| Salmonellosis refers to the diseases caused by Salmonella.It is a notifiable disease of the World Organisation for Animal Health(OIE).In China,salmonellosis is one of the second-class animal diseases prescribed by the government,and it has been identified the priority prevention disease in the ’National Long-term Animal Disease Prevention and Control Plan(2012-2020)’.Salmonella-infected livestock,poultry and their contaminated products are important sources of foodborne pathogenic bacteria,which does not only hinder the development of livestock and poultry-related industries,but also poses a great threat to human health.At present,there are few studies on the prevalence of Salmonella in breeder farms of China.The epidemic status of Salmonella in different periods of breeder breeding is not completely clear.In this study,seven breeder chicken farms in different regions of China were chosen to investigate the epidemic status,serotype distribution and antimicrobial resistance of Salmonella.One farm was chosen as a model,using CRISPR and PFGE molecular typing techniques to analyze the internal relationship of Salmonella strains isolated from different periods.The aim of this study is to investigate the prevalence and transmission characteristics of Salmonella in domestic breeder farms,and to find out the key points of prevention and control in the process of breeding chickens.1.Identification and drug resistance analysis of Salmonella strains isolated from breeder farmsFrom June 2015 to March 2017,a total of 2,185 samples were collected from 7 breeder chicken farms(named A,B,C,D,E,F and G farm),and 326 Salmonella strains were isolated.The overall positive rate of Salmonella was 14.92%.Statistical analysis showed that the positive rate of Salmonella was different from each farms(P<0.05),the highest positive rate was up to 18.37%,while the lowest one was 0%.The results showed that the positive rate of Salmonella was 24.66%and 24.52%in the laying and hatching period respectively,while the level in brooding period was relatively low(6.67%).The results showed that the positive rates of Salmonella were increased during brooding,rearing and laying periods,and the prevalence during the laying and hatching periods was maintained at a high level.During hatching and post hatching periods,the positive rate of Salmonella was decreased.Salmonella Enteritidis(S.Enteritidis)was the main serotype,followed by Salmonella Gallinarum-Pullorum.Antimicrobial resistance results showed that the rate of nalidixic acid resistance was highest(94.62%),followed by ampicillin(56.05%)and streptomycin(39.91%).Most of the strains are resistant to 1-3 kinds of antibiotics(76.23%),17.94%of the strains are resistant to 4-6 kinds of antibiotics,and there is few strain resistant to 7 or more kinds of antibiotics.The antimicrobial resistance rate was relative high in laying period,and 11 of 16 tested antibiotics got the highest resistance rate in this period.2.Molecular subtyping of Salmonella strains isolated from the breeder farmsIn this study,115 strains of S.Enteritidis isolated from different breeding periods of F farm were classified by CRISPR typing method.The results showed that all S.Enteritidis isolates were classified into three CRISPR types,which indicated that there were different S.Enteritidis genotypes in F farm.In addition,the same kind CRISPR subtype of S.Enteritidis was often isolated from a number of separation stages,indicating the possibility of Salmonella transmission in this farm.During brooding period,SET1 and SET2 type of S.Enteritidis strains were found,the same CRISPR types can also be found in the subsequent periods,suggesting that brooding period can be a key point of the prevention and control of Salmonella transmission.The association analysis of CRISPR type and drug resistance phenotype indicated that CRISPR typing technique can provide a good classfication of subgroup in a single serotype.A total of 60 S.Enteritidis strains isolated from G and F farms(supply and demand relationships between the two farms)were classfied into 4 PFGE clusters.PFGE cluster A contained 17 strains of G farm and 32 strains of F farm,indicating that brooding period is one of Salmonella transmission routes in breeder farms.At the same time,the rest of the strains were also divided into several other clusters,in which the strains number was less than that in PFGE cluster A.Analysis of each PFGE clusters,showing that PFGE cluster C strains were derived from F farm,PFGE cluster B and D strains were all derived from G farm.These results indicated that there were other ways in the transmission of Salmonella in breeder farms,and the transmission ability of each Salmonella subtype may be different. |
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