Bluetongue virus model
1. Risk assessment for Austria
Abstract
Bluetongue is an arboviral disease of ruminants causing significant economic losses. Our risk assessment is based on the epidemiological key parameter, the basic reproduction number. It is defined as the number of secondary cases caused by one primary case in a fully susceptible host population, in which values greater than one indicate the possibility, i.e. the risk, for a major disease outbreak. In the course of the Bluetongue virus serotype 8 (BTV-8) outbreak in Central European 2006 we developed such a risk assessment for the location of the University of Veterinary Medicine Vienna, Austria. Basic reproduction numbers were calculated using a well-known formula for vector-borne diseases considering the population densities of hosts (cattle and small ruminants) and vectors (biting midges of the Culicoides obsoletus spp.) as well as temperature dependent rates. The latter comprise the biting and mortality rate of midges as well as the reciprocal of the extrinsic incubation period. Most important, but generally unknown, is the spatio-temporal distribution of the vector density. Therefore, we established a continuously operating daily monitoring to quantify the seasonal cycle of the vector population by a statistical model. We used cross-correlation maps and Poisson regression to describe vector densities by environmental temperature and precipitation. Our results comprise time series of observed and simulated Culicoides obsoletus spp. counts as well as basic reproduction numbers for the period 2009-2011. For a spatio-temporal risk assessment we projected our results from the location of Vienna to the entire region of Austria. We compiled both daily maps of vector densities and the basic reproduction numbers, respectively. Basic reproduction numbers above one were generally found between June and August except in the mountainous regions of the Alps. The highest values coincide well with the locations of confirmed BTV cases.
Simulated midges
Daily maps of the simulated midges population for May to October in 2010 (left) and 2011 (right), respectively.
Risk assessment
Daily maps indicate the potential risk of a BTV-outbreak for May to October in 2010 (left) and 2011 (right), respectively. Green areas (R0<1) depict areas with minor risk, red (R0>1) with moderat risk and dark red (R0>>1) with high risk.
R code
CrossCorrelationMap.R
MidgesPopulationModel.R
BasicReproductionNumber.R
midges2009-2011.txt
climate2008-2011.txt
The model was developed using the R statistical computing environment (R Development Core Team, 2012).
References
Brugger, K. and F. Rubel, 2013:
Bluetongue disease risk assessment based on observed and projected Culicoides obsoletus spp. vector densities.
PLoS ONE, 8(4): e60330.
Acknowledgments
We are grateful to C. Rath for supporting the Culicoides monitoring, to K. Wimmer (Vetmeduni Vienna) for servicing the black-light trap, to A. Schmalwieser (Vetmeduni Vienna) and S. Schneider (ZAMG) for providing weather data sets, and to K. Fuchs (AGES) for providing gridded data of animal density. The research was partly supported by the PL2 Start-up project Bluetongue disease model of the University of Veterinary Medicine, Vienna.