WASHINGTON — An obscure mosquito-borne virus that has already prompted warnings in Central America to avoid getting pregnant and is thought responsible for thousands of birth defects in Brazil has now reached the United States, according to health officials.

The Centers for Disease Control and Prevention said U.S. doctors should test newborns who show signs of the Zika virus, especially in states such as Florida, where mosquitoes are a daily nuisance. The advisory came the same day that Arkansas officials confirmed that they had diagnosed someone there with the virus and warned it was possible that the virus had infected the local mosquito population.

Zika has already been found in 21 countries and territories in the Western Hemisphere.