Last year's Gulf of Mexico oil spill may be contributing to a rash of dead baby dolphins along the Gulf Coast, federal scientists said yesterday.

National Oceanic and Atmospheric Administration investigators said they have identified Brucella bacteria in five bottlenose dolphins found on beaches in the northern Gulf.

Environmental stressors could have inhibited the dolphins' immune systems, making them more vulnerable to Brucella and diseases such as pneumonia and meningitis.

But NOAA researchers were careful to emphasize that they cannot definitely say oil that spewed from a ruptured BP PLC well after a rig explosion in April 2010 contributed to the dolphin deaths.

"We don't know if this is indeed related to the BP oil spill," said Teri Rowles, the coordinator of NOAA's National Marine Mammal Health and Stranding Response program.

However, NOAA is operating under two hypotheses in testing dolphins now that it has identified Brucella as one cause of the deaths. One theory is the spill weakened their immune systems. Another is that the Brucella pathogen somehow advanced, making it more toxic to the animals.

As of Oct. 23, there have been 580 dolphin strandings in the northern Gulf, far more than typically occur in a year. Ninety-five percent of those animals were dead when they washed up on shore, and a significant portion have been babies, prenatal or still-born.

The strandings led NOAA to announce an "unusual mortality event" earlier this year and begin investigating the cause of the deaths (Greenwire, March 10).

The results released yesterday were based on testing 21 dolphin carcasses. NOAA found Brucella was the cause of death for five but said it may have also contributed to the others.

Investigators also said they are screening another 33 dolphins for Brucella but that the strandings are typically decomposed when they arrive at their lab, making testing difficult.

Stephanie Venn-Watson, a veterinary epidemiologist and chairwoman of NOAA's working group, emphasized that the Brucella finding is unique.

"It is very unusual for Brucella to be associated with an outbreak in dolphins," she told reporters on a conference call.

Still Venn-Watson said they are also looking at other causes of the outbreak, such as infectious diseases and marine biotoxins.

"With this unusual mortality event including a high number of fetuses, we included Brucella testing due to its known ability to infect dolphins and cause their pregnancies to fail," she said.

Brucella often affects cattle, goats, dogs and pigs and can lead to illnesses and failed pregnancies. It is rare that Brucella causes illness in humans, only about 100 to 200 people are infected with brucellosis each year, according to the Center for Disease Control and Prevention.

Even so, NOAA recommended that people should not attempt to handle stranded dolphins.