Aedes aegypti mosquitoes are the primary way the Zika virus is spread. (Ricardo Mazalan/AP)

Federal health officials on Wednesday urged pregnant women to consider postponing travel to Brownsville, Tex., because of five local cases of Zika virus infection that have been reported.

The Centers for Disease Control and Prevention sent an advisory to clinicians in its health alert network saying that the CDC is designating Brownsville, on the border with Mexico, a Zika cautionary area for testing and travel guidance. The advisory applies to pregnant women, women of reproductive age and their sexual partners who live in or visited Brownsville on or after Oct. 29.

In addition to urging pregnant women who live outside the city to consider postponing travel there, the CDC is recommending that local pregnant women and their partners be aware of Zika transmission and take strict precautions to prevent mosquito bites.

Texas health authorities reported the first case, involving a 43-year-old woman, on Nov. 28. On Friday, health officials reported four additional cases involving people who live near the first one. Only those five cases are known to have been acquired in the Brownsville area, and the CDC said there is no evidence so far of widespread, sustained local transmission.

But because it's still warm enough in South Texas for mosquitoes to spread the virus, the CDC and state and local authorities are continuing to investigate and to urge pregnant women to take precautions. Pregnant women who live in, traveled to or had sex without using a condom with someone who lives in or traveled to Brownsville on or after Oct. 29 should be tested for Zika infection, the CDC said.

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