Alison Young

USA TODAY

Federal health investigators are back at the Tulane National Primate Research Center in Louisiana this week seeking the source of the lab accident that has somehow exposed at least seven monkeys to a deadly bioterror bacteria. State officials, meanwhile, are planning to test wildlife and domestic animals in the surrounding area for possible exposure.

Investigators from the Centers for Disease Control and Prevention began arriving Monday at the sprawling laboratory and monkey breeding compound north of New Orleans. The CDC will be taking a closer work at worker safety and health practices in the center's veterinary hospital, said agency spokesman Jason McDonald. The hospital has been suspected as the site of the monkeys' exposure last fall to the bacterium, Burkholderia pseudomallei, which is not found in the United States.

Despite several weeks of investigation, officials still don't how the bacteria escaped a secure lab elsewhere at the 500-acre research complex, nor do they know the extent of the contamination — including whether bacteria have colonized soil or water in the huge outdoor primate breeding colony cages on the property. The outdoor monkey cages are near a school, homes, wetlands and a river.

The monkeys were not part of experiments and should have never crossed paths with the bacterium, which can cause serious and fatal illness in humans and animals that may not develop for days to years after exposure. The bacterium is highly regulated as a research material because of its potential to be used in a bioweapon. The bacterium is found primarily in soil and water in Southeast Asia and northern Australia.

CDC staff this week will participate in discussions about environmental issues that will be led by the Environmental Protection Agency or state officials, McDonald said. Louisiana state officials last month formally asked for EPA's help in assessing whether the lab breach has resulted in the bacteria contaminating the environment around the primate center.

USA TODAY previously reported that studies of the bacteria in the wild indicate that the number of soil samples taken by the EPA and Tulane last month was too small to detect the pathogen if present. Tulane officials cite the testing, which involved 39 soil and 13 water samples, as evidence the bacteria never got outside and say it was adequate.

"CDC has found no evidence to date to suggest the organism was released into the surrounding environment and therefore it's unlikely there is any threat to the public health," McDonald said late Monday. "The agency is continuing to work with federal and local officials to assess the situation."

On Friday, tests by CDC identified two more monkeys at the center that show signs of antibodies to the bacteria, indicating they had been exposed to it. That brings to seven the number of monkeys that ongoing testing found to be either infected with or showing signs of exposure to the bacteria. Three monkeys became so ill they had to be euthanized.

The two newly identified monkeys are healthy and do not show any signs of illness, according to a statement Tulane released late Friday. Both were in the same area of the veterinary hospital around the same time as the other animals that tests show were exposed. Because their levels of antibodies are low, the CDC has recommended they undergo additional testing, according to the Tulane statement.

Tulane on Monday declined to answer USA TODAY's questions seeking details of the two monkeys' history at the primate center and whether the animals are among 177 that were treated in the hospital before contamination was suspected and released into large outdoor enclosures on the primate center's South Campus. The outdoor enclosures hold a breeding colony of about 4,000 rhesus macaque monkeys.

If infected and showing signs of illness, the CDC says, animals can shed the bacteria through their urine and feces, potentially contaminating the soil and groundwater. People and animals can become infected through direct contact with contaminated areas.

Tulane primate center Director Andrew Lackner, in a community situation update posted on the center's website late last week, said that Louisiana's state agriculture and state wildlife departments "are currently preparing a plan for testing wildlife and domestic animals, both on and off [primate center] grounds."

State officials said that testing and surveillance plans are still being developed. In addition, the state agriculture department said it is working with the U.S. Department of Agriculture on a fact sheet for animal owners, veterinarians and laboratories on what signs to look for in the event there is an exposure to animals outside the Tulane facility.

Lackner, in recent interviews with USA TODAY, has questioned whether the bioterror bacterium possibly already is naturally present in Louisiana's soil and water — but just hasn't been recognized because nobody has systematically looked for it before. In his community update last week, Lackner wrote that "various Burkholderia species have been present in domestic animals in Louisiana since at least 2004, long before any scientific study of the organism began at [the primate center]."

It's unclear what organisms Lackner is referring to as having been present in Louisiana prior to Tulane's lab accident. Tulane declined Monday to answer questions about this as well.

The CDC has said that the strain Tulane was studying in its high-containment laboratory in Covington, La., is known as Strain 1026b. It was originally recovered from a rice farmer who was sickened in Thailand in 1993. Tulane's laboratory strain is identical to the strain of bacteria that sickened the first two of the primate center's macaques last November, the CDC has said.

Jay Gee, a CDC expert on Burkholderia pseudomallei, told USA TODAY in an interview last week that the bacteria has never been found in nature in the continental United States. Although some related types of bacteria have been found in North America, they do not routinely cause the kind of serious and potentially fatal disease produced by Burkholderia pseudomallei.

For full coverage of the Tulane lab incident and other articles in USA TODAY's ongoing investigation of lab safety, go to: biolabs.usatoday.com

Follow USA TODAY investigative reporter Alison Young on Twitter: @alisonannyoung