Innovative tools for assessing risks for severe adverse events in areas of overlapping Loa loa and other filarial distributions: the application of micro-stratification mapping

Abstract

Background: The wide distribution of Loa loa infection (loiasis) throughout the Democratic Republic of Congo (DRC) is a major obstacle to the plans to eliminate onchocerciasis and lymphatic filariasis (LF) because the standard drug regime is dependent on ivermectin, which cannot be used in co-endemic areas due to the risk of severe adverse events (SAEs). A better understanding of the micro-epidemiology, overlapping low and high risk areas, and how they relate to SAEs is critical to ensure safe and effective treatment.

Findings: Based on published data from the Bas Congo Province in DRC, this study used geographical information systems (GIS) to re-map and analyse onchocerciasis and loiasis prevalence (<20%, 20 to 40%, >40%) at 144 sites in relation to health district areas reporting SAEs. The new maps highlighted the contrasting patterns of the high prevalence sites, and significant geographical overlap between low onchocerciasis and high loiasis sites. Statistical analyses found that sites with medium to high loiasis prevalence were 10 to 16 times more likely to be in a SAE area than those with low prevalence of loiasis. Sites where both onchocerciasis and loiasis prevalence was >20% were also associated with SAE areas.

Conclusions: Collaborative efforts between the national onchocerciasis and LF programmes are critical as plans to scale interventions are moving forward and thus, alternative strategies needed in loiasis co-endemic areas which may include the new L. loa test and treat strategy using the Cellscope, or interventions such as integrated vector management, or anti Wolbachia therapy using doxycycline.

Figure 1

Geographical patterns of loiasis and onchocerciasis prevalence levels in relation to health districts reporting SAEs. Grey shading indicates SAE areas. Yellow dots indicate loiasis prevalence A. <20% B. 20 to 40% C. >40%. Brown dots indicate onchocerciasis prevalence D. <20% E. 20 to 40% F. >40%.

Figure 2

Geographical patterns of overlapping high and low loiasis and onchocerciasis prevalence levels in relation to historical co-endemicity and health districts reporting severe adverse reactions. Grey shading indicates SAE areas. Yellow dots with brown perimeter indicate overlapping loiasis and onchocerciasis prevalence A. <20% B. >20%. Historical prevalence and co-endemic zones are outlined in C, and the geographical relationship between historical co-endemic zones, overlapping prevalences of >20% and SAE areas highlighted in D.