Onchocerciasis as a risk factor for night blindness.

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.