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. [10] 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. [42]
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. [43] 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 for 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.
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.
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. [44] 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.
Filariasis. Appearance of microfilariae after concentration of venous blood with a Nuclepore filter.
Microfilariae may be absent in the following cases:
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Patients with ADL or late chronic lymphatic disease
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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. [45]
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. [42] 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. [46]
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. [47]
Complete blood cell count
Eosinophilia is marked in all forms of patent filarial infection.
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 for 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.
Imaging Studies
The following imaging studies can be used in the evaluation of filariasis:
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Chest radiography - Diffuse pulmonary infiltrates are visible in patients with tropical pulmonary eosinophilia (TPE)
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Ultrasonography - Can be used to demonstrate and monitor lymphatic obstruction of the inguinal and scrotal lymphatics
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Lymphoscintigraphy [11]
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. [48] In addition, deep onchocercomas and vitreous changes in the eye can sometimes be detected with ultrasonography.
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.
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.
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.
Filariasis. Adult Onchocerca volvulus contained within onchocercomas of the skin.
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Filariasis. This figure displays the life cycle of Wuchereria bancrofti in humans and mosquito vectors (ie, Aedes, Anopheles, Culex, Mansonia species). Life cycles of other lymphatic nematodes (ie, Brugia malayi, Brugia timori) are identical, while the life cycles for other filariae differ in the body location of adult worms, the microfilariae present, and the arthropod intermediate hosts and vectors.
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Filarial abscess scar on the left upper thigh in a young male who is positive for Wuchereria bancrofti microfilariae
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Lymphatic filariasis resulting from Wuchereria bancrofti infection may result in limb lymphedema, inguinal lymphadenopathy, and hydrocele. Photograph taken by Professor Bruce McMillan and donated by John Walker, MD.
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Filariasis. Unilateral left lower leg elephantiasis secondary to Wuchereria bancrofti infection in a boy.
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Filariasis. This is a close-up view of the unilateral lower leg elephantiasis shown in the previous image. Note the lymphedema and typical skin appearance of depigmentation and verrucous “warts.”
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Filariasis. Lateral view of the right outer aspect of a leg affected by elephantiasis secondary to Wuchereria bancrofti infection.
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Filariasis. Inner aspect of the lower leg of the male patient in the previous image, showing gross elephantiasis secondary to Wuchereria bancrofti infection.
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Filariasis. Unilateral left hydrocele and testicular enlargement secondary to Wuchereria bancrofti infection in a man who also was positive for microfilariae.
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Filariasis. Bilateral hydrocele, testicular enlargement, and inguinal lymphadenopathy secondary to Wuchereria bancrofti infection.
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Filariasis. Adult worms of Wuchereria bancrofti in cross section isolated from a testicular lump.
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Filariasis. Microfilaria of Wuchereria bancrofti in a peripheral blood smear.
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Filariasis. Appearance of microfilariae after concentration of venous blood with a Nuclepore filter.
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Filariasis. Onchocercomas of the forearm skin (sowda) in a Sudanese man.
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Filariasis. Adult Onchocerca volvulus contained within onchocercomas of the skin.
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Filariasis. Microfilariae of Loa loa detected in skin snips.
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Filariasis. Microfilariae of Mansonella perstans in peripheral blood.
