What factors determine when epidemics occur in the Mediterranean? Prediction of disease risk through time by climate-driven models of the temporal distribution of outbreaks in Israel.
Determination of the temporal relationships between climate and epidemics of Culicoides-borne viral disease may lead to control measures and surveillance being implemented earlier and more efficiently. Although Israel has reported few cases of bluetongue (BT) during the recent Mediterranean epidemic, outbreaks have occurred almost annually since the disease was first confirmed there (1950) with severe episodes occurring periodically. The south Mediterranean location and intensive farming of BT-susceptible European sheep breeds make the area ideal for investigation of the effect or role of climatic factors versus other potential host or virus factors in governing the timing of severe BT episodes. The authors present regression analyses of 20-year time-series of BT outbreaks versus four remotely sensed climatic variables. Low temperatures and high moisture levels (relative to average levels) in the preceding autumn coincident with the seasonal peak of vector abundance and outbreaks had a positive effect on the number of outbreaks the following year. The positive effects of high moisture levels are postulated to increase breeding site availability and refugia for adult C. imicola vectors (from desiccation) in autumn whilst low temperatures may increase fecundity, offspring size and survival through adulthood in winter by increasing initial vector population size the following year. The proportion of variance in the annual BT outbreak time series accounted for by climate factors was relatively low (approximately 20%), probably because most BT virus (BTV) circulation occurs silently, due to the circulation of non-virulent BTV strains, combined with the prevalence of relatively resistant local sheep breeds. Thus, the level of BTV transmission is poorly correlated with the rate of outbreak notification.
Braverman
Y
Y
Kimron Veterinary Institute, Beit Dagan, Israel.
Baylis
M
M
Tatem
A J
AJ
Rogers
D J
DJ
Mellor
P S
PS
Purse
B V
BV