Using Community-Level Prevalence of Loa loa Infection to Predict the Proportion of Highly-Infected Individuals: Statistical Modelling to Support Lymphatic Filariasis and Onchocerciasis Elimination Programs

Lymphatic Filariasis and Onchocerciasis (river blindness) constitute pressing public health issues in tropical regions. Global elimination programs, involving mass drug administration (MDA), have been launched by the World Health Organisation. Although the drugs used are generally well tolerated, individuals who are highly co-infected with Loa loa are at risk of experiencing serious adverse events. Highly infected individuals are more likely to be found in communities with high prevalence. An understanding of the relationship between individual infection and population-level prevalence can therefore inform decisions on whether MDA can be safely administered in an endemic community. Based on Loa loa infection intensity data from individuals in Cameroon, the Republic of the Congo and the Democratic Republic of the Congo we develop a statistical model for the distribution of infection levels in communities. We then use this model to make predictive inferences regarding the proportion of individuals whose parasite count exceeds policy-relevant levels. In particular we show how to exploit the positive correlation between community-level prevalence and intensity of infection in order to predict the proportion of highly infected individuals in a community given only prevalence data from the community in question. The resulting prediction intervals are not substantially wider, and in some cases narrower, than the corresponding binomial confidence intervals obtained from data that include measurements of individual infection levels. Therefore the model developed here facilitates the estimation of the proportion of individuals highly infected with Loa loa using only estimated community level prevalence. It can be used to assess the risk of rolling out MDA in a specific community, or to guide policy decisions.Author Summary: Lymphatic Filariasis (LF) is caused by parasitic worms which live in the lymphatic system. Though several body parts may be affected, LF characteristically leads to the enlargement of limbs, causing pain, physical disability and social stigma. Onchocerciasis (river blindness) is caused by similar worms which inhabit the subcutaneous tissue and lead to disturbing skin lesions, visual impairment and blindness. Onchocerciasis and LF affect over 146 million people, and have been targeted for elimination as public health problems by the World Health Organisation, using mass drug administration (MDA). Although the drugs used in MDA are generally well tolerated, individuals who are highly co-infected with Loa loa are at risk of experiencing serious adverse events. In order to inform decisions on whether MDA can be safely administered in a community, we investigate the relationship between community level prevalence of Loa loa and individual infection levels through the development of a statistical model. We find a strong positive correlation and show how this can be exploited to estimate the proportion of individuals highly infected with Loa loa without the need for data on individual infection levels. The model therefore provides a method to assess the risk of rolling out MDA in a specific community, or to guide policy decisions.