To the Editor: Despite improved public health, serious infections with nontyphoidal Salmonella enterica remain a major clinical and public health concern in Thailand and worldwide (1,2). Life-threatening Salmonella infections resistant to fluoroquinolones, extended-spectrum cephalosporins, or both, have been increasingly reported (3). Use of antimicrobial drugs for disease prevention and growth promotion in food animals has been implicated in this increase in drug resistance (4). Because of extensive global travel, such increases affect the medical community domestically and internationally (5). We report a pilot survey of drug resistance in Salmonella spp. in Thailand.

We studied archival nontyphoidal Salmonella isolates from bacteremic patients at King Chulalongkorn Memorial Hospital from January 2003 to October 2005 and from bacteremic patients in Thailand sent to the World Health Organization National Salmonella and Shigella Center in Bangkok during the first half of 2005. The isolates from these archives were nonoverlapping and were kept frozen at −80°C. Isolates were divided into Salmonella serovar Choleraesuis and other nontyphoidal Salmonella (non-Choleraesuis) because we observed that Choleraesuis isolates show a higher frequency of resistance to fluoroquinolones and extended-spectrum cephalosporins than non-Choleraesuis isolates. A standard Etest method (AB Biodisk, Solna, Sweden) was used to evaluate MICs for nalidixic acid, ciprofloxacin, and ceftriaxone. Susceptibility was defined according to the 2005 criteria for Salmonella of the Clinical Laboratory Standards Institute (CLSI, formerly NCCLS) (6).

Isolates showed high frequencies of antimicrobial drug resistance (Figure). All S. Choleraesuis isolates with ceftriaxone resistance also showed high levels of resistance to nalidixic acid (MIC ≥256 µg/mL); most of these also had reduced susceptibility to ciprofloxacin (MIC ≥0.125 µg/mL). Of 73 nalidixic acid–resistant Salmonella isolates, 55 (75%) required a ciprofloxacin MIC ≥0.125 µg/mL, 14 (19%) required an MIC of 0.094 µg/mL, and 4 (6%) required an MIC of 0.064 µg/mL. One patient with aortitis caused by ceftriaxone-resistant S. Choleraesuis died of a ruptured mycotic aneurysm.

In the food animal industry, the effect of using antimicrobial drugs has long been a subject of concern (7–9). Evidence from molecular epidemiologic studies (9) suggests that these concerns are genuine and that serious problems must be addressed. This concern is also supported by reports of fatal, invasive, nontyphoidal Salmonella infections resistant to quinolones or extended-spectrum cephalosporins (7,10). In Thailand, enrofloxacin, a veterinary fluoroquinolone, is used in animals in the poultry, swine, and seafood industries. Ceftiofur, a third-generation cephalosporin, is used extensively in swine for treatment and prevention of disease and for growth promotion. When compared with previous susceptibility patterns (5), current nontyphoidal Salmonella infections in humans in Thailand are more resistant to quinolones and cephalosporins. Susceptibility to nalidixic acid correlates well with reduced susceptibility to ciprofloxacin. An alarming increase in ceftriaxone resistance in S. Choleraesuis may be associated with inappropriate cephalosporin use in swine farming. Major revisions in current policies for use of antimicrobial drugs in food animals in Thailand are warranted.