In fairness to Louisiana, Naegleria fowleri is all over the place ;P Actually, its prevalence in LA is lower than in many other water-logged states, since their coastal marshes are bracken water, and N. fowleri doesn’t like the ocean (though clearly it’s fine with salt, given the neti pot incidents).
As for its history, N. fowleri is a relative newcomer to the stage of Things That Can Kill You, at least in terms of our knowledge of it. R.F. Carter and M. Fowler discovered it in 1965, down in Australia. It was actually used as the example to prove the hypothesis that there were highly adaptive amoebo-flagellates out there that could live both 100% freely in the environment, as well as establish themselves within the human body, when given the opportunity and loss of original environment. You know, like when one introduces a liquid into the sinuses and the amoeba gets caught on something, and it doesn’t get washed out. It’s lost its initial environment (which it would prefer to stay in, since adaptation to a new environment is physically demanding), but it can still survive just fine.
Meningitis from amoebas (Primary Amoebic Meningitis - PAM) like what N. fowleri causes hasn’t been known for too long, either. Though the pathogenicity of Entamoeba histolyca (the source of amoebic dysentery) was established back in 1875, the fact that it could establish itself in the central nervous system and cause such rapid death wasn’t known until I believe the Korean war, though I’m not positive on that fact. Either way, we didn’t know amoebas were such nasty bugs (even if they DID already kill us with dysentery) until the mid-20th century or so.