Lymphatic filariasis can be diagnosed clinically and through laboratory techniques.
Demonstration of microfilarae in the peripheral blood
Collection of blood specimen
The microfilariae that cause lymphatic filariasis circulates in the blood at night (nocturnal periodicity). So the optimal time for drawing blood to detect nocturnal periodic W. bancrofti infections is between 10 pm to 4 am. Blood used to detect subperiodic W. bancrofti may be drawn anytime.
Sites of blood collection: Finger prick, earlobe or venous blood (using EDTA anticoagulant)
- Thick blood smear: The standard method for diagnosing active infection is the identification of microfilariae in a blood smear (multiple thin and thick blood smear) by microscopic examination. Blood collection should be done at night to coincide with the appearance of the microfilariae, and a thick smear should be made and stained with Giemsa or hematoxylin and eosin. Giemsa stain does not stain the microfilarial sheath adequately. Sheathed microfilariae often lose their sheath when drying on thick films. At least 2 thick smears and 2 thin smears must be prepared.
For increased sensitivity, concentration techniques can be used.
- Thin blood smear: Examination of a thin blood film for microfilariae should include a low-power review of the entire film, not just the feathered edge.
- Membrane filtration method: It is the concentration technique used to trap microfilariae on the polycarbonate filter after red blood cells are lysed. 1-2 ml intravenous blood filtered through 3µm pore size membrane filter and the filter paper may be examined directly on a microscope slide (filters are transparent when wet).
- DEC (diethylcarbamazine) provocative test (2mg/Kg): It is somewhat impractical to obtain sample blood from a patient at late night. DEC provocation test is done to bring microfilariae in the periphery during daytime. After consuming DEC, microfilariae enter into the peripheral blood in day time within 30 – 45 minutes.
- Quantitative Blood Count (QBC):
Filarial infection can be diagnosed rapidly by the detection of microfilaria using a microhematocrit tube coated with acridine orange. QBC will identify the microfilariae and will help in studying morphology. Though quick it is not sensitive than blood smear examination. Major disadvantage of QBC method is the necessity of the fluorescence microscope.
Immuno Chromatographic Test (ICT): Antigen detection assay can be done by card test and through ELISA. Circulating filarial antigen detection is regarded as “Gold Standard” for diagnosing Wuchereria bancrofti infection. Specificity is near complete, sensitivity is greater than all other parasite detection assays, will detect the antigen in amicrofilaraemic as well as with clinical manifestations like lymphoedema, elephantiasis.
Serological tests: Wide varieties of serological tests are available for diagnostic and epidemiological purposes. Serologic techniques provide an alternative to microscopic detection of microfilariae for the diagnosis of lymphatic filariasis.
Patients with active filarial infection typically have elevated levels of antifilarial IgG4 in the blood and these can be detected using routine assays. The detection of IgG4 antibodies also reduces the cross reactivity of non-filarial antibodies.
IgG2 level appears to be increased in patients with elephantiasis.
Limitation: Except for patients not native to the area of endemicity, immunodiagnostic tests are of limited value.
PCR Amplification methods:
Polymerase chain reactions (PCR) assays are useful diagnostic tools for lymphatic filariasis as they can discriminate between past and present infection and can be used to monitor therapy and to detect and differentiate multiple filarial infections. Currently these procedures are often limited to research facilities.
Ultrasonography using 7.5 MHz or 10 MHz probes can locate and visualize the movements of living adult worms of W.b. in the scrotal lymphatics of asymptomatic males with microfilaraemia. The constant thrashing movements described as “filaria dance sign” can be visualized.
X-ray are helpful in the diagnosis of tropical pulmonary eosinophilia. X-ray picture will show interstitial thickening, diffused nodular mottling.
Hematology: Increase in eosinophil count. Patients with tropical pulmonary eosinophilia have a marked IgE response in addition to elevated eosinophilia.