Several reports out today detail the bacterial infection behind a string of dolphin deaths that have occurred since last year’s Gulf of Mexico oil spill. Brucellosis, a bacterial infection, has been identified in at least five of 21 tests of stranded dolphins.

A representative of the National Marine Mammal Health and Stranding Response Program for the National Oceanic and Atmospheric Administration told CNN that the dolphins could be dying because the bacteria has become more lethal, or it could simply “be more severe, because the dolphins are more susceptible to infection.” In either scenario, the root cause seems to be severe environmental stress, which could have been brought on by the BP oil spill.

Teri Rowles, coordinator of the National Marine Mammal Health and Stranding Response Program, told the St. Petersburg Times’ Craig Pittman that the oil “could have impaired the dolphins’ ability to respond to the bacterial infection.”

Investigators, however, still haven’t officially pinpointed the cause.

The deaths have been labeled an “unusual mortality event,” which is defined under the Marine Mammal Protection Act as “a stranding that is unexpected; involves a significant die-off of any marine mammal population; and demands immediate response.”

Between February 2010 and October 2011, there have been at least 580 “strandings” of dolphins and whales in the gulf. Most of them have been bottlenose dolphins. According to a chart on NOAA’s website, 114 were stranded prior to the oil spill response, 122 were either stranded or reported dead offshore during the initial response phase and 344 were stranded after the initial response phase had ended. The worst month for dolphin and whale strandings were March 2010 and March 2011 — which saw 62 and 73 strandings, respectively. (By comparison, the March average between 2002 and 2009 was 17.9.)

BP’s Deepwater Horizon exploded on April 20, 2010, leading to the worst oil spill the country has ever seen. The well was capped on July 15, 2010, but oil has been spotted in the gulf as recently as last monh.

According to the Centers for Disease Control, humans can be infected in one of three ways: “eating or drinking something that is contaminated with Brucella, breathing in the organism (inhalation), or having the bacteria enter the body through skin wounds.” Researchers have recommended that humans and their pets stay away from any stranded marine life.

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