Monitoring antimicrobial susceptibility of animal pathogens

For the past two years, researchers have been collecting bacterial isolates from animal species in various states. Their aim has been to build a surveillance system to monitor the antimicrobial susceptibility of certain pathogens. Researchers are now in the process of analyzing the data, and while preliminary findings are enlightening, funding problems may put an end to the surveillance.

"It's a shame, since we have the system up and running already," said Dr. Ching Ching Wu, head of the bacteriology and mycology section of Purdue University's Animal Disease Diagnostic Laboratory. "This should be a system where the samples and data continue to come in so you can look at retrospective and real-time data."

Several years ago, Dr. Wu proposed the monitoring project to the AVMA Steering Committee on Antimicrobial Resistance, and the AVMA, seeing its merit, provided funding of $70,000. Directors and microbiologists from American Association of Veterinary Laboratory Diagnosticians-accredited laboratories endorsed the project, and agreed to provide labor and technical resources.

Initially, 26 laboratories, representing 18 states, participated. With the difficulties in state funding, some labs have been unable to participate in the second year. Currently, 15 laboratories are regularly sending samples. Researchers are monitoring five pathogens each in bovine and swine species; four pathogens in canine and equine species; and three pathogens in poultry. In poultry, for example, researchers collect isolates of Escherichia coli, Salmonella spp, and Pasteurella multocida. For bovine species, they collect Mannheimia haemolytica, Salmonella spp, Staphylococcus aureus, Arcanobacterium pyogenes, and E coli.

Since the project began in February 2001, participating laboratories have collected a maximum of 10 isolates monthly for each pathogen in each species. Because the project represents a true clinical scenario, more isolates have been collected from some species than for others. Samples are then sent to Purdue University for testing, using 17 antimicrobials. Along the way, various state animal diagnostic laboratories have contributed resources.

So far, researchers have collected minimum inhibitory concentration data from over 7,584 isolates.

At press time, researchers had finished analyzing two years worth of canine data and were progressing with data from other species. They discovered that the MIC for two-thirds of the antimicrobials tested in canine species during the two-year study did not change when looking at 50 percent and 90 percent of the samples. They also found that the MIC 50 for roughly one-fourth of the drugs increased; however, only 10 percent of the drugs had an increased MIC 90. Dr. Wu says this indicates that veterinarians need to increase the antimicrobial dose to maintain efficacy, and the pathogens, while showing increased MIC, are not yet resistant to these antimicrobials.

Dr. Wu said that perhaps the most interesting finding is that a few antimicrobials (enrofloxacin, gentamicin, and tetracycline) showed decreasing MICs, revealing that the bacteria are becoming more susceptible to the drugs. Increased judicious use of a drug could cause this effect.

"One of the goals for this program is to monitor subtle changes in antimicrobial susceptibility," Dr. Wu said. She noted that if veterinarians are aware of increasing MIC to a particular antimicrobial, they can use other drugs to allow changes in the population dynamic, which may preserve the effectiveness of antimicrobials.

Another important finding was that antimicrobial resistance varied from state to state. "We see differences among states participating, so the national average data may not represent a true picture of your own state," Dr. Wu said.

At the moment, participants and other stakeholders have access to the data, which are stored in a Web-based database. When the project began, researchers had envisioned that veterinarians would be able to access data for their state, which would help them make treatment decisions. The money to maintain this Web site and continue the surveillance, however, has dried up. The AVMA provided the initial funding, but the project's leaders were tasked with securing funding to continue it. This has proven difficult. Dr. Wu says the government is increasingly funding bioterrorism and homeland security projects and, therefore, money available for other endeavors has dwindled. In early September, for example, the U.S. awarded $350 million to eight institutions for biodefense projects.

The AVMA has tried to help by lending grant-writing assistance and sending letters in support of funding to various agencies, including the national Centers for Disease Control and Prevention, Food and Drug Administration, and Department of Agriculture. Dr. Lyle P. Vogel, director of the AVMA Scientific Activities Division, says that the project could safeguard public health and, therefore, justifies public funding. "Some public health individuals theorize that genetic material encoding for antimicrobial resistance can transfer from animal pathogens to human pathogens," he said. "Therefore, it is in the public's interest, as well as for the health and welfare of animals, that a monitoring system for resistance development in animal pathogens be established."

If the project is continued, the surveillance system could serve as an early detection tool for changes in antimicrobial susceptibility at local, state, and national levels, providing direct feedback to veterinarians and an opportunity for timely intervention. At press time, however, no agency had stepped forward to provide the necessary funding.