Case #502 – October, 2019

Iron-hematoxylin stained thick blood films from an 18-year-old male resident of Equatorial Guinea were examined as a part of a field study. Two objects were detected in one blood film. The object at the top of the field shown in Figure A (magnified in Figure B) measured approximately 246 µm; the bottom object in Figure A (magnified in Figure C) measured approximately 198 µm. What is your diagnosis? Based on what features?

Figure A

Figure B

Figure C

This is a case of dual infection by the filarial nematodes Loa loa and Mansonella perstans. These two species have overlapping distributions in some regions of Western and Central Africa. Adults of L. loa and M. perstans are typically found in subcutaneous tissues and the peritoneal cavity, respectively, and both species produce microfilariae which circulate in peripheral blood. Microfilariae are morphologically distinct and may be readily differentiated in stained blood films.

Key diagnostic features shown include:

Size of microfilariae in stained blood films (L. loa: 230—250 µm; M. perstans: 190—200 µm)
Presence or absence of a sheath (L. loa: present; M. perstans: absent)
Tail morphology (L. loa: tapering, nuclei extending to tip; M. perstans: bluntly rounded, nuclei also extending to tip)