Examination with a dissecting scope (40x original magnification) revealed the following. Some of them were moving!

1. Diphyllobothrium latum, the broad fish tapeworm
2. Dracunculus medinensis, the Guinea worm
3. Spirometra spp., the cause of sparganosis in humans
4. the agents of gnathostomiasis: Gnathostoma spinigerum and Gnathostoma hispidum

Answer, Part II: What is the most common source of infection worldwide? Domestic pigs. What about in the United States? Wild game

According to a publication by the Centers for Disease Control and Prevention (Roy et al.”Trichinellosis Surveillance — United States, 1997–2001.” Centers for Disease Control and Prevention), trichinellosis has been steadily decreasing in the United States due to tightened regulations on pig farming and pork processing. They state:

Although trichinellosis was associated historically with eating Trichinella-infected pork from domesticated sources, wild game meat was the most common source of infection during 1997–2001. During this 5-year period, 72 cases were reported to CDC. Of these, 31 (43%) cases were associated with eating wild game: 29 with bear meat, one with cougar meat, and one with wild boar meat. In comparison, only 12 (17%) cases were associated with eating commercial pork products, including four cases traced to a foreign source. Nine (13%) cases were associated with eating noncommercial pork from home-raised or direct-from-farm swine where U.S. commercial pork production industry standards and regulations do not apply.

The following is an excellent source of information on trichinellosis with beautiful photos: http://www.trichinella.org/index_synopsis.htm

To make the diagnosis of trichinellosis on tissue section, one needs to recognize the classic appearance of the coiled larvae within tissue (typically skeletal muscle). Here is an image from this case that nicely demonstrates the larva, nurse cell (derived by the host) and stichosome (column of large rectangular cells):

Note that in this case, the larva is located in the skeletal muscle of the tongue, right below the tongue’s epithelium.

It is easiest to appreciate the coiled nature of the larvae by pressing non-fixed infected muscle between 2 slides and examining the tissue under the microscope. Here is an image from a previous case of the week that demonstrates a ‘squash’ prep:

Finally, for those of you non-pathologists who are having a hard time envisioning how a 3-dimensional coiled worm became a series of circles and ovals in tissue section, I’ve created the following diagram. The top image shows a coiled worm that is being cut along its longitudinal axis while making a slide. The bottom image shows how the worm would appear if you are only looking at the part that was cut and put on a slide.

100x original magnification

200x original magnification

200x original magnification

Diagnosis?

What is the most common source of infection worldwide? What about in the United States?

Splenic aspiration is an excellent way of diagnosing visceral leishmaniasis. However, the procedure carries the risk of splenic rupture and should only be undertaken by a skilled practitioner. An alternative procedure for diagnosis of visceral leishmaniasis is bone marrow biopsy.

Visceral leishmaniasis is prevalent in some parts of India and Africa, the Mediterranean, and South America.

Congratulations to all of the viewers who wrote in with the answer – you recognized that the morphologic features and size were consistent with these two closely related protozoa. E. histolytica is a recognized pathogen, although it only causes disease in approximately 10% of the people it infects. E. dispar, on the other hand, is generally considered a non-pathogen. Unfortunately, the two are morphologically indistinguishable, and require isoenzyme, antigen, or molecular methods to distinguish them. The only exception to this is when E. histolytica trophozoites are seen invading the bowel wall on histologic section, or contain ingested RBCs on ova and parasite exam. No ingested RBCs are seen in this case, so it is not possible to differentiate between the two organisms.

The answer to the second part of the question is that an asymptomatic host could still be infected with either E. histolytica or E. dispar (remember that most E. histolytica infections are asymptomatic).

Finally, just to make life difficult for clinical parasitologists, there is now a THIRD species of Entamoeba which is morphologically indistinguishable from E. histolytica and E. dispar. Entamoeba moshkovskii is now recognized to be a wide spread environmental organism and occasional human parasite. Therefore, I suppose that the most correct answer to this case is “Entamoeba histolytica/dispar/moshkovskii!!

The following object was seen on stool ova and parasite examination and measures approximately 15 micrometers (Modified Trichrome stain, 1000x).

The following was also seen, measuring approximately 20 micrometers in greatest dimension:

Congratulations to everyone who got this right! Also in the differential would be proglottids of Taenia spp. and artifacts such as onion skin which may be passed relatively intact and mimic tapeworm segments. However, the diagnosis is easily made by examining the central proglottid structures which form a rosette-structure, or so-called “Chinese character” as shown here:

When stained, this structure is more apparent (as seen from a previous case of the week):