U.S. officials are starting a massive effort to protect American athletes from Zika, even as they are learning more about the mysterious disease in real time. Some members of the U.S. Summer Olympics delegation are already being monitored for the virus as they begin to go to Brazil, the epicenter of the rapidly evolving outbreak.

The Centers for Disease Control and Prevention is also assessing the threat that athletes and other travelers to the Games may increase the global spread of the mosquito-borne virus when they return to their home countries.

The measures highlight how seriously officials are taking the Zika threat, especially its potentially devastating effect on the fetuses of pregnant women. There is no vaccine or treatment for the disease, which has been associated with paralysis, blindness, and severe birth defects.

On Monday, 11 senators sent a letter to the U.S. Olympic Committee asking for information on how it plans to protect athletes during the Games and after they return. In response, the USOC said the special infectious disease advisory group it created in March is providing real-time information about Zika as well as other infectious diseases. Athletes have been informed about mosquito-bite prevention, will be living in air-conditioned housing, and will be given long-sleeved shirts and pants, the USOC said. They will also be given condoms for up to six months following their trip to Brazil, based on CDC recommendations for certain individuals, the letter said.

In recent weeks, the CDC and and USOC have been in round-the-clock discussions about ways to test athletes and members of the delegation for Zika during the Olympics, which begin Aug. 5, and the Paralympics, which start Sept. 7. The U.S. delegation is expected to number about 2,630, including about 815 athletes. Of the 550 Olympic athletes, more than half will be women.

“We want to make sure if there are members of the U.S. delegation who get symptoms of Zika, that they are accurately and promptly tested,” said CDC Director Tom Frieden in an interview.

Still, he stressed that because the Games will take place during Brazil’s winter, when there are fewer active mosquitoes, “for the Olympians, the risks are really very, very low.”

CDC and health officials across the globe have advised women who are pregnant to avoid traveling to regions where Zika is prevalent, and they have advised women who are trying to become pregnant, and their male partners, to use condoms or abstain from sex during their stay and after returning home.

Zika infections during pregnancy can cause serious fetal brain abnormalities, including microcephaly, which is characterized by abnormally small heads and severe developmental problems. Zika is primarily transmitted through mosquitoes but can also spread through sexual contact.

Exactly how testing of U.S. athletes and staff will take place is still being worked out. One study underway at the University of Utah is monitoring potential Zika infections in about 100 participants. The volunteers include Olympic athletes, coaches and USOC staff. Researchers have been testing their blood, urine and saliva on a regular basis since early March as they travel to and from Brazil for preparatory visits.

Sixteen have completed travel. No one has tested positive so far, said Carrie Byington, a pediatric infectious disease specialist at the university who is leading the study. Byington also chairs the USOC’s Infectious Disease Advisory Group. She hopes to expand the pilot study to 1,000 individuals, including athletes, coaches and staff who will be attending the Olympics and the Paralympics.

“We expect at least 2,000 to be traveling and we hope to enroll at least half of them,” Byington said in an interview. Researchers want to monitor those who test positive for Zika for up to two years and also follow any pregnancies and their outcomes. About one-third of participants in the pilot study have said they plan to become pregnant within a year after the Olympics, she said.

The information from this group could be particularly valuable for learning about the risks the virus poses in pregnancy. A big plus for researchers: Participants are knowledgeable about their bodies, concerned about their health and committed to the research. That’s likely to produce good responses for the study, Byington said.

Everything you ever wanted to know about the Zika virus and its spread across North and South America. (Daron Taylor, Claritza Jimenez/The Washington Post)

For athletes who get sick at the competitions, it isn’t yet clear what kind of facilities or testing will be available. It’s possible that blood and urine specimens would be collected and sent back to U.S. labs, said Martin Cetron, director of the CDC’s Division of Global Migration and Quarantine. Cetron is also a member of the USOC’s special infectious disease panel.

It’s also possible that a rapid diagnostic test could be ready by then. Many research teams are trying to develop such a test.

In most people, Zika does not even produce symptoms. Only 1 in 5 infected individuals experiences the most common symptoms: fever, joint pain, rash and conjunctivitis, or pink eye.

“We’re still exploring what the locally available diagnostic testing opportunities for athletes who may get sick,” Cetron said in an interview. “If a gap is there, is that where CDC might be able to play a role.”

The International Olympic Committee insists that the Games will go forward as planned. U.S. Olympic Committee officials have said the decision to attend the Games is up to individual athletes.

While many Olympic athletes have expressed unease about being exposed to the Zika virus in Brazil, few – at least publicly — have gone as far as withdrawing from the Games for that reason. Australia’s Marc Leishman and Fiji’s Vijay Singh – both of whom were likely to make it to Rio in golf, which is being contested as a medal sport this year for the first time since 1904 – announced this spring they would skip Rio because of Zika.

Northern Ireland’s Rory McIlroy, the world’s third-ranked golfer, said recently that he is “monitoring” the spread of the virus. He acknowledged there is a chance he could skip the Olympics – but is still planning to participate, as of now.

Earlier this month, U.S. women’s soccer goalie Hope Solo told CNBC she would “begrudgingly” participate in the Games, but added she probably would not leave her hotel room except for practices and matches.

Last week, USA Swimming decided to move its pre-Olympic training camp from Puerto Rico to Atlanta because of Zika concerns. Puerto Rico, a U.S. territory, has been the hardest-hit place in the United States, with more than 1,100 cases, including 139 pregnant women.

A public-health expert writing in the Harvard Public Health Review argued this month that the Games should be canceled or moved, because visitors infected with the Zika virus in Brazil will return home and accelerate the spread of the virus.

“Rio is not on the fringes of the outbreak, but inside its heart,” wrote Amir Attaran, a public-health and law professor at the University of Ottawa. He cited Brazilian government data showing 26,000 suspected infections in Rio, the highest of any state in Brazil.

And while it is true that the Games will take place during Brazil’s winter, he cited Rio city government data that cases of dengue, a related virus spread by the same mosquitoes, spiked 600 percent in the first quarter of 2016 compared to the previous year.

“Dengue is an outstanding proxy for Zika,” he said in an interview. Yes, there will be fewer mosquitoes in winter. “But it’s got to come down from six times the level of last year.”

U.S. health officials said the risk posed by the Olympics needs to be placed in the broader context of global travel. Even without the Games, there is an enormous amount of travel between the United States and Brazil. An average of 235,000 people already travel from the United States to Brazil every month, based on last year’s data, CDC’s Cetron said. U.S. Embassy officials estimate that as many as 200,000 Americans will visit Rio for the Games.

An estimated 500,000 to 600,000 foreign visitors are expected to travel to the Olympics, Cetron said. That accounts for only 1 to 2 percent of the total global air travel to the more than 40 countries with active Zika transmission, he said.

Still, the CDC is conducting a risk assessment for each of the approximately 180 countries with known athlete delegations. More than 10,500 athletes are expected at the Olympics and about 4,350 are likely to be at the Paralympics. Analysts are looking at the expected number of athletes and travelers likely to go to Brazil, the susceptibility of those countries to epidemic diseases, and the potential for how added travel might accelerate the spread of Zika.

The analysis, which just started, is intended to help target resources to global disease control, Cetron said.  “Do they need more diagnostics? What’s their status around vector control? It’s to get a better understanding of the public health response,” he said.

Dave Sheinin contributed to this story.

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