Answer to the Parasite Case of the Week 742: Wuchereria bancrofti microfilariae.
I really enjoyed reading the comments on this case. FloridaFan provided this excellent description of his approach to microfilariae identification: "First the width of the worm is about the same as that of the surrounding neutrophils. Second, its length is greater than 200 micrometers. Third, it has a sheath. Now we know we are dealing with Loa loa, Wuchereria, or the Brugia ones, not with any little pesky Brugia. The next consideration is that the column of nuclei is continuous, the terminal nuclei are not separate from the immediate anterior nuclei. This eliminates the possibility of the Brugia malayi and timori, over that these are “far fetched” geographically. We are left with two candidates Loa loa and Wuchereria bancrofti. Though the tail is not so obvious, the Carrazi stain did show that the nuclei column terminates well short from the end of the tail. This rules out Loa loa. The only culprit left is Wuchereria bancrofti.
An anonymous commenter also noted that "Its nuclear column is relatively loose, and individual nuclei can be visualized throughout the column. The sheath is does not stain pink with Giemsa stain (as in Brugia malayi)."
These features can be nicely visualized in the photo of the Carazzi-stained Knott's concentration. Note that the Carazzi stain is a hematoxylin-based stain (rather than conventional Giemsa) that helps to demonstrate the sheath. It is a very useful stain to have!
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