Analysis Of Contamination Key Points And Quantitative Microbial Risk Assessment Of Salmonella Isolated From Swine Slaughterhouse In Jiangsu Province

Salmonella,one of the most important zoonotic pathogens,is sostenuto to threaten human health because of salmonellosis it can result in.The contaminated pork is major source can propagate Salmonella and lead to salmonellosis.At present,rare study about Salmonella contamination in swine slaughterhouse and Quantitative Microbial Risk Assessment(QMRA)was reported.Here,we investigated the Salmonella contamination situation and serotyping distribution in a semi-auto swine slaughterhouse in Huai an,Jiangsu province.MLST,PFGE and CRISPR typing was used to conduct gene-typing study.Additionally,QMRA model was built by using @risk software.Our mission is to investigate the major contaminated steps and contaminated way about Salmonella,and to predict the key control points,which provide data support for controlling Salmonella in swine slaughterhouse.1.Isolation,identification and quantification of Salmonella in swine slaughterhouseA total of 1116 samples were collected from 9 visits in swine slaughterhouse at Huai an from March 2016 to May 2017,the total prevalence was identified to be 30.6%.Samples were collected from carcass-swab,equipment,environment and intestinal content.Carcass-swab samples included cotton swabs from 8 slaughtering steps.Exsanguination,as first steps to detect,presented a high prevalence(46.9%).Through dehairing and flaming,the prevalence decreased to lowest(12.2%).After splitting,Salmonella prevalence increased to highest(64.6%);then,the carcasses were washed and prevalence substantially decreased to 32.7%.Through dressing step,Salmonella prevalence increased to 41.8%,then,decreased in both second-washing and chilling.Among equipment sample,splitter was identified to be detected frequently(38.5%).Among environment samples,both lairage and slaughtering area were found to be contaminated by Salmonella(4000%and 40.7%).The prevalence of intestinal content was 23.9%.The result of serotyping showed that S.Derby(39.8%)was the most prevalent serovars,followed by S.Typhimurium(25.4%)and S.Rissen(23.1%).These three serot)ypes were presented frequently in each sampling visit.Most possible number(MPN)was used to detected the contamination number of Salmonella.Both exsanguination and splitting steps presented high contamination number(2.50± 0.94 和 2.24 ± 0.72 logMPN/m2),the rest steps were low contamination level.The dynamic change of Salmonella contamination number was consistent with prevalence.2.Molecular subtyping of Salmonella isolated from swine slaughterhouseA total of 216 Salmonella were used to conduct MLST and 10 different sequence typing(ST)was distinguished,of which ST40,ST469,ST34 and ST155 were dominant.These four STs presented among carcass-swab,equipment,environment and intestinal content samples.Additionally,STs were consistent with serovars.112 S.Derby and 55 S.Typhimurium were used to conduct PFGE.The result showed,S.Derby and S.Typhimurium were distributed among 2 different clusters.Salmonella isolated from same or different visit were all distributed in same cluster.The clusters that included isolates from lairage,splitter and fleam also contained isolates from multiple visits.Salmonella isolated from splitter and different steps were highly resemble.For CRISPR typing,47 S.Derby and 44 S.Typhimurium were analysed and showed the similar results with PFGE.So,the results above showed,(1)introduced Salmonella(infected pig)and housing Salmonella(lairage,splitter and fleam)was all the major source of swine carcass contamination;(2)this contamination mostly occurred in steps from polishing to splitting where the splitter and grainer were conducted;(3)carcass of scalding,dehairing,washing and chilling can decrease the contamination prevalence and number.3.Quantitative Microbial Risk Assessment(QMRA)in swine slaughterhouse@RISK was used to build QMRA model.Exsanguination and chilling was regarded as input and output of model respectively,other steps between this two steps included scalding,dehairing,flaming,polishing,evisceration,splitting and washing.In each step,the relative risk factors and their calculating were intoduced.Parameters of risk factors were identified according to previous studies and production experience.We used method of Monte Carlo to simulate Salmonella number in each slaughtering step and 10000 iterations were conducted.The simulation result showed,the output(mean value)of dehairing,flaming,splitting and chilling were 53.83,27.83,534.82,34.28 and 46.24 MPN/m2,respectively.Compared with our practical detection value(61.79、32.46、1074.44、42.26和32.67 MPN/m2),the results were similar,which indicated the QMRA model accurately reflected dynamic change of Salmonella contamination in swine slaughterhouse.For sensitivity analysis,the method of variance contribution was conducted to analyse the importance of factors.The result showed that major factors included intestinal damage rate(42.7%),Salmonella occurrence(3.89%)and concentration(7.19%)of intestinal feces in evisceration step,washing time(29.64%)in washing step,and Salmonella concentration(7.35%)in exsanguination step.The result of sensitivity analysis and conclusion above were of mutual verification and supplement.For example,Salmonella originated from infected pig was major sources in swine slaughterhouse,which was correspondent with B(initial Salmonella load of pig carcasses).Salmonella contamination occurred during steps from polishing to splitting.The evisceration was just right between polishing and splitting,and this step included three risk factors(intestinal damage rate(α6),Salmonella occurrence(Pmpn6)and concentration of intestinal feces(β6)).Washing of carcasses could reduce the Salmonella contamination;washing time(Tmpn7)proved this point meanwhile.