Introduction

Salmonella is a group of bacteria that can cause salmonellosis in both animals and humans. Many infections are due to ingestion of contaminated foods. Eating raw or under-cooked foods of both plant and animal origins could lead to a high risk of salmonellosis infections. It was reported that salmonellosis associated with consumption of pork products accounts for approximately six to nine per cent and 10 to 15 per cent of the total number of foodborne salmonellosis cases in the US and Denmark, respectively.

Salmonella control continues to be a challenge for both food processors and producers.

Currently, there is no effective control method for this organism at the farm level. One of the control methods which appears to be promising is vaccination. In fact, there are commercial Salmonella vaccines available to swine producers in Canada.

The question is, do they work in Ontario? A recent study conducted by Drs Farzan and Friendship of the Ontario Veterinary College provides some information to answer this question.

Here is a brief summary of their findings presented at the recent University of Guelph Swine Research Day.

Materials and Methods

The study was carried out on a farrow-to-finish swine operation which has a history for both clinical and sub-clinical salmonellosis.

Nine cohorts of about 350 weaned pigs each were assigned to one of the three treatment groups: 1) S. typhimurium bacterin (an autogenous vaccine prepared from the S. typhimurium var. Copenhagen DT 104 isolated from the study farm; 2) S. choleraesuis vaccine (a commercial, oral vaccine) and 3) Control (no vaccine).

Body weights and faecal samples were collected for analysing growth performance and Salmonella prevalence.

Results

Salmonella shedding

The S. choleraesuis-vaccinated group had the highest Salmonella shedding, followed by the S. typhimurium bacterin and the control groups, respectively.

The Control group had the lowest Salmonella shedding among the three groups.

The results from this study indicated that vaccinating pigs with S. typhimurium bacterin had no effect on Salmonella reduction and pigs vaccinated with an oral live S. choleraesuis vaccine failed to produce cross-protection immunity against S. typhimurium and other Salmonella strains.

Weight gain

Pigs in the control group showed the best growth performance compared to the two vaccinated groups.

The results showed that the more Salmonella the pigs shed, the slower the pigs grow.

The study also demonstrated that sub-clinical salmonellosis can slow down a pig's growth, suggesting that sub-clinical Salmonella infection not only poses a food safety risk to consumers but also has a negative impact on producers' bottom line.

In summary, vaccinating pigs against Salmonella may not achieve the desired results as hoped. There are many factors that could affect the success of vaccination.

This study demonstrates that there is no silver bullet for on-farm control of Salmonella at the present time and perhaps within the foreseeable future. Good production practices such as thorough cleaning, effective sanitation and workable biosecurity should continue to be the basic and cost-effective strategy for on-farm Salmonella control. Other control methods such as the uses of vaccines, essential oils, liquid feeding and acidification of drinking water and/or feed may achieve good but inconsistent control results.

July 2010