U.S. health officials on Tuesday warned doctors to be alert for dengue cases as the tropical disease breaks international records. In barely six months, countries in the Americas have already broken calendar-year records for dengue cases. Watch NBC6 free wherever you are WATCH HERE WATCH HEREThe World Health Organization declared an emergency in December, and Puerto Rico declared a public health emergency in March. There are four types of dengue virus, simply known as 1, 2, 3 and 4. So far this year, there have been nearly 1,500 locally-acquired U.S. cases, nearly all of them in Puerto Rico.