Dermatologic Manifestations of Onchocerciasis (River Blindness) Workup
- Author: Jason F Okulicz, MD, FACP, FIDSA; Chief Editor: William D James, MD more...
The traditional standard diagnosis of onchocerciasis is based on the acquisition of 3- to 5-mg skin snips from an affected area. These snips are immediately immersed in sodium chloride solution and placed under a microscope. The emerging microfilariae are then counted. This method is specific, and it has been the most accurate. However, the use of skin snips is not sensitive for detecting early or mild infections, and this method is becoming increasingly unacceptable to people in endemic communities because of its invasiveness.
The Mazzotti test, now seldom used, involves the administration of 6 mg of diethylcarbamazine (DEC). DEC inhibits neuromuscular transmission in nematodes. Within 2 hours, a positive result produces pruritus and, sometimes, intense inflammation in the areas of dying microfilariae. Other possible effects such as vomiting, conjunctivitis, albuminuria, hypotension, and sudden death (rare) limit its usefulness. Oral DEC was formerly used in treatment of the disease.
The DEC patch test (ie, Mazzotti patch test) involves a topical application of DEC, which produces a local reaction to dying microfilariae at the patch site.[5, 6] This noninvasive test is specific, but it is less sensitive than the skin-snip test. In the future, this test may be more valuable in detecting the recrudescence of infection in onchocerciasis-free zones than in diagnosing the disease.
Enzyme-linked immunosorbent assays (ELISAs), which require only a finger-stick sample, are more sensitive and less invasive than skin-snip tests. This test is used to recognize specific microfilarial antigens. However, ELISA results cannot be used to distinguish past and current infections, a considerable problem in endemic areas.
The polymerase chain reaction (PCR) amplifies repetitive parasite DNA sequences in skin-snip specimens. Compared with skin-snip tests, this method has greater sensitivity in patients with low-level infections. The return of microfilariae to the skin after drug treatment can also be identified earlier with this test. The major disadvantages of PCR are its high cost and invasiveness. A minimally invasive and painless alternative to the use of skin snips is the collection of skin scratches. The superficial layer of the epidermis is removed by carefully scraping the skin with the blunt edge of a disposable lancet. PCR studies of skin scratches, instead of skin snips, have yielded similar results and may be used more often in the future.
Rapid-format antibody card tests are being developed to diagnose onchocerciasis.[8, 9] In these tests, serum samples can be used to detect antibodies, such as immunoglobulin G4 (IgG4) antibodies to recombinant O volvulus antigen Ov16. Tests for additional antibodies are currently under development. Initial studies show good sensitivity and specificity in small numbers of samples and controls using various methods. Some tests can be modified to permit the use of blood samples obtained from less invasive finger sticks. This modification increases the utility of the test for field use. Additional studies are needed to determine the value of these tests in the diagnosis of onchocerciasis.
Microfilariae can often be directly observed by means of slit lamp examination of the cornea and anterior chamber of the eye.
An oncho-C27 antigen detection dipstick assay using urine and tears has been developed. The test can be completed in as short as 3 hours, and the strips maintain reactivity when kept at room temperature for up to 8 months. Sensitivity and specificity were good in an initial study.
In early untreated cases, tissue biopsy samples may show a mild chronic inflammatory infiltration; eosinophils, lymphocytes, and histiocytes may surround the microfilariae. Microfilariae are often present without a surrounding cellular reaction. Later cases show hyperkeratosis, parakeratosis, tortuous dermal vessels, dilated lymphatics, and pigment incontinence. An increased number of dermal fibroblasts leads to perivascular fibrosis. In more advanced cases, hyalinized scar tissue replaces the collagen and elastic fibers in the dermis.
The microfilariae have pointed tails, elongated posterior nuclei, paired anterior nuclei, and large spaces between the tip or tail and the first nuclei. They lack sheaths.
Onchocercomas are made up of an outer vascular fibrous stroma embedded with groups of perivascular leukocytes. The inner layer is composed of hyaline connective tissue intermingled with coiled adult worms. A dense cellular infiltrate composed of eosinophils, lymphocytes, macrophages, and giant cells surrounds the worm. The coiled appearance of the worm, the presence of microfilaria in gravid females, and the presence of a gut help in identifying the worm. See the images below.
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