Study On The Synergistic Pathogenesis Between MPAIV And Avian E.coli With Low Pathogenicity And The Impact Of Different Inoculation Routines On The Pathogenicity Of MPAIV Evaluated In Chickens

Mildly pathogenic avian influenza virus (MPAIV) with subtype H9N2 is widely distributed and recently has resulted in considerable economic losses to poultry industry in China. Clinically, mildly pathogenic avian influenza virus infection is often complicated with avian colibacillosis. But there are few reports about the synergism between these two agents currently. We investigated their synergistic pathogenesis from the following aspects in the synergism model: the mortality rate in the synergism; the antibodies to outer membrane proteins (OMPs) and lipopolysaccharides (LPS) of E. coli in specific pathogen free (SPF) chickens; MPAIV isolation from the rectal swabs of living inoculated birds. The impact of different inoculation routines on the pathogenicity of MPAIV was also evaluated in SPF chickens.1. The synergistic pathogenesis between MPAIV and avian E. coli with low pathogenicity10-day-old specific pathogen free (SPF) chickens were inoculated intratracheally with lowly virulent E. coli and /or mildly pathogenic avian influenza virus (MPAIV). The results were revealed as follows: (1) The mortality rate of chickens inoculated with 2×107 colony-forming units (CFU) E. coli and 2×1043 ELD50 MPAIV simultaneously per bird was 39%. Chickens inoculated first with 2×104.3 ELD50 MPAIV and then 2×107 CFU E. coli at an interval of 48 hours experienced themortality rate of 67%, whereas those inoculated with 2 × 107 CFU E. coli and then 2 × 104.3 ELD50 MPAIV at the same interval exhibited the mortality rate of 17%, the group infected with MPAIV alone experienced the mortality rate of 11% while the group infected with E. coli alone had no death. These results suggested that MPAIV might longthen the periods that E. coli colonized the trachea and lung of inoculated chickens, and it might facilitate E. coli invasion into the respiratory tract of the chickens resulting in more severe pathologic abnormalities. There was a pathogenic synergism between E. coli and MPAIV. (2) An indirect enzyme-linked immunosorbant assay was used for the detection of antibodies to OMPs and an indirect hemagglutination assay for LPS of pathogenic E. coli from chicken origin, respectively. The antibodies to OMPs and LPS of the birds inoculated first with 2 × 104.3 ELD50 MPAIV and then 2 × 107 CFU E. coli were lower than those of other groups. This results indicated that the pathogenic synergism between E. coli and MPAIV seemed to be related to the immunosuppression exhibited in MPAIVinoculated birds against OMPs. (3) Samples of rectal swabs were taken from chickens of each group at different intervals postinoculation for isolation of challenged MPAIV in 10-day-old embryonating chicken eggs. The results showed that the duration for virus shedding in the birds infected with both MPAIV and E. coli seemed to be slightly longer than those infected with MPAIV alone, among these birds, those inoculated first with MPAIV and then with E. coli exhibited the longest virus shedding duration.2. The impact of different inoculation routines on the pathogenicity of MPAIV 10-day-old SPF chickens were inoculated intratracheally or intravenously with 2× 104.3 ELD50 mildly pathogenic avian influenza virus (MPAIV), respectively. They were euthanatized at some intervals post-inoculation and examined for macroscopic and microscopic lesions. At 14 day post-inoculation, the body weights of all experiment chickens were measured. The dynamics of MPAIV infection in SPF birds was also studied by immunohistochemistry. The results were showed as follows: (1) The mortality rate of chickens inoculated MPAIV(H9N2) by intratracheally was 17%, whilethat of birds inoculated by intravenously had no death. (2) At 14 day post-inoculation, the mean body weight of the chickens inoculated by intratracheally was 159g, and that of birds inoculated by intravenously was 229g, both of which were less than that of non-inoculation birds significantly, the mean body weight of the latter was 248g. (3) The birds inoculated with MPAIV by intratracheally exhibited more severe inflammation in the t...