Human salmonellosis has remained a considerable challenge for the U.S. food industry, regulatory agencies (both USDA and FDA), and public health agencies over the last decade. The latest estimates on the prevalence of human salmonellosis suggest that foodborne Salmonella infections cause around 1 million domestically acquired disease cases a year in the U.S., including 350–400 cases that result in deaths.
There are a number of reasons why it has been so difficult to reduce Salmonella transmission in the U.S. For one, non-typhoidal Salmonella (meaning all Salmonella except those that cause Typhoid fever), can be transmitted to humans through a variety of transmission routes, including a number of routes other than contaminated foods. The difficulties with accurate and reliable source attribution have lead to a situation where control efforts typically focus on food vehicles that have been identified as sources of large or multiple outbreaks. While valuable, this approach has problems, including the fact that cases linked to detected outbreaks only represent a small proportion of all salmonellosis cases that occur.
In addition, most source attribution efforts use approaches that identify the primary source (e.g., poultry, ducks, cattle), but do not identify the transmission pathway. This is important as the transmission pathways from a given source (e.g., poultry) to humans can differ significantly and may involve direct contact (e.g., with live chicks) or undercooked poultry, to just give two examples. While eradication or control at the primary source should be effective regardless of transmission route, very often more effective interventions can be developed and implemented when targeting specific transmission routes. Further improvements in our understanding of Salmonella sources and, importantly, transmission routes are thus critically needed to address this issue.
A second important issue and challenge is that non-typhoidal Salmonella represents a large variety of different serovars. Overall, more than 2,500 Salmonella serovars (e.g., Newport, Typhimurium, Heidelberg, Enteritidis, etc.) have been identified. Many of these serovars are rarely involved in human illness cases and outbreaks. Nevertheless, the serovars that are important in human disease and food contamination can differ considerably in different parts of the world and different serovars may also be associated with different animal hosts, reservoirs, and transmission routes.
While some current concerns in the U.S. appear to focus on the presence of multi-drug resistant Salmonella in raw meat and poultry, it is important to keep in mind that there is increasingly clear evidence that a number of genetic characteristics of Salmonella (in addition to whether they carry and express genes that confer resistance to antibiotics) may determine how likely a given Salmonella strain is to cause human disease. While strain and serotype-specific approaches to pathogen control are increasingly seen with regard to E. coli (the news is full of reports on new approaches to control and regulate the “big six”), similar strain-specific approaches to control of Salmonella are still limited.
While improved science-based and science-informed approaches to Salmonella control require efforts in a number of scientific disciplines, recent advances in genomics tools provide one clear route towards an improved ability to detect and characterize disease outbreaks as well as an improved understanding of serotypes diversity.
A recent collaborative study my research group was involved in reported genome sequences for 16 Salmonella serotypes, for which no full genome sequences have previously been reported (Den Bakker et al., 2011. BMC Genomics 12:425; freely available at http://www.biomedcentral.com/1471-2164/12/425/abstract). Analyses on these 16 newly sequenced as well as 30 existing publicly available genomes showed a subdivision of Salmonella enterica subsp. enterica (the most common Salmonella subspecies, which includes the vast majority of serotypes that affect warm blooded animals) into at least two subpopulations.
While these data and analyses are still far from providing a definitive classification of Salmonella into subgroups with distinct disease-causing capabilities, they provide critical information that not only leads us towards a path of better classification of Salmonella into subgroups that may differ in human health risk, but also provide the first genome sequence data for a number of Salmonella serotypes that have recently caused human disease outbreaks.
Department of Food Science