Divide the recorded case history by the years it spans and the American base rate comes out near two infections a year [13], against a worst year on record of eight [2]. That is what a health department outside the South has to work with. A state that logs its first case has not learned that its lakes changed; it has learned that one clinician ordered the right test on one patient. The northern confirmations cited by the Bulletin of the Atomic Scientists [6] are the whole count-based argument for a range shift, and the scientists quoted alongside them put it no stronger than suspicion [7].
The mechanism is not the weak part of that argument. The counting is, because heat moves the other variable too. Hotter weather puts more people into lakes and rivers [9], so exposure opportunities rise whether or not the organism's usable range moves an inch. Both effects push the same number, the annual case tally, in the same direction, and the tally cannot tell a public health office which one it is watching. Water temperature series and environmental sampling from named lakes could.
Surveillance is a poor early-warning system here anyway, given how the disease runs. Symptoms open as headache, fever, nausea and vomiting, and most patients die between one and 18 days after they start [5]. With more than 97 percent of known patients dying [3], at most five of the 180 U.S. patients on record survived [15]. A confirmed case is a post-mortem finding, useful to the next swimmer and to nobody else. The reporting is careful to note that overall risk stays extremely low [16], which is exactly why no jurisdiction gets a second look at the same lake.
So the burden falls on advisory work, which the CDC frames as individual and pre-exposure: nose clips in warm freshwater, no stirring of shallow sediment, head above water in hot springs, and distilled, sterile or boiled water for sinus rinses [10]. It works only if a swimmer hears it before wading in. Because the amoeba lives naturally in soil and warm freshwater and feeds on bacteria there [11], there is no lake-scale remediation to fund and nothing to dose. The exception is engineered water: it has turned up in inadequately chlorinated pools and splash pads [11], which is a maintenance failure an inspector can find. Everywhere else the tool is a sign at a boat ramp, written by an office that has never had cause to write one.
