(CNN) -- Scientists investigating the stranding of hundreds of dolphins in the northern Gulf of Mexico since early last year reported Thursday that they have identified Brucella bacteria in five of 21 tested and are trying to determine whether the deaths may be linked to last year's BP oil spill.
"We believe these five dolphins died from brucellosis," said Teri Rowles, coordinator of the National Marine Mammal Health and Stranding Response Program for the National Oceanic and Atmospheric Administration.
"Die-offs from bacterial infections could be occurring because the bacterium has become more lethal, but they could also be occurring, or be more severe, because the dolphins are more susceptible to infection."
Severe environmental stress, such as exposure to oil, could have reduced the animals' ability to fight infection, she said, adding that investigators were trying to pinpoint the cause.
Another possibility is that the bacterium itself has changed so that it causes more serious disease, she said.
The BP spill is "only one of several things you could look at as a possible cause of reduced immunity or increased susceptibility," she said.
BP did not immediately return a call and an e-mail seeking comment.
The government scientists are investigating what they call an unexplained mortality event -- the stranding of 580 dolphins and whales, most of them bottlenose dolphins -- between February 2010 and October 23. That vast majority of the animals were found dead or died shortly after stranding, the scientists said.
NOAA researchers focused on Brucella because of its association with failed pregnancies; the stranded dolphins included a higher-than-expected number of neonatal deaths, said Stephanie Venn-Watson, a veterinary epidemiologist and chairwoman of the working group that advises NOAA on the investigation.
But little is known about marine Brucella, she said. Separate forms of the bacteria can infect dogs, pigs, cattle, goats and other animals.
"All of these have the potential to infect people," she said, with the species in cattle being more virulent.
She said NOAA researchers were working with the Centers for Disease Control and Prevention and the National Institute of Occupational Safety and Health to determine the occupational risk to people working with marine mammals. The researchers recommended that people keep themselves and their pets away from any stranded marine mammals.
Of the 21 animals tested, nine were from waters off Louisiana, 10 from Mississippi and two from Alabama, Venn-Watson said. The five that tested positive for Brucella were all from Louisiana. Two were fetuses that died in utero from bacterial pneumonia caused by Brucella; one adult died from brucellosis affecting the lungs; and two adults died from meningitis, an infection of the tissues that encase the brain that is caused by the bacteria.
"What makes this interesting is the fact that not only was the Brucella identified, but it is associated with pathology consistent with what this bacterium can do," Venn-Watson said.
An additional fetus that died with the death of its mother, which had meningitis, also had Brucella in its lung, she said. Tests on the mother's tissues are pending.
The researchers urged the public to alert NOAA scientists if they see stranded dolphins or whales in the Gulf of Mexico.
Previous reports have identified Brucella in marine mammals off the U.S. coast, but before now, such reports have been sporadic, the scientists said.
Their ability to find more cases has been hampered by the decomposed condition of many of the bodies. "Only about 15% of dolphins stranding are what we consider ideal for a diagnostic workup," Venn-Watson said.
About 100 to 200 cases of brucellosis occur in the United States each year, according to a CDC spokeswoman. It is more common in much of the rest of the world, where animal-disease programs may not be as robust, including Portugal, Spain, North Africa, Italy, Greece, South and Central America, Eastern Europe, Asia and the Middle East, spokeswoman Lola Scott Russell said.
Symptoms in humans can include fever, sweats, headaches, back pain and an inflammation of the lining of the heart. It is fatal in less than 2% of cases, she said.
The most common mode of transmission is by eating or drinking contaminated milk products, especially those that are not pasteurized, breathing in the bacteria or having them enter through a skin wound, she said.