The city is dedicating $21 million over the next three years to combat the Zika virus before even a single case has been transmitted here, Mayor de Blasio announced Monday.

The funds will go toward increased testing, control of the mosquito population and an education campaign in the wake of 40 cases in which city residents have acquired the virus while traveling outside the country.

They include six pregnant women, who are at greater risk because the virus can cause birth defects.

“We want a proactive plan in place so we can get ahead of this situation,” de Blasio said at a public-health lab in Kips Bay. “We do not see a scenario in this city or in this country where there’ll be a widespread outbreak, but that does not for a moment stop us from wanting to address even more limited problems.”

City health officials acknowledged that the type of mosquito known to carry the disease in Central and South America, as well as in the Caribbean, isn’t found in the five boroughs.

But they worry that a similar type of mosquito found here could become a carrier.

“That’s why we want to do a lot more testing,” the mayor said.