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These findings support those of a 2011 health assessment of live dolphins in Barataria Bay, Louisiana, a heavily oiled area during the spill which showed those resident dolphins had poor health, adrenal disease, and lung disease.

The timing, location, and nature of the detected lesions support that contaminants from the Deepwater Horizon oil spill caused these lesions and contributed to the high numbers of dolphin deaths within this oil spill's footprint. Increased dolphin deaths following the oil spill are part of the northern Gulf of Mexico unusual mortality event investigation.

"This is the latest in a series of peer-reviewed scientific studies, conducted over the five years since the spill, looking at possible reasons for the historically high number of dolphin deaths that have occurred within the footprint of the Deepwater Horizon spill," said Dr. Teri Rowles, veterinarian and one of 22 contributing authors on the paper, and head of NOAA's Marine Mammal Health and Stranding Response Program, which is charged with determining the causes of unusual mortality events, also known as UMEs. "These studies have increasingly pointed to the presence of petroleum hydrocarbons as being the most significant cause of the illnesses and deaths plaguing the Gulf's dolphin population. This study carries those findings significantly forward."

Direct causes of death, during this period, likely included:

• Chronic adrenal insufficiency resulting from adrenal gland effects;

• Increased susceptibility to life-threatening outcomes due to adrenal insufficiency, especially when challenged with pregnancy, cold temperatures, and infections; and

• Increased susceptibility to primary bacterial pneumonia, possibly due to lung injury, or alterations in immune function.

Animals with untreated adrenal insufficiency are at risk of life-threatening adrenal crises. The adrenal gland produces hormones -- such as cortisol and aldosterone -- that regulate metabolism, blood pressure and other bodily functions.

"Animals with adrenal insufficiency are less able to cope with additional stressors in their everyday lives," said Dr. Stephanie Venn-Watson, the study's lead author and veterinary epidemiologist at the National Marine Mammal Foundation, "and when those stressors occur, they are more likely to die."

Dolphins are seen swimming through the oil spilling from the Deepwater Horizon oil well at the height of the spill in 2010. (Credit:NOAA)

Since early 2010, there has been an ongoing cetacean unusual mortality event involving primarily bottlenose dolphins in the northern Gulf of Mexico. Three out of four groupings of elevated dolphin strandings identified within this event followed the Deepwater Horizon oil spill.

This ongoing die-off, with the highest number of dead bottlenose dolphin strandings on record in the northern Gulf of Mexico, coincided with the largest marine-based oil spill in the United States.

Barataria Bay, Louisiana, was one of the most heavily oiled coastal areas from the Deepwater Horizon oil spill, and the new study shows that half of the dead dolphins examined from Barataria Bay that stranded between June 2010 and November 2012 had a thin adrenal gland cortex, indicative of adrenal insufficiency. One in every three dolphins examined across Louisiana, Mississippi and Alabama had this lesion. In comparison, only 7 percent of the dead stranded reference dolphins, collected from other coastal regions outside the Deepwater Horizon oil spill area and time frame, had a thin adrenal cortex.

In fact, almost half of the dolphins with this otherwise rare adrenal lesion appeared to have died without another clear explanation for their death.

In addition to the adrenal lesions, the scientific team discovered that more than one in five dolphins that died within the Deepwater Horizon oil spill footprint had a primary bacterial pneumonia. Many of these cases were unusual in severity, and caused or contributed to death.

"These dolphins had some of the most severe lung lesions I have seen in the over 13 years that I have been examining dead dolphin tissues from throughout the United States," said Dr. Kathleen Colegrove, the study's lead veterinary pathologist based at the University of Illinois. In comparison, only 2 percent of reference dolphins had these lesions.

In other mammals, exposure to petroleum-based polycyclic aromatic hydrocarbons, known as PAHs, through inhalation or aspiration of oil products can lead to injured lungs and altered immune function, both of which can increase an animal's susceptibility to primary bacterial pneumonia. Dolphins are particularly susceptible to inhalation effects due to their large lungs, deep breaths and extended breath hold times.

The prevalence of Brucella and morbillivirus infections, which were investigated as potential alternative causes for increased dolphin deaths, was low in UME dolphins after the oil spill and was no different compared to the reference dolphins. Additionally, biotoxin levels were either low or below the detection limit in the UME dolphins.

Ongoing studies assessing changes in these lung and adrenal gland lesions over time will help to address questions regarding how long these chronic conditions may last.

The study team included representatives from NOAA's National Marine Fisheries Service, NOAA's National Ocean Service, the National Marine Mammal Foundation, the University of Illinois, the University of Georgia, the Dauphin Island Sea Lab and University of South Alabama, the Institute for Marine Mammal Studies, Gulfport, Mississippi; the Louisiana Department of Wildlife and Fisheries, the Audubon Aquarium of the Americas, New Orleans; the Mote Marine Laboratory, Sarasota, Florida; the Texas Marine Mammal Stranding Network, Galveston; and the Marine Mammal Pathology Services, Olney, Maryland.

This work was completed as a part of the Northern Gulf of Mexico unusual mortality event investigation and a part of the Deepwater Horizon Natural Resource Damage Assessment being conducted cooperatively among NOAA, other federal and state trustees, and BP.