St. Louis Encephalitis peaks in which time of year?

• A. Early July
• B. Mid July
• C. Late August
• D. October

Answer: (C)

St. Louis Encephalitis transmission is driven by how many mosquitoes are active and how much virus is circulating in birds, so the risk to humans climbs as the summer progresses and peaks in the late summer. In warm, late-summer conditions, Culex mosquitoes are abundant and have had ample time to pick up the virus from infected birds, amplifying it within the local cycle. That combination—maximum vector populations plus high levels of viral amplification in birds—creates the highest chance of human infections, which is why the peak occurs around late August. Earlier in the summer, mosquito numbers and virus amplification are still building, so the peak hasn’t been reached yet. By October, cooler weather reduces mosquito activity, and transmission typically declines.