Short-term holding of market hogs at packing plants may be a major source of salmonella infection.

Researchers following pigs from farm to slaughter found 40% of slaughter pigs showed contamination with salmonella organisms.

Studies show while nearly 40% of market hogs are contaminated with salmonella when held for just a few hours at slaughter, the on-farm salmonella infection rate was from 4% to 7%. These findings are from the collaborative work of H. Scott Hurd, DVM, USDA's National Animal Disease Center (NADC), and James McKean, DVM, Iowa State University (ISU), both of Ames, IA.

Slaughter Prevalence

Transportation and long-term holding of hogs before slaughter have been linked to an increase in salmonella prevalence at the packing plant, Hurd says. This increase in prevalence has been attributed to the effects of stress, increased commingling and shedding of salmonella organisms.

However, little has been said regarding the environment as a source of salmonella infection, Hurd observes in a talk at the American Association of Swine Veterinarians annual meeting in Nashville, TN.

Most plants try to avoid holding hogs more than six to eight hours. But holding hogs at least two hours is advised to improve meat quality.

Studies Point to Problems

Some studies are starting to suggest that even short-term holding of hogs (two to three hours) at slaughter may provide a major avenue of salmonella infection, relates Hurd.

In an NADC/ISU study discussed at last year's swine veterinary conference, Hurd and McKean reported a 10-fold increase in the number of pigs positive for salmonella at slaughter vs. on-farm trials.

“This increase occurred in groups that were housed for an additional 18 hours at an off-site, cleaned and disinfected holding facility, as well as those shipped 85 miles directly to slaughter,” Hurd says. The lack of holding effect and the much larger variety of salmonella serotypes found at slaughter vs. the farm led him to believe that factors beyond stress were more important to increased infection rates.

A second joint study addressed differences in samples collected at slaughter vs. the farm using herds enrolled in the Accelerated Pseudorabies Eradication Program. Fifty market hogs were sampled at the farm, and another 50 were tested at the plant after transport in disinfected trailers and two to three hours in holding pens.

In five of the six farms tested, isolation of salmonella organisms was five times higher for pigs necropsied at the plant.

In a third NADC/ISU study, Hurd evaluated whether hogs could become infected with salmonella in a two- to three-hour holding period from a contaminated environment.

Forty market hogs were exposed to salmonella typhimurium from contaminated feces of hogs intranasally exposed four days earlier. Pigs were autopsied at two, three and six hours after exposure to the contaminated floor.

In just two to three hours, most of the animals were positive for the salmonella strain, reports Hurd. After six hours, all hogs had at least one tissue sample test positive.

To match results, the exact same tissues (intestinal contents and lymph nodes) and procedures were used for pigs necropsied at the farm and at the packing plant, he points out. More tissues were tested in the plant necropsies, which could add to the salmonella numbers, he admits.

But he still believes the three tests bear out a startling trend — that salmonella infections seem to be magnified at the plant.

Hurd concludes: “If pigs are re-infected in their gut at the holding pen at slaughter, then that increases the probability that an accidental gut laceration will result in a contaminated carcass.”