An explanation might be that human exposure to the virus suddenly increased greatly. There have been reports that people in the eastern Turkey sometimes bring their domestic birds into their houses to protect them from the cold weather, which can be severe in eastern Anatolia.
The Figure shows the minimum daily temperature in the four cities of Erzincan, Erzurum, Kars and Van in eastern Turkey for the period 20 December 2005 to 16 January 2006 (Data missing for Erzincan, 21 and 29 December). Starting from a temperature near to 0°C the days before Christmas, there was a marked drop in temperature on 25 and 26 December, reaching below -30°C in Erzurum on 29 December. According to tables produced by the World Meteorological Organization [1] , the average daily temperatures for January in these four cities, during the period 1961-1990, were -3.3°C, -9.8°C, -9.9°C and -4.2°C, respectively. Minimum daily temperature is on average some 3 to 6 degrees lower, so the figures indicate an unusually cold spell during the week after Christmas.
Figure. Minimum daily temperature (in degrees Celsius) in four cities in eastern Turkey, 20 December 2005 – 16 January 2006. (Data missing for Erzincan on 21 and 29 December).
If the severe weather caused people to bring their domestic birds into their houses, this could explain why humans in the area were not heavily exposed until a few months after the introduction of the virus in the birds–. This would also explain why the outbreak seems to have started in several different cities at around the same time.
Acknowledgements:
Data for the figure was kindly supplied by Hans Alexandersson of the Swedish Meteorological and Hydrological Institute. Dr Guena?l Rodier, leader of the joint WHO-led mission in Turkey, was the first to voice the idea that cold weather might be one explanation for the outbreak.