Filariasis laboratory findings

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]Associate Editor(s)-in-Chief: Ahmed Elsaiey, MBBCH [2]

Overview

The diagnosis is made by identifying microfilariae on a Giemsa stained thick blood film. Blood must be drawn at night, since the microfilaria circulate at night, when their vector, the mosquito, is most likely to bite. There are also PCR assays available for making the diagnosis.

Laboratory Findings

Identification of microfilariae by microscopic examination is the most accuarte diagnostic procedure. Examination of blood samples allows the identification of microfilariae of Wuchereria bancrofti, Brugia malayi, Brugia timori, Loa loa, Mansonella perstans, and M. ozzardi. It is important to time the blood collection with the known periodicity of the microfilariae. The blood sample can be a thick smear, stained with Giemsa or hematoxylin and eosin. For increased sensitivity, concentration techniques can be used. Concentration techniques include centrifugation of the blood sample lyzed in 2% formalin (Knott's technique), or filtration through a Nucleopore® membrane.

Antigen detection

Detection of the filarial antigens has become one of the sensitive and specific diagnostic procedures to diagnose filariasis.^[1]
The antigen detecting tests include the following:
- ELISA
- Immunochromatographic technique (ICT) and it includes:
  - Alere Filariasis Test Strip
  - Card-based assay for qualitative results

Special Procedures for Detecting Microfilariae

Blood microfilariae:

Capillary (fingerstick) blood: Since microfilariae concentrate in the peripheral capillaries, thick and thin smears prepared from fingerstick blood are recommended.

Anticoagulated (EDTA) venous blood (1 ml) should be concentrated by one of the following methods:
- Centrifugation (Knott’s technique)
  - Prepare 2% formaldehyde (2 ml of 37% formaldehyde + 98 ml H2O).
  - Mix 9 ml of this 2% formaldehyde with 1 ml of patient’s venous blood. Centrifuge at 500 × g for 10 minutes; discard supernatant. Sediment is composed of WBCs and microfilariae (if present).
  - Examine as temporary wet mounts.
  - Prepare thick and thin smears; allow to dry; dip in absolute methanol before Giemsa staining to enhance staining of microfilariae.
- Filtration
  - Place Millipore® or Nucleopore® membrane filter (5 µm pore) in filter holder with syringe attachment.
  - Mix 1 ml of venous blood (in EDTA) with 10 ml of 10% Teepol® 610 (Shell Co.); allow to stand for several minutes to allow lysis; transfer to a 10 ml Luer-Loc® syringe; attach the filter apparatus.
  - Force the solution through the 5 µm pore filter, followed by several syringes of water to wash out the remaining blood, then 1 or 2 syringes full of air to clear excess fluid.
  - Prepare a temporary wet mount by removing the filter and placing it on a glass slide, adding a drop of stain or dye and a coverslip.
  - For permanent preparations, pass 2 to 3 ml of methanol through the filter while it is still in the holder; remove filter and dry it on a glass slide; then stain it with Giemsa stain, horizontally (so that the filter does not wash off the slide); coverslip filter before examining.

Gallery

Micrograph of the posterior end of a Brugia malayi microfilaria in a thick blood smear using Giemsa stain. From Public Health Image Library (PHIL). ^[2]
Micrograph of the posterior end of a Wuchereria bancrofti microfilaria in blood smear using Giemsa stain. From Public Health Image Library (PHIL). ^[2]
Micrograph of a Wuchereria bancrofti microfilaria in a thick blood smear using Giemsa stain technique. From Public Health Image Library (PHIL). ^[2]
Micrograph of the internal structure of a Wuchereria bancrofti microfilaria using Giemsa stain. From Public Health Image Library (PHIL). ^[2]
Photomicrograph reveals morphologic details at the anterior end of a Wuchereria bancrofti microfilarial parasite in a blood smear using Giemsa stain (1000X mag). From Public Health Image Library (PHIL). ^[2]