The Geographic Distribution of Loa loa in Africa: Results of Large-Scale Implementation of the Rapid Assessment Procedure for Loiasis (RAPLOA)

Abstract

BackgroundLoiasis is a major obstacle to ivermectin treatment for onchocerciasis control and lymphatic filariasis elimination in central Africa. In communities with a high level of loiasis endemicity, there is a significant risk of severe adverse reactions to ivermectin treatment. Information on the geographic distribution of loiasis in Africa is urgently needed but available information is limited. The African Programme for Onchocerciasis Control (APOC) undertook large scale mapping of loiasis in 11 potentially endemic countries using a rapid assessment procedure for loiasis (RAPLOA) that uses a simple questionnaire on the history of eye worm.Methodology/Principal FindingsRAPLOA surveys were done in a spatial sample of 4798 villages covering an area of 2500×3000 km centred on the heartland of loiasis in Africa. The surveys showed high risk levels of loiasis in 10 countries where an estimated 14.4 million people live in high risk areas. There was a strong spatial correlation among RAPLOA data, and kriging was used to produce spatially smoothed contour maps of the interpolated prevalence of eye worm and the predictive probability that the prevalence exceeds 40%.Conclusion/SignificanceThe contour map of eye worm prevalence provides the first global map of loiasis based on actual survey data. It shows a clear distribution with two zones of hyper endemicity, large areas that are free of loiasis and several borderline or intermediate zones. The surveys detected several previously unknown hyperendemic foci, clarified the distribution of loiasis in the Central African Republic and large parts of the Republic of Congo and the Democratic Republic of Congo for which hardly any information was available, and confirmed known loiasis foci. The new maps of the prevalence of eye worm and the probability that the prevalence exceeds the risk threshold of 40% provide critical information for ivermectin treatment programs among millions of people in Africa.

Filaria zoogeography in Africa: ecology, competitive exclusion, and public health relevance

“…This separation would avoid the potential interactions of two microfilarial species entering an inappropriate vector in the same blood meal causing potentially lethal damage as non-compatible microfilariae enter the vector mid-gut. In Central Africa and parts of West Africa the five filarial species are regarded as co-endemic [16,25,[33][34][35][36][37][38][39], although more refined mapping may show that there is less overlap than previously thought, with environmental factors playing a key role in determining vector distributions and transmission potential [21].…”

“…Examination of the distribution of M. streptocerca in the DRC showed a wide distribution in the northeastern, central, and western regions of the Congo River basin area, but not in the mountainous southeast and extreme east where altitudes were high and ecologically distinct [39]. Although no continental African map for M. streptocerca is available, the recent distribution maps of the other four filarial species indicate that throughout West Africa, west of Benin, W. bancrofti, O. volvulus, and M. perstans are broadly co-endemic, whereas east of Benin to the borders of Uganda and the DRC all species are co-endemic and sympatric to varying degrees [34][35][36][37][38]. These differing distributions could be related to the well-defined Dahomey Gap in West Africa, which is a savanna corridor that extends to the Atlantic coast in Benin, Togo, and Ghana, and separates the West African rainforest into two regions.…”

“…Here we present Uganda as an example, where L. loa prevalence is low or absent despite high endemicity in the neighbouring DRC [38,60] (Figure 2). This could be related to the mountain range dividing the two countries, the relatively high elevations, and lack of dense tropical rainforest suitable for Chrysops in Uganda [54,56].…”

Clinical Features of Imported Loiasis: A Case Series from the Hospital for Tropical Diseases, London

“…Loiasis is transmitted by adult female Chrysops flies and its epidemiology is largely dependent on the distribution of the fly: loiasis is confined to forest areas of central and west Africa. 1 More than 10 million people are estimated to be infected, 2 and in some areas loiasis is the most common reason for visiting hospital. 3 Loiasis is also seen as an imported infection among visitors (travelers and expatriates) and migrants.…”

Absence of an Association Between Plasmodium falciparum Infection and Post-Ivermectin Loa-Related Non-Neurologic Serious Adverse Events

“…Although rare, these potentially fatal SAEs continue to impede the progress of the African Program for Onchocerciasis Control and the Global Program to Eliminate Lymphatic Filariasis in areas where loiasis is endemic. 1 The major risk factor for the development of post-ivermectin SAEs is a high L. loa microfilarial (mf) density. 2 It has been estimated that the relative risk of developing non-neurologic marked adverse events or SAEs, as defined previously, 3 is significantly increased when the Loa density exceeds 8,100 or 30,000 mf/mL blood, respectively.…”