We reviewed final reports of nontyphoidal Salmonella outbreaks investigated by CDC from 1984 to 2002. We excluded outbreaks that occurred in a healthcare setting or outside the United States, including on cruise ships. When antimicrobial susceptibility was not recorded in the final report, we searched CDC microbiology records for susceptibility test results on isolates collected as part of the outbreak. Outbreaks were only included if susceptibility data were available for >1 isolate. Because different laboratories performed susceptibility testing, the antimicrobial agents tested and the methods used varied between outbreaks. We classified outbreaks as resistant when the outbreak strain was resistant to ≥1 antimicrobial agent; other outbreaks were considered pansusceptible. Outbreaks caused by resistant strains were additionally classified as R-type AC/KSSuT when the outbreak strain was at least resistant to ampicillin, chloramphenicol or kanamycin, streptomycin, sulfamethoxazole, and tetracycline. Outbreaks were additionally classified as resistant to a clinically important agent when the outbreak strain was at least resistant to ampicillin, trimethoprim-sulfamethoxazole, aminoglycosides, fluoroquinolones, or a third-generation cephalosporin.
Data from the final investigative reports were used for the analysis. When analyzing data according to outbreaks, we calculated the medians for percentage hospitalized and percentage who died and compared medians with the Wilcoxon rank-sum test. When analyzing data according to ill persons, we pooled data from the reports, calculated proportions, and compared proportions with chi-square or, when appropriate, Fisher exact test. The denominators for percentage hospitalized and died varied depending on the number of persons in whom outcome data were ascertained. All p values were 2-tailed. Data were analyzed by using SAS v.9 (SAS Institute, Cary, NC, USA).
From 1984 to 2002, CDC investigated 48 community outbreaks of nontyphoidal Salmonella
strains in the United States. Of these, 47 (98%) had a final report available for review (). We restricted our analyses to the 39 (83%) outbreaks in which data about antimicrobial susceptibility were available. These 39 outbreaks affected 23,206 persons. The largest outbreak occurred in 1985, in which culture-confirmed S
. Typhimurium infection associated with milk consumption developed in 16,659 persons (11
Strains from 11 (28%) outbreaks were resistant, and 28 (72%) were pansusceptible. The 11 outbreaks caused by resistant strains involved 18,698 persons. Of these 11 outbreaks, 7 (64%), involving 17,182 persons, had strains that were at least R-type AC/KSSuT, and 9 (82%), involving 17,919 persons, had strains that were resistant to a clinically important agent.
Hospitalization data were available for 32 outbreaks involving 21,702 ill persons. The hospitalization rates were higher for each type of outbreak caused by resistant salmonellae compared with outbreaks caused by susceptible salmonellae (p<0.01 for each comparison) (). To account for differences in the size of outbreaks, we compared the median proportion of persons hospitalized and compared hospitalization rates after excluding a large Salmonella outbreak that occurred in 1985. The median proportion hospitalized for each type of outbreak caused by resistant strains (26%) was >2.5 times higher than the median proportion hospitalized for outbreaks caused by pansusceptible strains (10%, p<0.05 for all resistance patterns). The difference in hospitalization rates between outbreaks caused by resistant and susceptible strains was similar after we excluded the large 1985 outbreak of resistant S. Typhimurium, in which the percentage hospitalized was 22%. The results also remained similar after excluding S. Enteritidis outbreaks, in which rates of hospitalization and isolate resistance were low.
Hospitalization and death rates among nontyphoidal Salmonella outbreaks by resistance pattern, 1984–2002*
Mortality data were available for 24 outbreaks involving 21,927 persons. A greater proportion of persons died in resistant outbreaks than in pansusceptible outbreaks, but the difference was not significant (0.1% in outbreaks caused by resistant strains vs. 0.06% in outbreaks caused by pansusceptible strains, p = 0.57) ().
The 8 outbreaks in which no susceptibility data were available involved 1,914 ill persons. Three (38%) outbreaks were due to S. Enteritidis and 2 (25%) to S. Typhimurium. In the 6 outbreaks for which hospitalization data were available, 70 (20%) of 353 persons were hospitalized. In the 4 outbreaks for which mortality data were available, 7 (0.4%) of 1,708 persons died.