40+ horses at the barn I am at, all it takes is 1, to infect the whole populous - and many of those horses compeate regularily in the LMHJA circuit.
Blood could be drawn in 1 day by 1 vet for the whole populous, before the mosquito shows up. Negative blood is taken with the vet, then *poof* a horse gets bitten....
Scarey if you think about it.
Being from Canada and being involved in the equestrian world troughout the majority of my life there, never having to do coggins, nor never hearing about it - I've never heard of horses being wiped out by this "disease". Coggin's isn't done there. Interesting.