Why did researchers suspect that oil contamination made cetaceans more vulnerable to disease?

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Fireboats battle a blaze on the Deepwater Horizon drilling rig in the Gulf of Mexico, April 2010. — Deepwater Horizon oil spill Fireboat response crews attempt to extinguish the blaze aboard the Deepwater Horizon oil rig in the Gulf of Mexico, April 21, 2010.

Researchers suspected that oil contamination made cetaceans more vulnerable to disease because typical causes of widespread fatalities, such as morbillivirus and toxins from red tides, were ruled out as the primary drivers of the spike in strandings and deaths. Instead, investigators observed an unusual incidence of Brucella infection in stranded dolphins.

This finding, combined with evidence from a 2013 study of living dolphins in Barataria Bay—which revealed that many animals suffered from lung and adrenal disorders linked to oil exposure—led scientists to conclude that contaminants from the Deepwater Horizon oil spill had compromised the animals' health, leaving them more susceptible to other environmental dangers.1