Prevalence of Salmonella Species and Serovar Diversity in Swine Finishing Farms in Alberta

January 28, 2005 — Adam Moeser (ed. Scott Hurd)
  [ES]

Salmonella is recognized as one of the most important swine-related zoonotic diseases. In the past, methods of control of Salmonella have focused predominantly on the processing stage of pork production. However, as pathogen reduction performance standards in the packing plant increase, more pressure is placed on preventative strategies at the farm level. The objective of this longitudinal study was to determine the prevalence and source (pigs and/or environment) of Salmonella spp. in Alberta finishing farms and to identify the serovar diversity that exists between farms.

Ten Alberta swine veterinary practitioners selected 90 Alberta pig farms to be included in the study. Selected farms were geographically representative of the major swine producing areas in Alberta and accounted for approximately 25% of market swine production in Alberta. Selection criteria included annual production of 2,000 or more market pigs and willingness of producer to participate in the study. The trial was conducted over a 5-month period (May through September, 2000). A total of 20 samples were collected over three farm visits from each farm (15 fecal and 5 environmental) by assigned veterinarians. Fecal samples were obtained from randomly selected finisher pens within 45-60 days of market. Environmental samples were also collected and consisted of swabs from empty pens, boots, building dust, and main drains. Isolation of Salmonella from fecal and environmental samples was conducted through standard culture protocols as described by Sorensen et al., 2002 (J. Food Prot. 65: 484-491).

The number of Salmonella positive samples ranged from 1 to 19 on farms. Salmonella were detected in 14% of fecal samples and 20% of environmental samples. Approximately 67% of farms had at least one positive Salmonella sample (CI = 57-77%). Over three farms visits, Salmonella status changed from positive to negative or vice versa on 27% of the farms and 20% of farms became positive for the first time suggesting the importance of multiple sampling for determination of farm status. The number of environmental samples collected from farms that were positive for Salmonella ranged between 1/5 and 4/5. Salmonella positive rates for environmental samples were highest for boots (39%, 34/88 samples), main drain (32%, 27/85 samples) and empty pens (11.6%, 18/155 samples) and lowest for dust samples (6%, 5/89 samples).

A total of 22 serovars were detected on the 60 farms. Identification of only 1 serovar was detected in 85% of positive samples, whereas 2 and 3 serovars were recovered from 13% and 1.1% of samples, respectively. For positive farms, 64% had only 1 serovar recovered whereas 17, 11, 4, and 3% of farms had 2, 3, 4, and 5 serovars recovered from samples, respectively. Similar serovars were detected between pen and environmental samples. The serovars most commonly detected were Typhimurium (78 isolates), Derby (71 isolates), and Infantis (47 isolates).

Reference:

Longitudinal study of Salmonella species in 90 Alberta swine finishing farms. Rajic A, Keenliside J, McFall ME, Deckert AE, Muckle AC, O'connor BP, Manninen K, Dewey CE, McEwen SA. Vet Microbiol. 105:47-56, 2005.

Editor's comment:

This type of on farm sampling (~15 fecal samples) is interesting but likely to severely underestimate the true on-farm prevalence for multiple reasons; infected pigs shed periodically, the size of fecal sample affects ability to detect positive sample and 15 samples will not detect if the prevalence is low. References available upon request.