Morphology: Size

Adult worms are not the focus of this continuing education module; however - for comparison - mature nematode filarial worms may reach 20-100mm in length to provide perspective.¹⁴ (Note: Male and female worms are of different sizes.)

Microfilariae observed microscopically may be grouped by general size, as noted in the table below.¹⁵ Size may be measured using a microscope containing a calibrated ocular micrometer.¹⁶

Large (>200 μm in length): Wuchereria, Loa, Brugia, and Onchocerca

Small (<200 μm in length): Mansonella

Width: Large species tend to be as wide as a red blood cell (RBC), about 6-8 μm in diameter; small species tend to be about half this width.

(Note: Size is helpful in identifying microfilariae in skin snips. *Recall that Mansonella streptocerca and Onchocerca volvulus may be seen in skin snips. Also, beware of artifacts such as fungi; artifacts are usually smaller than microfilariae and lack necessary nuclei.)

15. 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

16. CDC. "Calibration of the ocular micrometer." Division of Laboratory Services job aid. Public domain. No date. Accessed April 6, 2023. https://www.cdc.gov/labtraining/docs/job_aids/basic_microscopy/Calibration_Ocular_Micrometer_508.pdf