In this Tuesday, March 8, 2016 file photo, a health agent scoops water from a boat docked at the Jurujuba beach during an operation to eradicate the Aedes aegypti mosquito, in Niteroi, Brazil. On Friday, March 16, 2016, U.S. health officials are revising their Zika travel warnings, saying it's OK for pregnant women to travel to Mexico City and other destinations at high elevations. (AP Photo/Leo Correa)

The government Friday revised its Zika travel warnings, saying it's OK for pregnant women to travel to Mexico City and other places at high elevation in outbreak regions.

The kind of mosquito that spreads the Zika virus is rare at higher elevations because of the lack of humidity and other conditions, according to the Centers for Disease Control and Prevention. Infections from Zika or the related dengue virus haven't been seen that high up.

Since mid-January, the health agency has advised pregnant women to avoid travel to areas with Zika outbreaks because the virus may be linked to a surge of birth defects in Brazil. The list has grown to about three dozen destinations, most in Latin America or the Caribbean.

Friday's revision excludes any part of those countries above 6,500 feet (2,000 meters).

CDC officials insist science drove the change, but acknowledge there were concerns that overly broad travel warnings may unnecessarily hamper trade and tourism.

"I suspect several countries will be quite pleased" by the revision, said Dr. Martin Cetron, who leads CDC's division of global migration and quarantine.

Five of the listed countries have large cities or sizeable areas at high elevations, including Mexico City, Bolivia's La Paz and Colombia's Bogota.

An estimated 40 million U.S. travelers went to Zika outbreak destinations last year, including about a half million , CDC officials estimate.

Experts think most people infected with Zika virus don't get sick. And those that do usually develop mild symptoms—fever, rash, joint pain, and red eyes. But the unexpected numbers of in Zika outbreak areas of Brazil have raised alarms.

More information: CDC maps: