Measurement of circulating filarial antigen levels in human blood with a point-of-care test strip and a portable spectrodensitometer

The Alere Filariasis Test Strip (FTS) is a qualitative, point-of-care diagnostic tool that detects Wuchereria bancrofti circulating filarial antigen (CFA) in human blood, serum, or plasma. The Global Program to Eliminate Lymphatic Filariasis employs the FTS for mapping filariasis-endemic areas and assessing the success of elimination efforts. The objective of this study was to explore the relationship between the intensity of positive test lines obtained by FTS with CFA levels as determined by enzyme-linked immunosorbent assay (ELISA) with blood and plasma samples from 188 individuals who live in a filariasis-endemic area. The intensity of the FTS test line was assessed visually to provide a semiquantitative score (visual Filariasis Test Strip [vFTS]), and line intensity was measured with a portable spectrodensitometer (quantitative Filariasis Test Strip [gFTS]). These results were compared with antigen levels measured by ELISA in plasma from the same subjects. qFTS measurements were highly correlated with vFTS scores (p = 0.94; P < 0.001) and with plasma CFA levels (p = 0.91; P < 0.001). Thus, qFTS assessment is a convenient method for quantifying W bancrofti CFA levels in human blood, which are correlated with adult worm burdens. This tool may be useful for assessing the impact of treatment on adult filarial worms in individuals and communities

Risk factors for lymphatic filariasis in two villages of the Democratic Republic of the Congo

Abstract Background Little is known regarding risk factors for lymphatic filariasis (LF) in Central Africa. To expand on what is known, we studied the epidemiology of LF in two endemic villages in the Democratic Republic of the Congo. Methods Dependent variables were Wuchereria bancrofti antigenaemia detected with filarial test strips (FTS) and microfilaraemia detected by night blood smears. The following factors were investigated: sex, age, the use of bednets, the use of latrines, hunting, fishing and agricultural activities, history of treatment with anthelmintic drugs, overnight stays in the bush, population density, the number of household members, and distance to rivers. Mixed multivariate logistic regression models were used. Results Two hundred and fifty nine out of 820 (31.6%) of subjects aged ≥ 5 years had W. bancrofti antigenaemia and 11.8% (97/820) had microfilaraemia. Multivariable analysis of risk factors for antigenaemia demonstrated increased risk for males (aOR = 1.75, 95% CI: 1.20–2.53, P = 0.003), for older individuals (aOR = 9.12 in those aged > 35 years, 95% CI: 4.47–18.61, P < 0.001), for people not using bednets (aOR = 1.57, 95% CI: 1.06–2.33, P = 0.023), for farmers (aOR = 2.21, 95% CI: 1.25–3.90, P = 0.006), and for those who live close to a river (aOR = 2.78, 95% CI: 1.14–6.74, P = 0.024). Significant risk factors for microfilaraemia included age, male gender, overnight stay in the bush, and residence close to a river (aOR = 1.86, 2.01, 2.73; P = 0.011, 0.010, 0.041; for the three latter variables, respectively). People who reported having taken levamisole (n = 117) during the prior year had a significantly decreased risk of having filarial antigenaemia (aOR = 0.40, 95% CI: 0.21–0.76, P = 0.005). Conclusions Age, sex, not using bednets, and occupation-dependent exposure to mosquitoes were important risk factors for infection with W. bancrofti in this study. The association with levamisole use suggests that the drug may have prevented filarial infections. Other results suggest that transmission often occurs outside of the village. This study provides interesting clues regarding the epidemiology of LF in Central Africa

Results of the models exploring individual factors associated with ICT-miss (ICT-negative results in persons with positive FTS results).

<p>Results of the models exploring individual factors associated with ICT-miss (ICT-negative results in persons with positive FTS results).</p

Cohen's Kappa score estimate obtained from a random-effects model.

<p>LH: Liberia Harper District. LF: Liberia Foya District. CDI: Côte d'Ivoire. One village (Wetchoken, in the Harper district) from Liberia was not added in this analysis because all the ICT were negative. "RE model" represents the pooled Cohen's Kappa scores by group from the random-effects models.</p

A multi-center field study of two point-of-care tests for circulating <i>Wuchereria bancrofti</i> antigenemia in Africa

<div><p>Background</p><p>The Global Programme to Eliminate Lymphatic Filariasis uses point-of-care tests for circulating filarial antigenemia (CFA) to map endemic areas and for monitoring and evaluating the success of mass drug administration (MDA) programs. We compared the performance of the reference BinaxNOW Filariasis card test (ICT, introduced in 1997) with the Alere Filariasis Test Strip (FTS, introduced in 2013) in 5 endemic study sites in Africa.</p><p>Methodology</p><p>The tests were compared prior to MDA in two study sites (Congo and Côte d'Ivoire) and in three sites that had received MDA (DRC and 2 sites in Liberia). Data were analyzed with regard to % positivity, % agreement, and heterogeneity. Models evaluated potential effects of age, gender, and blood microfilaria (Mf) counts in individuals and effects of endemicity and history of MDA at the village level as potential factors linked to higher sensitivity of the FTS. Lastly, we assessed relationships between CFA scores and Mf in pre- and post-MDA settings.</p><p>Principal findings</p><p>Paired test results were available for 3,682 individuals. Antigenemia rates were 8% and 22% higher by FTS than by ICT in pre-MDA and in post-MDA sites, respectively. FTS/ICT ratios were higher in areas with low infection rates. The probability of having microfilaremia was much higher in persons with CFA scores >1 in untreated areas. However, this was not true in post-MDA settings.</p><p>Conclusions/Significance</p><p>This study has provided extensive new information on the performance of the FTS compared to ICT in Africa and it has confirmed the increased sensitivity of FTS reported in prior studies. Variability in FTS/ICT was related in part to endemicity level, history of MDA, and perhaps to the medications used for MDA. These results suggest that FTS should be superior to ICT for mapping, for transmission assessment surveys, and for post-MDA surveillance.</p></div

Results for antigenemia and microfilaremia according to age class and history of MDA.

<p>Results for antigenemia and microfilaremia according to age class and history of MDA.</p

Cross-tabulation of filarial antigen test results (qualitative) obtained by the ICT and the FTS.

<p>Cross-tabulation of filarial antigen test results (qualitative) obtained by the ICT and the FTS.</p

Probability of microfilaremia by CFA score.

<p>The lines indicate the probabilities in the pre-MDA sites, and the dashed lines the probabilities in the post-MDA sites with 95% confidence intervals.</p

Relationship between the ICT prevalence rate and the FTS/ICT ratio.

<p>All the data were included in this figure: Liberia (Foya and Harper), Côte d'Ivoire (Soribadougou and Yadio), Congo, DRC, Sri Lanka, and Indonesia (see <a href="http://www.plosntds.org/article/info:doi/10.1371/journal.pntd.0005703#pntd.0005703.t001" target="_blank">Table 1</a> for details). Foya and Harper study areas comprised 9 and 15 villages, respectively. Black symbols (post-MDA IVM+ALB), blue symbols (post-MDA DEC+ALB), green symbol (post-MDA ALB alone), and pink symbols (pre-MDA).</p