Salmonella Control: What Is Happens Upstream Is Important, By Clint Lichty from South West Ontario Veterinary Services

Salmonellosis in humans continues to be one of the most commonly reported gastrointestinal infections in both the European Union (EU) and North America. Pork is one of a number of sources for human salmonellosis outbreaks. It is well understood that when market hogs have a higher prevalence of Salmonella within the farm population there is an increased risk of transmission of Salmonella bacteria to consumers. What is not always well understood is that piglets originating from some sow units may have a higher infection prevalence. When weaned piglets are infected and carrying the Salmonella bacteria in their mesenteric lymph nodes they can predispose the downstream pig flow to increased Salmonella infection. As the initial infection spreads from one growing pig to another the number of potentially contaminated carcasses increases. It is, therefore, important to identify sow farms where the infection pressure with Salmonella is very high. This increased infection in piglets may be a result of increased numbers of sows and gilts that are shedding Salmonella at farrowing. Even if shedding is low the infection rate can be high when the sow farm environment itself is contaminated with Salmonella bacteria.

Salmonella monitoring of the sow farm tries to assess shedding and environmental contamination. “Sock” swabs that are pulled over farm boots can be used to conveniently sample manure on the floor while walking around the pen. Photo 1 Sock samples are a cost effective, labour efficient and sensitive technique for identifying the presence of Salmonella. At the same time that sock samples are taken additional environmental swab samples are taken from areas of the room environment other that the floors. In one study, 23 Salmonella suspect sow farms had samples taken in the supposedly “cleaned and disinfected” areas of the building. In 14 (60.9%) of these farms, the allegedly cleaned and disinfected areas were positive for Salmonella in 35.9% (n = 23) of samples. This can occur even if there are no notable clinical signs of Salmonellosis in breeding females or their piglets. Sampling of some of these compartments led to detection of visible contamination such as old feed in feeders, droplets of feces in the corners or heavy feed dust on the water or feed pipes.  Farrowing areas in Salmonella suspect sow farms should be sampled in a thorough manner to assess the risk for the infection of piglets. Sock or swab testing results in bacterial isolation and then serovar identification such that the serovars of Salmonella that are circulating can be defined. This helps to “connect the dots” of contamination at all levels of production from upstream to downstream. These German researchers wanted to monitor the Salmonella status of sow farms in farms that  finished their own finishers or were delivering piglets to finishing farms with a known high serological prevalence identified within the QS Salmonella monitoring system. There were 105 piglet-producing sow farms from across Germany that were included in the study.

The researchers found the following:

  • 92.4% of the sow farms supplying piglets to high seroprevalence finishers tested positive in at least one sow barn sample.
  • Salmonella was detected in 38.2% of the sock samples and 27.1% of the environmental swab samples. There was generally good agreement between these two types of samples but on occasion sow farms were positive on only 1 of the sampling types.
  • S. Typhimurium was the most frequent serovar detected in this study.

Take Home Message:

  • Sock and environmental swab samples work well for Salmonella detection in farrowing units.
  • Sock and environmental swab samples are a cost effective, labour efficient and sensitive technique for identifying the presence of Salmonella.
  • Salmonella problems in the nursery or finisher can occur even when the number of piglets infected with Salmonella at weaning is very low or zero. ( the main source of infection is downstream) If, however, the farrowing unit is “seeding” the production flow with high numbers of piglets at weaning it will be more difficult to control Salmonella in the downstream barns.
  • “Never underestimate the importance of what is happening upstream!”

Reference: Kathrin Lillie-Jaschniski , Christoph Wähner , Miriam Viehmann , Silke Hauf , Christina Gale , Judith Rohde , Isabel Hennig-Pauka   Sock and Environmental Swabs as an Efficient, Non-Invasive Tool to Assess the Salmonella Status of Sow Farms  Animals (Basel) . 2023 Mar 11;13(6):1031. doi: 10.3390/ani13061031.