Taiwan study shows spread of fluoroquinolone-resistant Salmonella from pigs to humans

Feb 11, 2002 (CIDRAP News) – A study from Taiwan shows a rapid increase in fluoroquinolone resistance in Salmonella enterica serotype choleraesuis in humans over the past 2 years and suggests that the resistant strain spread from pigs. The findings prompted the researchers to urge a ban on fluoroquinoline use in food animals.

In the third quarter of 2001, 60% of S enterica serotype choleraesuis isolates in two associated hospitals were resistant to ciprofloxacin, according to the report by Cheng-Hsun Chiu, ME, and colleagues in the Feb 7 issue of the New England Journal of Medicine. Molecular typing indicated that pigs were the source of the resistant strain.

The authors examined data collected from 1987 to 2000 on Salmonella infections at a large general hospital and an affiliated children's hospital and investigated fluoroquinolone resistance in S enterica serotype choleraesuis isolates from 2000 and 2001. Forty-eight clinical isolates of this strain were subjected to molecular microbiologic examination, as were 17 isolates collected in 1997 and 26 isolates from pigs that had dysentery. The isolates were tested for antimicrobial susceptibility and genotyped by infrequent-restriction-site–polymerase-chain-reaction (IRS-PCR) analysis, and their plasmid profiles were determined.

A total of 501 S enterica serotype choleraesuis isolates were collected from 1987 through 2000. As a proportion of all Salmonella isolates, S enterica serotype choleraesuis increased from 2.7% for 1996 through 1998 to 5.0% for 1999 and 2000. Before 1991, most S enterica serotype choleraesuis isolates were susceptible to ampicillin, chloramphenicol, and trimethoprim-sulfamethoxazole, the report says. But by 2000, 78% of isolates were resistant to the three antibiotics, and resistance to ciprofloxacin began to emerge. "The proportion of clinical isolates of S enterica serotype choleraesuis that were fluoroquinolone-resistant increased rapidly and dramatically from none through 1999 to 60% in the third quarter of 2001," the report states.

The PCR analysis included amplification of a fragment of the gene for DNA gyrase A that contains the fluoroquinolone-resistance–determining region of the gene. This showed that all the ciprofloxacin-resistant isolates from humans and swine had mutations that led to identical amino-acid substitutions at the same two positions on the gene. Other evidence suggests that these two mutations are equally important in facilitating fluoroquinolone resistance, the report says.

"All of the isolates from humans and swine that we examined had similar IRS-PCR and plasmid patterns, suggesting that human S enterica serotype choleraesuis infections were acquired from pigs," the authors say. None of the patients infected by ciprofloxacin-resistant S enterica serotype choleraesuis had previously been treated with quinolones, the authors note.

The researchers say that third-generation cephalosporins are now the only antibiotics with reliable activity against S enterica serotype choleraesuis in Taiwan, since most isolates are resistant to ampicillin, chloramphenicol, trimethoprim-sulfamethoxazole, and now ciprofloxacin. "In view of the severe adverse consequences for human health of the use of fluoroquinolones in food animals, we suggest that such use should be prohibited," they conclude.

Chiu C-H, Wu T-L, Su L-H, et al. The emergence in Taiwan of fluoroquinolone resistance in Salmonella enterica serotype choleraesuis. N Engl J Med 2002;346(6):413-9
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