Monday, October 26, 2009

Case of the Week 90

The following photos are from a Giemsa-stained slide of corneal scrapings from a patient with a painful red eye.

Sunday, October 25, 2009

Answer to Case 90

Answer: Acanthamoebic keratitis

This is a case of keratitis caused by Acanthamoeba species. These protozoan parasites are free-living amoebae found widely in the environment, such as in fresh water pools, soil, dust, and even chlorinated tap water. Keratitis usually occurs in patients that wear contact lenses, since the lens provides a nidus for irritation and infection. The classical history is that of a patient who wears contact lenses and rinses them in tap water or home-made saline solution.

The diagnosis is made by identifying classic cysts and/or trophozoites in corneal scrapings or biopsies, (histopathology, cytopathology, or microbiology preparations). Acanthamoeba spp. can also be grown in culture or detected using fluorescent and molecular methods. Trophozoites measure approximately 15-45 µm, and produce many spine-like processes (acanthapodia) in culture. They differ from Entamoeba histolytica trophozoites by having a large nucleus, large, centrally-located karyosome, and no peripheral chromatin. The cysts are typically 10-25 µm in diameter, and have a classic morphology, with a double-walled cysts. The outer wall (exocyst) is typically wrinkled, while the inner wall (endocyst) is hexagonal, spherical, or star-shaped. Like the trophozoites, the cysts contain one nucleus. CLICK ON IMAGES TO ENLARGE

Culture on tap water agar - See Previous Case of the Week.

Another important differential diagnosis in this scenario is herpes keratitis caused by herpes simplex virus. One viewer commented that the organisms appeared to have the classic "marginated chromatin, glassy/cleared-out nuclei" of HSV-infected cells. This is particularly relevant in this case, since the cysts do not have visible nuclei. However, they can still be differentiated from herpes-infected cells by the presence of the wrinkled double walls. Also, the classic features (the 3 "M's") of herpes are not present - Multinucleation, nuclear Molding, and Margination of nuclear chromatin. Below is a classic case of a multinucleated herpes simplex virus-infected cell.

This patient with acanthamoebic keratitis did well after treatment, with little residual scarring. However, this is not always the case, and many cases of keratitis are very refractory to treatment.

A more deadly presentation of Acanthamoeba spp. is chronic granulomatous encephalitis, a slowly progressive, often fatal infection of immunocompromised patients. Disease usually disseminates from a primary dermal or pulmonary source, cysts may be found in the skin, lungs and other organs.

Thanks to all who wrote in with comments!

Monday, October 19, 2009

Case of the Week 89

A farmer presented with bloody diarrhea and abdominal cramps. Endoscopy revealed multiple ulcers, and the following biopsies were obtained:


Answer to Case 89

Answer: Balantidium coli
This parasite is unique in 2 aspects: it is the largest protozoan parasite and the only ciliated parasite to infect humans. Like amebiasis, the site of infection is typically the large intestine, where it can cause invasive disease, bloody diarrhea, fever, and abdominal pain. This case is a good example of invasive disease with mucosal ulceration (below).

Within the ulcer and intestinal mucosa, the large ciliated trophozoites can be seen.

The trophozoites are large (typically > 50 microns) and have a vacuolated cytoplasm and a classic dark "kidney-bean" shaped macro-nucleus. These features allow these trophozoites to be distinguished from those of Entamoeba histolytica, which are smaller (20 microns) and have a smaller, less distinctive nucleus. In the image below, only a partial cross-section of a trophozoite is seen, and so the full diameter is not appreciated.

A second trophozoite shown below is clearly larger and is surrounded by a "fuzzy" layer, which represents the cilia.

Here is an image from a trichrome-stained fecal smear that more clearly shows the "kidney-bean" shaped macronucleus and surrounding cilia.

Finally, Dr. W. reminds us that Balantidium coli trophozoites have a particular type of motility. She states "When I first encountered this parasite as a path resident and learned that its motility was "boring", I thought that meant that the motility was uninteresting". Indeed, the motility is "rotary" or "boring". Great story!

Thanks to everyone who wrote in with comments.

Monday, October 12, 2009

Case of the Week 88

Here is an unusual case for you.
The following objects were seen in a bowel resection:

I'll give you a hint - the organisms look the same as they normally would, but they are not where you would expect them to be.

Answer to Case 88

Answer: Enterobius vermicularis (pinworm)

Shown here are cross-sections of adult pinworms surrounded by neutrophilic inflammation in the wall of the colon. This is an unusual, but well-documented manifestation of pinworm infection. Typically, the adult worms do not penetrate the intestinal wall; the males pass out with the feces after maturation, while the females remain in the cecum and travel to the anus at night to lay eggs on the perianal skin. However, adult worms and eggs can occasionally get trapped in the intestinal mucosa and cause a suppurative or granulomatous reaction.

The diagnostic features include the adult nematode structures (cuticle, gut, musculature) and the classic lateral alae (spines) that are present at all levels of the body (see below). E. vermicularis is the only ADULT nematode the infects humans to have lateral alae. Of note, several larval nematodes also have lateral alae, but lack the size and well-defined structures of the adults.

Congratulations to all the viewers who got this one correct! Also, special thanks to S.C. who raised an important differential diagnosis for this case: the eggs of Schistosoma mansoni. These eggs can appear as oval, round or elongated in cross-section, and are often seen in the submucosa of the large intestine, surrounded by a granulomatous host reaction. Not that the thin eggs shell walls are typically wrinkled or collapsed (see image below). The eggs are also much smaller (>100 microns in length) than the 20 to 50 mm cross-section diameter pinworm adults. Finally, the spine on a S. mansoni egg is singular and lateral, rather than bilateral. The image below shows a classic example of the lateral spine. (CLICK ON IMAGE TO ENLARGE)

Monday, October 5, 2009

Case of the Week 87

The following is a giemsa-stained air-dried smear of CSF from a comatose 14 year body. He had been in his normal state of health up until the day previous, when he experienced rapidly declining mental status. The forms seen were the only ones present, and measure approximately 20 micrometers in diameter.

Sunday, October 4, 2009

Answer to Case 87

Answer: Naegleria fowlerii trophozoites

Congratulations to Alasdair, Salbrent, and Anonymous who all got this correct!

The key to the identification is recognizing the small nuclei with large karyosomes in the trophozoites. Note also the classic bubbly cytoplasm. These are characteristic features of the free-living amoebae. (CLICK ON IMAGE TO ENLARGE)

The differential diagnosis includes the other free-living amoebae Acanthamoeba spp. and Balamuthia mandrillaris. However, these 2 amoebae are typically seen within brain parenchyma (instead of the CSF) and also produce characteristic cysts (the dormant stage). The history also fits best for infection with N. fowleri, given the rapid state of deterioration in an otherwise healthy boy. Infection with the other two free-living amoebae is typically in immunocompromised adults, and has a subacute or chronic presentation. Of note, N. fowleri does not produce cysts in humans - only the trophozoite form is seen.