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.

KEYWORDS::

• Onchocerciasis
• Reported night blindness
• Optic nerve disease

ACKNOWLEDGEMENTS

This report is based on OB’s MSC Epidemiology dissertation at the London School of Hygiene and Tropical Medicine. The author would like to thank the following who were active participants in the original study: Professor Adenike Abiose (The Principal Investigator), who gave permission to use the data, Dr. Ian Murdoch, fellow ophthalmologist on the project who also critiqued this paper, Professor Simon Cousens, supervisor at the LSHTM, the late Professor Barrie Jones, who was one of the originators of the ivermectin trials in Kaduna. The ophthalmic nurses who participated in the project included Messers. Liman, Sati and Danboyi. Logistics support was provided by Mr. Iro Nuhu, the late Babatunde Asha, Mr. Sule and co. Thanks also to the good people of these areas of Kaduna state without whose participation this project would not have been possible.

Declaration of interest: The author wishes to state that there are no personal or financial interests or relationships that could bias or influence this study.