Always take into consideration the size, nuclear column (pattern), headspace, tail morphology, and sheath when attempting to identify microfilariae from the stained smear.
Note: Mixed infections are possible. It is helpful to have detailed information on the patient's travel history, as several of the parasite genera/species share the same geographical distribution.
Summary of the General Morphologic Characteristics of the Microfilariae23, 24
| Filarial Agent | Headspace | Tail | Sheath on Giemsa |
| Wuchereria bancrofti | Short | No nuclei | Present: Colorless |
| Brugia malayi | Long | Terminal and subterminal nuclei, large gaps separate these nuclei | Present: Bright pink |
| Brugia timori | Long | Terminal and subterminal nuclei, large gaps separate these nuclei | Present: Colorless |
| Loa loa | Short | Nuclei randomly distributed to tip of tail | Present: Colorless |
| Onchocerca volvulus* | Long | Tapered tail, flexed, no nuclei | Never present |
| Mansonella ozzardi | Slightly longer than wide | Tapered and pointed tail, no nuclei | Never present |
| Mansonella streptocerca* | Usually short, but difficult to discern because of slender body | Hooked tail, nuclei all the way to the tip | Never present |
| Mansonella perstans | Short | Blunt tail, nuclei densely packed all the way to the tip | Never present |
(*Seen on skin snips, not in peripheral blood)
23. Mathison BA, Couturier MR, Pritt BS. Diagnostic identification and differentiation of microfilariae. J Clin Microbiol. 2019;57(10):e00706-19. Published 2019 Sep 24. doi:10.1128/JCM.00706-19
