Ophthalmologic Manifestations of Onchocerciasis Workup

Updated: Oct 16, 2014
  • Author: Debora E Garcia-Zalisnak, MD; Chief Editor: Hampton Roy, Sr, MD  more...
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Workup

Laboratory Studies

Serologic testing for antifilarial antibodies to immunoglobulin G (IgG) and IgG4 via enzyme-linked immunosorbent assay (ELISA): A positive IgG result indicates likely exposure. A positive IgG4 results indicates active filarial infection.

Mazzotti test: An oral test dose of 5 mg DEC is administered, which inhibits the neuromuscular transmission in nematodes. A positive result yields intense pruritus within 2 hours secondary to dying parasites. Although itching can be controlled with corticosteroids, severe systemic reactions can occur.

ELISA and immunochromatographic test (ICT): ELISA, the ubiquitous biological technique, uses multiple recombinant antigens. ELISA is useful to differentiate O volvulus from its cousins but requires a specialized laboratory. ICT, a rapid format antibody card test, uses an individual antigen. The sensitivity of ELISA and ICT has been estimated as 97% and 86%, respectively. Both ELISA and ICT are more sensitive than the skin snip technique and the patch test using topical diethylcarbamazine (DEC). ICT compares with ELISA and is inexpensive.

Polymerase chain reaction (PCR): An advanced molecular technique using DNA or RNA probes specific to O volvulus can be used to assay the blackfly vector and human host infection using PCR technology. These probes are sensitive very early in the disease. Although highly specific, PCR requires meticulous laboratory technique to avoid contamination and false-positive results. In addition to requiring specialized skills and being expensive, PCR-based diagnostic methods of onchocerciasis still depend on skin snips.

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Imaging Studies

Ultrasonography can be used to visualize subcutaneous nodules. However, it is not part of the usual workup.

Onchocerciasis is diagnosed based on clinical and laboratory data rather than imaging studies.

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Other Tests

DEC patch test: A 10% DEC anhydrous lanolin patch is applied to the skin and later checked for local dermatitis caused by the dying microfilariae. It is safer than the Mazzotti test but not as sensitive as the skin snip test.

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Procedures

Skin snip test: This is the test of choice. Punch biopsies are taken from multiple skin sites and incubated at room temperature to allow for microscopic observation of microfilariae emerging from the skin samples. However, this method has low sensitivity low-transmission areas and in areas where long-term use of the microfilaricidal ivermectin has resulted in the significant reduction of individual and community microfilariae loads. Also, the procedure is painful and involves a high risk of blood-borne infections (eg, HIV).

Nodulectomy: This involves removal of skin nodules to inspect for adult worms. This is a diagnostic and therapeutic technique. However, it is very invasive.

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Histologic Findings

Ovolvulus can be visualized with phase-contact microscopy. It is coiled while living and straightens as it dies. Female worms live from 8-10 years, releasing millions of first-stage larvae throughout their lifetime. These microfilariae are 320-360 µm long and 5-10 µm in diameter and live from 6-30 months. In hyperendemic areas, an infected individual may have as many as 150 million microfilariae in his or her body.

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Staging

Onchocerciasis is not classified according to different stages.

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