Cross-Reactivity of Filariais ICT Cards in Areas of Contrasting Endemicity of <i>Loa loa</i> and <i>Mansonella perstans</i> in Cameroon: Implications for Shrinking of the Lymphatic Filariasis Map in the Central African Region

<div><p>Background</p><p>Immunochromatographic card test (ICT) is a tool to map the distribution of <i>Wuchereria bancrofti</i>. In areas highly endemic for loaisis in DRC and Cameroon, a relationship has been envisaged between high <i>L</i>. <i>loa</i> microfilaria (Mf) loads and ICT positivity. However, similar associations have not been demonstrated from other areas with contrasting levels of <i>L</i>. <i>loa</i> endemicity. This study investigated the cross-reactivity of ICT when mapping lymphatic filariasis (LF) in areas with contrasting endemicity levels of loiasis and mansonellosis in Cameroon.</p><p>Methodology/Principal Findings</p><p>A cross-sectional study to assess the prevalence and intensity of <i>W</i>. <i>bancrofti</i>, <i>L</i>. <i>loa</i> and <i>M</i>. <i>perstans</i> was carried out in 42 villages across three regions (East, North-west and South-west) of the Cameroon rainforest domain. Diurnal blood was collected from participants for the detection of circulating filarial antigen (CFA) by ICT and assessment of Mf using a thick blood smear. Clinical manifestations of LF were also assessed. ICT positives and patients clinically diagnosed with lymphoedema were further subjected to night blood collection for the detection of <i>W</i>. <i>bancrofti</i> Mf. Overall, 2190 individuals took part in the study. Overall, 24 individuals residing in 14 communities were tested positive by ICT, with prevalence rates ranging from 0% in the South-west to 2.1% in the North-west. Lymphoedema were diagnosed in 20 individuals with the majority of cases found in the North-west (11/20), and none of them were tested positive by ICT. No Mf of <i>W</i>. <i>bancrofti</i> were found in the night blood of any individual with a positive ICT result or clinical lymphoedema. Positive ICT results were strongly associated with high <i>L</i>. <i>loa</i> Mf intensity with 21 subjects having more than 8,000 <i>L</i>. <i>loa</i> Mf ml/blood (Odds ratio = 15.4; 95%CI: 6.1–39.0; p < 0.001). Similarly, a strong positive association (Spearman’s rho = 0.900; p = 0.037) was observed between the prevalence of <i>L</i>. <i>loa</i> and ICT positivity by area: a rate of 1% or more of positive ICT results was found only in areas with an <i>L</i>. <i>loa</i> Mf prevalence above 15%. In contrast, there was no association between ICT positivity and <i>M</i>. <i>perstans</i> prevalence (Spearman’s rho = - 0.200; p = 0.747) and Mf density (Odds ratio = 1.8; 95%CI: 0.8–4.2; p = 0.192).</p><p>Conclusions/Significance</p><p>This study has confirmed the strong association between the ICT positivity and <i>L</i>. <i>loa</i> intensity (Mf/ml of blood) at the individual level. Furthermore, the study has demonstrated that ICT positivity is strongly associated with high <i>L</i>. <i>loa</i> prevalence. These results suggest that the main confounding factor for positive ICT test card results are high levels of <i>L</i>. <i>loa</i>. The findings may indicate that <i>W</i>. <i>bancrofti</i> is much less prevalent in the Central African region where <i>L</i>. <i>loa</i> is highly endemic than previously assumed and accurate re-mapping of the region would be very useful for shrinking of the map of LF distribution.</p>