Microfilaria persistent foci during post MDA and the risk assessment of resurgence in India

Pondicherry, a union territory in India, is an endemic district for bancroftian lymphatic filariasis transmitted by Culex quinquefasciatus where eight rounds of mass drug administration (MDA) were completed in 2011 (annually once from 2004 to 2011).The objectives of this study were to conduct a focal survey to assess microfilaria and antigen (Ag) prevalence among young adults and to assess vector infection and infectivity through a focal entomological survey.

Mosquitoes were collected using gravid traps in Sedurapet village of Pondicherry and dissected to enumerate W. bancrofti larvae stage first larval stage (L1), second larval stage (L2), and third larval stage (L3). Microfilarias (Mf) were detected using blood smears collected from inhabitants.

A total of 360 individuals from 67 houses were enrolled in this study of which 290 (80.6%) were surveyed for the presence of Mf. Two Mf carriers were detected yielding an overall prevalence of 0.69% and two out of 85 (2.35%) were Mf antigen positive. Of the 2875 mosquitoes collected by gravid trap, Culex quinquefasciatus (93.9%) was the predominant species, followed by Anopheles subpictus (2.3%) and Culex vishnui (3.8%). The density of Cx. quinquefasciatus was 28.1 per trap-night. A total of 2429 Cx. quinquefasciatus were dissected and microscopically examined for abdominal conditions (gravid 85%, semi-gravid 9.4%, unfed 3.8%, and fully fed 1.9%) and filarial infection. One mosquito (infection rate equal to 0.04%) was found to harbor a second stage filarial larva, and none of the mosquitoes had infective stage larva.

Our results show no reappearance of infection of lymphatic filariasis in Sedurapet village of Pondicherry after MDA, and thus, no further intervention is required in that area for possible resurgence of lymphatic filariasis. However, monitoring should be continued as part of post MDA activities until the endpoint of complete elimination is achieved. We demonstrated that xenomonitoring can be used to monitor the post MDA situation for possible risk of transmission to initiate control measures.