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Answer to Case of the Week 24

Answer: Naegleria fowlerii trophozoites

Congratulations to all who 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.

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.

Thanks for sharing. Where was this case worked up and what is the pathogenesis? Seems like a terrible thing to get!

Yes, the case was worked up, but unfortunately it was too late to save the patient. Most of these cases are universally fatal. The amoebae enter the nose while diving/swimming and travel up the olfactory nerves, through the cribiform plate, and into the brain, causing rapidly progressive amoebic meningocephalitis. I’m guessing that death is due to CNS destruction and edema.

thanks for writing!

I should also mention that certain water locations are of particular concern – especially warm, stagnant, fresh water ponds and lakes. You can find warning notices posted on some lakes in Florida that caution you again alligators, snakes, and amoebae!