Julie Funk, DVM, MS, PhD
Ohio State University
Although the estimated $0.15 – 3 billion dollar public health cost from antimicrobial resistance is primarily attributed to human use of antibiotics, antimcrobials used for livestock may also have an impact. Currently, antibiotics in agriculture do not require veterinary prescriptions and are used to 1) treat individuals for disease (therapeutic); 2) treat a herd to stop the spread of disase (metaphylactic); 3) prevent an outbreak (prophylactic); and 4) promote growth. As the percentage of healthy animals treated increases, so does public concern* that the antibiotics will be less effective in treating human diseases. To assess the risk of antibacterial use in animals on human microbial resistance, the probabilities of the presence of resistant bacteria in the animal, human ingestion of it and adverse health consequences must be evaluated. Some studies seems to indicate that there is an impact on humans but more data on in vivo transfer is needed. For example, while fluoroquinolones were approved in 1986 to treat campylobacter infections in humans, resistant isolates did not appear until two years after they were approved for use in chickens (1995). However, the data may be influenced by the increased efforts to identify resistance in the past few years. In addition, the extent of human resistance attributed to antibacterial use in animals is difficult to quantify especially as there is a large variablity in estimating the proportions of human, therarpeutic animal and non-therapeutic animal anitmicrobial use. Analysis of resistant isolates in European countries where some antibiotics have been banned for growth promotion, indicate that resistant eteroccoci have decreased but there has been no impact on human health. There have been no changes in resistant salmonella and campylobacter, but pigs have shown increased diarrhea and decreased growth in the nursery while there has been no effect on the growth performance of finishers. Current research at OSU includes evaluating the effect of sub-therapeutic chlortetracycline on the reservoir of antimicrobial resistance in swine.
Additional research is underway to compare the source, diversity and resistance of three foodborne bacteria for swine reared in both conventional and antibiotic-free environments.
* The US Food and Drug Adminiatration maintains a website with links to guidance documents and other information regarding antimicrobial resistance and the use of antibiotics for livestock at: http://www.fda.gov/cvm/antimicrobial/antimicrobial.html.
Discussion:
Fred Michel (OSU) asked why more resistant isolates have been found in humans than in animals. If transfer is from animals to humans, wouldn’t you expect more in the animals? Funk agreed but noted that a Danish study indicated that there may be more than one source of the isolates. Also, animals are surveyed at slaughter rather than when infection is apparent. However, isolates are detected in humans when diagnosing illness.
A participant in Columbus questioned data on resistance to tetracycline. Funk noted that in the literature, swine seem to harbor a higher percentage of fecal e. coli resistant to tetracycline but it is not clear why.
Mo Saif (OSU) noted that tetracycline is used in poultry but rarely for growth. Funk noted that it also is not used for growth promotion in swine. The research underway used tetracycline because the grant funded research for assessing the impact of antimicrobials for growth promotion on foodborne pathogens. However, most antimicrobial used for growth are active against gram positive bacteria and most foodborne pathogens are gram negative. Thus, tetracycline was chosen and administered at 50 grams per ton because at that concentration there are no labeled claims for therapeutic use.
Linda Saif (OSU) asked whether the studies in Denmark and Sweden found an increase in other diseases when specific antimicrobials were eliminated. Funk responded that there have been some increases but they are primary viral. The increased incidence of diarrhea in nursery swine was treated with copper and zinc but those before environmental concerns were prevalent. Claims have been made that bedding nursery swine on deep straw is effective in reducing diarrhea; however, US studies indicate other problems with deep straw.
L. Saif asked about other studies on whether antibiotics used for growth promotion are effective. Funk noted that there are not many and none are observational. One in Thailand evaluated the DNA of bacterial communities in feces. It looked at shifts in population in response to antibiotics. Current thought of how antibiotics promote growth is that they shift the bacterial population in the gastrointestinal tract which increases the feed efficiency. There is some evidence to support this.
L. Saif asked about the use of antibiotics for parasitic control (e.g., cryptosporidia). Funk replied that they are still used for such control but data of the effectiveness is not clear.
Discussion notes by Mary Wicks
The OCAMM (Ohio Composting and Manure Management) Program is funded by OARDC/OSU.