Twin Cities blood banks are taking steps to prevent the Zika virus from entering the region’s blood supply, another sign that health officials are monitoring the exotic disease closely.

Although health officials stress that the risk of Zika transmission is low in Minnesota, both Memorial Blood Centers and the Minnesota Region of the American Red Cross will ask people who have traveled to Zika-infected regions to delay blood donations until they have been on American soil for at least four weeks.

Also this week, the list of infected countries grew to 30. Jamaica and Tonga, an island nation in the South Pacific, were identified by the U.S. Centers for Disease Control as countries to be avoided by pregnant women or those trying to become pregnant.

Zika has been linked to cases of microcephaly, a birth defect that causes abnormally small heads. It is spread primarily through mosquito species that are common in tropical areas. Most people who contract the virus never develop symptoms, which can include rash and joint pain, and most never require hospital treatment.

Most of the Zika cases in the United States, including two so far in Minnesota since 2014, have been linked to travelers returning from infected areas internationally.

Still, the CDC has not ruled out the possibility that the Zika virus could spread within the United States. That most likely could occur in southern regions where the mosquito responsible for Zika transmission, Aedes aegypti, has been found. And a Texas case investigated by the CDC showed this week that Zika can be transmitted sexually. The Texas case is actually the third to indicate that sexual transmission is possible, but researchers remain unsure how much risk sexual contact poses.

Which mosquitoes?

The recent spread of Zika, which hadn’t even reached the Western Hemisphere until last year, has triggered international anxiety and a string of unanswered questions.

But for Minnesota, authorities say, the risk of transmission is low.

“There’s nothing that would support a concern about transmission now here, other than sexual transmission or blood transfusion,” said Michael Osterholm, an epidemiologist and director of the Center for Infectious Disease Research and Policy at the University of Minnesota.

“We hope that it never happens, but we just have to be prepared,” he said.

Thanks in part to its cold winters, Minnesota is inhospitable to the type of tropical mosquito that carries Zika from person to person.

“They just are not really hardy enough to survive here in the long term,” said Elizabeth Schiffman, an epidemiologist at the Minnesota Health Department who tracks mosquito-borne diseases. “It really does matter what kind of mosquito that you have.”

Zika joins a group of other mosquito-borne exotic diseases that have been imported to Minnesota through travel, yet have shown no signs of spreading locally.

Some, like malaria and dengue fever, have been recorded in the state for years. Between 2010 and 2014, there were 270 cases of malaria and 55 cases of dengue reported in Minnesota. All were cases brought into Minnesota by people who traveled abroad.

Other diseases are new. Chikungunya, a disease that causes severe join pain, first appeared here in late 2013 and later grew to 28 cases in 2014, all of them brought into the state.

Like Zika, all of these diseases are transmitted by mosquitoes that are not located here. Still, the Health Department investigates all mosquito-borne infection cases to determine the cause and to ensure that transmission patterns are not changing.

“If we got a case of malaria in someone who never left Minnesota, that would be a big deal because that might mean we were having local transmission,” said Schiffman. “We never say it is impossible, but it is extremely unlikely.”

West Nile virus is an example of a disease that came into Minnesota and later spread. A disease that originated in Africa, it was not present in the west until 1999, when an outbreak struck New York City. It eventually spread throughout the country, reaching Minnesota in 2002.

West Nile’s spread was enhanced because it was carried by several species of mosquito, including those that are among the 50 varieties of mosquitoes in Minnesota.

But West Nile is not a model for the Zika virus in Minnesota, researchers say. Not only are the mosquitoes different types, they behave differently. Mosquitoes that carry West Nile favor wetlands and wooded areas and bite at night. The mosquitoes associated with Zika are adapted to tropical urban areas, bite mainly during the day, and can breed in small containers of water — even bottlecaps.

Nonetheless, Zika is among an increasing number of diseases that are catching the attention of health care workers.

“There certainly have been more relatively explosive epidemics of mosquito-borne diseases in this hemisphere in the last several years,” said Dr. Patricia Walker, medical director of the HealthPartners Travel and Tropical Medicine Center. “When epidemics occur, we will see more disease in the U.S. primarily because we are seeing more people traveling.”

Walker said her clinic has been fielding a lot of calls about Zika.

“Mostly it has been pregnant women because of all the media coverage,” she said. “They aren’t really sick, but just curious if they should be tested for Zika.

“We are learning as we go along, just as we did with Ebola or chikungunya,” she added.

“The American public should remember that there is no such thing as a disease over there and a disease over here,” Walker said. “If we always are thinking globally at addressing emerging threats around the world at the source, then that will be better not only for those countries but for us as well.”