Superbugs are a growing public health concern in the United States. According to the Center for Disease Control and Prevention, at least 2 million people are infected with antibiotic-resistant infections each year, 23,000 of who will die as a result. Now, scientists worry seagulls, a wild bird that has an “enormous migratory reach,” could make matters worse.

The authors of a new study published in the Journal of Antimicrobial Chemotherapy have found a highly drug-resistant form of Escherichia coli in bird droppings they sampled in Lithuania and Argentina. This form of E. coli is marked by the mcr-1 gene, which is what makes it resistant to the powerful, "last-resort antibiotic" called colistin. Study authors believe that the seagulls picked up the superbug after eating sewage or medical waste.

"The lifestyle of gulls allows them to carry and disseminate pathogenic and resistant microorganisms despite country borders," the researchers wrote, according to Newser. “Water contaminated by feces of birds should be foreseen as important risk factor for transmission of resistant bacteria.” Because these are migratory birds, Newser added that they serve as great vehicles for spreading the deadly drug-resistant bacteria around the world —the fear of every scientist.

Drug resistance occurs when microbes such as bacteria, viruses, or parasites evolve over time. It adapts to the environment in a way that enables species to reproduce, thrive and spread quickly and efficiently. Antibiotics work to stop bacterial infections by hindering their ability to grow, but if the microbes undergo genetic changes that allow them to thrive regardless, then prescribing antibiotics is useless. Part of the reason for resistance has to do with overprescribing antibiotics in the first place — the more people take a certain medication, the less effective it is.

While colistin is widely used in animal production it's becoming increasingly popular in human medicine due to the spread of multidrug-resistant bacteria that cause potentially deadly infections, like pneumonia. Its use, though, is still limited, “which is why it’s still powerful enough to kill many stubborn bacteria,” Fortune reported. 

That said, in 2013, there was one case of colistin in a pig living in Shanghai. Since this announcement, scientists all over the world have examined their collection of bacterial isolates to see whether they contain the mcr-1 gene. Newser cited at least 100 cases of colistin resistance in nearly two-dozen countries, including bacteria that were in storage for at least five years. The Centers for Disease Control and Prevention also tested more than 55,000 samples of bacteria they collected from humans, animals and food for the gene, finding it in just two samples from pigs slaughtered in Illinois and South Carolina, according to Fortune

It's not just animals. In May, a 49-year-old woman in Pennsylvania seeking treatment for a urinary tract infection became the first person in the U.S. to become infected with colistin-resistant bacteria. While she has recovered from the infection, she, like the seagulls, is still a carrier for the superbug.

According to The National Geographic, colistin resistance can lurk in the gut bacteria, mainly E.Coli, for an unknown period of time. It’s possible someone can carry the superbug without knowing it and pass it on to others. This gene can go undetected until someone develops an infection.

Officials are currently on alert, continuing to track down this colistin-resistant strain and building a surveillance system  to help locate and hopefully contain it.