The CDC action was prompted by tests that found Zika, a mosquito-borne illness, in fetal and newborn tissue of Brazilian babies affected with microcephaly. The agency said additional studies are needed.
The advisory lists Brazil, Colombia, El Salvador, French Guiana, Guatemala, Haiti, Honduras, Martinique, Mexico, Panama, Paraguay, Suriname, Venezuela and Puerto Rico.
"Out of an abundance of caution, pregnant women (are) advised to consider postponing travel to areas where Zika virus transmission is ongoing," the CDC said.
The CDC suggested pregnant women who must travel talk to health care providers beforehand and follow steps to prevent mosquito bites. Women trying to become pregnant should also consult, it said.
Ecuador had previously counted four cases, but the infected people had returned from travel to affected areas, but the health ministry announced two new cases on Friday. And the infected had not left the country. The ministry suspects the patients contracted Zika locally.
Microcephaly is a neurological disorder that results in babies being born with abnormally small heads, causing severe developmental issues and sometimes death.
"We now have an accumulating number of cases in babies from miscarriage or who were born with microcephaly with evidence of Zika," said Dr. Lyle Petersen, director of the Vector-Born Disease division of the National Center for Emerging and Zoonotic Infectious Diseases at the CDC.
"That suggests a stronger and stronger relationship of Zika and microcephaly."
Other travelers are now under a level two travel alert and are asked to practice enhanced precautions. That includes using EPA-registered repellents containing DEET, using permethrin-treated clothing and gear, and staying inside screened or air-conditioned rooms.
Petersen said people often don't heed the warnings to protect themselves against mosquitoes while traveling. "Pregnant women need to take the best precautions possible when traveling where this [Zika] virus is spreading," Petersen said.
The Zika virus is transmitted when Aedes mosquitoes bite an infected person, and then spread the obtained virus by biting others. There is no vaccine to prevent it and no medicine to treat a Zika virus infection.
The Zika virus is part of the flavivirus family, which contains the deadly yellow fever virus, as well as West Nile, chikungunya, and dengue. There is no vaccine to prevent it and no medicine to treat a Zika virus infection. It's transmitted when a mosquito bites an infected person, and then spreads the obtained virus by biting others. In theory, the Zika virus could be spread through blood transfusion, but as yet there are no documented cases, the CDC said. There is one case of possible virus transmission via sexual contact.
'Pandemic in progress'
Last November, Brazilian health officials advised Brazilian women not to become pregnant after they discovered a connection between the Zika virus and an alarming increase in microcephaly. In 2014, there were only 147 cases of the neurological impairment in the country. According to the latest numbers from Brazil's Health Ministry, 3,530 cases of microcephaly and 46 infant deaths may be linked to the virus.
"That's a pandemic in progress," said Dr. Anthony Fauci, director of The National Institute of Allergy and Infectious Diseases at NIH, who wrote a recent Zika-related editorial in the New England Journal of Medicine
. "It isn't as if it's turning around and dying out, it's getting worse and worse as the days go by."
"And it's determined by officials in Brazil that peak mosquito transmitting time is in April, so we haven't begun to see the full impact of this outbreak," said Fauci.
Infectious disease control will need to step up its game if that reality comes to pass, said Fauci. "We don't have vaccines or therapeutics, and we need better diagnostics that can be done at point of care. We're aggressively working on a Zika vaccine but the best thing right now is avoidance or mosquito control."
Impact on the United States
Earlier this week health officials in Harris County, Texas, confirmed a case of Zika virus in a person who recently returned from Latin America, where the virus is endemic.
According to CDC spokesman Tom Skinner, there have been 14 imported cases of Zika virus among returning U.S. travelers from 2007 to 2014. At least another eight imported cases were confirmed by CDC in 2015 and 2016, and they are still running tests on specimens from returning U.S. travelers who became ill last year and this year, so that number could rise.
The concern, of course, is whether these cases imported to the United States could result in locally transmitted cases. The Aedes mosquito, which transmits Zika virus, is present in many areas of the United States.
"However, recent chikungunya
outbreaks in the United States suggest that Zika outbreaks in the U.S. mainland may be likely limited in size," Petersen said. Given this, it is important that we maintain and improve our ability to identify and test for Zika and other mosquito-borne diseases."