Onchocerciasis as a Risk Factor for Night Blindness

Abstract

Purpose: In the Kaduna State Nigeria Onchocerciasis focus, the prevalence of reported night blindness (RNB ) was 12.9%, higher than the national average (1%), and a control non-onchocercal community (0.83%, P < 0.0001). Risk factors for RNB were explored.Methods: This was an analysis of baseline data from the phase three ivermectin trials in Kaduna (1988–1995). 6831 subjects in the onchocercal zone and 1563 in the control zone were examined. Ordinal logistic regression was used to assess the relationship between microfilaria load (uninfected, low (<10 mf/mg), moderate (10–49 mf/mg) and high (50+ mf/mg)) and likelihood of RNB.Results: Ocular evidence of vitamin A deficiency (Bitot spots or xerophthalmia) was absent in both populations. The excess risk of nightblindness attributable to domicile in this onchocerciasis-endemic area was 11.9% with a population attributable fraction of 92.2%. The prevalence of RNB and age-adjusted odds ratio increased with higher microfilaria load (P < 0.0027.) Subjects with onchocerciasis-related ocular lesions such as optic nerve disease (OND; age-adjusted OR 2.29, 95% confidence interval, CI, 1.86–2.83), sclerosing keratitis (OR 2.75, 95% CI 2.10–3.50), and onchocercal chorioretinitis (OR 1.66, 95% CI 1.22–2.26) were significantly more likely to report night blindness.Overall, subjects with a primary diagnosis of ‘ocular onchocerciasis’ were 50% more likely to report night blindness. OND, cataract and trachoma together accounted for 52% of all RNB but OND (onchocerciasis-related in 80% of cases) emerged as the single most common associated pathology in 30% of cases.Conclusions: Onchocercal infection probably accounted for the excess of RNB in this focus.