HPAI can occur in all four seasons of the year, but is mostly seen in winter and spring. This is largely on account of two different mechanisms:

Avian influenza viruses are particularly temperature sensitive and their span of survival declines with rising ambient temperature. Moreover in summer and fall the greater intensity of ultraviolet radiation from sunshine plays a potent disinfecting role.

Henhouses are more thoroughly ventilated in summer and fall than in winter and spring. This considerably reduces virus levels and lowers the chance of infection. In addition good ventilation is beneficial to the respiratory tract mucous membrane and by lowering irritation has an important role to play in maintaining the resistance of the respiratory tract membranes.

(China.org.cn February 13, 2004)