Filariasis Workup

Updated: Jun 22, 2018
  • Author: Brian F Lich, MD; Chief Editor: Pranatharthi Haran Chandrasekar, MBBS, MD  more...
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Workup

Approach Considerations

Traditionally, the diagnosis of filariasis requires demonstrating microfilariae in the peripheral blood or skin. However, circulating filarial antigens (CFA) are now routinely used to diagnose W bancrofti infection. The microfilariae of all species that cause lymphatic filariasis and the microfilariae of L loa, M ozzardi, and M perstans can be detected on a blood smear. [9] Broadly, the diagnostic approach varies by group of filariasis.

Lymphatic filariasis

Microfilariae on blood smear examination: Draw blood at night, when levels of parasitemia are generally highest. The three lymphatic filarial species can also be distinguished based on their morphologic characteristics on light microscopy.

Circulating filarial antigen (CFA) detection: These assays are regularly available for only W bancrofti detection in lymphatic filariasis.

Adult worms can be seen in the lymphatics.

Additional testing in lymphatic filariasis includes PCR and serology. PCR is not widely available and is mostly used in a research setting. Serology testing for filarial antibodies cannot distinguish between past and present infection and are not typically specific for filarial infections; however, specificity can be improved via assays based on certain recombinant antigens, such as Wb123 in W bancrofti. [41]

Cutaneous filariasis

Definitive diagnosis of O volvulus and M streptocerca infections occurs when microfilariae are detected in multiple skin snip specimens taken from different body sites. In addition, microfilariae of O volvulus may be detected in the cornea or anterior chamber of the eye, using slit-lamp examination. O volvulus may also be detected with antigen testing, although this is not regularly available. [42] Additional testing with serology and PCR have similar application in these cases, as noted above. Of note, the Mazzoti test (detailed below) should not be routinely used in the diagnosis of onchocerciasis owing to its risk of severe adverse reactions.

Loa loa infection can be definitively diagnosed by observing microfilariae on blood smear examination or by detecting migrating adult worms in the subcutaneous tissue or conjunctiva. For travelers to endemic areas, serology can be useful to detect exposure to Loa loa. Sensitivity and specificity of such testing varies depending on the assay used.

Body cavity filariasis

M ozzardi and M perstans infections can be definitively diagnosed by observing microfilariae on blood smear examination. Additional testing with serology and PCR have similar application in these cases, as described above.

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Detection of Microfilariae in Blood

The microfilariae of all species that cause lymphatic filariasis and the microfilariae of L loa, M ozzardi, and M perstans are detected in blood. (See the image below.)

Filariasis. Microfilariae of Mansonella perstans i Filariasis. Microfilariae of Mansonella perstans in peripheral blood.

Species that cause lymphatic filariasis have microfilarial levels that tend to peak at night, so it is recommended to collect samples between 10:00 pm and 2:00 am. For loiasis, microfilariae levels peak between 10 am and 2 pm. Capillary finger-prick or venous blood is used for thick blood films. Venous blood also can be concentrated or passed through a Nuclepore filter before being examined microscopically to improve sensitivity. [43] The organism species can be determined based on the microscopic appearance. W bancrofti and Brugia species have an acellular sheath. W bancrofti has no nuclei in its tail, whereas B malayi has terminal and subterminal nuclei. (See the image below.)

Filariasis. Appearance of microfilariae after conc Filariasis. Appearance of microfilariae after concentration of venous blood with a Nuclepore filter.
Filariasis. Microfilaria of Wuchereria bancrofti i Filariasis. Microfilaria of Wuchereria bancrofti in a peripheral blood smear.

Microfilariae may be absent in the following cases:

  • Patients with ADL or late chronic lymphatic disease
  • Typically, patients with loiasis, unless the infection has been present for many years

Detection of filarial antigen

The presence of circulating filarial antigens in the peripheral blood, with or without microfilariae, is diagnostic of filarial infection and is useful in monitoring response to therapy. Commercial kits are available for W bancrofti to test venous blood and can be quantitative (Og4C3 monoclonal antibody-based ELISA) or qualitative (immunochromatographic). These assays have all demonstrated superior sensitivity over microscopy. [44]

Detection of filarial antibodies

The use of recombinant antigens for the diagnosis of certain filarial species has improved sensitivity and specificity of these tests over the years. For W bancrofti, an IgG4 assay has been developed for the recombinant antigen Wb123 and demonstrates superior sensitivity and specificity in the diagnosis of bancroftian filariasis. [41] In addition, ICD card tests for IgG4 antibodies against recombinant antigen Ov-16 in onchocerciasis have improved the sensitivity and specificity of serologic testing in these cases. [45]

Serum immunoglobulin concentrations: Elevated serum IgE and IgG4 occur with active filarial disease. A multiplex bead assay to monitor serial levels of serum antibody during treatment has been proposed. [46]

Complete blood cell count

Eosinophilia is marked in all forms of patent filarial infection.

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Detection of Microfilariae in the Skin, Eye, and Urine

Skin

O volvulus and M streptocerca infections are diagnosed when microfilariae are detected in multiple skin snip specimens from different body sites.

Preferred skin snip sites vary regionally. In suspected cases of African onchocerciasis, the recommended sites for skin snips are the gluteal and thigh regions. For American onchocerciasis, the scapula and iliac crest areas are preferred.

Mazzotti test

Owing to the risk of severe adverse reactions, the Mazzotti test is not regularly used in the diagnosis of onchocerciasis, especially in individuals with a high disease burden. In certain cases, the test may allow for a presumptive diagnosis of cutaneous filariasis when skin snips are negative for microfilariae. To perform the test, a single dose (50-100 mg) of DEC is given, and, if the patient is infected, he or she will experience an intense pruritic rash with fever and edema. Steroids may be necessary to control this inflammatory reaction. Alternatively, a patch test with 10% DEC solution can be placed on the skin, resulting in a more localized reaction.

Eye

Microfilariae of O volvulus may be detected in the cornea or anterior chamber of the eye using slit-lamp examination.

Urine examination and microscopy

Microfilariae may also be observed in chylous urine and hydrocele fluid. If lymphatic filariasis is suspected, urine should be examined macroscopically for chyluria and then concentrated to examine for microfilariae.

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Imaging Studies

The following imaging studies can be used in the evaluation of filariasis:

  • Chest radiography - Diffuse pulmonary infiltrates are visible in patients with tropical pulmonary eosinophilia (TPE)
  • Ultrasonography - Can be used to demonstrate and monitor lymphatic obstruction of the inguinal and scrotal lymphatics
  • Lymphoscintigraphy [10]

Ultrasonography has also been used to demonstrate the presence of viable worms, which are seen to be in continuous motion (ie, "filarial dance" sign). This imaging characteristic has been used to monitor the effectiveness of treatment. [47] In addition, deep onchocercomas and vitreous changes in the eye can sometimes be detected with ultrasonography.

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Biopsy

Biopsy specimens should be obtained only in patients with cutaneous filariasis, as excising nodes may further impede lymphatic drainage in patients with lymphatic filariasis. Adult worms of O volvulus and L loa are found in the nodules and fibrotic tissue of the skin. L loa worms occasionally can be dissected from the conjunctiva of the eye or bridge of the nose as they migrate through subcutaneous tissue.

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Histologic Findings

Lymphatic filariasis

Affected lymph nodes demonstrate fibrosis and lymphatic obstruction with the creation of collateral channels. The skin of individuals with elephantiasis is characterized by hyperkeratosis, acanthosis, lymph and fatty tissue, loss of elastin fibers, and fibrosis. (See the image below.)

Filariasis. Adult worms of Wuchereria bancrofti in Filariasis. Adult worms of Wuchereria bancrofti in cross section isolated from a testicular lump.

Onchocerciasis

Two areas are evident in onchocercomas: (1) a central stromal and granulomatous, inflammatory region where the adult worms are found and (2) a peripheral, fibrous section. Microfilariae in the skin incite a low-grade inflammatory reaction with loss of elasticity and fibrotic scarring. (See the image below.)

Filariasis. Adult Onchocerca volvulus contained wi Filariasis. Adult Onchocerca volvulus contained within onchocercomas of the skin.
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