As concern grows over the Zika virus, major airlines are offering to re-assign flight crew concerned about contracting the disease from routes to affected countries.

U.S. carriers United and Delta are joining Europe's Lufthansa and Air France in allowing pilots and cabin crew to switch assignments that would reduce possible contact of the mosquito-borne virus, linked to thousands of birth defects in Brazil.

Reuters obtained an internal memo on Jan. 28 from United that 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, Charles Hobart, spokesman for parent United Continental Holdings Inc, told Reuters on Wednesday.

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

The current Zika virus outbreak originated in Brazil in May of 2015. Since then, it has spread to many countries in Central and South America and to several Caribbean islands.

The most common symptoms of Zika are fever, rash, joint pain, and conjunctivitis.   The illness is usually mild with symptoms lasting for several days to a week.  But the virus has also been linked to more serious conditions of microcephaly, which is believed to cause birth defects, and Guillian-Barré syndrome,  a nerve disorder that can lead to paralysis.

Women who are pregnant have been advised to postpone travel to areas where Zika virus is present.  In addition, this week's confirmation of a case in Texas where the Zika virus was transmitted via sexual contact has also raised the concern for all travelers.

It's suggested that those who are traveling to the affected areas protect themselves against the mosquito-borne virus by wearing long-sleeved shirts and long pants, using EPA-registered insect repellents, treating clothing and gear with permethrin, and sleeping under a mosquito net if they must be outdoors.

Meanwhile, companies selling insect repellent gear are responding to the crisis.

A company called Insect Shield that makes permethrin-treated clothing has launched a new program that enables consumers to send their own clothes to them for treatment. Insect Shield, which started in 2001, began by making permethrin repellent adhere to military uniforms to protect soldiers against Lyme Disease.

And GBaby has released a line of infant clothing called “Babty Protect” containing micro-capsules that release a natural repellent made from citronella oil.

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

However, airlines have already offered refunds to customers who have booked flights to countries affected by the virus, for which there is currently no viable vaccine.