Impact of MDA and the prospects of elimination of the lone focus of diurnally sub periodic lymphatic filariasis in Nicobar Islands, India

Highlights

• Six rounds of MDA significantly reduced Mf prevalence in all age classes but continue to persist above 1%.

• Persistence of infection above 1% could result in the risk of resurgence.

• Follow up of cohorts, indicates acquisition of new infections.

• This area is not eligible for TAS despite six rounds of MDA as the Mf prevalence was above 1%.

• Mass DEC fortified salt can be a potential supplementary measure to hasten the process of elimination.

Abstract

Mass Drug Administration is being carried out in Andaman and Nicobar Islands, India since 2004. Cross sectional microfilaria (Mf) survey was conducted in Nancowry group of islands, the lone foci of diurnally sub periodic form of bancroftian filariasis in Nicobar district, to examine its eligibility for Transmission Assessment Survey (TAS). A total of 2561 individuals (coverage: 23.9%) were screened from five islands. The overall Mf prevalence was 3.28%. Except one island, all other islands recorded Mf prevalence >1%, ranging from 2.5% to 5.3%, indicating persistence of infection despite six annual rounds of MDA. Mf prevalence was age dependent and was higher among males, but not significantly different between genders. Age and gender specific analysis showed a significant reduction in all the age classes among females vis a vis pre-MDA prevalence while the reduction was significant only in 21–30 and 41–50 age classes in males. Exposure to day biting and forest dwelling Downsiomyia nivea can be attributed for the persistent infection besides non-compliance for MDA. Based on fits of modified negative binomial distribution, true prevalence of Mf carriers in the community was estimated to be 4.74%, which is markedly higher (about 24%) than the observed prevalence of 3.28%. Follow up of cohorts showed evidence of continued persistence of infection and acquisition of new infections post six rounds of MDA. As the Mf prevalence was above >1% in four of the five islands, this area is not eligible for TAS, warranting continuation of MDA. Mass DEC fortified salt is suggested as an adjunct to hasten elimination of infection.

Introduction

Lymphatic filariasis (LF) is prevalent in tropical and sub-tropical countries, and an estimated 120 million people are affected in 73 endemic countries (WHO, 2012), with an estimated 1.393 billion people residing in areas where filariasis is endemic, necessitating mass drug administration (MDA). In India, LF is endemic in 20 states with about 610 million people residing in endemic areas (Raju et al., 2010, WHO, 2012) and continues to be an important public health problem in India contributing about 44.3% of the global burden (WHO, 2012). As early as 1942 (Wilcock, 1942), Nicobar group of islands were identified to be endemic with about 5.8% infection rate. Subsequent studies have shown the prevalence of diurnally subperiodic Wuchereria bancrofti (DspWB), on the remotely located Nancowry group of islands in Nicobar district (Kalra, 1974, Russel et al., 1975, Tewari et al., 1995) while the rest of the islands with periodic form of W. bancrofti. This is the lone foci of infection with DspWB in India vectored by Downsiomyia nivea (earlier known as Ochlerotatus niveus). Reports are available on the prevalence, distribution, and assessment of endemicity status (Russel et al., 1975, Tewari et al., 1995, Shriram et al., 2002), vector incrimination (Tewari et al., 1995), clinical epidemiology (Shriram et al., 2002), host feeding activity (Shriram et al., 2005), transmission dynamics (Shriram et al., 2008), and vector population dynamics and bioecology (Shriram and Krishnamoorthy, 2011a).

Filariasis elimination programme through MDA with DEC and albendazole (alb) was launched in 2004, covering all the 250 districts including Andaman and Nicobar Islands, targeting LF elimination by 2015. Ending 2011, six rounds of MDA have been completed covering three districts viz. North and Middle, South Andaman and Nicobar district with a population of 384,032. Monitoring Mf prevalence is carried out as a part of programme implementation in sentinel and spot check sites and all the sites have been reported to have <1% Mf prevalence. However, none of sentinel or spot check sites represent DspWB endemic islands and therefore the impact of MDA on DspWB is not known. As per the Transmission Assessment Protocol (TAS) protocol (WHO, 2011), the threshold of antigen prevalence among the children in the age class 6–7 years in areas where W. bancrofti is endemic and transmitted by Aedes vectors is half of that in areas where Culex/Anopheles are the main vectors. This is based on the fact that Aedes vectors are known to be more efficient vectors compared to the others. Nicobar district being endemic for both Culex transmitted periodic and Aedes transmitted sub periodic filariasis, this implementation unit has to be divided into two evaluation units for TAS to make a decision on cessation of MDA. The present study is to assess the impact of MDA in areas with Aedes transmitted bancroftian filariasis which can be used for checking the pre-TAS criteria for this evaluation unit. Also the results are discussed with the view point of identifying appropriate measures to hasten the process of elimination of this lone focus before its spread to other areas.