Landscape, demographic, entomological, and climatic associations with human disease incidence of West Nile virus in the state of Iowa, USA

Abstract

Background: West Nile virus (WNV) emerged as a threat to public and veterinary health in the Midwest United States in 2001 and continues to cause significant morbidity and mortality annually. To investigate biotic and abiotic factors associated with disease incidence, cases of reported human disease caused by West Nile virus (WNV) in the state of Iowa were aggregated by census block groups in Iowa for the years 2002-2006. Spatially explicit data on landscape, demographic, and climatic conditions were collated and analyzed by census block groups. Statistical tests of differences between means and distributions of landscape, demographic, and climatic variables for census block groups with and without WNV disease incidence were carried out. Entomological data from Iowa were considered at the state level to add context to the potential ecological events taking place.

Results: Numerous statistically significant differences were shown in the means and distributions of various landscape and demographic variables for census block groups with and without WNV disease incidence. Census block groups with WNV disease incidence had significantly lower population densities than those without. Landscape variables showing differences included stream density, road density, land cover compositions, presence of irrigation, and presence of animal feeding operations. Statistically significant differences in the annual means of precipitations, dew point, and minimum temperature for both the year of WNV disease incidence and the prior year, were detected in at least one year of the analysis for each parameter. However, the differences were not consistent between years.

Conclusion: The analysis of human WNV disease incidence by census block groups in Iowa demonstrated unique landscape, demographic, and climatic associations. Our results indicate that multiple ecological WNV transmission dynamics are most likely taking place in Iowa. In 2003 and 2006, drier conditions were associated with WNV disease incidence. In a significant novel finding, rural agricultural settings were shown to be strongly associated with human WNV disease incidence in Iowa.

Figure 1

Hot-spot analysis result at five km distance threshold. Results of the hot-spot analysis (Getis-Ord Gi*) for human WNV disease rate from 2002–2006 by census block group with a 5 km threshold analysis distance. The values above 2.0 represent statistically significant hot-spots while below -2.0 are statistically significant cold-spots. The majority of census block groups fall in the statistically insignificant middle categories.

Figure 2

Mean population density in census block groups with zero to three WNV human disease cases (2002–2006). On the x-axis is the mean population density (persons/ha²) for census block groups calculated according to the number of WNV disease cases (i.e. 0, 1, 2, or 3 cases). The number of census blocks which had that number of cases is shown above the point in the graph.

Figure 3

Centroids of census block groups with WNV disease overlain on Iowa population density. The centroids of those census block groups with WNV disease are overlain on a layer of populations density spit into three classes (Low = 0 – 0.5 persons/ha, Medium = 0.5 – 4 persons/ha, and High > 4 person/ha).

Figure 4

Absolute t-values for landscape variables based on Students t-test. This graphic represents the absolute t-value from the Students t-test comparing the means of landscape variables for census block groups with and without human WNV disease incidence. Those landscape variables to the left of the vertical line are ones in which there were lower means for census block groups with WNV disease incidence (negative) and those to the right were ones in which there were higher means for census block groups with WNV disease incidence (positive). All except grassland, water, and slope were significant (P <= 0.006). Those labelled with * used the 'equal variance not assumed' version of the t-test in SPSS. The agriculture class was reclassified from alfalfa, corn, soybeans, and other row crops.

Figure 5

Census block groups with and without WNV disease overlain on Iowa 2002 land cover. The census block groups with WNV disease are highlighted with a darker outline. The land cover dataset was produced by the Iowa Department of Natural Resources and metadata can be found at [43].

Figure 6

Human WNV disease cases in Iowa (2002–2005) compared to state mean NDVI scores. Total human WNV disease cases in Iowa by year (2002–2005) plotted against scaled mean NDVI scores from 2002–2005. The mean NDVI scores were calculated by calculating an average NDVI raster based on 14 separate scenes from March until September of each year. The mean NDVI score for each year was then calculated from this raster and plotted against the number of WNV disease cases.

Figure 7

The proportion of Culex mosquitoes and the WNV disease incidence rate in Iowa counties. The mosquito proportions were derived from the databases at the Iowa State University Medical Entomology Laboratory IowaMosquito.net website while the WNV disease incidence rates were derived by dividing the number of cases in a county by the population and multiplying by one million to get the rate per million people.