Diagnostic tools for lymphatic filariasis (LF) elimination programs are useful in mapping the distribution of the disease, delineating areas where mass drug administrations (MDA) are required, and determining when to stop MDA.

The prevalence and burden of LF have been drastically reduced following mass treatments, and the evaluation of the performance of circulating filarial antigen (CFA)-based assays was acknowledged to be of high interest in areas with low residual LF endemicity rates after multiple rounds of MDA.

Scientists at the University of Yaoundé (Cameroon) studied an area delimited by four health districts known as an historical focus of LF, with prevalence ranging from 0.4% to 22.1%, but where no mass antifilarial intervention was ever performed before the outset of the survey. All volunteers underwent clinical examination for LF clinical signs, and blood sample collection for prevalence and intensity of the infection.

Calibrated thick blood smears (CTBS), immunochromatographic test (ICT) and Og4C3 enzyme-linked immunosorbent assay (ELISA) were carried out by night to identify Wuchereria bancrofti microfilarial and circulating filarial antigen carriers. A threshold determination assay regarding ICT and ELISA was performed using serial plasma dilutions from individuals with positive microfilarial counts. The ICT is a rapid card test manufactured by BinaxNow (Scarborough, ME, USA) for the detection of 200 kDa W. bancrofti circulating filarial antigen (CFA). The Og4C3 ELISA test kit is specific for W. Bancrofti CFA, and is manufactured by TropBio Pty Ltd. (Sydney, NSW, Australia).

All 404 individuals were tested for ICT, ELISA and CTBS. CFA was detected in 11 (2.7%;) enrollees using ICT and in 28 (6.9%) individuals using ELISA, the difference being significant. CTBS revealed that seven (1.7%) of these 404 individuals harbored microfilariae (mf), with an intensity of infection of 7.49 mf/mL. All individuals harboring microfilariae as indicated on a positive blood films were detected by ICT and ELISA, but among individuals positive for ELISA, only 35.7% of them were detected using ICT indicating a moderate agreement between both tests. Threshold determination analyses showed that ICT was positive at some dilution factors for different plasma samples, but ELISA was still positive at the last plasma dilution for which ICT yielded a negative result.

The authors concludes that their findings suggest a loss of sensitivity for ICT in low endemicity settings, especially in people exhibiting low levels of circulating filarial antigen, raising serious concern regarding the monitoring and evaluation procedures in the framework of LF elimination program. The study was published on December 23, 2015, in journal BMC Infectious Diseases.

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University of Yaoundé 
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TropBio Pty Ltd.