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Estimating Salmonella's Prevalence

A national survey of salmonella's presence in finishing hogs estimated the organism was "shed" on more than a third (38%) of hog operations.

Shedding refers to transmission or spread of an organism. Salmonella is shed by pigs through their feces, according to Eric Bush, DVM, veterinary epidemiologist at USDA's Centers for Epidemiology and Animal Health (CEAH).

NAHMS Survey CEAH manages USDA's National Animal Health Monitoring System (NAHMS), which conducted a 16-state survey to detect presence of salmonella and other food-borne pathogens and to identify good farm production practices to reduce the risks associated with salmonella, according to Bush.

To identify the presence of salmonella in the NAHMS Swine '95 study, 152 producers each provided 50 fecal samples from pens of late finishing hogs on their farms. From those collections, a total of 6,655 usable samples from 988 pens were tested, revealing evidence of salmonella on 58 of the operations.

Some 35 of the 58 positive farms (60%) had a positive sample in less than half of the finisher pens tested. But a fourth of the operations had active salmonella "shedders" in over two-thirds of their finisher pens.

The major risk factors linked to shedding of salmonella in finishers encompass feed management, pig exposure in the herd and environment. The strongest risk factor was for finisher feed form.

He observes, "While the pelleting process is thought to eliminate salmonella from feed, this does not preclude recontamination of pellets or prohibit their influence on the likelihood of a pig shedding salmonella already present in the pig."

Other significant factors include: * A different but also significant effect was seen by use of feed mixed off farm. Bush suggests this may reflect differences in the byproducts used, processing techniques or added risk of contamination from trucking feed.

* "While farms with single sex pens were at higher risk of being classified as salmonella positive (than mixed sex), the highest risk category was for farms with both single sex and mixed sex pens," explains Bush. Besides potential effect of split sex feeding itself, increased sorting to form single-sex pens may harm the pig's gut, increasing transmission within a herd or serving as an important stressor which induces shedding, he suggests.

* The greater odds that hog farms in the southeast were positive for salmonella may be due to climatic or environmental factors.

The NAHMS Swine 2000 study will again look at salmonella, to formulate a trend in prevalence and to better understand the risk factors that have been identified. The study will also look at other food-borne pathogens to describe their prevalence and identify risk factors.

Bush reminds the success of these surveys all depends on producer participation.