Demonstration of the Adult Parasite:- A coil of hair like white worms discovered in an excised nodule in the appropriate epidemiologic setting is diagnostic of onchocerciasis on visual inspection.
Most nodules have only three to five adults in them (less in hypoendemic areas), though nodules containing as many as 50 have been found, which can be liberated from tissue by collagenase digestion
Serologic testing for O.volvulus IgG is not generally available and when done usually utilizes a crude antigenic preparation of a non-onchocercal filarial parasite.A positive result cannot differentiate between the eight filarial species and may cross-react, albeit at lower titters, with other nematode infections such as strongyloidiasis.
Despite the lack of specificity, the sensitivity of this type of serology is almost 100%.Most people resident in endemic areas will have antibodies whether or not they are currently infected. Thus, serologic evaluation in filarial disease is helpful only in two situations: in persons exposed to or infected with filarial parasites who are originally from nonendemic areas and were presumably seronegative initially; and (Remme, 2004) to detect a quantitative decrease in antibody levels that may occur as a response to definitive therapy.
Eosinophil Count In Adult Parasite
The total eosinophil count is not helpful diagnostically as it is often but inconstantly elevated in onchocerciasis.
Serum IgE levels are usually elevated in onchocerciasis, with highest levels in sowda patients.
Adult worms in suspected onchocercomas will produce a central, relatively homogeneous echogenic area containing echo-dense particles with a lateral acoustic shadow. Within the onchocercomas, calcifications or fluid may be identified. Occasionally, worm movements are observed, regularly so in cystic nodules. Particularly in these nodules, sequential ultrasonographic monitoring of nodules after macrofilaricidal therapy can provide information on drug effects.
A 50 mg dose of diethylcarbamazine (DEC), the Mazzotti test, can be given to patients suspected of harboring O.volvulus but in whom no microfilariae can be detected. Since severe reactions in the skin and eye are not totally avoidable, the systemic test has now been replaced by a patch test performed by applying topical DEC (10% in body solutions such as Nivea) to a small area of skin (10 × 10 cm) in order to provoke a localized Mazzotti reaction.
This is a highly sensitive and specific diagnostic test (pruritus and pustular onchodermatitis develop after 24–48hrs) and is considered for detecting recrudescence of O.volvulus infection in a sentinel population of children and young adults within the onchocerciasis-free zone created by the control programs. (Toe, Adjami , Boatin , et al., 2000).
DNA Diagnosis of Individual Patients
Detection of parasite DNA in routine skin snips by PCR amplification of O-150, an O.volvulus -specific 150 bp repeated genomic DNA family ,is more sensitive for detecting low-level infection in individual patients than classical skin snipping, and similar in sensitivity to the DEC patch test. (Boatin, Toe, Alley, et al., 2002) PCR assays, available in research settings, are of importance in diagnosing expatriates.
Several methods (standard qualitative protocols followed by gel electrophoresis, PCR-ELISA, (Pischke , Buttner , Liebau , et al., 2002) quantitative PCR with probes) are applied to DNA extracted from skin snips or less painful skin scratches that are fresh, frozen, or preserved in ethanol. PCR is the method of choice in the detection of O.volvulus in the Simulium vector in large pooled samples. (Yameogo , Toe , Hougard, et al., 1999).
Serology Demonstration of the Adult Parasite