Multinational outbreak of Salmonella enterica serotype newport infections due to contaminated alfalfa sprouts

Context In December 1995, reported Salmonella enterica serotype Newport (SN) infections increased sharply in Oregon and British Columbia but not elsewhere in North America. Similar unexplained increases had been noted in 6 other states in the fall of 1995. Objective To determine the source of the outbreak(s). Design Case-control studies, environmental investigations, bacterial subtyping, and surveillance information review. Settings Oregon and British Columbia communities (winter 1995-1996) and Georgia, Oklahoma, Pennsylvania, Vermont, Virginia, and West Virginia (fall 1995). Participants Oregon and British Columbia residents with culture-confirmed SN infections and onset from December 1, 1995, through February 29, 1996, and healthy community controls. Main Outcome Measures Odds ratio (OR) of illness associated with exposures; distribution patterns and culture of alfalfa seeds and sprouts; subtyping of SN isolates. Results We identified 133 cases in Oregon and British Columbia; 124 (93%) occurred in patients older than 18 years; 87 (65%) were female. Case patients were more likely than community control subjects to report having eaten alfalfa sprouts in the 5 days preceding illness (41% [17/41] vs 4% [3/75]; OR, 17.0; 95% confidence interval, 4.3-96.0). Case isolates shared a distinctive pulsed-field gel electrophoresis (PFGE) pattern. The SN was grown from seeds and alfalfa sprouts. The distribution of 1 seed lot to multiple growers corresponded to the distribution of cases. Distribution of a second seed lot from the same European wholesaler corresponded to the location of the fall outbreak, which was characterized by a similar demographic profile. The PFGE pattern of fall outbreak isolates and confiscated sprouts and seeds was indistinguishable from the Oregon and British Columbia outbreak and differed from background isolates. Conclusions The SN-contaminated alfalfa seeds were distributed to multiple growers across North America in 1995 and resulted in a protracted international outbreak scattered over many months. Current sprouting methods are inadequate to protect consumers from such events. Alfalfa sprouts may be an elusive but important vehicle for salmonellosis and other enteric infections.