Lymphatic Filariasis in 2016 in American Samoa: identifying clustering and hotspots using non-spatial and three spatial analytical methods

Background: American Samoa completed seven rounds of mass drug administration from 2000-2006 as part of the Global Programme to Eliminate Lymphatic Filariasis (LF). However, resurgence was confirmed in 2016 through a community-based survey and a WHO-recommended school-based transmission assessment survey (TAS). This paper uses data from the 2016 community survey to compare different spatial and non-spatial methods to characterise clustering and hotspots of LF.Method: Non-spatial clustering of infection markers (antigen [Ag], microfilaraemia [Mf], and antibodies (Ab [Wb123, Bm14, Bm33]) was assessed using intra-cluster correlation coefficients (ICC) at household and village levels. Spatial dependence, clustering and hotspots were explored using semivariograms, Kulldorf’s scan statistic and Getis-Ord Gi* statistics at locations of surveyed households.Results: The survey included 2671 persons (750 households, 730 unique locations in 30 villages). ICCs were higher at household (0.20-0.69) than village levels (0.10-0.30) for all infection markers. Semivariograms identified significant spatial dependency for all markers (range 207-562 metres). Using Kuldorff’s scan statistic, significant clustering was observed in two previously known locations of ongoing transmission: for all markers in Fagali’i and all Abs in Vaitogi. Getis-Ord Gi* statistic identified hotspots of all markers in Fagali’i, Vaitogi, and Pago Pago-Anua areas. A hotspot of Ag and Wb123 Ab was identified around the villages of Nua-Seetaga-Afao. Bm14 and Bm33 Ab hotspots were seen in Maleimi and Vaitogi-Ili’ili-Tafuna.Conclusion:Our study demonstrated the utility of different non-spatial and spatial methods for investigating clustering and hotspots, the benefits of using multiple infection markers, and the value of triangulating results between methods.Funding Statement: This work received financial support from the Coalition for Operational Research on Neglected Tropical Diseases (COR-NTD), which is funded at The Task Force for Global Health primarily by the Bill & Melinda Gates Foundation, by UK aid from the British government, and by the United States Agency for International Development through its Neglected Tropical Diseases Program. CLL was funded by an Australian National Health and Medical Research Council Fellowship (1109035). The funders had no role in interpretation of results, decision to publish, or preparation of the manuscript. MS is funded by a fellowship from the Westpac Scholars Trust.Declaration of Interests: The authors declare that they have no competing interests.Ethics Approval Statement: This study was approved by the American Samoa Institutional Review Board and the Human Research Ethics Committee at the Australian National University (protocol number 2016/482).