The sturdy reusable grocery bag traveled with the girls' soccer team from Beaverton to Seattle for a weekend tournament, where it picked up something much less sweet than the cookies inside.

But the team members didn't know highly contagious viruses were on the bag as they passed it around during Sunday lunch, plucking out the chocolate goodies.

The next day six of the girls fell violently ill in a mysterious outbreak of norovirus, the leading cause of severe gastroenteritis in the United States.

It took Oregon scientists about five days of intensive sleuthing to pinpoint the bag as the likely culprit and lab tests to confirm its role.

That confirmation marked a breakthrough: Scientists have long known that this hardy virus is transmitted from person to person but never before have they been able to prove that transmission from an inanimate object caused an outbreak.

"In other outbreaks, we have been able to isolate the virus from door handles or keyboards, but we have never been able to show it was the keyboard or door handle that made people sick," said Kimberly Repp, epidemiologist with the

.

The investigation also highlighted how hardy the noroviruses are -- and the challenge public health officials have combating them.

The outbreak occurred in October 2010, but the investigative report was just published today in

.

The authors -- Repp and William Keene, senior epidemiologist with

-- detail how they tracked the outbreak to the contaminated bag.

Seventeen girls, ages 13 and 14, and their chaperones traveled to Seattle on a Friday afternoon in five cars to play in a weekend soccer competition pitting 120 teams from Oregon and Washington. On Saturday, one of the teens started to feel sick so she went to a chaperone, asking if she could stay in her room. The girl ended up spending six hours in the chaperone's bathroom, throwing up and suffering from diarrhea. The woman whisked her out of the hotel and drove her back to Oregon. The team played on Saturday and enjoyed Sunday lunch together in a room at the hotel before returning home that afternoon.

On Monday, six more girls came down with acute gastroenteritis. One of the mothers called public health authorities in Washington state who alerted Oregon Public Health.

Repp spent the next several days interviewing and re-interviewing the girls who got sick, trying to figure out how the virus had spread.

Clearly, the outbreak started with the first girl. But Repp and Keene couldn't figure out how she infected the others. She stayed in a room by herself on Friday, and once she felt ill had no contact with anyone besides the chaperone.

Norovirus

The highly contagious virus is the top cause of U.S. foodborne disease outbreaks and the most common cause of acute gastroenteritis in the U.S., causing about 21 million illnesses each year and contributing to roughly 70,000 hospitalizations and 800 deaths. Symptoms include vomiting, diarrhea and stomach cramps.

Most outbreaks happen when infected people spread the virus to others through close contact. But it can also spread by consuming contaminated food or water and touching surfaces or objects that have the virus on them.

Leafy greens, fresh fruits and shellfish are most commonly involved in foodborne outbreaks.

The best prevention: thorough hand washing and cleaning contaminated surfaces with a bleach-based solution.

Source: Centers for Disease Control and Prevention

Norovirus -- which caused 139 of 213 outbreaks of gastroenteritis in Oregon in 2010 -- is often transmitted through direct contact with an ill person, though contaminated water or food can cause an outbreak as well. That makes it difficult to track transmission.

"In outbreaks, it can be difficult to discern what exposure causes disease because norovirus can spread through many means," said Aron Hall, an epidemiologist with the

.

Repp and Keene questioned the girls about what car they traveled in, what they ate in Seattle, what rooms they stayed in and with whom.

"Every single thing we could think of," Repp said.

All of the six girls had eaten at the Sunday lunch. They kept mentioning the cookies, which had been passed around in the bag. The cookies were bought at a store and hadn't been opened. The only common denominator was the bag.

The first sick girl never touched the bag, but it was in the chaperone's bathroom when the girl got sick. Another team member saw it and took it to the Sunday lunch.

"It was a perfect vehicle for transmission," Repp said.

But suspicion is not proof. The Oregon scientists needed the bag to test for the virus. Two weeks later, the owner, who traveled a lot, turned the bag over at a soccer practice. Repp showed up, the bleachers packed with parents, wearing blue latex gloves and carrying a plastic bag.

"I didn't want to get sick," Repp said.

The owner of the reusable grocery bag -- until then unaware it could be contaminated -- ran to the restroom to wash her hands.

Repp and Keene sent the bag -- made out of laminated woven polypropylene -- to the state's public health laboratory in Hillsboro for testing. Three days later, the results came back. Two samples from the sides of the bag below the handle tested positive for the same norovirus strain that caused the outbreak.

The tests solved their mystery and illustrated how robust these viruses are. They can live on surfaces for weeks and survive in water up to two months, Hall said.

"Norovirus is in a group that tends to be more resistant, more environmentally stable" than other viruses, he said. "It's challenging to evaluate because we can't actually grow norovirus outside their human hosts."

The investigation also demonstrates why noroviruses are so difficult to combat, Hall said.

"What this report does is it helps raise awareness of the complex and indirect way that norovirus can spread," Hall said. "It highlights the challenge we face in trying to control this problem."

Repp does not recommend that consumers ditch reusable grocery bags. But she says they should be cleaned with sanitizing wipes or in the washing machine after traveling to a store.

"You wash your clothes after you wear them," she said. "Wash your bag after you use it."

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