Five of the more than 500 dolphins that have washed ashore dead along the Gulf Coast from Florida to Texas over the past two years died from a bacterial infection called Brucella, federal scientists announced Thursday.
And last year's Deepwater Horizon oil spill may have played a role, they said.
"The working hypothesis is that the oil negatively impacted the dolphins' immune system," said Teri Rowles, coordinator of the National Marine Mammal Health and Stranding Response Program. "It could have impaired the dolphins' ability to respond to the bacterial infection."
What got scientists' attention, Rowles said, is that while some previous dolphin deaths have been attributed to Brucella infections, those have never amounted to more than one at a time.
"To see it happen in a cluster . . . is not expected," she said.
However Rowles cautioned that there could have been other reasons why the rest of the dolphins died in the gulf and pointed out that the investigation is continuing.
"We cannot rule in, or out, a role for the Deepwater Horizon exposure," she said. Another possibility is that the Brucella has become stronger for some reason — but they don't know why that would be. Or the loss of available food may have played a role.
They do think there are likely to be more bodies washing up.
"We don't consider this event over," Rowles said.
The infection afflicting the dolphins is not the first such event that scientists suspect is tied to last year's oil spill.
Over the winter, anglers began pulling in red snapper that didn't look right. The fish had dark lesions on their skin. Many had fins rotting away, and discolored or even striped skin. Inside, they had enlarged livers, gallbladders and bile ducts.
Jim Cowan, an oceanographer at Louisiana State University who examined them, said in April, "There's no doubt it's associated with a chronic exposure to a toxin."
Cowan said he thought the toxin to be oil from the BP spill. The investigation is continuing.
The number of dead dolphins began to surge last year, leading to fears that they were being killed by the BP oil spill.
The surge actually began at least a month before the explosion of the Deepwater Horizon oil rig off the Louisiana coast on April 20, 2010. But the acceleration in deaths has pointed fingers at the oil spill.
From March 2010 to last week, 580 bottlenose dolphins and sperm whales have died and washed ashore in Florida, Alabama, Mississippi, Louisiana and Texas. The total for last year was 265.
The number has increased this year, with 290 dolphins having washed ashore dead along the Gulf Coast from Texas to Florida so far. That figure greatly exceeds the average for the previous decade, according to records kept by the National Oceanic and Atmospheric Administration.
In this year's surge of deaths, March was the worst month, with 67 dead dolphins washing up onto the beaches. In February, when 57 dolphins washed up on the beaches dead, 34 of them were premature newborns or stillborn dolphins.
In the past, the average number of newborn and stillborn dolphins to wash ashore in that area in February was just two.
One dolphin that washed ashore last December was coated in oil that matched what spewed out of the Deepwater Horizon rig, which may be another indication that the two are connected. NOAA officials noted that only a fraction of dead dolphins and whales in the gulf wash ashore, so the actual number of deaths is almost certainly higher than what can be confirmed.
Many of the dolphin carcasses were too decomposed for further study, NOAA officials said. But they were able to test 21 of the dead dolphins for Brucella, and five tested positive, indicating that's what killed them.
The cause of death for the 16 dolphins that did not test positive for Brucella is still under investigation. Scientists are also looking at additional samples from more dolphins. No whales showed signs of infection.
One dolphin that washed ashore prior to the oil spill was tested, and it did not show signs of Brucella, said Stephanie Venn-Watson, a veterinary epidemiologist working with federal officials to study the deaths.
Since 1991, when federal officials began keeping records, there have been 15 such die-offs of large numbers of marine mammals in the gulf region. In the past, the die-offs have been found to be caused by Red Tide and other biotoxins, or by infectious diseases. Sometimes no cause was found.