Our planet’s oceans have become a reservoir of antibiotic genes – the remnants of waste from our homes, hospitals, and farms washing from shore into our waters. It’s no secret humans use a lot of antibiotics, so much so we’re developing resistance to their powerful ability to treat bacterial diseases. However, seeing the upward trend in antibiotic use mirrored in dolphins is concerning, say the authors of a new study published in the journal Aquatic Mammals. 

"Unlike humans, wild dolphins are not taking antibiotics and seeing an increase in resistance among the dolphin isolates was not expected," said lead author Adam Schaefer, an epidemiologist at Florida Atlantic University’s (FAU) Harbor Branch, to IFLScience. "It is concerning because it means that antibiotic-resistant bacteria and antibiotics are getting into the marine environment. Once in the environment, the resistance genes are being exchanged between bacteria in the water, some of these bacteria are potential human pathogens."

Based on their findings, the trend suggests the source is entering the marine environment via human activities or terrestrial discharges. Swab samples taken from the blowhole, gastric fluid, and feces of 171 bottlenose dolphins (Tursiops trun­catus) between 2003 and 2015 in Florida’s Indian River Lagoon (IRL) were analyzed, with more than 730 isolates obtained during this time.

The overall prevalence for resistance to at least one antibiotic was 88.2 percent. Erythromy­cin (used to treat a variety of bacterial infections) took the lead with 91.6 percent prevalence, followed by ampicillin (a penicillin-type antibiotic used to treat bacterial infections such as ear or bladder infections) at 77.3 percent, and cephalothin (a cephalosporin antibiotic) at 61.7 percent.

"Erythromycin is a very commonly used antibiotic, even in veterinary medicine and agriculture," said Schaefer. "Its widespread use contributes to increases in antibiotic resistance in the environment."

During the 13-year assessment, resistance to ciprofloxacin among E. coli nearly doubled, with quite a few other antibiotics increasing significantly as well. "They are likely coming from terrestrial sources. The IRL where the study was done is surrounded by a larger human population. Inputs from wastewater treatment, canals, and agriculture all impact the health of the lagoon in general," said Schaefer. 

"Our study could not confirm if these resistant bacteria are causing any infections in dolphins. Instead, as an apex predator at the top of the food chain, dolphins can be a barometer for the health of the environments they live in. The same waters that humans use for fishing, swimming, and recreation. Therefore, these animals can also identify potential public health threats."

The Centers for Disease Control and Prevention estimate that over 2 million antibiotic-resistant infections occur in the United States each year, resulting in around 23,000 human deaths. On the flip side, antibiotics also save lives, which is why professionals urge caution but not disuse of them until alternatives can be found. 

Just recently, a study measured some of the highest concentrations of mercury found in the skin of bottlenose dolphins in the English Channel.