Surveillance of lymphatic filariasis after stopping ten years of mass drug administration in rural communities in south India.

BACKGROUND: While various studies provided insight into the impact of mass drug administration (MDA), information on the dynamics of the post-MDA threshold level lymphatic filariasis (LF) infection facilitates understanding its disappearance pattern and determining the duration of post-MDA monitoring and evaluation.
METHODS: The changes in microfilaraemia (Mf) prevalence and vector infection rates were monitored for four (2005-2008) and six years (2005-2010) respectively after stopping ten rounds of annual mass diethylcarbamazine (DEC) administration in a group of five villages located in South India. Four years after stopping MDA, circulating filarial antigenaemia (Ag) status among children and adults was also assessed in two villages.
RESULTS: Overall Mf prevalence (n = 700) and vector infection rates (n=803-3520) showed a declining trend. Two villages maintained zero Mf status in each of the four years, vector infection rate was zero from the third year onwards and Ag prevalence in adults was 0.4% (n = 226). In two other villages despite persistence of Mf and vector infection there was zero vector infectivity rate during the third to sixth year and Ag prevalence among children (n = 50) was nil. In the fifth village Mf prevailed at <1.0% and Ag prevalence among 1-7 year old children was 4.6% (n = 44) and vector infectivity rate during the sixth year was 0.1% (n = 852).
CONCLUSION: The incidence of sporadic new infections is evident in highly endemic communities such as the fifth village. However, there is uncertainty on the potential of the Ag positive children to reestablish infection. Six years of post-MDA monitoring and evaluation appears to be adequate to discern the status of transmission interruption and appropriate decision making.