Isolation of Salmonella Enteritidis phage type 30 from a single almond orchard over a 5-year period

Abstract

In 2001, Salmonella Enteritidis phage type (PT) 30 was isolated from drag swabs of 17 61-ha almond orchards on three farms linked to an outbreak of salmonellosis associated with consumption of raw almonds. The objective of this study was to evaluate the long-term persistence of Salmonella Enteritidis PT 30 in one of the almond orchards associated with the outbreak. Swabs (gauze saturated with full-strength sterile evaporated skim milk and attached to string) were pulled along the orchard floor in a standardized manner for 55 m. At each sample time, two pooled samples (four swabs each) were collected from each orchard quadrant. Swabs were enriched for Salmonella using a delayed secondary enrichment procedure developed for isolation of Salmonella from poultry houses. Suspect Salmonella isolates were selected, confirmed, serotyped, and phage typed, and pulsed-field gel electrophoresis (PFGE) patterns were determined after cleavage with XbaI and BlnI. Salmonella was recovered infrequently from pooled samples collected from January through July (3 of 56 samples, 5.3%). In general, Salmonella isolation frequency per sample time increased during and immediately after the harvest, when large amounts of dust were generated in or near the orchard: August, 4 (20%) of 20 samples; September, 13 (20%) of 64 samples; October, 27 (42%) of 64 samples; November, 4 (25%) of 16 samples; and December, 2 (25%) of 8 samples. All 53 Salmonella isolates during the 5 years were identified as Salmonella Enteritidis PT 30, and two PFGE patterns that differed by the presence of an approximately 40-kb fragment were identified. These data demonstrate the potential for long-term environmental persistence of Salmonella in almond orchards.