Seven staff at the Oregon Zoo were infected with tuberculosis following an outbreak starting in 2013 among three bull elephants - Packy, his son Rama and Tusko.

The seven people who developed a latent form of the disease without symptoms had close contact with the elephants, according to a report published Thursday by the Centers for Disease Control and Prevention. An eighth person, a volunteer, also developed a mysterious case of tuberculosis.

None of those people were infectious and nobody in the public was at risk, said Dr. Jennifer Vines, deputy health officer for Multnomah County.

The report adds to the somewhat thin knowledge about the transmission of tuberculosis from elephants to people, Vines said. The good news is that even though TB is highly contagious, the three infected elephants at the zoo did not spread the disease to visitors, including those who attended one of Rama's painting parties in which he created splatter paintings.

TB is a respiratory disease that's spread through the air when a person - or in this case elephant -- coughs, sneezes or otherwise spreads the bacteria. About 5 percent of the captive Asian elephants in North America are infected. The disease can be deadly to elephants. Three pachyderms at an exotic animal farm in Illinois died from the disease between 1994 and 1996, according to the CDC. One handler in that outbreak got sick as well.

At the Oregon Zoo, the first case popped up in May 2013, when a test on Rama was positive. At the time, the animals were checked annually for TB by testing secretions from their trunks. In December, Packy tested positive, followed by Tusko in June 2014.

All three animals were put on a months-long round of treatment and the zoo enacted safety measures, according to Bob Lee, the zoo's elephant curator.

He said staff with close contact with the elephants have donned protective masks that are form fitted to their faces. Employees also stopped using a pressure washer to disinfect the areas and switched to a regular hose to avoid blasting bacteria around. The infected elephants were kept away from the other pachyderms but still could see, hear and smell the other elephants.

"There was a lot of contact," Lee said. "We just don't want them sharing the same air space."

The infected elephants were also kept at least 100 feet away from the public, four times more than the recommended distance, Lee said.

Tusko completed his 18-month long treatment and was reintroduced to the herd. Rama had just finished treatment when he was euthanized last March. Tusko was put to sleep in December. Tusko had mobility issues caused by a decades-old foot injury and Rama was hobbled by an old leg injury.

Lee said they were euthanized to put them out of pain not because of the infection.

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

While zoo staff tended to the animals, county epidemiologists launched an investigation to identify any human cases. Health officials found 118 people who they felt may have potentially been at risk. They included zoo staff with close contact with the animals and volunteers and members of the public who may be have exposed to trunk secretions or elephant feces.

Only seven people among the group of staff with close contact with the pachyderms tested positive. The report indicates that six of those people were likely infected by Rama. Investigators were not able to determine how the seventh became infected. None had any symptoms and was not contagious but they were offered free medication anyway to prevent them from getting sick.

The treatment is voluntary but the staff members will be screened periodically and none has developed symptoms, Vines said.

The eighth person is a bit of a mystery. That person was a volunteer and only around the elephants during an orientation.

"This person was a volunteer at the zoo but only had an hour of time spent in the elephant barn," Vines said.

That person had the exact same bacterial strain as the one that infected Rama. But the patient was diagnosed and treated in 2012, before any of the positive elephant tests.

"There is not a tidy bottom line to this," Vines said. "There are three possibilities."

She said perhaps it was coincidental that the strains matched, though that's highly unlikely. They were tested by whole genome sequencing. She said it was also possible that there was another animal or person who infected that patient, though the investigation didn't turn any up. The third possibility is that an elephant was infected in 2012 but that zoo staff didn't know about it. A negative elephant test is not conclusive, according to Oregon Zoo staff.

Nineteen people who attended the orientation and were tested came out negative, Vines said.

Nevertheless, People for the Ethical Treatment of Animals said in a statement that the infections confirm "that the risk to human health posed by captive elephants is serious."

The outbreak has changed testing among zoo staff. Lee said employees with close contact with the elephants are now tested more than just once a year, which is standard for all 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.

The new elephant space was designed to accommodate infected elephants, with Packy in an area with its own ventilation. Though he's had some troubles with the treatment, going off medication when his liver values went up, he's now doing well on the drugs, Lee said.

If all goes well, the 53-year-old bull elephant will be back with the herd when his treatment is over.

-- Lynne Terry

lterry@oregonian.com; 503-221-8503