Liz Szabo

USA TODAY

The Zika virus could hit Puerto Rico hard in the coming months, with the potential for tens of thousands of cases, a federal official said Wednesday.

The Centers for Disease Control and Prevention (CDC) reported the first local case of the virus on the island in December. The CDC says a total of eight Zika cases spread by local mosquitoes have been reported in Puerto Rico so far. Doctors have diagnosed one case in the U.S. Virgin Islands. Zika is not yet spreading among local mosquitoes in the continental USA.

Puerto Rico is battling outbreaks of two other diseases spread by the Aedes aegypti mosquito that also transmits Zika, said Thomas Frieden, director of the Centers for Disease Control and Prevention. Frieden said 80% to 90% of adults there have been infected with dengue virus, also known as "breakbone fever" because of its potential to cause excruciating joint pain. Hawaii also is battling a dengue outbreak.

A virus called chikungunya infected one in four adults on Puerto Rico in the first year the disease — which can cause debilitating joint pain — arrived in the Western Hemisphere, he said. Chikungunya originated in Africa. Doctors first diagnosed it in the Americas in 2013. Zika outbreaks are known to follow outbreaks of chikungunya.

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Frieden said he's concerned about the risk to pregnant women and babies in Puerto Rico. Health officials in Brazil report the Zika outbreak there is coinciding with a surge in cases of microcephaly, a birth defect in which babies are born with abnormally small heads and incomplete brain development.

The CDC refrains from saying Zika definitely causes microcephaly, but the evidence linking the birth defect to the virus is growing stronger, Frieden said. Scientists found genetic material from Zika in the brains of dead babies and fetuses with microcephaly.

More than 34,000 babies are born on Puerto Rico each year, Frieden said. Scientists don't yet know how often Zika infections in pregnant women result in birth defects.

About two dozen CDC staff are stationed in Puerto Rico, in addition to the 50 staff who normally work at the agency's dengue research center located on the island, Frieden said. A total of 500 CDC staff are working on Zika worldwide.

The CDC recommends pregnant women avoid traveling to places with Zika outbreaks. If they cannot avoid travel, they can protect themselves by wearing long sleeves and pants when outdoors and using mosquito repellent.

Travel has the potential to spread Zika infections, especially in the spring and summer, when mosquitoes become active. About 40 million people a year travel between the USA and places with Zika outbreaks, Frieden said. Spring break, a popular time to visit Puerto Rico, is only about a month away.

Although people infected with Zika could spread the virus to mosquitoes on the U.S. mainland, researchers say large-scale outbreaks are less likely than in South America or the Caribbean. That's partly because the climate is cooler in the USA and the Aedes mosquito's range is limited.

Cities in the continental U.S. are less densely populated than in the Americas and Caribbean, with less poverty and trash on the roads. American homes are more likely to have window screens and air conditioning, which keep mosquitoes out.

The USA limited previous outbreaks of dengue and chikungunya, suggesting Zika outbreaks on the mainland could be contained with aggressive mosquito control, said Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, part of the National Institute of Health.

There is no approved vaccine or treatment against Zika. Scientists are hoping to create a vaccine based on existing ones against dengue and yellow fever, which is also spread by the Aedes mosquito, Fauci said.

Early human trials of a Zika vaccine could begin by August or September, although it could take another three to four months to know if experimental vaccines are effective and safe, Fauci said. Larger human studies could begin in 2017.

If Zika continues to be a major public health threat, the Food and Drug Administration could speed the approval of an emergency vaccine. The priority for vaccinations would be for women of child-bearing age, Fauci said. At least 15 companies are working on a Zika vaccine, according to the World Health Organization.

Developing a safe Zika vaccine could be complicated, because the virus is associated with Guillain-Barre syndrome, a disorder where the immune system attacks the nervous system, resulting in paralysis, said Michael Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota.

"If you have an infectious agent that causes Guillain-Barre syndrome, you have to be concerned that the vaccine could cause Guillain-Barre syndrome," Osterholm said.

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