I'm sure it's been there for a while. A few months ago, I was watching the national news and they put up a map of Central and South America with the effected countries in red and the uneffected in white. Belize was in white but was completely surrounded by Mexico, Guatemala and Honduras in red. Given the number of mosquitos in Belize, especially in the jungle, I just rolled my eyes.
I chalked it up to the usual Belize's lack of resources to track and furnish that sort of information.
Luckily, it won't be an issue for the vast majority of tourists--even if they get it they probably won't know it--but for those who are pregnant they should think long and hard before visiting Central or South America. I don't know how the locals are going to cope with this.