Characterization Of Antimicrobial Resistance And Serovar Distribution Of Haemophilus Parasuis Isolates

Haemophilus parasuis is the causative agent of swine meningitis,polyserositis and polyarthritis.As the development of the pig industry,the disease has become one of the most important bacterial diseases of pigs worldwide.The study reported here focused on the serovar distribution and the antimicrobial susceptibility of Haemophilus parasuis in China,which also provided very important materials for Haemophilus parasuis research and contributed for diagnosis,preventing and control of Haemophilus parasuis.The main research included as below:1.Isolation,identification and serological characterization of Haemophilus parasuis isolates from ChinaFrom thirteen provinces and cities in China,478 strains of Haemophilus parasuis were isolated from samples of nasal cavity,lung,brain,heart blood and joints of pigs from 2007 to 2008,based on morphologic,cultural and biochemical characteristics.All Haemophilus parasuis isolates were serotyped by Kieletein-Rapp-Gabriedson(KRG) Gel Diffusion(GD) test.Thirteen Haemophilus parasuis serovars were identified.Serovar 5(26.4%) and serovar 4(14.6%) were the most prevalent serovars,followed by serovars 13(10.9%) and 12(9.1%),while 25.5%of the isolates could not be assigned to a serovar(nontypable).The serovars 1,2,4,5,12,13 and 14 could be isolated from nasal cavity and other organs of swine simultaneously,but the serovars 6,7 and 9,could only be isolated from samples of nasal cavity.2.Study on the antimicrobial resistance of Haemophilus parasuis isolates2.1 Study on drug sensitivity of Haemophilus parasuisIn vitro susceptibility test of the 110 H.parasuis isolates was performed by the agar dilution method.Using the breakpoints given,all isolates were susceptible to cefotaxime, ceftiofur,azithromycin,chloramphenicol,florfenicol and tilmicosin.Most of them were susceptible to tiamulin(99.1%),erythromycin(98.2%),penicillin(93.6%),ampicillin (90.9%),cefaclor(91.8%),levofloxacin(93.6%),tetracycline(96.4%),gentamicin (93.6%),spectinomycin(90.0%) and ciprofloxacin(82.7%).However,a high level of resistance was observed for enrofloxacin(70.9%) and trimethoprim/sulphamethoxazole (44.5%).2.2 The antimicrobial antibiograms of Haemophilus parasuis There were 32 antimicrobial antibiograms observed in this study.Only nine isolates (8.2%) were susceptible to all drugs tested;however,most of Chinese isolates(68.2%) being resistant to only one or two antibiotics,multidrug resistant strains(≥3 antimicrobial agents) were only observed in 23.6%of all isolates.5 isolates(4.5%) were resistant to at least five antimicrobials;and only one isolate(0.9%) was resistant to seven antimicrobials simultaneously.2.3 The correlations between antimicrobial resistance and serovars or virulenceThe comparison of the distribution of antimicrobial resistance with the distribution of serovars revealed that there was no statistically significant correlation between the antibiotic resistance and the serovars or virulence of H.parasuis(x~2-test,P＞0.05).2.4 The variability in resistance rates of Haemophilus parasuisIn vitro susceptibility test of the 188 H.parasuis strains isolated from 2002 to 2004 was performed by the agar dilution method.The antimicrobial susceptibility of these isolates was compared with the H.parasuis strains isolated from 2007 to 2008.The results showed that the resistance to chloramphenicol,streptomycin and kanamycin were descending,oppositely,the resistance to trimethoprim/sulphamethoxazole,enrofloxacin and other antibacterial agents were adverse tendency.