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New Data on Antimicrobial Resistance a Mixed Bag

While some Salmonella and Campylobacter strains grew in resistance, others fell, finds NARMS

The federal government has released its 2010 data on antibiotic resistance among Salmonella and Campylobacter in both food animals and humans. While some strains, such as Salmonella Heidelberg, became more resistant to certain drugs between 2009 and 2010, resistance among many serotypes has decreased or remained steady over the past few years.

The figures were published by the National Antimicrobial Resistance Monitoring System (NARMS), housed at the U.S. Food and Drug Administration’s Center for Veterinary Medicine. The program, which tracks trends in resistance among foodborne bacteria, was launched in 1996 as a collaborative effort between FDA, the Centers for Disease Control and Prevention and the U.S. Department of Agriculture.

The 2010 findings varied widely from strain to strain and drug to drug, but a few trends emerged.

Resistance in Salmonella

Between 2009 and 2010, multidrug resistance – resistant to three or more antibiotics – dropped or stayed the same among most non-Typhoidal Salmonella, which are the second most common source of foodborne illness and the leading cause of hospitalization among foodborne pathogens. Overall, multidrug resistance in human isolates was at an all-time low since 1996.

The strain most commonly resistant to three or more drugs was Typhimurium (a non-Typhoidal serotype, contrary to what its name suggests); 44 percent of these isolates were multidrug resistant.

The two strains that grew in resistance between 2009 and 2010 were Salmonella Heidelberg and Salmonella Serotype I 4,[5],12:i:- (some serotypes are not named). The latter serotype has been discovered more and more frequently in humans and meat over the past 10 years, according to Dr. Patrick McDermott, Director of NARMS.

Interestingly, the presence of Salmonella Dublin in food animals has steadily increased since 1997, and this strain accounts for 55 percent of multidrug resistant Salmonella found in cattle at slaughter, which rose 6 percent between 2007 and 2009. The presence of Heidelberg in meat animals rose between 2009 and 2010.

Scientists also found that resistance to ceftriaxone – an antibiotic used to treat human Salmonella infections – was higher in 2010 among Salmonella Heidelberg isolates from both humans and poultry than it had been in 2009, with the exception of isolates from retail chicken breasts.

Ceftriaxone is a member of the cephalosporin class of antimicrobials, which the FDA limited for use in food animals in April of this year in order to “preserve the effectiveness of cephalosporin drugs for treating disease in humans.”

The action prohibits the “extra-label” use of these drugs, meaning that they may not be used at improper dosages or to prevent disease, and only those cephalosporins that are not intended for human or companion animal use may be used in food animals.

“Serotype Heidelberg is an important poultry-associated serotype where ceftriaxone resistance has gone up,” explains McDermott. “FDA will continue to monitor resistance in this serotype following implementation of the extralabel use prohibition.”

According to the NARMS data, ceftriaxone resistance among human strains rose from 8 percent in 2008 to 21 percent in 2009 and again to 24 percent in 2010. Among isolates from chickens at slaughter, resistance to the drug increased from 8.5 percent in 2008 to 18 percent in 2009 and then again to 32 percent in 2010. Resistance in isolates from retail ground turkey and turkeys at slaughter increased from 3.5 percent and 13 percent, respectively, in 2008 to 10 and 33 percent in 2009, and then rose to 24 and 36 percent in 2010.

Among isolates from retail chicken breast, resistance rose from 17 percent in 2008 to 32 percent in 2009 before declining to 24 percent in 2010.

The highest prevalence of ceftriaxone resistance among these meats was found among Typhimurium strains, 81 percent of which were resistant to the drug. Indeed ceftriaxone-resistant Typhumurium has increased in overall prevalence when isolated from chicken breasts, rising from 44 percent in 2007 to 61 percent in 2010.

A similar rise in ceftriaxone resistance was observed in samples taken from animals at slaughter. Resistance in isolates from cattle and turkeys was at its highest since 1997.

In total, the number of samples tested for Salmonella in 2010 included 2,474 samples from humans, 400 from retail meats and 1,073 from healthy food animals at slaughter.

Resistance in Campylobacter

A total of 2,136 Campylobacter isolates were tested, including 1,310 from humans, 518 from retail meats (505 from chicken breasts and 13 from ground turkey) and 308 from chickens at slaughter.

Ciprofloxacin resistance in Campylobacter jejuni isolated from retail chicken breasts rose from 17 percent in 2006 to 23 percent in 2010, but in Campylobacter coli resistance to the drug fell from 22 percent to 14 percent. C. jejuni is known to be a more common source of foodborne illness than C. coli.

Conversely, resistance to two important antibiotics used to treat human campylobacter infections – erythromycin and azithromycin – remained low or even declined.

Erythromycin resistance in C. jejuni isolated from humans, retail chicken breasts, and chickens at slaughter remained below 4 percent, as it has since testing began, said the report.

Resistace of C. jejuni to gentamicin, another drug used to treat illness Campylobacter, among other severe infections, remained at below 1 percent in human isolates in 2010. Among C. coli this number grew from 0 percent to 11 percent among human isolates.

What does it mean?

Overall, “the increasing proportion of human isolates that are not resistant to any tested antimicrobial has been an encouraging general trend in NARMS since testing began,” says McDermott.

And while this data reflects some trends in resistance among strains, it does not provide definitive evidence of which drugs lead to resistance in human strains when used in animals.

In order to be able to use NARMS data to figure out which drugs are contributing to resistance in human bacteria, and therefore better inform animal agricultural policy, FDA needs more information, says McDermott.

“We know that antimicrobial use selects for antimicrobial-resistant bacteria,” he says. “In the absence of detailed drug exposure information, however, it is challenging to draw conclusions about how resistance in a single randomly collected isolate evolved as a consequence of specific drug use.”

“FDA is working to get better information to help correlate use practices with the development of resistance,” he says. The agency is currently asking for input on possibly changing its data reporting policy.

So far the program has made some steps in this direction.

“The NARMS data have been used to help establish FDA’s basis for withdrawing approval for poultry fluoroquinolones and the extra-label prohibition of cephalosporins,” notes McDermott of NARMS role in past policy decisions.

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