Laboratory Studies
Pathological diagnosis
Traditionally, a diagnosis of onchocerciasis requires demonstration of microfilariae in a skin-snip biopsy sample (see Procedures). This technique yields high specificity (100%) in experienced hands but low sensitivity (20%-50%) in early stages of infection.
The diagnosis may also be made by direct examination of surgical specimens obtained by excision of nodules.
Immunodiagnosis
Antibody detection does not distinguish between active and past infections. Various antibodies have been tested, as follows:
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Ov16 card test: Antibodies against this antigen have been shown to yield high sensitivity (approximately 80%) and specificity (approximately 85%) and may yield positive results in early infections when skin-snip results are negative. [30, 31] Capillary blood samples are collected by finger prick. The immunochromatographic card test is used to detect the presence of immunoglobulin G4 (IgG4) antibodies to recombinant Ov16 antigen.
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Recombinant hybrid proteins (OvH2 and OvH3): This test is based on hybrid proteins of two separate Onchocerca proteins (Ov20 and Ov33). High sensitivity (>95%) and specificity (>95%) has been described in this enzyme-linked immunoassay (ELISA)–based antibody detection test. [32]
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An ELISA-based test using a cocktail of 3 antigens (Ov7, Ov11, Ov16) has also been used to detect antibodies. A comparison study showed that a mixture of these 3 proteins yielded a sensitivity of approximately 97% and a specificity 100%, superior to those of the recombinant Ov16 card test. [33]
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Testing for a low–molecular-weight antigen fraction of female O volvulus parasite yields sensitivity and specificity similar to those of skin-snip testing. [34]
Antigen detection
Oncho-27 antigens have been studied in the diagnosis of Onchocerca infections. The advantage of this test is that it uses urine or tears for testing. In a study of 456 patients in a hyperendemic area of Cameroon, this technique yielded a sensitivity and specificity of 100%. [35]
Nucleic acid amplification tests
Polymerase chain reaction (PCR) using material from skin-snips or skin scratches provides high sensitivity and specificity, superior to older methods. [36, 3] However, the limited availability of technical expertise, as well as the high cost of the test, restricts its use in resource-limited settings.
Imaging Studies
Ultrasonography may reveal nonpalpable nodules, although this is not useful as a screening test. Ultrasonography of an adult worm in a nodule reveals a homogeneous echogenic area containing echodense particles with a lateral acoustic shadow.
Other Tests
An older technique that is more of historical interest is the test dosing with diethylcarbamazine (DEC) in oral form to observe the reaction that suggests the presence of onchocerciasis. More recently, a patch test using DEC, which decreases risk, has been used. This testing is based on the principle of the Mazzotti reaction and involves the topical application of DEC in a cream base (DEC patch). It elicits localized cutaneous reactions (pruritus, maculopapular eruptions, dermal edema) in response to dying microfilariae. [37] Earlier studies reported varying degree of sensitivity (30%-92%) in patients with positive skin-snip results. Severe cutaneous reactions may require steroid therapy or hospitalization. Higher concentrations of DEC and longer patch times increase the sensitivity. [38] However, false-positive reactions may occur in patients with other filarial diseases such as Loa loa infection.
Procedures
Skin snip
In this technique, a razor blade is used to remove tiny skin samples (3-5 mg) from multiple sites (iliac crests, shoulder blades); they are then placed in saline to observe microfilariae emerging from the skin samples. Alternatively, for the purpose of quantitation of microfilarial burden, sclerocorneal punch samples can be used to obtain a specific amount of skin (usually 5 micrograms) for each sample.
The sensitivity is low in the prepatent disease stage of disease, in geographic areas of low prevalence, and in areas of mass ivermectin use.
Skin-snip biopsy specimens can also be used to detect microfilariae using nucleic acid amplification.
Taking a skin snip from the hip with a 5-microgram iris scissors for quantitatively diagnosing Onchocerca volvulus infection (Rio de Micay, Colombia). Courtesy of D Scott Smith, MD.
Taking a skin snip from the hip with a 5-microgram iris scissors for quantitatively diagnosing Onchocerca volvulus infection (Rio de Micay, Colombia). Courtesy of D Scott Smith, MD.
Nodule resection (nodulectomy, onchocercomectomy)
Although this is the most invasive method, surgical removal of nodules has both diagnostic (if adult worm is observed) and therapeutic potential (elimination of the adult worms in that nodule). It was later shown that the public health value of performing nodulectomy campaigns is of little value due to occult nodules that may enable disease transmission to persist. [39, 40]
Histologic Findings
Microscopic examination of excised onchocercomata reveals cross-sections of adult worms with eosinophils and lymphocytes at the periphery of the nodule.
Microfilaria of Onchocera volvulus stained from skin biopsy. Courtesy of Cali Colombia, D Scott Smith, MD, and Gloria Palma, MD, PhD.
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Simulium fly (black fly).
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Histopathology of an Onchocerca volvulus nodule. Courtesy of the CDC and Dr. Mae Melvin.
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Simplified life cycle of Onchocerciasis volvulus.
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Subcutaneous nodule on hip caused by encysted Onchocerca volvulus.
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Man blinded by microfilariae of Onchocerca volvulus.
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Bleeding on skin due to scratching of itchy skin (onchodermatitis).
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One anatomic quarter of body (leg) markedly affected by edema and papule formation of onchodermatitis.
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Skin of a West African person with leopard spot depigmentation.
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Patient from Cameroon with sowdalike lesions.
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Microfilaria of Onchocera volvulus stained from skin biopsy. Courtesy of Cali Colombia, D Scott Smith, MD, and Gloria Palma, MD, PhD.
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Taking a skin snip from the hip with a 5-microgram iris scissors for quantitatively diagnosing Onchocerca volvulus infection (Rio de Micay, Colombia). Courtesy of D Scott Smith, MD.
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Taking a skin snip from the hip with a 5-microgram iris scissors for quantitatively diagnosing Onchocerca volvulus infection (Rio de Micay, Colombia). Courtesy of D Scott Smith, MD.
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Adults blinded by onchocerciasis. This is the WHO/New York Times photo that inspired World Bank funding for River Blindness control programs and then the bronze statue in front of the WHO in Geneva Switzerland. Courtesy of Otis Historical Archives, National Museum of Health and Medicine.
