中缅边境猪群感染猪链球菌2型的病原特性初探

Pathogenic Characterization of Streptococcus suis Serotype 2 in Infected Pig Groups across China-Myanmar Border

2016年10月中旬,在中缅边境多个猪场暴发以呼吸困难、关节肿胀、部分仔猪拉黄水样粪便,后期伴有神经症状、败血症而死为特征的传染病。为查明病因,对采集自病死猪只的全血、肺组织、脓包及脓汁于血琼脂上划线培养,对所获溶血性菌落采用革兰染色、生化试验、药敏试验和基于慢性病例血清的凝集试验做初步鉴定;随后用细菌16S rRNA基因通用引物扩增该菌株DNA片段,并对PCR扩增产物进行TA克隆及测序分析。结果发现,该菌株为革兰阳性球菌;葡萄糖、蔗糖、乳糖、麦芽糖、水杨苷、棉实糖利用试验呈阳性,山梨醇、七叶苷、马尿酸盐、甘露醇、65g/L NaCl、肌醇和触酶试验均呈阴性;对头孢噻肟、头孢哌酮、恩诺沙星和复方新诺明具有较高敏感性,对青霉素、阿莫西林、庆大霉素、环丙沙星等具有中度敏感性,但对四环素、多西环素、卡那霉素、新霉素、氟苯尼考、阿米卡星、林可霉素等药物具有较强耐药性;凝集试验呈阳性且血清抗体效价为1∶256;PCR扩增出约1 500bp的单一条带,其核酸序列与猪链球菌(Streptococcus suis)第1核酸序列型(ST1)同源性最高(达99%),据此判定为猪链球菌2型。鉴于被调查猪群曾接种猪链球菌2型灭活疫苗,试验结果表明,当地猪群发生多重耐药性猪链球菌2型的跨境传播与感染,提示应加强对该菌抗原性、致病性及耐药性改变的监测及严格控制猪群的跨境流动。

In mid-October 2016,there was an outbreak of a contagious disease in several pig farms in the China-Myanmar border,which was characterized by dyspnea,joint swelling,discharge of yellow stool in partial piglets,neurological symptoms at the late stage and sepsis before death.In order to investigate its etiology in detail,the whole blood,lung tissue,pustule and pus were collected from the diseased pigs,then cultured by streaking on blood agar,followed by apreliminary identification of the resulting hemolytic colony using Gram staining,biochemical test,drug susceptibility test and agglutination test based on sera from chronic cases;apair of universal primers for bacterial 16S rRNA genes were subsequently used to amplify the isolates corresponding DNA fragment by PCR,and the PCR products were subjected to TA cloning as well as sequence analysis.Consequently,the hemolytic strain was spherical and positive for Gram staining.The biochemistry tests of glucose,sucrose,lactose,maltose,salicylic acid and raffinose were positive,but tests of sorbitol,esculoside,hippurate,mannitol,65 g/L NaCl,inositol and catalase were negative.It had a high sensitivity to cefotaxime,cefoperazone,enrofloxacin and sulfamethoxazole,a moderate sensitivity to penicillin,amoxicillin,gentamicin and ciprofloxacin,and a strong resistance to tetracycline,doxycycline,kanamycin,neomycin,florfenicol,amikacin,lincomycin and other drugs.Agglutination test was positive with a seral antibody titer of 256.PCR yielded a single 1 500 bp band that had the highest sequence identity up to 99% with Streptococcus suis type 2 sequence type 1（ST1）,thus the isolated strain was determined to be S.suis type 2.Since these piglets were once inoculated with S.suis type 2 inactivated vaccine,our findings here demonstrated that this cross-border infectious disease in the local herds was caused by multi-drug resistant S.suis type 2,indicating the importance of monitoring changes of the bacterial antigenicity,pathogenicity and drug resistance and strict control of cross-border flow of pigs.