Eight years out from the 2010 Deepwater Horizon oil spill, bottlenose dolphins remained more susceptible to infection due to crude oil's detrimental effects on their white blood cells, the latest sign that the Gulf of Mexico population is struggling to recover from the environmental disaster.

The findings, published Thursday in Environmental Toxicology and Chemistry, also suggested that the immune system issues are being passed onto dolphins born after the spill, raising concerns of multigenerational harms.

An explosion on BP’s Deepwater Horizon drilling rig in 2010 killed 11 people and dumped about 206 million gallons of oil into the Gulf of Mexico over two years, creating the second-largest oil spill in history. About 1,300 bottlenose dolphins are estimated to have died in the following five years, with many more suffering from lung disease, impaired reproduction and other effects associated with the spill.

Scientists have been monitoring the impacted dolphin population in Barataria Bay near Louisiana since 2011, including Sylvain De Guise, the study’s lead author and a professor of pathobiology at the University of Connecticut. He and his coauthors documented the results of blood and spleen samples through 2018 and compared them with a reference population from Florida’s Sarasota Bay not exposed to the oil spill.

The most recent samples showed that compared with the Sarasota Bay population, the Barataria Bay dolphins had three times as many regulatory T cells, a type of white blood cell that suppresses immune responses. The levels were similar to those measured in 2011, showing little long-term improvement. They dipped, however, in 2013 and 2014 — possibly due to unrepresentative samples during that period, De Guise said, although why the dolphins appeared to be getting better before reversing course is not fully clear.

The researchers also measured an imbalance in the regulatory T cells’ response to immune signals that rendered them less fit to combat pathogens. 

“When the leak is plugged and you don’t see those slicks of oil and you don’t see those birds all covered that they clean up with Dawn, it kind of fades off memory,” De Guise said, “but it’s pretty dramatic that 10 years later, we still see health effects in the dolphins.”

A similarly altered response to immune signals was found when regulatory T cells from the healthy Florida dolphins were exposed to sweet crude oil in a petri dish, directly tying the immune-system problems to the oil spill.

“It demonstrated a clear cause and effect,” De Guise said. “There's no confounding factor in a petri dish.”

The researchers also exposed mice to the crude oil via injection and bred them to measure its effects between generations. Regulatory T cells in the blood of exposed mice were twice as high as in the control group, although no significant difference was found in the spleen samples or in either sample of their offspring.

The later generation of oil-exposed mice did have a higher proliferation of other T cells than the control progeny, a condition also found in the Barataria Bay dolphins.

The long-term dysregulation found in the bottlenose dolphins’ immune systems and the possibility of issues being passed down generations “raises significant concerns for the recovery of dolphin stocks affected by the Deepwater Horizon oil spill,” the researchers wrote. The Barataria Bay dolphins may take longer to recover than the 39 years estimated previously.

Previous research has found that the oil spill detrimentally impacted the reproduction of bottlenose dolphins in the Gulf of Mexico. Only two of the 10 pregnant dolphins observed in Barataria Bay during a 2011 health assessment birthed a surviving calf, compared with an 83% pregnancy success rate in reference populations. And between March 2010 and July 2014, 211 premature, stillborn or young dolphins were found stranded onshore, well above the average of 11 found per year before the spill.

The study was funded by the Gulf of Mexico Research Initiative, an independent scientific research organization set up in the wake of the oil spill with $500 million from BP. The initiative is wrapping up its 10-year mission, so this study is the last one currently planned to follow up on the Barataria Bay population of dolphins and the effects of the oil spill, according to De Guise. He and his team plan on continuing to research the health of dolphins in different populations.

“It's rare that you can get an opportunity to follow up up to a decade after a natural disaster or a semi-natural, half-manmade disaster,” De Guise said.

The article, “Long‐Term Immunological Alterations in Bottlenose Dolphin a Decade after the Deepwater Horizon Oil Spill in the Northern Gulf of Mexico: Potential for Multigenerational Effects,” was published Feb. 18 in Environmental Toxicology and Chemistry. The authors of the study were Sylvain De Guise, University of Connecticut and Connecticut Sea Grant Program; Milton Levin and Lindsay Jasperse, University of Connecticut; Jean Herrman, Companion Animal Dental Services; Randall Wells, Chicago Zoological Society; Teresa Rowles, National Marine Fisheries Service; and Lori Schwacke, National Marine Mammal Foundation. The lead author was Sylvain De Guise.