From 1995 to 2000, 13,334 S
. Enteritidis infections were recorded in Denmark, accounting for 62% of all zoonotic salmonella infections. To monitor drug resistance (2
), we examined a random sample of 2,546 isolates, of which 82 (3.2%) were resistant to the quinolone nalidixic acid. These data showed that quinolone resistance increased from 0.8% (3 of 384 isolates) in 1995 to 8.5% (31 of 366) in 2000 (). Resistance to other antimicrobial agents was infrequent, and quinolone resistance was mainly present as a single resistance.
Annual proportion of quinolone resistance in isolates of Salmonella Enteritidis, Denmark, 1995–2000.
Quinolone resistance was related to foreign travel as well as S. Enteritidis phage type (PT). In isolates from patients with a known history of foreign travel, 8.9% were resistant, compared with 2.4% in domestically acquired infections (p<0.0001; Table). In 157 patients who had returned from a European destination (excluding Scandinavia), 18 (11.5%) had resistant isolates. Resistance was highest in patients returning from Spain: 12 (19.7%) of 61 isolates were resistant. Five (7.5%) resistant strains were found in 67 isolates from Asia (mainly Turkey and Thailand), but no resistant strains were recovered from 25 persons who had traveled to Africa. Five patients had visited other countries (1 resistant strain), and we had no information about the destination for the remaining 48 patients (3 resistant strains).
The major sources of domestically acquired S.
Enteritidis infections are raw or undercooked eggs produced in Denmark (3
, unpub. data). The most common phage types in Danish layer hens are PT 6 and PT 8, which accounted for 65.1% of the domestically acquired infections in our study. Resistance in these two phage types remained low (), as were the rates of resistance in PT 13A, PT 25, and PT 34. These types also originate from layer hens. In contrast, the proportion of resistant isolates was highest in phage types PT 1, PT 4, PT 6A, PT 14B, and PT 21, which are often associated with infections from imported poultry products, including imported broiler chickens.
Prevalence of quinolone resistance in human isolates of Salmonella enterica serotype Enteritidis, by phage type and history of foreign travel. Denmark, 1995–2000.
From 1994 to 1997 in England and Wales, quinolone resistance in S.
Enteritidis increased from 0.4 % to 1.3%. As in our study, resistance was highest in PT 1 (19%) and PT 6A (14%) (4
). These types were mainly associated with foreign travel. In a recent study from Spain, 31% of 385 S.
Enteritidis isolates overall but 80% of PT 1 isolates were reported to be quinolone resistant (5