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Answer to Case of the Week 59

Answer, Part I: Trichinella spp. Although Trichinella spiralis is the most common species to infect humans in the United States, it is generally not possible to speciate based on morphologic features. The exception is Trichinella pseudospiralis whose larvae are not encapsulated compared to T. spiralis, T. nativa, T. nelsoni, T. britovi, and T. murrelli which do have encapsulated larvae. These species are the predominant to infect humans.

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.

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The (un)reliability of medical research

There is an interesting article in this month’s Atlantic Monthly regarding the “flexible” nature of medical statistics and the way that researchers (often unknowingly) massage statistical analyses to support favored hypotheses. The article is essentially a layman’s overview of the work of John P. A. Ioannidis, whose paper “Why Most Published Research Findings Are False” is the most downloaded article in the history of PLoS Medicine. From the article:

“The studies were biased,” he says. “Sometimes they were overtly biased. Sometimes it was difficult to see the bias, but it was there.” Researchers headed into their studies wanting certain results—and, lo and behold, they were getting them. We think of the scientific process as being objective, rigorous, and even ruthless in separating out what is true from what we merely wish to be true, but in fact it’s easy to manipulate results, even unintentionally or unconsciously. “At every step in the process, there is room to distort results, a way to make a stronger claim or to select what is going to be concluded,” says Ioannidis. “There is an intellectual conflict of interest that pressures researchers to find whatever it is that is most likely to get them funded.”

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Case of the Week 59

The following were an incidental finding at autopsy. Shown are hematoxylin and eosin stained sections of human tongue:

100x original magnification

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Answer to Case of the Week 58

Answer: Leishmania spp. amastigotes. Note the small (2-5 micron) oval-shaped objects within and outside of macrophages, with a nucleus and classic rod-shaped kinetoplast. The large purple objects are macrophage nuclei.

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Case of the Week 58

The following were seen on splenic aspirate. Diagnosis?

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Answer to Case of the Week 57

Answer: Entamoeba histolytica/dispar

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!!