| Infectious coryza(IC)is an acute upper respiratory tract disease caused by Avibacterium paragallinarum(Apg).The main clinical symptoms are nasal discharge,facial swelling,lacrimation,and conjunctivitis,which causes slowed growth and decreased egg production.In recent years,the incidence of IC in domestic large-scale farm is significantly rising.In this study,epidemiological investigation were carried out in some intensive chicken farms in China where IC occurred,and the pathogenicity of epidemic strains was determined.Drug sensitivity tests were conducted in vitro to investigate the drug resistance of Apg isolates in order to prevent the prevalence of IC.1.Isolation,identification and pathogenicity of Avibacterium paragallinarum in some intensive chicken farms during 2019 to 2020This study conducted epidemiological investigations from intensive chicken farms in Jiangsu,Hebei,and Ningxia during 2019 to 2020.Samples were collected for isolation and identification of Apg.The serum typing results showed that three serotypes A,B,and C were prevalent in the investigated chicken flocks.A total of 40 strains were isolated and identified.There were 11 isolates identified as serotype A,10 isolates identified as serotype B and 19 isolates identified as serotype C,indicating that serotype C has become the most prevalent serotype.This result is quite different from previous domestic investigation.Combined with the epidemiological information,it seems that IC mainly occurs at the end of spring and early summer and the change of seasons in autumn and winter.Chickens were infected mainly in the laying period,also in early brooding stage with 30?40 age.Some flocks were found to be infected with two or three serotypes.The outer membrane protein Hmtp210 has been reported extremely different in different serotypes.In this study,40 isolates of Hmtp210 gene hypervariable region sequences were further cloned,sequenced,and performed by genetic evolution analysis.It is found that strains with the same serotype are homologous,and strains with different serotypes are located in their own independent evolutionary branches on the evolutionary tree.It indicates that the comparison of the Hmtp210 gene hypervariable region have a good correspondence with Page serotyping.Therefore,it is expected to be an alternative to serological typing for Apg.In view of the fact that serotype C has been widely prevalent in the flocks,this study carried out the pathogenicity test of C-type isolate.The 32-day-old SPF chickens were challenged with 2020/JS80.The clinical symptoms and pathological change,and the replication of Apg were detected.The results showed that 2020/JS80 strain possessed strong pathogenicity,and chickens showed obvious depression in 24 hours after the challenge.On the 3rd day after challenge,the peak of the disease occurred,with the incidence reaching 100%.The facial swelling of the infected chickens was observed,and the food intake and water intake of the chickens were significantly reduced.On the 5th day,the clinical symptoms decreased,the intake of food and water increased,and the facial swelling gradually subsided.On the 7th day,mental status and appetite of chickens returned to normal.Yellow jelly-like objects in the nasal cavity,bleeding spots in the trachea,and enlarged spleen and bursa of Fabricius were found.Pathological change showed infiltration of macrophages and heterophils in the nasal and infraorbital sinuses,partial shedding of tracheal cilia,and inflammatory cell infiltrations in the submucosa of the trachea.The qPCR test results showed that the Apg were mainly colonized and replicated in the nasal cavity and trachea,but the chickens showed obvious bacteremia from the 3rd to the 5th day after the challenge.Apg can be detected in the heart,liver,spleen,lungs,and kidneys.2.Analysis of drug resistance of Avibacterium paragallinarumThe commonly used drug including ?-lactams,sulfonamides,tetracyclines,amido alcohols,aminoglycosamines,fluoroquinolones,lincomycin hydrochloride/spectinomycin sulfate and macrolide drug sand carried out the drug susceptibility test in vitro.The results showed that all of Apg isolates showed strong multi-drug resistance.(1)All of isolates were resistant to compound sulfamonomethoxine sodium,with a MIC value of up to 256 ?g/mL.(2)The resistance to different drugs of tetracycline antibiotics is different.39/40 isolates were resistant tooxytetracycline.Nearly half of the isolates were resistant to doxycycline.(3)The isolates were highly sensitive to ?-lactam antibiotics,especially amoxicillin and ceftiofur sodium.In summary,this study found that:(1)The current prevalence of Apg in large-scale layer farms is relatively complicated.Three serotypes A,B,and C are all prevalent,and mixed infections with more than two serotypes are more common.C-type strains are highly pathogenic and turns to be the prevalent serotype;(2)The multi-drug resistance of isolates is very common,but most of them were sensitive to ?-lactam antibiotics,and ?-lactam antibiotics can be chosen as the preferred drug for treatment. |
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