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Food Chemical News
The Animal Health Institute responded to a report by FDA’s National Center
for Toxicological Research in an Aug. 15 letter to NCTR Director Daniel
Casciano. Rich Carnevale, vice president of regulatory, scientific and
international affairs at AHI, told Casciano that the article provides no
balance about the impacts of human versus animal use. “We believe that this
article falls into the trap of assuming a cause and effect between animal
use and human health,” he said.
“[The article] talks about antibiotic resistant foodborne organisms
‘straining the health care system’, and the importance of curtailing farm
use because of an ‘emerging public health issue,’” Carnevale said. “This is
a gross overstatement of the likely impact of food animal use and ignores
the real human health impact from resistant nosocomial infections in
hospitals, which account for 10,000 human deaths a year, according to press
reports last year. These infections have nothing to do with animal use.”
In addition to outlining current research efforts (see FCN, Aug. 13, Page
18), the NCTR report highlighted key regulatory issues for FDA’s Center for
Veterinary Medicine.
NCTR’s Casciano had no comment on AHI’s letter.
Specific criticisms
Carnevale said NCTR was incorrect to state that 50 million pounds of
antibiotics are used in animals in the United States. “The reality is that
data is not available for the total amount of antibiotic used in the U.S.,
or what percentage animal use represents,” he said.
Carnevale also sharply criticized NCTR’s statement that use of
antimicrobials in food-producing animals contributes to the widespread
prevalence of antimicrobial resistance.
“Upon what scientific basis is this statement made?” he said. “What is
meant by widespread prevalence? In what organisms? In what species? Where
is the data?”
Also of concern to the drug industry were NCTR’s statements that antibiotic
residues are known to linger in farm products, and that very little is
known about their long-term effects.
“The implication that residues in food contribute to antimicrobial
resistance concerns is unfounded,” Carnevale said. “There is no scientific
evidence that ingestion of occasional and minute amounts of residues that
may be in meat or poultry [primarily organ tissues] has any effect on
conferring resistance to enteric bacteria of consequence to human health,”
he added.
Multiple resistances
AHI also criticized the research center’s statement that attributes
numerous drug-resistant bacteria to use of drugs in animals, calling it
“greatly overstated.”
Data collected as part of the National Antibiotic Resistance Monitoring
System show that nearly 90% of all Salmonella slaughter plant isolates are
susceptible to a panel of 17 antimicrobials, he said.
Carnevale said NCTR incorrectly implied there is genetic linkage between
the animal drugs fluoroquinolone and vancomycin with regard to resistance
determinants.
“There is no connection to our knowledge,” he said. “Vancomycin use in food
animals has been prohibited by the CVM, and no approvals for glycopeptides
have ever been granted by the FDA, thus it cannot be a source of concern.
Vancomycin is not effective against gram-negative bacteria for
physiological reasons unrelated to genetic resistance genes,” he said.
Carnevale also took issue with NCTR’s position that there are questions
about human consumption of residues in food animals and their effects on
the indigenous human intestinal microflora.
“While the evaluation of microbiological endpoints has been the subject of
scientific debate for a number of years, the possible consumption of low
levels of residues in meat and poultry with the selection of resistant
organisms harmful to human health has not been identified as a real concern
of the FDA,” Carnevale said.
Virginiamycin
A key regulatory issue for CVM, the NCTR report said, is use of
virginiamycin in food-producing animals. For more than 20 years,
virginiamycin has been used as an animal growth promoter in Europe and
North America, NCTR said.
The scientific problem is that virginiamycin may select for resistant
enterococci, which can contaminate food, enter the human gastrointestinal
tract and pass the new resistance genes to enterococci of the human
intestinal microflora, NCTR said.
FDA in 2000 approved a structurally related antimicrobial, Synercid®
(quinupristin/dalfopristin), for use in humans as a last-resort drug
against antibiotic-resistant enterococci and other bacteria. “However,
Synercid®-resistant bacteria are being found in patients who have never
been exposed to this drug combination before, suggesting a non-human
source,” NCTR said.
The NCTR report related agricultural use of virginiamycin with resistance
in organisms that have no known animal reservoir. Carnevale said that
relationship is “pure speculation, unsupported by any relevant data.”
To tackle this issue, CVM is conducting a risk assessment on the
plausibility of a link between the use of virginiamycin in animals and
quinupristin/dalfopristin resistance in humans, and the human health impact
attributable to use of virginiamycin (see FCN, Aug. 13, Page 17).
The NCTR report is available at
http://www.fda.gov/nctr/science/journals/Default.htm.
— Rebecca Osvath
Food Chemical News,
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