Federal health officials on Friday advised pregnant women to postpone traveling to 13 Latin American or Caribbean countries and Puerto Rico where mosquitoes are spreading the Zika virus, which has been linked to brain damage in babies.
Women considering becoming pregnant were advised to consult doctors before traveling to countries with Zika cases, and all travelers were urged to avoid mosquito bites, as were residents of Puerto Rico and the United States Virgin Islands.
“We believe this is a fairly serious problem,” said Dr. Lyle R. Petersen, chief of vector-borne diseases for the Centers for Disease Control and Prevention. “This virus is spreading throughout the Americas. We didn’t feel we could wait.” The C.D.C. advisory applies to 14 Western Hemisphere countries and territories: Brazil, Colombia, El Salvador, French Guiana, Guatemala, Haiti, Honduras, Martinique, Mexico, Panama, Paraguay, Suriname, Venezuela, and the Commonwealth of Puerto Rico.
This appears to be the first time the Centers for Disease Control and Prevention has advised pregnant women to avoid a specific region. The warning is expected to affect the travel industry and could affect the Summer Olympics, set for Brazil in August.
Officials at Brazil’s Health Ministry were not available for comment Friday night. Hours earlier, Philip Wilkinson, a spokesman for the Rio 2016 organizing committee, said that Olympic venues “will be inspected on daily basis during the Rio 2016 Games to ensure there are no puddles of stagnant water and therefore minimize the risk of coming into contact with mosquitos.”
On Friday, the agency delayed and then rescheduled its announcement three times, without explanation, and finally made it in a 7 p.m. telephone news conference.
The disease is expected to begin spreading in this country as the weather warms. More than a dozen isolated cases of Zika infection have been found in the continental United States, including one recently in Texas. But all were in travelers who had just returned from overseas. No local transmission has been found.
A C.D.C. epidemiologist, Dr. Erin Staples, said earlier this month that she expected Zika to follow the same pattern as other mosquito-borne viruses like dengue and chikungunya: large outbreaks in Puerto Rico followed by smaller ones in Florida, Gulf Coast states and possibly Hawaii.
A study published online by The Lancet on Friday reached similar conclusions after analyzing mosquito species ranges and air-travel patterns from Brazil.
Only one case has been confirmed in Puerto Rico, but because testing is rare and many cases show no symptoms, doctors assume there are many more. How far it spreads, officials said, will depend on which mosquitos prove adept at transmitting it and how aggressive mosquito control efforts are. The yellow fever mosquito, Aedes aegypti, is common in Florida and the Gulf Coast and is an efficient Zika transmitter. The Asian tiger mosquito, Aedes albopictus, ranges as far north as New York but it is not yet known whether it transmits Zika.
The virus first appeared on the South American continent in May. Although only one person in five ever gets symptoms, and even then it often causes only mild rashes, red eyes and fevers, women who have had it, particularly in the first trimester of pregnancy, appear to be much more likely to have children with small heads and damaged brains, a condition called microcephaly.
Zika virus has been found in brain tissue and amniotic fluid from babies who died in the womb or were born with microcephaly by both Brazilian and American scientists. Microcephaly has several other causes, including genetic defects, alcohol exposure in pregnancy, or rubella or cytomegalovirus in the mother during pregnancy.
Scientists do not know why or how Zika crosses the placenta and enters the fetal brain to do damage. It is not related to rubella or cytomegalovirus, but is related to yellow fever, dengue and West Nile virus, which are not widely known to harm embryos.
The C.D.C. advisory came after several days of consultation with outside experts. Some virologists have been urging the health authorities to issue such a warning as the threat from Zika has grown. Officials in Brazil said Tuesday that they were investigating more than 3,500 cases of microcephaly in newborns. Until last year, the country normally had about 150 cases each year.
There is no vaccine for Zika, but the National Institute of Allergy and Infectious Diseases has been working on one for the past month, said Dr. Anthony S. Fauci, the institute’s director. Because the disease is closely related to yellow fever and Japanese encephalitis, for which there are vaccines, experts think that making one should not prove as difficult as, for example, vaccines against H.I.V. and malaria have proven. Epidemiologists estimate that more than 1.5 million Brazilians have been infected.
Earlier this month, a leading Brazilian health official suggested that women in the hard-hit northeastern region postpone having babies.
Although the C.D.C. advisory appeared to meet the definition of a Level 3 alert — “avoid nonessential travel” — it was issued as a Level 2.
Levels 1 and 2 are “practice usual precautions” and “practice enhanced precautions.” A C.D.C. spokesman said it was kept at Level 2 because Level 3 indicates danger to all travelers while Level 2 means danger only to travelers with “specific risk factors” — in this case, pregnancy.