Top U.S. airlines United (UAL.N) and Delta (DAL.N) have offered to re-assign certain flight crew concerned about contracting the Zika virus from routes to Latin America and the Caribbean, the companies told Reuters on Wednesday.

The previously unreported policies show how the mosquito-borne virus, linked to thousands of birth defects in Brazil, looms as an issue not just for airline passengers but for flight attendants and pilots as well.

In an internal memo on Jan. 28, seen by Reuters, United said expectant flight attendants as well as those seeking to become pregnant could switch routes to avoid Zika-affected regions without repercussions. The airline has similar options available for pilots, said Charles Hobart, spokesman for parent United Continental Holdings Inc.

Delta Air Lines Inc has also let flight attendants and pilots switch assignments since Jan. 17, and "a small number of crew members have swapped trips to date," spokesman Morgan Durrant told Reuters.

"We have immediate concern about our members' health," said Sara Nelson, international president of the Association of Flight Attendants-CWA, when asked for comment on United's memo.

"This issue is changing at a fairly rapid pace, (and) it's important that those updates are ongoing," she said, adding that airlines appeared to be responding faster to employee concerns than they did during past outbreaks, such as the spread of Ebola in 2014.

Airlines and hotel chains have said it is too early to tell if the Zika epidemic is affecting bookings.

However, top U.S. carriers, including United and Delta, are offering refunds for flights to impacted areas. Travel agents also say "babymooners" - parents-to-be taking last-hurrah vacations - have backed out of trips and changed itineraries.