Until recently, diagnosis of filarial infection depended on the direct demonstration of the parasite (almost always microfilariae) in blood using relatively cumbersome techniques and having to take into account the periodicity (nocturnal or diurnal) of microfilariae. Alternative methods based on detection of antibodies by immunodiagnostic tests did not prove satisfactory since they both failed to distinguish between active and past infections and had problems with specificity owing to their cross-reactivity with common gastrointestinal parasites and other organisms.
Antigen detection: Circulating filarial antigen (CFA) detection should now be regarded as the 'gold standard' for diagnosing Wuchereria bancrofti infections. The specificity of these assays is excellent, and the sensitivity is greater than that achievable by the earlier parasite-detection assays. Essentially all individuals with microfilaraemia also have detectable circulating antigen, as well as do a proportion of those amicrofilaraemic individuals with clinical manifestations of filariasis (e.g., lymphoedema or elephantiasis) but no circulating microfilariae. In addition, some individuals who appear normal also have detectable circulating antigen that disappears after effective treatment with DEC for these cryptic infections. Two commercial configurations of this assay are available, one based on ELISA methodology that yields semi-quantitative results, and the other based on a simple card (immunochromatographic) test, yielding only qualitative (positive/negative) answers. Thus, this new diagnostic approach is equally applicable to clinical or field evaluation of bancroftian filariasis infections. Unfortunately, no such test is currently available for brugian filariasis.
Many lymphatic filariasis patients are amicrofilaraemic, and because no serologic test other than that detecting CFA is specific, in the absence of antigen testing the diagnoses of these infections must be made 'clinically' (i.e., on circumstantial evidence) with support from antibody or other laboratory assays. Most secure of these clinical diagnoses is that of the tropical eosinophilia syndrome; in addition to its distinctive clinical presentation such patients have extraordinarily high levels of total serum IgE (almost always in excess of 10 000 ng/ml), and their levels of specific antifilarial antibodies (both IgG and IgE) are extremely high, the absolute levels depending on the specific tests used. For other amicrofilaraemic syndromes serologic findings based on detecting IgG4 antibodies have proven helpful, since this subclass has greater diagnostic specificity and is stimulated by the presence of active infection. Such antibody analyses are also especially helpful in diagnosing the expatriate syndrome' where 'background (i.e., pre-exposure) levels' of IgG and especially IgG4 antibodies to filarial antigens will be very low, so that elevated levels have significant diagnostic implications in association with the clinical presentations.
Eosinophilia is a frequent concomitant of all filarial syndromes, but only when the levels are extremely high (as in tropical eosinophilia or the expatriate syndrome ) are they diagnostically helpful.
Conventional X-rays are rarely helpful in diagnosing lymphatic filarial infection, except in the case of tropical eosinophilia where the picture can be variable but characteristically includes interstitial thickening and diffuse nodular mottling in the lung fields. Ultrasound examination of the lymphatics (especially scrotal lymphatics in men, and the breast and retro-peritoneal lymphatics in women) can reveal rapidly moving ("dancing") adult worms (see 'Animated documentation of the filaria dance sign (FDS) in bancroftian filariasis' in Filaria Journal), and lymphoscintigraphy, though not diagnostic of filarial infection, can identify lymphatic functional and gross anatomical abnormalities.
In situations of lymphadenopathy with or without accompanying inflammation of the nodes or lymphatic vessels, biopsy can often detect adult worms, but this approach is rarely used as a diagnostic procedure.
Prior to the development of the CFA assay, detection of microfilariae in blood was the standard approach to diagnosing lymphatic filarial infection (or loiasis), and it is the one still required today for both brugian filariasis and those situations where the antigen detection test is not available for bancroftian filariasis. For such assessments one must take into account the parasites' possible nocturnal periodicity in selecting the optimal blood drawing time (10 p.m.-2 a.m. for most brugian filariasis and bancroftian infections). The simplest technique for examining blood or other fluids (including hydrocele fluid, articular effusions and urine) is to spread 20 microliters evenly over a clean slide that is dried and then stained with Giemsa or a similar stain. A wet smear may also be made by diluting 20-40 microliters of anti-coagulated blood with water or 2% saponin, which will lyse the red blood cells but allow the microfilariae to remain motile and thus more readily identifiable. The larger the blood volumes examined, the greater will be the likelihood of detecting low parasitaemias. Knott's concentration technique has been used to examine 1 ml volumes of anti-coagulated blood by mixing the blood with 10 ml of 2% formalin, centrifuging the preparation and examining the sediment either unstained or fixed and stained; the microfilariae are non-motal and generally straight, and they can be easily missed if the viscous sediment is not searched diligently. More recently, membrane filtration has been advanced as the most sensitive technique for detecting and quantitating microfilariae in blood, urine or other body fluids. Polycarbonate (Nuclepore®) filters with a 3 µm pore size have proved most satisfactory. A known volume of anti-coagulated blood or other fluid is passed through a Swinnex holder containing the filter, followed by a large volume (about 35 ml) of pre-filtered water that lyses the red blood cells. A volume of air then follows the water, and the filter is removed, placed on a slide and stained. Morphology of the parasite is much more difficult to assess than when specimens are prepared initially on slides, but detection and quantitation are very straightforward.