Fig 1.
Spatial distribution of anthrax cases relative to hippo population size in 2004/5 outbreak.
Symbols represent case locations. Colour ramp from light to dark on gridded rectangle cells represent increasing number of at-risk hippos within a 6 km radius buffer zone. Hippopotamus amphibious live in water during day, congregating in social groups called schools. They walk out at night an average of 3–6 km to graze [4]. A buffer of 6 km radius was generated around each geo-referenced school to map high risk areas for hippo anthrax outbreaks. The study area was overlaid with 6 x 6 km grid cells and clipped to the buffers. Twenty-seven (n = 27) grid cells were used to map spatial overlap between cases and hippo populations. Cases were mostly recorded in areas with at-risk hippo populations exceeding 324.
Fig 2.
patial distribution of anthrax cases relative to hippo population size in 2010.
Table 1.
Spearman’s bivariate correlations for linear associations between hippo cases and population size in 2004/5 and 2010 outbreaks.
Fig 3.
Spatial and temporal distribution of Hippopotamus anthrax clusters in 2004/5.
Spatio-temporal data for clusters including centroid coordinates, radii, and time span were generated using the retrospective space-time permutation model of the spatial scan statistics and mapped using QGIS (Version 2.18.9). The first two clusters 1 & 2 had overlapping dates and were considered origins for this outbreak.
Fig 4.
Spatial and temporal distribution of Hippopotamus anthrax clusters in 2010.
Clusters 2 & 4 occurred upstream, relative to cluster 1 and against direction of water flow.
Table 2.
Temporal characteristics of case clusters in Hippopotamus anthrax outbreaks of 2004/5.
Table 3.
Temporal characteristics of case clusters in Hippopotamus anthrax outbreaks of 2010.
Fig 5.
Epidemic curve showing patterns of epidemic spread for weekly anthrax cases in Hippopotamus.
This outbreak occurred in clustered waves (epidemic generations), lasted 42 weeks from 25 July 2004 to May 2005 and died after 3 generations. Clusters that spanned over 20 days were considered protracted single outbreaks beyond anthrax incubation period or multiple outbreaks (Tables 2 and 3). Dashed line shows time in week 9 of outbreak when management intervention for carcass disposal was initiated. Number of cases did not drastically decline but epidemic peaks in the second and third epidemic generations dropped below the peak in the first wave instead of becoming successively larger.
Fig 6.
Epidemic curve showing patterns of epidemic spread for weekly anthrax cases in Hippopotamus in2010.
Outbreak occurred in less distinct waves, lasted 27 weeks from 11 June to December and died after 3 generations. Dashed line shows time at onset of the outbreak in week 1 when management intervention for carcass disposal was initiated. Number of new cases drastically declined immediately following response.
Fig 7.
Location specific patterns of epidemic spread for weekly Hippopotamus anthrax cases in 2004/5.
Epidemic curves were characterized by geographical locations at Lakes George and Edward (Panel A); Kazinga channel and R. Kyambura (Panel B) to examine possibilities of occurrence of a series of sporadic point source outbreaks. Epidemic waves detected were numbered 1–6 according to the sequence of occurrence of waves at the respective locations. Epidemic patterns remained clustered, progressing successively and waves lasted beyond one incubation period of anthrax (average 1–14 days) except at Kyambura River.