Microfilariae, i.e. the larval stage of Filaria spp., may be found in the peripheral blood after white blood cell concentration by centrifugation and, occasionally, also in bone marrow preparations and other fine-needle aspirates. Morphological findings differ according to the species and form the basis for diagnosis. In buffy coat smears, Loa loa microfilariae appear as primitive serpentine-shaped organisms containing many nuclei with a head space, a sheath unstained with Giemsa and the tapering of the tail (top image). In the lower left image, a thick film shows the size of a Loa loa microfilaria in relation to white blood cells and the coiled tail; note also the row of nuclei through the whole body of the parasite right to the end of the tail (bottom right image).1 Eosinohilia is often an associated feature.
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