RESEARCH Table 3. Trend analysis of the proportion of fluoroquinolone- resistance among Campylobacter, National Antimicrobial Resistance Monitoring System, 1997–2001 Y Unadjusted ORa (95% CI) Adjusted ORb (95% CI) 1997c 1.0 1.0 1998 1.0 (0.6 to 1.7) 1.3 (0.7 to 2.4) 1999 1.4 (0.9 to 2.3) 2.1 (1.2 to 3.9) 2000 1.1 (0.7 to 1.8) 1.5 (0.8 to 2.8) 2001 1.6 (1.0 to 2.5) 2.5 (1.4 to 4.4) aOR, odds ratio; CI, confidence interval. bAdjusted odds ratios were calculated by using logistic regression model, which accounted for site-to-site variation in prevalence. c1997 was the reference value. 16 C. coli isolates came from retail chickens purchased in Minnesota. A ciprofloxacin-resistant strain of Campylo- bacter was identified in 10% of the 180 retail chicken products tested, and an erythromycin-resistant strain was identified in 2% of chicken products (Table 4). The distri- bution of ciprofloxacin MICs in Campylobacter species of retail chicken and human isolates was similar. For both human and poultry Campylobacter isolates, MICs were predominantly <0.5 or >32 µ g/mL with few intermediate phenotypes (Figure 1). Discussion Fluoroquinolone-resistant Campylobacter have emerged over the last decade in the United States. In 1990, no ciprofloxacin-resistant human isolates were identified in a national sentinel county-based survey. From 1997 to 2001, the prevalence of ciprofloxacin-resistant Campylobacter increased significantly from 13% to 19%. These data are consistent with four prior surveillance stud- ies from humans conducted in the United States: 1) a hos- pital-based study in Pennsylvania conducted from 1982 to 1991 found no fluoroquinolone resistance among C. jejuni isolates (29), 2) a second study at the same Pennsylvania hospital found a sharp increase in ciprofloxacin resistance among C. jejuni from 8% in 1996 to 40% in 2001 (30), 3) a study conducted in Wisconsin between 1992 and 1995 found 12% of the C. jejuni to be ciprofloxacin-resistant (31), and 4) a study in Minnesota showed an increase in quinolone-resistant C. jejuni isolates from 1.3% in 1992 to 10.2% in 1998 (Figure 2) (18). The emergence of fluoro- quinolone resistance among Campylobacter isolates in the 1990s has occurred while resistance to other antimicrobial agents has remained stable. Specifically, resistance to the macrolides, azithromycin and erythromycin, which are commonly used antimicrobial agents in humans (32), has remained low (1%–3%). Our retrospective case-comparison study showed that patients with ciprofloxacin-resistant C. jejuni infections were more likely to be hospitalized compared to patients with ciprofloxacin-susceptible infections. These results, however, are based on a small number of patients, and age could have been a confounder. Other studies have found that patients infected with fluoroquinolone-resistant Campylobacter have a longer duration of diarrhea than patients with fluoroquinolone-susceptible isolates, although no difference in hospitalization rates have been reported (18,33). These findings may have clinical impli- cations. Ciprofloxacin is commonly used to treatment severe Campylobacter infections and other intestinal infections in adults, so the rise of fluoroquinolone resist- ance may result in ineffective treatment when fluoro- quinolones are used. Macrolides, which are efficacious in treating Campylobacter (5,34), should still be considered the first-line drugs for severe Campylobacter infections, as resistance to this class remains low. Our study also identified foreign travel, particularly to Europe, to be associated with ciprofloxacin-resistant C. jejuni infection. High rates of fluoroquinolone-resistant Campylobacter have been reported from southern Europe and other regions of the world (13). Studies in northern Europe have associated fluoroquinolone use in food ani- mals, particularly poultry, as a source for human infection with fluoroquinolone-resistant Campylobacter (13). Nevertheless, while foreign travel was a risk factor in our study, over half of the ciprofloxacin-resistant infections were domestically acquired. Ciprofloxacin resistance was not associated with use of fluoroquinolones before speci- men collection, which suggests that fluoroquinolone- resistant organisms did not result from individual use of fluoroquinolones. A more recent, larger case-control study of patients infected with ciprofloxacin-resistant Campylo- bacter infections found similar results to our study; 58% of illnesses were domestically acquired, and none of the patients took fluoroquinolones after illness onset and before specimen collection (35). Our 1999 survey of retail chicken sold in selected supermarkets provided ecologic evidence that chicken Table 4. Antimicrobial resistance among Campylobacter isolates from retail chicken, by species, National Antimicrobial Resistance Monitoring System, 1999 % resistant C. jejuni C. coli Othera Antimicrobial agent (n = 62) (n = 16) (n = 2) Azithromycin 6 0 0 Chloramphenicol 0 0 50 Ciprofloxacin 24 19 50 Clindamycin 5 0 0 Erythromycin 6 0 0 Gentamicin 0 6 0 Nalidixic acid 29 37 50 Tetracycline 69 50 50 aOne isolate was undetermined (i.e., hippurate-negative Campylobacter that could not be further speciated by 16S polymerase chain reaction study), and one isolate was an unknown Campylobacter that could not be further characterized. 1106 Emerging Infectious Diseases • www.cdc.gov/eid • Vol. 10, No. 6, June 2004