West Nile virus monitoring programs started detecting this year’s first batch of disease-carrying mosquitoes and infected birds earlier this month in Illinois, California, Tennessee and other states. Forecasting how many people may catch the disease presents a challenge for officials, according to the Centers for Disease Control, because many local factors influence the disease along with larger scale weather patterns.
A frigid winter and chilly spring may have worked to knock the insect population down in many areas of the U.S. However, the rainy spring that helped break the drought in many areas also increased the number watery breeding sites for disease-spreading mosquitoes. High temperatures also increase the rate of mosquito reproduction, according to the CDC.
Social and economic factors also play a part in West Nile outbreaks, according to the CDC. Higher risk of West Nile exists in places where people spend more time outdoors, have less access to enclosed, air-conditioned areas or lack public health initiatives to destroy mosquito breeding areas.