Answer to Case 809

Answer to the Parasite Case of the Week 809: Fly larva, most likely a member of the family Calliphoridae. As noted by Idzi Potters, "This is an L2 larva, as there are only two slits. This makes ID very challenging. The cute hand-with-too-many-fingers-like anterior spiracles make me believe that this is a larva from the family Calliphoridae (Calliphora sp. or Lucilia sp.), causing facultative myiasis." Here is the image of the anterior spiracles (hand with too many fingers) that Idzi is referring to:

Unfortunately, this is as far as we can comfortably go with our identification in this case. It is especially important to note is that most taxonomic keys such as the CDC Pictorial Keys or the manuscript that Blaine Mathison and I wrote for Clinical Microbiology Reviews are for third instar larvae and cannot be reliably used for most of the second instar larvae. If you had used these keys in this case, you may have ended up with an aberrant identification such as Cochliomyia. This genus includes C. hominivorax, the New World screwworm, and finding it could have important epidemiologic and clinical implications since it causes destructive infestations and has been eliminated from the United States.

To better understand the significance of this case, we called the outside provider and discovered that the larva was found in the toilet of an asymptomatic woman from the midwestern United States with no travel history. It was therefore considered a case of environmental contamination and not true myiasis.