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St. Louis encephalitis virus has a geographic range running **from Canada to
Argentina**. Human cases have **almost exclusively occurred in the United
States**.
That mismatch is the interesting fact about this disease. The virus is spread
across two continents; the illness is, in practice, an American one. Whatever
explains it — the mosquito species present, the bird hosts, how people live
alongside both — it is not the range of the virus.
Where and when
Most cases have occurred in eastern and central states, where
urban-centred outbreaks have recurred since the 1930s. In more recent
years, sporadic cases and outbreaks have occurred in the Southwest.
The annual number of reported neuroinvasive disease cases **fluctuates
widely** — this is not a disease with a steady baseline.
| Region | Season |
|---|---|
| Temperate areas of the United States | Primarily late summer or early autumn |
| Southern states, where the climate is milder | Year-round |
Who is at risk
**Everyone who lives in or visits an area where the virus has been
identified**, and particularly **people who work or take part in recreational
activities outdoors**. There is no group the disease selects for beyond
exposure to mosquitoes.
How it is counted
St. Louis encephalitis is a nationally notifiable condition. State and
territorial health departments report cases to the CDC using standard case
definitions, and the CDC recommends the national surveillance case definition
be applied consistently by every state — which is what makes the year-to-year
fluctuation above meaningful rather than an artefact of who counted what.
Case data is updated regularly in the summer and autumn, with historic data
available from 2003 onward.
Source: Centers for Disease Control and Prevention.
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