Scientists investigating a plague of 580 marine-mammal deaths, nearly all bottlenose dolphins, in the northern Gulf of Mexico have identified a specific bacterial infection as a possible culprit.
The deaths, declared an "unusual mortality event" requiring intensive study by the National Oceanic and Atmospheric Administration, began in early 2010 and continue to this day.
Although the timeline coinicides significantly with the BP Deepwater Horizon oil spill and its aftermath, the pathology experts have not conclusively linked the spill to the deaths.
Nor have they eliminated the oil spill out as a possible "stressor" that could have weakened the dolphins to the point where the common bacteria killed them.
"We cannot rule out the role of Deepwater Horizon exposure," said Dr. Terri Rowles, director of NOAA Fisheries Marine Mammal Health and Stranding Response Program.
"Various chemical contaminants may have lowered [the dolphins'] immune response, making them more susceptible" to infection, Rowles said.
"Nutritional stress" caused by the lack of food species also could have weakened the animals, she said. It also could be that a deadlier and more virulent strain of Brucella bacteria, unrelated to the spill, has manifested itself, scientists noted.
All of the dolphin deaths took place along the Gulf coastlines of Louisiana, Mississippi, Alabama, and Florida's panhandle.
There is no evidence the "mortality event" has spread to southern Gulf of Mexico waters off South Florida, the scientists said in a Thursday press conference.
The marine species of the Brucella bacteria has been found in the remains of five dolphins from 21 dead animals studied (many other remains were decomposed to the point where they could not be studied).
That rate of nearly 25 percent of Brucella infection is is unusually high, said Stephanie Venn-Watson, a veterinary epidemiologist heading the investigation.
Typical Brucella deaths in marine mammals occur "sporadically," usually in individual cases, she said. "To see it happen in a cluster is not expected," Venn-Watson said.
The Brucella virus can cause lung and brain absesses, and caused trigger deaths of calves through premature birth.
There were 39 dolphin deaths recorded in the study area in the months of March during 2010 and 2011. The historical average would be 14 for the same two-month period.
In February 2011, there were 34 reported deaths. The historic average for February is two deaths. Additional investigation into the mortality event will continue.