Researchers identify antimicrobial resistance patterns in terrestrial and marine species
In a sampling of 50 sharks captured off the coast of Louisiana, scientists have found that roughly 60 percent harbor multidrug-resistant bacteria. Inland, in a study of 78 raptors brought to a Louisiana veterinary hospital, 64 had Escherichia coli isolates, and 13 percent of the isolates were antimicrobial-resistant. These animals may be contributing to a problem that is on the minds of many people these days.
"We are looking at antimicrobial resistance in wildlife that, theoretically at least, never should have been exposed to some of these synthetic drugs," said Dr. Mark Mitchell, director of Louisiana State University's Wildlife Hospital of Louisiana. According to Dr. Mitchell, who spoke July 21 at the AVMA Annual Convention, researchers need to determine the extent of the problem and health professionals need to modify their actions. Otherwise, the problem is only going to snowball.
"A lot of these animals undergo extreme migration patterns," he said. "Think about birds that travel from Wisconsin and Minnesota to Louisiana and Brazil. Think of sharks that travel from the Gulf of Mexico up to Massachusetts. The potential is there to serve as additional exposure to other animals."
Up until now, only a few studies have focused on antimicrobial resistance in wildlife. A 1990 study of 19 starlings in Iran identified widespread antimicrobial resistance and determined that resistance patterns were similar to those in poultry housed near the birds. A few other reports found resistance in some commercially important fish.
"We know that antimicrobial resistance is out there, but one of my concerns is what happens when the wildlife is exposed to it," said Dr. Mitchell. "Are these animals being exposed to this, and their own flora is developing these resistance patterns? Will they serve to disseminate it?"
To examine the issue, Dr. Mitchell launched several studies. In the first, he evaluated injured raptors that were brought by concerned citizens to the Wildlife Hospital of Louisiana, between January and December of 2000. "Raptors are at the top of the food chain in our area," he said. "These are animals that could be potentially feeding on many of the things lower in the food chain that could get exposed to antibiotics, either (via the) watershed or contact with antibiotic-impregnated livestock feed."
In addition to the E coli finding, his team found eight isolates of Klebsiella spp, and five had antimicrobial resistance patterns. Proteus organisms were isolated from four birds, and 75 percent of the samples had antimicrobial resistance patterns. Other bacteria also had multidrug resistance.
In a second study, researchers looked at all adult wildlife that arrived at the hospital from May 2001 through August 2001. The findings were similar.
Dr. Mitchell says all isolated bacteria were resistant to doxycycline, but this didn't surprise him—the drug is popular in veterinary and human medicine, and antimicrobial resistance is common. The findings might also be expected with gram-negative organisms that have intrinsic resistance factors. What did surprise him, however, was resistance to some aminoglycosides, sulfonamides, and fluoroquinolones, as well as chloramphenicol.
Fluoroquinolones have been used extensively since the 1970s, Dr. Mitchell said, and bacteria are supposed to have difficulty becoming resistant to these drugs. Fluoroquinolones interfere with the normal function of DNA gyrases and topoisomerases, enzymes required for proliferation and viability of bacteria. Other antimicrobials, such as chloramphenicol, are used infrequently.
Dr. Mitchell thinks that, most likely, bacteria are sharing antimicrobial knowledge with each other to survive. "They are incorporating DNA from bacteria with knowledge of what is happening with these antimicrobials," he said.
In a third study, the Louisiana researchers analyzed sharks in the waters off Belize, Louisiana, and Florida, and in dogfish off Massachusetts' shores. Again, they found multidrug-resistant bacteria, and the amount of resistance was the same, whether the marine life was close to the shore or farther out in the ocean. "The solution to pollution is dilution, so you would think the likelihood of exposure would be minimized," he said. Even in the supposedly pristine area of Belize, five of 12 sharks had multidrug-resistant bacteria, and four other sharks had bacteria resistant to at least one drug.
Dr. Mitchell said antimicrobials that find their way into rivers and waterways are most likely the cause of the problem. For example, the Mississippi River acts as a point source, and a sewage treatment plant is located near the Belize study location.
"The fact that we found antimicrobial resistance patterns in both terrestrial and marine species shows there is something going on out there," he said. "Whether it has to do with mutations or some kind of exposure as the result of contact from antibiotics from humans or from livestock—we need to know about it."
Dr. Mitchell admits that some of the sample sizes are small, but says that the findings are still a pretty good indicator of a problem. He says veterinarians have to think about how their actions could contribute to the problem.
When you give a drug to get rid of E coli, do you really think you are just killing the E coli? "There are tens of millions of other bacteria in there that are surviving the (antimicrobial) exposure, taking that information, and plugging it into their DNA," he said.
"The way we actually treat bacterial diseases in human and veterinary medicine is archaic. We give an antibiotic that is typically broad in spectrum, and we treat a specific pathogen."
Until this dilemma is addressed, however, health professionals need to reassess their actions. Human and veterinary health professionals routinely prescribe antimicrobials empirically, without confirming bacterial infection or antimicrobial susceptibility, Dr. Mitchell said. "We, as veterinarians, have a very important role because we administer half of the antibiotics. We are guilty of distributing antimicrobials when we shouldn't, and we have to be part of the solution."
In addition, health care workers need to make sure they are not physically spreading resistant bacteria. Dr. Mitchell, in another experiment, collected environmental samples at his clinic and examined them for bacteria with antimicrobial resistance patterns. It was everywhere—on employees' hands, the gloves used to handle the animals, and even in the air.
"We really need to determine what the environmental risk is, both in the environment, outside of our practices, and inside," he said.