Development and evaluation of an antigen detection dipstick assay for the diagnosis of human onchocerciasis.

Summary To improve on the diagnosis of onchocerciasis, especially light infections, we developed and evaluatedan oncho-dipstick test based on the detection of Onchocerca volvulus specific antigens in urine and tears.The test was able to detect as little as 25 ng/ml of parasite specific antigens in samples and took as littleas 3 h. Evaluation of the assay on 456 residents of an onchocerciasis hyperendermic area in Cameroonresulted in 408 (89.5%) positives in urine and 374 (82%) positives in tears. The prevalence ofonchocerciasis in the study area, as determined by Rapid Epidemiological Mapping of Onchocerciasis(REMO) and skin snip methods, was 52 and 36.8%, respectively. The sensitivity of the oncho-dipstickassay was 100% in urine and 92% in tears; its specificity was 100% in both. Concordance between urineand tear test results from the same individuals was 87%. The test strips were sufficiently reactive whenleft at room temperature for up to 8 months. The test would be useful for laboratory diagnosis ofonchocerciasis in low transmission zones and to ascertain successful treatment of patients inexperimental drug studies.keywords onchocerciasis, antigen-detection, dipstick test, oncho-C27 antigenIntroductionOnchocerciasis is a chronic and disabling disease caused bythe parasitic nematode Onchocerca volvulus. The parasiteis transmitted from one person to another through the biteof a blood sucking black fly of the genus Simulium.Anestimated 17.7 million people currently suffer from theinfection worldwide, the vast majority of whom live intropical Africa (Burnham (1998)). Infection withO. volvulus, if not treated, typically causes pruritus,papular eruption of the dermis, skin dyspigmentation,serious visual impairment/blindness, kidney disease,epilepsy and hyposexual dwarfism (WHO (1995)). There isno vaccine nor a suitable macrofilaricidal drug against theinfection. Onchocerciasis control programmes require anon-invasive, highly sensitive and specific diagnostic testfor use in low transmission areas and for monitoringrecrudescence of transmission in disease freed areas.Definitive proof of active infection due to O. volvulus is bymicroscopic demonstration of worms in skin snips or innodules surgically excised from suspects. Unfortunately,the commonly used skin snip method is insensitive in lowtransmission areas and in areas where long-term use of themicrofilaricidal ivermectin has resulted in significantreduction of both individual and community skinmicrofilariae loads (Taylor et al. (1989); Newell (1997)),with a consequential reduction in the prevalence of mostsigns and symptoms of the disease. Furthermore, the skinsnip procedure is painful and involves a high risk of blood-borne infections (e.g. HIV), which may result in refusal ofthe test by populations under investigation (Boatin et al.(1998)).To improve upon the diagnosis of onchocerciasis,alternative methods based on the detection of antibodies orantigens in body fluids have been developed (Mbachamet al. (1992); Lavebratt et al. (1994); Chandrashekar et al.(1996); Ngu et al. (1998); Weil et al. (2000)). Unfortu-nately, antibody detection assays, although sufficientlysensitive, are unable to distinguish between active and pastinfections (Lavebratt et al. (1994); Chandrashekar et al.(1996); Weil et al. (2000)). On their part, developedantigen detection assays are either not sufficiently sensitiveand specific, or are laborious and time consuming (Mba-cham et al. (1992); Ngu et al. (1998); Vincent et al.(2000)). Recently, the development of Polymerase ChainReaction (PCR) based methods for the detection of parasiteDNA in skin snips has greatly improved the diagnosis ofonchocerciasis given its high sensitivity and specificity(Meredith et al. (1991); Zimmerman et al. (1994)). ButPCR-based diagnostic methods in onchocerciasis still