Seven staff members at the Oregon Zoo tested positive for tuberculosis after an outbreak among three elephants started in 2013, according to a report from the Centers for Disease Control and Prevention.

The seven had a latent form of the respiratory disease and displayed no symptoms, according to the CDC report released Thursday.

The report adds to the somewhat thin knowledge about the transmission of tuberculosis from elephants to people, said Dr. Jennifer Vines, deputy health officer for Multnomah County.

According to the CDC, the illness is one of many that can spread from pets or wildlife to humans. Those include bird flu, mad cow disease and West Nile virus.

Roughly 5 percent of the captive Asian elephants in North America are infected with the disease, the report said.

People for the Ethical Treatment of Animals said in a statement Friday that the report confirms what the group has been saying for years about the risk of captive elephants to human health.

"People concerned about their own health as well as the elephants' should stay far away from circuses, elephant rides, and any other cash-grabbing stunts still featuring elephants," said Rachel Mathews, counsel with PETA's Captive Animals Law Enforcement unit.

The outbreak at the Portland-based zoo began in in May 2013, when a test on an elephant named Rama was positive. At the time, the animals were checked annually for TB by testing secretions from their trunks. Packy tested positive in December 2013 and Tusko did so in June 2014.

Bob Lee, the zoo's elephant curator, told The Oregonian (http://is.gd/XaWEX8) that the infected elephants were put on a months-long round of treatment and the zoo enacted safety measures, such as keeping the public at least 100 feet away.

While zoo staff tended to the animals, Multnomah County epidemiologists tried to identify human cases. Health officials found 118 people who might have been at risk for the disease that's generally spread by coughing and sneezing. They included zoo staff with close contact with the animals and volunteers and members of the public who may be have been exposed to trunk secretions or elephant feces.

The report said no one who tested positive had spent time in TB-endemic countries or had other risk factors, such as a history of injection drug use. Those with positive tests had chest radiographs, were evaluated for symptoms and were offered free medication.

Two of the three infected elephants were later euthanized because of painful injuries. Rama had an old leg injury, and Tusko had a decades-old foot problem.

"TB wasn't a factor in deciding to euthanize them," Lee said. "They had done really well with the treatment."

Packy, who's 53 years old, continues to receive medication and will rejoin the herd when his treatment is finished.

The outbreak has led to increased TB testing. Lee said employees who have close contact with elephants are now tested more than just once a year, which is standard for zoo employees across the country. The zoo's six elephants are also tested more frequently. The males are tested monthly and the females get checked once a quarter.

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