Answer: Loa loa
Florida Fan nicely describes the important features of this case:
"The microfilariae are sheathed which narrows the choice to only three: Wuchereria bancrofti, Loa loa and Brugia malayi/Brugia timori. The sheath did not stain pink, and the patient did not come from Southeast Asia or Polynesia, thus eliminating B. malayi" (although the pink sheath is not always seen with B. malayi). "The column of nuclei extends all the way to the end of the tail and this differentiates these microfilariae from those of W. bancrofti. We have one last choice: Loa loa."
Blaine will remind us that the sheath is not always seen, and therefore the size of the microfilariae should also be taken into consideration. The unsheathed microfilariae of Mansonella spp. are significantly narrower than the others, which is a helpful feature in eliminating this filarial worm from the differential.
Here are some of the key diagnostic findings seen in the images in this case: