| Salmonella is one of the most important zoonotic pathogens,which is widely distributed in most pork production chains worldwide.Pigs have thus become the main carrier of Salmonella.However,many Salmonella species are latent in healthy pigs.As a result,Salmonella is very likely to contaminate food and endanger human health and even endanger life.China is a large pork producer and consumer,and thus special attention should be paid to the prevention and control of Salmonella contamination in the pork production chain.Pig farm as the source of the pork production chain,it is the most important link for the prevention and control of Salmonella for food safety in China.In this study,a two-year Salmonella epidemiological survey was conducted on a large-scale pig farm in Shanghai,in order to clarify the prevalence of Salmonella in swine farms and put forward specific prevention and control measures.From April 2016 to March 2018,a total of 587 pig manure samples were collected and 158 Salmonella isolates were isolated.The total isolation rate was 26.9%.201 feed samples were collected and 10 Salmonella isolates were obtained,72 samples of environmental swabs were collected and 15 strains of Salmonella were isolated with isolation rate of 20.8%.The results of serotype distribution showed that Salmonella Typhimurium was the predominant serotype in pig farm(39%),and Salmonella London(24%)and Salmonella Kentucky(17%)also accounted for a large proportion;S.Typhimurium(30%)and Salmonella Rissen(40%)were the main isolates of feed samples.During the study,The Salmonella isolation rate from swine samples showed 2 dynamic changes,which divided the level of Salmonella isolation rates into 3 phases.From April to December 2016,Salmonella infection rate in pigs was 46.2%.The second phase was from March to September 2017,Salmonella isolation rate was 8.8%.During the third phase,isolation of Salmonella was 32.6%from October 2017 to March 2018.A follow-up survey of 12 sows was taken along the production chain on the farm,as a result,one pig was infected with Salmonella Kentucky in the delivery room,and two pigs were infected with Salmonella Kentucky and S.Rissen in the nursery,showed that the delivery room and nursery were the key production links for Salmonella infection.A total of 154 strains of Salmonella isolates from the farm were tested for resistance to 20 antibiotics.The results showed that the strains exhibited more than 70%resistance to lincomycin,tetracycline,oxytetracyline,ampicillin and nalidixic acid,three of them commonly used in the field except for ampicillin.In terms of multi-drug resistance,most of the strains exhibited resistance to 7 antibiotics.Several S.Typhimurium and Salmonella Kentucky strains were resistant to 18 antibiotics.In order to find out the relationship between the Salmonella isolated from feed and environment and pigs,a pulsed-field gel electrophoresis test was performed on 58 strains of S.Typhimurium and 20 strains of S.Rissen.The results showed that the majority of S.Typhimurium strains from the pig farm were divided into four major PFGE types(T1?T2?T3?T4),of which 37 isolates were always present in live pigs and in pig farm environments from October 2016 to November 2017 belonged to the main type T1.It was isolated from feed ingredients and feces from May 2016 to July 2016.The seven strains of S.Typhimurium were homologous,belonged to T2,indicating that Salmonella could enter the farm from the feed.One novel PFGE type T3 emerged in October 2017.And in November 2017,another new PFGE type T4 ofS.Typhimurium was isolated from the manure of a newly introduced pig.The PFGE test for S.Rissen showed that except for a few strains with different PFGE types,the PFGE patterns of S.Rissen strains were divided to 2 types(R1?R2),especially strains isolated from feed and fecal samples were both belong to type R2,further indicating that feed is very likely to be the source of Salmonella infection.According to the changes in the isolation rate of Salmonella,after September 2017 when the S.Rissen strain was isolated from the feed,the isolation rate of S.Rissen in swine manure samples increased significantly,and the feed and feces source isolates belonged to the same PFGE type,indicated that the contaminated feed was an important source of Salmonella in farms.It showed that the contaminated feed was the main cause of the change in Salmonella infection rate on the farm from the second stage to the third stage.Thirty representative strains,including twenty-three S.Typhimurium and seven S.Rissen strains were sequenced.The results showed that except for SH086,the similarity between the other S.Rissen strains was 99.0%?99.6%.Among them,the similarity between SH134(feed source)and SH136(feces origin)reached 99.6%;between S.Typhimurium strains,the similarity ranged from 99.8%to 100.0%.The similarity between SH035(feed source)and SH034(stool source)was as high as 99.95%,indicating contamination feed is a direct source of Salmonella on farms.SH073 was isolated from swabs from workers' hands with the same clone as SH071 and SH081 isolated from pig fecal samples,indicating that the workers were important carriers of Salmonella transmission in swine.Salmonella control measures were optimized on swine farm in November 2016,which contained 3 parts.The first was to strengthen the monitoring of bacteria in feed.The second is to improve the farm management model,improve the requirements of the staff disinfection,and enforce the relevant measures.The last was optimizing the disinfection process of pig farm,set the disinfection time at times when the herd was active,and extend the disinfection time by 30s.As the result,swine infection rate of Salmonella decreased from 43.0%to 8.8%after optimization,indicated that the improvement of prevention and control measures is the main reason leading to the prevalence of Salmonella in farm from the first phase to the second phase.The results of quantitative determination of Salmonella in the environment samples showed that the amount of Salmonella coverage before optimization was 6.1 MPN/g in the fence,and reduced to less than 3 MPN/g after optimization;the amount of Salmonella coverage also declined in the water nozzle after optimization.The results showed that strengthening the management of staff and rationally optimizing the use of disinfectants can significantly reduce the prevalence of Salmonella in farm. |
PDF Full Text Request