Identification and control of an isolated, but intense focus of lymphatic filariasis on Satawal Island, Federated States of Micronesia, in 2003

BACKGROUND There is very limited data available on the prevalence of Bancroftian filariasis in the Federated States of Micronesia (FSM). Considerable attempts to eliminate the disease had occurred in the Pacific region by the year 2003, and the prevalence in FSM was thought to be sufficiently low that the region was considered non-endemic. However, a survey conducted in 2003 on an isolated atoll of FSM, Satawal Island, challenged that assumption. METHODS Participants on Satawal Island were recruited and their blood tested for Wuchereria bancrofti antigen by the filariasis immunochromatographic test (ICT) card and circulating microfilaria by Knott's concentration technique. A survey for active cases of lymphoedema, elephantiasis and hydrocoele was performed and mosquitoes were trapped and dissected to detect larvae of W. bancrofti. RESULTS A total of 104 males and 149 females from early teens to mid-80s were tested. Men had a significantly higher prevalence of infection than women in both the ICT test (53 vs 28%; p < 0.001) and by Knott's concentration results (37 vs 11%; p < 0.001). Microfilaria prevalence was higher in subjects ≤25 than in those >25 years of age. All persons sampled were treated for LF. No cases of elephantiasis or hydrocoele were detected. No Aedes dissected were positive but three of nine culicine mosquitoes were positive for L1-3 larval stages of W. bancrofti by microscopy. In depth interviews were conducted with residents and chiefs. CONCLUSIONS This survey showed that even in regions thought to be close to elimination, isolated high intensity foci of lymphatic filariasis may occur. These need to be recognized and control measures instituted such as local MDA as in the current study.