Bacteriological evaluation of vaccination against Salmonella Typhimurium with an attenuated vaccine in subclinically infected pig herds
Introduction
Human salmonellosis is the second most reported foodborne zoonosis in Europe (EFSA, 2015b,a, 2016, 2017). Infections with Salmonella enterica subspecies enterica serovar Typhimurium (Salmonella Typhimurium) in pigs constitute a major risk for human salmonellosis. In 2016, pigs and pork were considered the source of 26.9% of the human infections caused by Salmonella Typhimurium and of 61.9% of the human infections caused by Salmonella Typhimurium monophasic variants (EFSA, 2017).
The high prevalence of subclinical infections with Salmonella Typhimurium in pig herds, together with the intermittent shedding pattern of infected pigs and the common occurrence of carriers and environmental reservoirs, hamper a proper control of infections at farm level (Gray and Fedorka-Cray, 1996; Funk et al., 2001; Pires et al., 2013; Lynch et al., 2018). Current on-farm control measures are mainly focused on biosecurity, cleaning and disinfection, all-in all-out management and acidification of feed and drinking water (Wales et al., 2011; Andres and Davies, 2015; Callegari et al., 2015; Hill et al., 2016). Vaccination against Salmonella in pigs can be an important tool to control Salmonella infections at farm level as well (Denagamage et al., 2007; de la Cruz et al., 2017; Wales and Davies, 2017).
The main goals of vaccination are to prevent clinical salmonellosis, to decrease shedding and colonization and to decrease the probability of contamination of carcasses (Haesebrouck et al., 2004; Meeusen et al., 2007; Boyen et al., 2008; Arguello et al., 2012; Wales and Davies, 2017). Several papers report beneficial effects of different vaccines against Salmonella infection in pigs, either under experimental or field conditions (Denagamage et al., 2007; de la Cruz et al., 2017; Wales and Davies, 2017). Most studies measured vaccine efficacy by comparing the presence of Salmonella in feces and/or organs between vaccinated and control groups (de la Cruz et al., 2017), additionally, the effect on weight gain is evaluated in several studies (Farzan and Friendship, 2010; De Ridder et al., 2014).
The attenuated histidine-adenine auxotrophic vaccine Salmoporc® (IDT Biologika) is currently the only Salmonella Typhimurium vaccine commercially available for pigs in Europe. The vaccine has previously been evaluated with promising results in sows and weaned piglets of three and six weeks of age (Lindner et al., 2007), in piglets of three and 24 days of age (Roesler et al., 2010; De Ridder et al., 2014), in weaned piglets of four and seven weeks of age (Theuß et al., 2017) and in sows and gilts (Davies et al., 2016; Smith et al., 2018). To our knowledge, the vaccine has not been evaluated in a field study applying several different vaccination strategies in one farrowing batch. Also, the vaccine has not yet been evaluated in fattening pigs at the start of the fattening period. Vaccination of fattening pigs may be beneficial compared to vaccination around weaning, as it avoids extra handling, stress and possible adverse reactions in the crucial transition period around weaning.
The general aim of the present study was to assess the effect of different vaccination strategies (1. vaccination of sows, 2. vaccination of sows and piglets, 3. vaccination of sows and fattening pigs, 4. vaccination of piglets, 5. vaccination of fattening pigs) with an attenuated Salmonella Typhimurium vaccine in subclinically infected pig herds. The specific objectives were: 1. to investigate the effect of the different vaccination strategies on the excretion of Salmonella Typhimurium field strains, 2. to investigate to which extent the different vaccination strategies reduce the number of pigs positive for Salmonella Typhimurium field strains in the ileocecal lymph nodes at slaughter, 3. to evaluate the possible persistence of the vaccine strain in ileocecal lymph nodes, 4. to evaluate the genetic variation between Salmonella Typhimurium isolates originating from vaccinated and non-vaccinated pigs and 5. to evaluate the effect of the different vaccination strategies on the daily weight gain during the fattening period.
Section snippets
Farm selection
Three Belgian pig farms (A, B, C) were selected based on the following criteria: 1. bacteriological detection of Salmonella Typhimurium on the farm and high Salmonella-specific antibody levels in fattening pigs (S/P-ratios ≥0.6) in the former Belgian Salmonella Surveillance Program, without clinical Salmonella infections during the year preceding the study, 2. a minimum of 72 sows per farrowing batch, 3. possibility to monitor the pigs from birth to slaughter, 4. willingness of the farmer to
Fecal and overshoe samples: sows
The bacteriological results of the fecal and overshoe samples from the sows are shown in Table 2. In total, 41/875 (4.7%) samples were Salmonella positive; 12/265 (4.5%), 24/299 (8.0%) and 5/311 (1.6%) samples from farm A, B and C, respectively. The vaccine strain was detected in 32/875 (3.7%) samples, collected in all three farms, both from vaccinated and from non-vaccinated sow groups. In total, 6/875 (0.69%) samples were Salmonella Typhimurium field strain positive; 1/265 (0.38%), 5/299
Discussion
The current study investigated the effect of different vaccination strategies against Salmonella Typhimurium with an attenuated vaccine on the excretion of Salmonella and the number of pigs positive for Salmonella in the ileocecal lymph nodes in three subclinically infected pig herds. In production cycle 2, the results showed a protective effect of vaccination on the number of Salmonella Typhimurium field strain positive lymph nodes when applying vaccination either in sows and piglets, sows and
Conclusion
The results of the current study should, because of limitations in the study design and the variability between the farms and production cycles, be extrapolated with care. Nevertheless, they provide evidence that applying vaccination against Salmonella Typhimurium in sows and piglets (preferred), sows and fattening pigs, and piglets only can support the control of Salmonella Typhimurium infections by decreasing the prevalence of Salmonella Typhimurium field strain positive lymph nodes at
Declaration of interest
None.
Acknowledgements
The research that yielded these results was funded by the Belgian Federal Public Service of Health, Food Chain Safety and Environment through the contract [SUSALVAC; RT14/3].
We thank the farmers for their motivation and help with this study. Also, we are grateful to the veterinarians of the farms, the slaughterhouse employees and all colleagues and students for their assistance during the study. The lab personnel, especially Nathalie Van Rysselberghe, is greatly appreciated for the help during
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