Animal & Veterinary
PulseNet, FoodNet, NARMS; Tools to Fight Disease, Protect Public Health
by Richard L. Arkin, J.D., Assistant Editor
FDA Veterinarian Newsletter 2007 Volume XXII, No VI
Foodborne illness outbreaks are shifting from the typical point source, or “church supper,” outbreak to more diffuse out-breaks. These can occur over many communities, with only a few illnesses in each, and therefore are difficult for public health authorities to track.
The nature of outbreaks has changed because food production and distribution have changed. Until recently, the food supply system consisted of local growers and local or regional processors. More recently, large food-producing facilities, often with nationwide distribution, have replaced smaller, regional facilities. Public health experts have difficulty detecting and dealing with this relatively new style of dispersed outbreak.
The Food and Drug Administration’s Center for Veterinary Medicine, the Centers for Disease Control and Prevention (CDC), and the U.S. Department of Agriculture (USDA) are working in partnership to detect and combat the problems of this new type of outbreak.
These three government agencies have established federal food safety programs to improve their ability to identify and investigate outbreaks and take appropriate action. These programs, “PulseNet,” “FoodNet,” and “NARMS,” use new laboratory, research, statistical, and analytical tools to help protect public health.
PulseNet is the National Molecular Subtyping Network for Foodborne Disease Surveillance. The PulseNet database consists of genomic DNA banding patterns (“fingerprints”) of bacteria generated using a technique called pulsed-field gel electrophoresis (PFGE). PulseNet laboratories determine the subtype of the bacteria collected locally, search their local databases for clusters of matching isolates, and forward the data to CDC. CDC does an epidemiologic evaluation of bacteria from across the country. Public health authorities can use that evaluation to determine if clustered cases of food-borne illness are caused by the same strain of bacterium, which is important in determining the source of the outbreak.
Data derived from PulseNet’s subtyping service network play an important role in the surveillance and investigation of foodborne illness outbreaks. Easier identification allows for epidemiologic investigations, product recalls, public health no-tifications, regulatory actions, industry improvements, and, ultimately, possible prevention of future disease. At the same time, PulseNet is a tool for developing and disseminating improved technologies for molecular fingerprinting.
PulseNet currently tracks Shiga toxin-producing Escherichia coli, Listeria, Salmonella, Shigella and Campylobacter. Scientists are developing protocols for two other organisms, Vibrio species and Clostridium botulinum.
One example of PulseNet in action involved 30 dog owners in Canada who developed salmonellosis. Many of the dog owners had recently given treats made from dried pig ears to their dogs. After the PulseNet data showed a connection between the illness and bacteria found on the pet treats, FDA issued a nationwide public health warning in the United States about dog-treat-related salmonellosis. Dr. Shaohua Zhao, a scientist with the Division of Animal and Food Microbi-ology (DAFM) in CVM’s Office of Research, isolated and serotyped Salmonella bacteria from various brands of dog treats and established PFGE profiles of the Salmonella serotypes. She also later determined the antibiotic susceptibility of the serotypes found in the dog treats.
Another example of PulseNet at work involved DAFM researchers who have used the National Antimicrobial Resis-tance Monitoring System (NARMS) network to share PulseNet data about Salmonella bacteria isolated from humans and animals. These data have helped broaden understanding of the development of resistance to antibiotics. These DNA fin-gerprinting techniques are now used to examine the presence of drug-resistant bacteria in retail chicken, turkey, beef, and pork.
CDC created PulseNet in 1995, in cooperation with the Association of Public Health Laboratories and state public health laboratories. By early 2000, PulseNet had grown to include 46 state public health laboratories, the public health laboratories in New York, N.Y., and Los Angeles, CA, and USDA’s Food Safety and Inspection Service Laboratory, as well as FDA’s laboratories in the Center for Food Safety and Applied Nutrition and CVM. In addition, six provincial labora-tories in Canada joined PulseNet in 1999 and 2000.
During the same period it created PulseNet, CDC, along with five state health departments, implemented an active foodborne disease surveillance network called the Foodborne Diseases Active Surveillance Network (FoodNet) as part of a response to emerging infectious disease threats.
FoodNet is designed to help public health officials better understand the epidemiology of foodborne diseases in the United States.
The objectives of FoodNet are to:
- Determine the burden of foodborne disease.
- Monitor trends in the burden of specific foodborne illness over time.
- Attribute the burden of foodborne disease to specific foods and settings.
- Interrupt transmission during an ongoing outbreak.
- Develop and assess interventions to reduce the burden of foodborne disease.
FDA and USDA continue to be active collaborative partners with CDC and state and local laboratories in FoodNet. Currently, California, Colorado, Connecticut, Georgia, Maryland, Minnesota, New Mexico, New York, Oregon, and Tennessee have FoodNet sites.
NARMS was established in 1996 as a collaborative effort among CVM, USDA, and CDC to address the continuing problem of food derived from animals commonly carrying organisms that are pathogenic to humans, but not animals. These bacteria can be resistant to antimicrobial drugs under the right conditions.
For example, Salmonella, Campylobacter, and Escherichia coli O157 may be found in the intestines of healthy food ani-mals. All three bacteria can cause foodborne illness in humans. These bacteria can develop resistance when exposed to antibiotics given to the animal. These resistant bacteria can contaminate meat at slaughter and then infect humans who eat undercooked or mishandled raw meat products.
CVM, CDC, and USDA use NARMS to monitor changes in antimicrobial drug susceptibility of select zoonotic bacterial pathogens (which are animal bacteria that can transmit disease to humans) in food-producing animals, retail meats, and humans. Under the NARMS program, these bacteria are tested for susceptibility to a specific set (panel) of antimicrobial drugs important in human and animal medicine.
The NARMS program is intended to help manage antimicrobial resistance by providing data to:
- Identify changes in antimicrobial resistance patterns in zoonotic foodborne bacterial pathogens and select commensal organisms.
- Respond to unusual or high levels of bacterial resistance to antimicrobials in humans, animals, and retail meats in order to contain or mitigate resistance dissemination.
- Assist FDA in making decisions about approving safe and effective drugs for humans and animals, as well as promote prudent and judicious use of antimicrobial drugs.
- Design follow-up epidemiology and research studies to better understand the emergence and transfer of antimicrobial resistance.
NARMS also provides a national source of enteric bacteria isolates for research in diagnostic test development, discovery of new genes and molecular mechanisms associated with resistance, and the study of mobile gene elements, virulence, and colonization.
How NARMS Operates
Each year, samples are taken from a variety of sources and tested for changes in the resistance of certain enteric bacteria to selected antimicrobial drugs. Public health laboratories in all 50 states collect isolates from people suffering enteric disease. USDA collects isolates from healthy farm animals, animal clinical specimens, and carcasses of food animals at slaughter. FDA collects isolates from meat products at processing plants. Retail meat samples are collected from grocery stores and other retail outlets that sell meat.
The human isolates are sent to CDC in Atlanta, GA, for microbiological and epidemiological analyses. Food animal iso-ates are collected from sites across the United States and sent to USDA’s Antimicrobial Resistance Research Unit in Athens, GA, for susceptibility testing. Additionally, bacterial isolates from retail meats are collected from grocery stores in 10 participating FoodNet states and sent from the FoodNet laboratories to CVM for antimicrobial drug susceptibility test-ing. CVM conducts the susceptibility testing of these samples at its Office of Research laboratory at the Agency’s Muirkirk Research Center in Laurel, MD.
NARMS is designed so that the same kits to test isolates are used in the human, animal, and retail meat testing pro-grams. For all isolates, testing bacteria for susceptibility to antimicrobials involves determining the minimum inhibitory concentration (MIC) for a panel of common antimicrobial agents. The MIC is the lowest concentration of a drug that will slow or stop the growth of the bacteria being tested. The higher the MIC number, the greater resistance the bacteria have.
Each year, FDA, CDC, and USDA evaluate the antimicrobial drugs included in the testing panels for relevance to the NARMS objectives, sometimes removing a drug from the panel and replacing it with another. CDC, USDA, and FDA are all currently testing Salmonella, Escherichia coli, Campylobacter, and Enterococcus for susceptibility to the same panel of antimicrobial drugs. The results of these tests are compared with results from previous years to look for emerging trends in resistance. NARMS reports are published annually by all three NARMS partners.
Role of DAFM
As part of the NARMS work, DAFM researchers obtain approximately 350 Salmonella isolates each year from retail meats. Researchers at DAFM subtype all of the isolates by PFGE and submit the DNA fingerprinting patterns to PulseNet. The patterns are compared to human clinical isolates through PulseNet. These studies reveal if there is a spread of resistant isolates between animals and humans or widespread dissemination of unrelated strains.
Researchers are also characterizing the genes that confer resistance. This research helps scientists understand the genetic diversity of Salmonella and the extent to which Salmonella strains move from animals to humans. It also helps scientists understand the effects antibiotic usage in animal husbandry can have on antimicrobial resistance in foodborne pathogens, as well as the mechanism of resistance-gene transfer between animal and human bacterial pathogens.
Scientists and other interested people can access NARMS data through links on the CVM home page. Human data are located on the CDC Web site. Animal data are on the USDA Web site.
In addition, the agencies participating in the NARMS program hold periodic public meetings to present results and pro-vide a forum for presentation of other related antimicrobial resistance research.
PulseNet, FoodNet, and NARMS are tools that continue to broaden our understanding of foodborne disease in a changing economy. These systems have played a key role in the multi-agency partnership to detect and combat food-borne illness.