Post-MDA Transmission Assessment Survey for Elimination of Lymphatic Filariasis in La Ciénaga, Dominican Republic

The Dominican Republic is one of four remaining countries in the Americas with lymphatic filariasis (LF). Annual mass drug administration (MDA) with albendazole and diethylcarbamazine was conducted in La Ciénaga, an impoverished urban barrio in Santo Domingo, from 2004 to 2006. Eight years after the last MDA, a transmission assessment survey (TAS) was conducted in November–December 2014 to determine if LF transmission remains absent. Of 815 first and second grade primary school students (mean age: 6.51 years; range 5–9) tested by immunochromatographic test (ICT), zero (0.0%) were positive. This is below the TAS critical cutoff of nine, indicating that the area “passed” TAS and that transmission remains interrupted in La Ciénaga. Importantly, this also provides evidence that three rounds of effective (> 65% coverage) MDA, likely aided by environmental improvements and periodic school-based albendazole monotherapy MDA, achieved interruption of LF transmission from a relatively low-transmission setting. The Dominican Republic (DR, population 9.5 million) is one of four remaining countries in the Americas region with lymphatic filariasis (LF)—a mosquito-transmitted parasitic disease that currently affects an estimated 67.88 million people in 73 countries. The island of Hispaniola, which the DR shares with Haiti, accounts for approximately 90% of cases in the Americas. LF in Hispaniola is caused by Wuchereria bancrofti with Culex quinquefasciatus the principal vector. Infection is not fatal, but 30–40% of individuals develop lymphedema, elephantiasis, and/or genital swelling (hydrocele in men) due to blockage of the lymphatic vessels by adult worms. Affected individuals often suffer impairment of daily activities and social isolation in addition to the pain and discomfort of severe disease. The Dominican Ministry of Health created the Program to Eliminate Lymphatic Filariasis (PELF) in 1998 to coordinate national LF elimination. Patterned after the global strategy, PELF targets elimination of LF through 1) annual mass drug administration (MDA) of albendazole (donated by GlaxoSmithKline) and diethylcarbamazine (DEC) to interrupt LF transmission by 2020; and 2) morbidity control to alleviate disability for those already infected. Baseline mapping, initiated in 1999, identified three focal areas of transmission in the DR: La Ciénaga, southwest, and east regions. La Ciénaga, literally “swamp,” is an impoverished urban barrio on the western banks of the Ozama River in the capital Santo Domingo that contains around 50,000 inhabitants. Surveys from the early 1980s identified La Ciénaga as a hot spot of LF transmission, with microfilaremia (mf) prevalence of 9.8% among individuals older than 5 years of age. In 2002, before the implementation of MDA for LF, sentinel site surveys of the same age group documented mf prevalence of 2.5% and 10.7% antigen prevalence by immunochromatographic test (ICT). Annual MDAwith albendazole and DEC was conducted in La Ciénaga in 2004 (May–June), 2005 (May–June), and 2006 (November–December), with population coverage rates of 67%, 92%, and 86%, respectively (Figure 1). In addition, distribution of albendazole monotherapy for treatment of soiltransmitted helminths (STH) in primary school children (ages 5–14 years) occurred nationally on an annual basis from 1995 to 1998 and from 2005 to 2007, and semiannually from 2008 to present. Sentinel site surveys conducted in November 2006 before the third round of LF MDA found that antigen and mf prevalence had been reduced to 0.0%. For this reason, PELF decided to stop MDA for LF after 2006. In 2011, 5 years after the final round of albendazole–DEC MDA, a communitybased post-MDA survey of 539 children aged 6–10 years old found only one (0.2%) ICT-positive individual—a girl who lived in the area for less than 2 years—further indicating that LF transmission had been interrupted. The purpose of this survey was to determine whether transmission remained absent in La Ciénaga using the World Health Organization (WHO) transmission assessment survey (TAS) protocol. Formalized in 2011, TAS is a lot quality assurancetype survey to determine whether LF prevalence is below thresholds under which recrudescence is unlikely to occur even in the absence of MDA (< 2% antigen prevalence in areas where W. bancrofti is transmitted by Culex or Anopheles mosquitoes). The target population for TAS is children 6–7 years old, as this population is born after the start of MDA and should be LF-free in the absence of local transmission. If the number of antigen positive individuals is less than a cutoff value corresponding to the antigen prevalence threshold for a given sample size, the survey area “passes” the TAS. In areas that have stopped MDA, two post-MDA TAS surveys are recommended 2–3 years following the last round of MDA, and 2–3 years apart to fulfill post-treatment surveillance for verification of transmission elimination. A systematic, school-based TAS was conducted in the three sub-barrios that comprise La Ciénaga (La Ciénaga, Los Guandules and Guachupita) from November 15 to December 5, 2014, with the three areas considered as a single TAS evaluation unit (EU). This design was selected based on high (> 95%) primary-school enrollment, estimated size of target population, and the feasibility of sampling all seven schools in the area. Assuming 3.5% of the area’s 50,000 inhabitants are 6–7 years old, a sample of 594 children in primary grades 1 and 2 (approximating the 6–7 year age group) was required. For this sample size, the corresponding critical cut-off of seven antigen-positive individuals provides at least a 75% chance of passing if the true antigen prevalence is 1.0% and no more than about a 5% chance of passing (incorrectly) if the true prevalence of antigenemia is ≥ 2%. Meetings were *Address corresponding to Gregory S. Noland, The Carter Center, 453 Freedom Parkway, Atlanta, GA 30307. E-mail: gregory.noland@ cartercenter.org