Answer: Wucheria bancrofti
Ok, I admit this was a hard case. There are several morphologic features present in these images to point you to an identification; unfortunately, some are conflicting. However, this is a real case and one that initially stumped us in the laboratory as well. Below are the steps I use for speciating microfilariae based on morphologic features:
First, I look for is the presence or absence of a sheath. Only 3 microfilariae have a sheath, so this immediately helps to focus your answer. For those of you that like memory aids, you can remember the 3 sheathed microfilariae by the following phrase: Wears Long Britches. The first letter of each word (W.L.B.) is the first letter of each organism: Wucheria bancrofti, Loa loa, and Brugia spp. Think of the "long britches" as the sheath over the worm's "leg".
If you identify the sheath, then look to see if the sheath stains with giemsa. The sheath of W. bancrofti and L. loa doesn't usually stain, and thus appears as a negative image, while the sheath of Brugia is usually bright pink. The sheaths of the first 2 organisms do stain pink with hematoxylin and eosin, so it's important to know what stain you're looking at (shame on me for neglecting to include this information).
This is where most will get out their atlases and look up the other specifics such as length of the worm, position of nuclei relative to the tail, characteristic of the nuclei and the tail, and the size of the cephalic space. Finally, travel history is essential, since the filarial worms have different geographic distributions.
So for this case, the overall length (approx. 300 microns), presence of a sheath, short cephalic space, and lack of nuclei in the narrow pointed tail indicated that this was W. bancrofti. Furthermore, the patient was from Africa, and had no history of travel to Asia, which would be expected to make a diagnosis of Brugia spp. The only feature that didn't quite fit was the fact that the sheath was stained pink with giemsa. Well, I guess you can't have it all....
Thank you to everyone who wrote in on this case. It just goes to show that real-life is rarely as easy as the textbooks.