Abstract
Background
Lymphatic filariasis (LF) is targeted for elimination by the year 2020. As of 2017, 67 of the 72 endemic countries have implemented annual Mass Drug Administration (MDA) for interrupting LF transmission. Transmission Assessment Survey (TAS) is the recommended protocol to evaluate the impact of MDA and to decide when to stop MDA in an Evaluation Unit (EU, population ≤2 million). As the human infection levels go down with repeated MDA rounds, it becomes a challenge to select the appropriate survey methods to assess transmission interruption. This study validates a standard protocol for molecular xenomonitoring of infection in vectors (MX) at an EU as a complementary tool for TAS to stop MDA and its utility for post-MDA or post-validation surveillance.
Methodology
The study was conducted in Cuddalore district, Tamil Nadu, India, which was found eligible for TAS after 15 annual rounds of MDA (4 with DEC alone and 11 with DEC plus albendazole). The district was divided into two EUs as per the TAS protocol and one EU was randomly selected for the study. A two-stage cluster design vector sampling, developed and validated at a sub-district level, was implemented in 30 randomly selected clusters in the EU. Female Culex quinquefasciatus were collected placing gravid traps overnight (1800–0600 hrs) inside the premises of systematically selected households. Pools of 20–25 blood-fed, semi-gravid and gravid Cx. quinquefasciatus were subjected to real-time quantitative PCR (polymerase chain reaction) assay for detecting Wuchereria bancrofti DNA. Pool infection rate (% of pools positive for W. bancrofti DNA), and the estimated prevalence of W. bancrofti DNA in mosquitoes and its 95% confidence interval were calculated. Additionally, in these 30 clusters, microfilaria (Mf) survey among individuals >5 years old was carried out. School-based TAS was conducted using Immunochromatographic Card Test (ICT) in the EU. Prepared itemized cost-menu for different cost components of MX survey and TAS were estimated and compared.
Results
MX survey showed that only 11 (3.1%) of the 358 pools (8850 Cx.quinquefasciatus females), collected from 30 clusters, were found positive for W. bancrofti DNA. The estimated vector infection rate was 0.13% (95% CI: 0.07–0.22%), below the provisional threshold (0.25%) for transmission interruption. Of 1578 children tested in the TAS, only four (0.25%) were positive for filarial antigenemia, and it is well below the critical cut-off (18 positives) for stopping MDA. Among 9804 persons tested in the 30 clusters, only four were found positive for Mf (0.04%; 95% CI: 0.01–0.1%). The Mf-prevalence was <1% threshold for transmission interruption in humans. The estimated costs for TAS and MX per EU were $14,104 USD and $14,259 USD respectively.
Conclusions
The result of MX protocol was in good agreement with that of TAS, providing evidence to recommend MX as a complementary tool to TAS to decide on stopping MDA. MX can also be a potential surveillance tool for post-MDA and post-validation phases as it could detect sites with residual infection and risk of resurgence of transmission. MX is economically feasible as its cost is slightly higher than that of TAS.
Lymphatic filariasis (LF), commonly known as “elephantiasis” is caused by filarial parasites and transmitted among humans by mosquitoes. This parasitic infection results in chronic diseases such as swelling of limbs and hydrocele. Global programme to eliminate lymphatic filariasis (GPELF), launched by the World Health Organization (WHO) in 2000 endorsed the mass treatment of all the people above 2 years of age in the endemic areas with a single dose of anti-filarial drugs administered annually for a minimum period of 5 years. WHO also recommended transmission assessment survey (TAS) protocol to assess the impact of mass treatment and to decide on stopping mass treatment. The protocol aims at screening young children who were born after the mass treatment for filarial infection. If the number of infected children is smaller than the pre-defined number, mass treatment can be stopped. The same protocol is followed for periodical assessment to verify whether there are any new infections. Alternatively, vector infection levels by molecular xenomonitoring (MX, detection of parasite DNA in the mosquitoes) can be used to verify whether there are any infected mosquitoes. This tool has been applied in many studies and there is a provisionally established mosquito infection threshold level (0.25%) below which transmission is interrupted. This can be an alternative tool for TAS. We validated this method at district level by collecting filariasis transmitting mosquitoes from 30 villages/wards and compared the results with those of TAS. There was good agreement between the decisions based on TAS and MX in our study. Though in the EU both vector and human infection levels were below their respective threshold levels, the mosquito infection in individual sites was above the threshold, indicating residual hotspots and risk of resurgence. In addition, we estimated the cost of conducting MX and TAS for their economic feasibility and found that the cost of MX is only marginally higher than that of school-based TAS. Thus, our study results provide recommendations to use MX as a tool complementary to TAS (i) for taking a decision on stopping MDA, (ii) for monitoring post-MDA and post-validation surveillance programme, and (iii) for remapping areas to initiate MDA.
Collapse