The Zika virus is spread through mosquito bites, and there is no medicine or vaccine for it. There have been outbreaks in the Caribbean and Latin America.

NEW YORK — U.S. health officials issued new guidance Tuesday for doctors whose pregnant patients may have traveled to regions with a tropical illness linked to birth defects.

Doctors should ask pregnant women about their travel and certain symptoms, and — if warranted — test them for an infection with the Zika virus, the Centers for Disease Control and Prevention said Tuesday.

If there are signs of an infection or there are other reasons to believe the fetus is affected, ultrasounds should be considered to monitor the baby’s development, the CDC advised.

The virus is spread through mosquito bites, and there have been outbreaks in parts of the Caribbean and Latin America. There is no medicine or vaccine for it.

Usually the infection only causes a mild illness, if at all. Most infected people don’t develop symptoms. In those that do, the worst of it involves fever, rash, joint pain, and red eyes — which usually lasts no more than a week.

But there’s mounting evidence linking the infection to a birth defect called microcephaly, in which the head is smaller than normal and the brain may not have developed properly.

Usually rare, more than 3,500 babies with the condition have been reported in Brazil since October. The connection to Zika is still being investigated, and health officials note there are many causes of the condition, including genetics, and exposure during pregnancy to alcohol and certain germs and toxic chemicals.

In the Brazil cases, most of the mothers apparently were infected during the first trimester, but there is some evidence the birth defect can occur later in pregnancy, CDC officials have said.

Last week, CDC officials said pregnant women should consider postponing trips to 14 destinations — Brazil, Colombia, El Salvador, French Guiana, Guatemala, Haiti, Honduras, Martinique, Mexico, Panama, Paraguay, Puerto Rico, Suriname and Venezuela.

They also advised women who are trying to get pregnant or thinking of getting pregnant to talk to their doctor before traveling to those areas.