SAN DIEGO, Sept. 27 (UPI) -- U.S. Navy dolphins carry bacteria resistant to the antibiotic ciprofloxacin, even though they are rarely treated with the drug, according to research presented Friday at a microbiology meeting.
The unexpected finding raises questions about how the resistance arose and whether it might have implications for human health, said veterinarian Stephanie Wong of the U.S. Navy Marine Mammal Program.
"Since Navy animals live and work in the open ocean," Wong told United Press International, "bacteria found on them can tell a story about what's happening in the ocean."
She added dolphins may be "sentinels" -- giving early warning that the oceans are a breeding ground for antibiotic resistance.
Physician Barbara Murray of the University of Texas Medical School, an authority on antibiotic resistance, told UPI Wong's finding is "intriguing," but its significance still is unknown.
Wong tested nearly 700 samples taken from dolphins, sea lions, and a beluga whale between 1988 and 2001 and found 10 of the animals -- all dolphins -- carried bacteria resistant to the drug ciprofloxacin.
Two of the dolphins actually suffered from a gastrointestinal disease caused by drug-resistant E. coli, Wong said, although most were simply carrying the resistant bacteria in their blowholes.
Although the animals are cared for by veterinarians, half never had been given ciprofloxacin or any related antibiotic, Wong said. As well, she found evidence of resistance to drugs rarely or never used on marine mammals.
Because the samples were taken from the animals' blowholes, Wong said, it is probable the resistant bacteria originate in the water, rather than from food or an existing infection. The blowhole, like the human nose, acts as a trap for bacteria from the outside world, she said.
One possible explanation is that the drugs -- or possibly resistant bacteria themselves -- are finding their way into the ocean from such human sources as sewage or farm run-off, Wong said.
"There is a need to further investigate how antimicrobial-resistant bacteria survive and move in the ocean, in relation to other sources," she said, adding it also is possible the ocean may be a "large reservoir that creates its own resistance to antibiotics."
For instance, she said, shrimp produce their own antibiotics against bacteria called vibrio, which in humans can cause cholera, and some vibrio bacteria evolve resistance to those antibiotics. Something similar could be happening with ciprofloxacin, Wong said, raising the fear such resistance might leap from bacteria in the sea to those on land.
The second scenario is unlikely, however, Murray said, because ciprofloxacin -- unlike penicillin and many other antibiotics -- is synthetic. It does not occur in nature, she said, so it is difficult to see how resistance to it could arise naturally.