Answer to Parasite Case of the Week 580: Loa loa microfilariae. As noted by several readers, the nuclei go to the tip of the tail, the microfilariae are relatively large, and there is faint evidence of a sheath, all of which are characteristic features for this species. As I teach my students, the nuclei "flow-a flow-a" (to the tip) in Loa loa. Another memory trick from William Sears is that the nuclei go "lower and lower" in Loa loa. Take your pick for your favorite! Remember that the sheath will not always be seen. Size is a more definitive feature in differentiating Mansonella spp. from the sheathed microfilariae.
William also mentioned that one should always check for the presence of Onchocerca volvulus co-infection, and get a microfilariae count before initiating therapy with the drug of choice for loiasis, diethylcarbamazine (DEC). In this case, the microfilaremia was calculated at 1,960 microfilariae/mL blood, which is well below the threshold of 8,000/mL at which DEC is contraindicated (thank you for calculating this Heather, and for the lovely photos in this case). DEC is also contraindicated in patients with concomitant onchocerciasis since the rapid death of O. volvulus microfilariae in the eye can lead to blindness.
What I found remarkable about this case was how long the microfilariae remained alive. They were still moving (albeit, somewhat choppily) at 10 days after blood draw, and the following image was taken of a smear make with 14 day-old blood. Not bad! According to filariasis expert, Shelly Michalski, the Project Liaison for the NIH Filariasis Research Reagent Resource Center, microfilariae are remarkably hardy, and can survive freezing -80°C. Wow!
And finally, to celebrate Valentine's day, here is this beautiful Brugia from Blaine Mathison: