A definitive diagnosis of filariasis is usually made by the demonstration of microfilariae in the peripheral blood. The exception to this is the diagnosis of Onchocerca volvulus which is diagnosed by demonstration of microfilariae in skin snips.
Blood filariasis:
A wet preparation is examined and samples are filtered using a Nuclepore membrane. If W.bancrofti is suspected blood collection time should be between 22.00 and 02.00 hours, for Loa loa diurnal blood sample (noon) is preferred.
Sample requirements: EDTA blood. The sample size is not critical but ideally between 5ml and 10ml should be sent, the larger the sample filtered the greater the chance of demonstrating the microfilariae.
Key factors affecting tests: sheathed microfilariae may ex-sheath if blood sample is not examined within 2 to 3 days of collection, all microfilariae may last up to 72 hours in EDTA blood before disintegration.
Onchocerca volvulus:
Please contact the laboratory before sending skin snips for microfilariae diagnosis, contact details on page one of this handbook.