Investigation Of PRDC Bacterial Pathogens Infection In Large-scale Pig Farms And Screening Of APP Virulent Clinical Strains

Porcine Respiratory Disease Complex(PRDC)is an important disease that hinders the development of pig industry in China.Primary or secondary infection of pathogenic bacteria is the main cause.The pathogens associated with PRDC mainly included Actinobacillus pleuropneumoniae(APP),Streptococcus suis(SS),Haemophilus parasuis(HPS),Pasteurella multocica(Pm),etc.APP can cause acute infection in pigs,which can spread rapidly once it becomes infected and often lead to acute death of large and medium-sized pigs.Therefore,it is of great significance to evaluate the infection of pathogenic bacteria in pigs and to study the biological characteristics of APP isolates in clinic.In this study,clinical investigation was conducted on pigs from 6 large-scale pig farms in Hubei,Guangxi and Anhui provinces.PCR technology and bacterial isolation and identification were used to analyze the carrying status of major bacterial pathogens in PRDC in the investigation area.At the same time,the pathogenicity of APP clinical strains was studied,and the APP clinical virulent strains were screened.The specific work is as follows:1.Investigation on pathogenic bacteria infection in large-scale pig farms in Hubei,Guangxi and AnhuiPathogens of APP,SS,Pm,HPS and Mhp were detected by 656 nasal swabs and 38tissue samples collected from 6 large-scale pig farms in Hubei,Guangxi and Anhui provinces.PCR results showed that the infection rates of APP,SS,HPS,Pm and Mhp were2.13%,17.38%,22.71%,4.27%and 0.15%,respectively.Among them,210 samples with single pathogen infection were detected,with a positive rate of 32.01%,and 47 samples with two or more pathogens mixed infection were detected,with a positive rate of 7.32%.In the mixed infection,46 samples were doubly infected,including 31 SS+HPS,9 HPS+Pm,4 APP+HPS,and 3 SS+Pm.One case of triple infection was SS+HPS+Mhp.Therefore,the situation of pathogen infection is very serious in large-scale pig farms,and the type of mixed infection is various and the number is huge,which should arouse vigilance.2.Isolation,identification and drug resistance analysis of bacteriaThe collected positive samples were isolated and identified,and 10 strains of SS,3strains of HPS and 1 strain of APP were isolated through routine bacterial isolation and identification combined with PCR identification.Serotyping showed that a total of 5 strains of serotype 9 SS were isolated,the other 5 strains of SS could not be classified,2 strains of serotype 4 HPS,1 strain of serotype 5 HPS and 1 strain of serotype 5 APP.At the same time,20 clinical strains suspected of APP were isolated and identified in the laboratory,and15 strains of APP were obtained.Serotyping showed that 3 strains of serotype 1 APP,1strain of serotype2 APP,3 strain of serotype 5 APP,2 strain of serotype 7 APP,1 strain of serotype 8 APP,2 strain of serotype 12 APP and 3 strain of serotype 15 APP.The drug susceptibility test of 16 strain APP,10 strains SS and 3 strains HPS showed that the drug resistance rate of 16 strains APP to tetracycline was 62.5%,and the drug resistance rate to penicillin G was 25%.The drug resistance of 10 strains of Streptococcus suis was complex,the drug resistance rate of tetracycline was 80%,the drug resistance rate of erythromycin was 70%,and the drug resistance rate of SS was 60%to minocycline,and some strains were resistant to neomycin,butylamicane and benxacillin.Three strains of Haemophilus parasuis were resistant to tetracycline and neomycin.All the above strains were sensitive to cephalosporin antibiotics.The results showed that cephalosporins should be selected in clinical drug use in pig farms,and tetracycline drugs should be avoided,and combined drug use and shuttle drug use should be emphasized to avoid the emergence of super-resistant strains.3.Study on the pathogenicity of APP in mice and screening of clinical virulent strains16 strains of APP were obtained by routine bacterial isolation and identification.According to the serotype and growth characteristics of APP,6 strains of clinical APP were selected to study their pathogenicity in mice.The strains were numbered as ZL-APP(serotype 1),AP-1(serotype 1),YC-15(serotype 5),GXYX(serotype 5),APP5-YXLM(serotype 5)and HB-16(serotype 7).The results showed that 6 clinical strains of APP could cause clinical symptoms such as dyspnea,lethargy and hair exploding in mice,and all of them were fatal to mice.Necropsy showed severe lung tissue injury,hemorrhage and adhesion in the lung.The results showed that the virulence of serotype 1 and serotype 5was significantly higher than that of serotype 7,and the virulence of different isolates of the same serotype was significantly different.Ap-1 and YC-15 showed the highest pathogenicity,with LD50 of 5×10~6 CFU and 7.94×10~6 CFU,respectively.In summary,APP virulent strains were screened,AP-1 and YC-15 respectively.