Answer to Case 519

Answer: Tachyzoites of Toxoplasma gondii. 

My accompanying request was to list other parasites that can be acquired from eating (undercooked) turkey. The two excellent responses I received from Bernardino Rocha were Gnathostoma spp. (likely, but we're not sure), and Trichinella pseudospiralis. Did we miss anything? Please write in if you can think of others.

Now, a few fun facts for the curious:
The tachyzoites are the rapidly-dividing form of T. gondii [tachy is from the ancient Greek ταχύς (takhús, “swift”)], and are the predominant form seen in acute and re-activated infections. In this case, the presence of numerous extracellular forms is evidence of an active infection. In contrast, tissue cysts containing bradyzoites, the slowly-replicating forms [brady is from the ancient Greek βραδύς (bradús, “slow”)] are seen during latent infection. The word zoite derives from the ancient Greek ζῷον (zôion, "animal"). I always make sure to point out word origins to my students when they are useful for remembering parasite names. For example, most medical students know the difference between tachycardia and bradycardia, and know to think of a 'zoo' as a place where animals are found. Helping them apply the knowledge they already have helps them learn these new, and often very foreign-sounding words.

T. gondii can infect any nucleated cell and tachyzoites are commonly seen within host cells during active infection. In this case, we can see both free and intracellular tachyzoites:

For those of you who like etymology, you may also be interested to know that the word Toxoplasma comes from the ancient Greek words τόξον (tóxon, "bow" or "arc") and  πλάσμα (plásma, “something molded”); thus the name nicely describes arc-shaped form of this parasite. I love when parasite names actually make sense!  You can especially appreciate this shape in air-dried touch preparations. Disappointingly, the arc shape is only rarely seen in sections of formalin-fixed, paraffin-embedded tissue, since the parasites tend to shrink and round up during fixation, taking on a more ovoid appearance. This makes it much trickier to differentiate them from small yeasts such as Histoplasma capsulatum and the amastigotes of Trypanosoma cruzi and Leishmania spp.

Transmission electron microscopy allows us to take a closer look at the T. gondii tachyzoites. As mentioned by Bernardino, the apical complex containing conoids and rhoptries is nicely seen:

HERE is an open access book chapter by the Global Water Pathogen Project that contains a reprint of the classic Dubey et al. 1998 figure showing the major structures of T. gondii tachyzoites and bradyzoites.