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:
- 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.