Answer to Case 739

 Answer toParasite Case of the Week 739:Loa loamicrofilariaeThanks to everyone who wrote in with comments. We received a lot of different responses including some of the sheathed and unsheathed microfilariae. Therefore, this is a great time to review my approach to identifying microfilariae in blood specimens. You can also readthis articleI wrote with Blaine Mathison and Marc Couturier that provides a diagnostic algorithm for microfilariae in blood. In this algorithm, we recommend first measuring the length of the microfilariae. If they are small (<200 micrometers long), then it is likely to be one of theMansonellaspecies.Mansonellaare not sheathed, small, and quite narrow. Their width is smaller than the diameter of a neutrophil or eosinophil. If, on the other hand, the microfilariae are relatively large (>200 micrometers), then you are dealing withWuchereria bancrofti, Loa loa,or one of the Brugiaspecies. These are sheathed microfilariae, but the sheath may not always be readily visible on Giemsa stain. Therefore, the size is a more reliable diagnostic feature. To better visualize the sheath, a hematoxylin stain can be performed such as the Carazzi or Delafield hematoxylin stain. (Note: the hematoxylin and eosin stain used in histopathology preparations can also be used, so if you don ' t have a hematoxylin stain for microfilariae, you can ask for help from your friends in anatomic pathology!)  These larger microfilariae can then be differentiated by ch...
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