Using kernel density estimates to investigate lymphatic filariasis in northeast Brazil

Abstract

After more than 10 years of the Global Program to Eliminate Lymphatic Filariasis (GPELF) in Brazil, advances have been seen, but the endemic disease persists as a public health problem. The aim of this study was to describe the spatial distribution of lymphatic filariasis in the municipality of Jaboatã o dos Guararapes, Pernambuco, Brazil. An epidemiological survey was conducted in the municipality, and positive filariasis cases identified in this survey were georeferenced in point form, using the GPS. A kernel intensity estimator was applied to identify clusters with greater intensity of cases. We examined 23 673 individuals and 323 individuals with microfilaremia were identified, representing a mean prevalence rate of 1.4%. Around 88% of the districts surveyed presented cases of filarial infection, with prevalences of 0-5.6%. The male population was more affected by the infection, with 63.8% of the cases (P,0.005). Positive cases were found in all age groups examined. The kernel intensity estimator identified the areas of greatest intensity and least intensity of filarial infection cases. The case distribution was heterogeneous across the municipality. The kernel estimator identified spatial clusters of cases, thus indicating locations with greater intensity of transmission. The main advantage of this type of analysis lies in its ability to rapidly and easily show areas with the highest concentration of cases, thereby contributing towards planning, monitoring, and surveillance of filariasis elimination actions. Incorporation of geoprocessing and spatial analysis techniques constitutes an important tool for use within the GPELF.

<p>Mapping and monitoring for a lymphatic filariasis elimination program: a systematic review</p>

“…The map so created showed that infection with W. bancrofti was more widespread than previously appreciated. 26 AfricaUgandaOnapa et al, 20058545 districts19 of 76 sites require MDA17,533 childrenAg prevalence5–19An Ag survey was carried out among school children (5–19 years) in 15 districts. The study suggested that screening of school children for Ag was a simpler and useful approach to mapping the geographical distribution of LF. 27 Eastern MediterraneanEgyptHassan et al, 199886Nile delta area covering 201 villages of 11 districtNot mentionedNot mentionedMf prevalenceAll agesThe first study on spatial analysis that looked into clustering of Mf prevalences (up to 2 km) at community level and showed heterogeneous pattern in filariasis transmission.Correlation between low Mf prevalence and higher humidity, low temperature and low rainfall were significant. 28 AfricaAfricaGyapong et al 20018787 communities selected using 25x25km grid sampling method; 30 communities of these were at 50x50km gridsNot mentionedNot mentionedAg prevalence>15A spatial sampling grid of 50km interval between villages was used for sampling villages for rapid assessment of filariasis endemicity using Ag. This approach of mapping was recommended to capture the cross-border foci, which was found to exist.…”

“…A few studies mapped the distribution of LF61,62,66,83,84 using Mf and or Ag surveys and showed that the LF was more widely spread than it was previously thought in those countries. Few studies went further to carry out spatial analysis of the filarial prevalence data collected for mapping through a spatial sampling grid with 50 km distance between sampled locations and predicted LF prevalence at 25 km distance 60,77,85–87…”

Urban lymphatic filariasis

“…Eighty percent of the districts had one or more positive cases of mf (with highest prevalence of 5.1 %), and a positive association was observed between mf prevalence and an established Social Deprivation Index for districts (Bonfim et al 2009b). The authors concluded these studies by utilising the established indicators/indices for spatial analysis and risk-mapping of LF in the urban environment (Bonfim et al 2011; Brandão et al 2011; Medeiros et al, 2012). …”

Towards global elimination of lymphatic filariasis: a systematic review of the application of spatial epidemiological methods to enhance surveillance and support elimination programmes

“…†Some studies reported more than one aim or outputs. ‡Two studies mapped the standardised parasite density ratio, 30 43 one study mapped the density of microfilaremia distribution 58 and seven studies mapped the risk of LF occurrence (based on having a seromarker above a threshold). 16 BMJ Public Health study of malaria, 3 which may have promoted the use of spatial methods in Africa, where endemic areas for both diseases often overlap.…”