The investigation into how two monkeys at the Tulane National Primate Research Center were infected with a bacterium the U.S. government considers a potentially severe threat to human or animal health has focused on the center’s veterinary clinic after two other monkeys showed signs of infection.

A federal investigator who visited the center in January to look into how the first two monkeys became infected reported symptoms consistent with infection by the bacterium Burkholderia pseudomallei, but subsequent testing has showed that the woman likely was not infected while at the center.

Last week, a parish spokesman said a third monkey had shown an immune response consistent with having been exposed to the bacterium but had not shown signs of melioidosis, or Whitmore’s disease, which is caused by infection with Burkholderia pseudomallei. A fourth monkey also has shown antibodies consistent with exposure to the disease, parish spokesman Ronnie Simpson said Thursday.

The fourth monkey’s only contact with the three others was at the center’s veterinary clinic, and the investigation by officials from the federal Centers for Disease Control and Prevention and the U.S. Department of Agriculture will focus there, Simpson said.

Burkholderia pseudomallei is common in southeast Asia and Australia, where it is most often found in contaminated soils and water. Infection can happen when people inhale contaminated soil or water droplets, drink water with the bacterium in it or come into direct contact with contaminated soil, according to the CDC.

The bacterium also can infect a number of animals, including sheep, goats, pigs, horses, cats, dogs and cattle. Symptoms of melioidosis mimic those of other diseases, often making it hard to diagnose. Once diagnosed, however, it is treatable with antibiotics.

Since the investigation began, numerous tests of the air, water and soil around the center, which is adjacent to a school and near three neighborhoods, have been conducted. Nine air samples have been tested; none have shown the presence of Burkholderia pseudomallei. Air and water testing is ongoing, Simpson said.

The CDC has ordered Tulane to suspend its work with so-called “select agents,” defined by the CDC as those that “have the potential to pose a severe threat to public, animal or plant health or to animal or plant products.” Other select agents include ricin, anthrax, tuberculosis and bird flu.

News of the first two monkeys’ infection was reported by WWL-TV on Jan. 16, though the infection was discovered in November. Tulane officials have stressed that they did not believe there was any threat to public health, but some public officials complained about not being told earlier of the problem.

At a rare Saturday news conference Feb. 7, CDC officials said an investigator who had been at the primate center had reported symptoms consistent with infection. Tests indicate her exposure to the bacterium was not recent, Simpson said.

The Tulane National Primate Research Center is one of seven such centers nationwide that receive the majority of their funding from the National Institutes of Health. The 500-acre, 300-employee center has about 5,000 monkeys, most of which are rhesus macaques. Scientists at the center have done research on ricin, HIV/AIDS, tuberculosis and Lyme disease.

Follow Faimon A. Roberts III on Twitter, @faimon.