Answer to Case 331

Answer:  Ascaris lumbricoides ova, fertile

What makes this case somewhat unique is that there is a larva inside of each egg. As noted by Anon and Dr. Leisure, you would not expect to see an embryonated egg in a fresh stool specimen. Instead, eggs are passed from the human host in the unembryonated form and then develop in the soil into the infectious embryonated form containing a larva.  However, if you have eggs in archived stool specimens - even in specimens preserved in formalin - the eggs can continue to develop as they would in the soil.  That is what happened in this case.  These eggs were seen in a stool specimen that had preserved in 10% formalin for several months, and larvae could be seen moving inside of the eggs!  This is why I always teach my students that formalin will not reliably kill all parasites and that it is important to treat all specimens as if they are potentially infectious.