River Water as a Reservoir for Salmonella enterica on the Maryland Eastern Shore (Delmarva)

Introduction: Foodborne illness outbreaks of Salmonella enterica have been associated with fresh produce cultivated on the Delmarva Peninsula.  Epidemiological investigations linked a 2002 and a 2005 outbreak to tomatoes grown on the eastern shore of Virginia, with recovery of the outbreak strain from irrigation pond water on the implicated farm.  The source of a 2014 Salmonella Newport outbreak in Maryland, associated with cucumber consumption, remains unknown.  These recurrent outbreaks suggest a persistent environmental reservoir increasing the likelihood of contamination of fields in this region. We currently do not have sufficient environmental data to completely understand the ecology of S. enterica on the Delmarva Peninsula. 

Purpose: The purpose of this study was to evaluate the prevalence of S. enterica in surface water from rivers on the eastern shore of Maryland (Delmarva).

Methods: Water samples were collected from a total of 23 sites along four major rivers (Choptank, Nanticoke, Wicomico, Pocomoke) on the eastern shore of Maryland during the summer and fall of 2015, to determine the presence of S. enterica in surface waters. At each sampling location, a 10 L volume of water was filtered through a Modified Moore Swab cartridge.  Salmonellae were isolated by pre-enrichment in lactose broth, enrichment in tetrathionate broth, and selective growth on R&F Salmonella chromogenic plating medium.  Presumptive S. enterica isolates were confirmed by PCR amplification targeting the invasion A (invA) and invasion gene regulator (hilA) genes.

Results: A total of 14 out of 23 (61%) samples were positive for S. enterica (1/6 Choptank, 5/5 Nanticoke, 5/6 Pocomoke, 3/5 Wicomico). From these samples, 110 presumptive Salmonella isolates were recovered (6 Choptank, 40 Nanticoke, 43 Pocomoke, 21 Wicomico). All isolates were PCR-positive for both the invA and hilA genes.

Significance: This study confirms that surface river water on the eastern shore of Maryland serves as an environmental reservoir for S. enterica.