as carpet beetles, book lice, ants and clothes moths. 

The main way to tell a pseudoscorpion apart from a true scorpion is that the former is much smaller (usually < 5 mm long) and lacks a telson (venom bulb) and aculeus (stinger) at the end of their abdomen. Of note, both pseudoscorpions and scorpions are arachnids and have 8 legs in their adult form - just like ticks and mites.

Pseudoscorpions may occasionally be submitted to the laboratory after concerned individuals find them in their home and bring them to their physician. That is probably how we received this one. However, Chuck Blend also noted that they are commonly found in the library, 'hanging out' between the pages of old books. You can also find them out in nature, under stones, leaf litter, tree bark and mulch. 

Monday, May 17, 2021

Case of the Week 639

This week's case was generously donated by Drs. Timothy Y. Chou and Roberta J. Seidman. The specimen is a corneal biopsy in an elderly woman who had been receiving ocular steroid treatment (but no systemic immunocompromising agents). The patient went on to develop bilateral punctate keratitis, and corneal biopsy showed the following objects:

H&E, 600x and 1000x

Tissue Gram stain, 1000x
What is the most likely diagnosis? What additional studies would you recommend to confirm the diagnosis?

Sunday, May 16, 2021

Answer to Case 639

Answer to Case of the Week 639: Microsporidia spores. 

As many of you noted, the differential diagnosis includes Toxoplasma gondii tachyzoites. 

Here are some of the key morphologic features:

  • Location in the cornea. [T. gondii is usually found in the posterior chamber of the eye (e.g., retina)]
  • Small oval shape with well-defined contours on H&E:

Unfortunately, T. gondii tachyzoites are much less defined on H&E - they can be very difficult to see. They often don't even have a nice crescent shape like we seen on Giemsa-stained air-dried impression smears. They often just look like little blobs in tissue.
  • Strong Gram-positivity on tissue Gram stain. (In comparison, T. gondii tachyzoites do not stain well, and may appear Gram-negative).
Other considerations would include small fungi such as Candida glabrata. Therefore, a panel of histochemical stains may also be useful. Microsporidia spores may be focally GMS positive, but aren't usually uniformly positive like yeasts are. Also, microsporidia spores will stain focally positive with an acid fast stain, strongly positive with warthin-starry stain, and have a polar dot-like positivity with PAS. They are also often birefringent with polarized light.

If you still wanted to do additional studies to confirm your diagnosis, I would recommend immunohistochemistry and PCR for T. gondii, and transmission electron microscopy. Microsporidia spores are beautifully detailed on TEM. This is still a conventional method for microsporida genus/species identification. If available, you could perform strong trichrome staining on the tissue or corneal scrapings. Finally, you could perform a broad range amplification and sequencing assay for microsporidia. There are a number of different species that infect the eye, so while you would start with a PCR for Encephalitozoon hellem (PCR for Encephalitozoon species are available in some reference labs), a negative result wouldn't rule out infection with other microsporidia (e.g., Vittaforma corneae, Nosema ocularum, Microsporidium spp., Trachipleistophora hominis).

So some of you may be asking why I included a case of microsporidiosis on my parasite blog. You are correct in that it is not longer considered a parasite! However, microsporidia are still routinely identified in clinical parasitology in many labs, so it's a good entity to be familiar with.  

Tuesday, May 11, 2021

Case of the Week 638

This week's case was generously donated from Drs. Cynthia Magro and Lars Westblade. The following object was identified in an hematoxylin and eosin stained tissue section of toenail in a patient with onychomycosis. Identification?

Sunday, May 9, 2021

Answer to Case 638

Answer to the Parasite Case of the Week 638: Not a human parasite; seed.

This is a small seed that likely became embedded in the nail. Botanist Dr. Mary Parker commented that "I would rule out a grass/cereal seed, as there is clearly an embryo (radicle and cotyledon) embedded in starchy endosperm, the whole surrounded by an epidermis and cuticle. The central vascular strand can be seen in the radicle, with pro-vascular cells in the cotyledon. It must be a small seed but in the absence of a scale bar, it is difficult to say more."  I learned from Dr. Cynthia Magro who donated this case that the patient has onychomycosis, so it is understandable how a small seed could have become trapped within a thick, roughened nail. 

Here are some of the morphologic details for those of you who are interested!

Monday, May 3, 2021

Case of the Week 637

Welcome back to the first Monday of the month! It's time for our monthly case from Idzi Potters and the Institute of Tropical Medicine in Antwerp. The following structures were found in a stool specimen from a patient originally from the Democratic Republic of the Congo. He presented with long standing diarrhea (occasionally bloody), intermittent abdominal pain, decreased appetite and unintentional weight loss. They are approximately 200 micrometers long.

What is the most likely identification?

Sunday, May 2, 2021

Answer to Case 637

 Answer to Parasite Case of the Week 637: Schistosoma intercalatum

This was definitely a tricky case! The eggs strongly resemble those of Schistosoma haematobium, but stool would be an unusual source for this parasite, and the eggs in this case are larger than what would be expected for S. haematobium (usual size is 110 to 170 micrometers long). In comparison, S. intercalatum eggs are most often found in stool and measure 140 to 240 micrometers long. They also characteristically have an equatorial bulge as seen in one of the images from this case:

Of note, Schistosoma intercalatum has a very limited geographic distribution, being found primarily in the Congo area. Therefore, getting a travel history can be extremely helpful. As Marc Couturier pointed out on Twitter, the other major consideration is S. guineensis due to the morphologic similarities of the eggs of the two species. If a location had not been provided, then reporting out "Schistosoma intercalatum/S. guineensis" would be best.