الفهرس 
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Abstract
Low pathogenic avian influenza (LPAI) H9N2, highly pathogenic avian influenza (HPAI) H5N1, and H5N8 circulate in Egyptian poultry and cause veterinary and public health significance. The co-circulation of such zoonotic viruses among poultry in Egypt for several years should render that country a hypothetical hotspot for the emergence of new reassortants, or may limit the detection of subsequent HPAIV infection in gallinaceous poultry by attenuating the severity of disease. Recent animal studies have contributed to the growing evidence that temporary non-specific innate immune responses may lead to interference among avian influenza viruses (AIV). In the case of antigenically similar variants of a pathogen such as LPAI-H9N2 and HPAI-H5N1, interference may similarly manifest at the host scale through immunological cross-protection (CP); the prevention or partial inhibition of an individual host to future infections may lead to altered transmission dynamics during pathogen co-circulating. In the case of influenza, cross-protection as a consequence of adaptive immunity is expected since neutralising antibodies and cross-reactive cell-mediated immunity are known to act across homo and hetero sub typically diverse viruses. The main objective of this study was designed to mimic the consequences of natural co-circulation of LPAI H9N2 and HPAI H5N1 viruses without the effect of different vaccination strategies used and circulation of other pathogens. The disease consequence of HPAI H5N1 in chickens that pre-infected by LPAI H9N2 in different time intervals were monitored and evaluated by collection of cloacal and tracheal swabs for studying virus shedding by RT-PCR test, blood samples to evaluate post challenge immune response using ELISA and HI, and organs collected for histopathological examination to study tissue tropism. Our results showed that there is cross-protection in the co-infected groups as shown in the decreasing percentage of morbidity, mortality and increasing the percentage of survivability in co-infected groups that challenged with LPAI H5N1 virus and HPAI H5N1 virus at different time intervals than the mono-infected group that challenged with HPAI H5N1 virus. It can be concluded that the change in the epizootiological pattern of HPAI H5N1 in the field is related to circulation of LPAI H9N2, beside the different vaccination strategies used and application of different levels of biosecurity. The current findings can be taken in consideration for the control of HPAI in poultry considering frequent co-circulation of both LPAI and HPAI.