الفهرس 
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Abstract
As avian influenza has become one of the biggest threats for poultry production and human health, vaccination is a powerful tool to control and eradicate the disease .so this study aimed to evaluate tow commercial available inactivated AT vaccines (H5N1 and H5N2) used in Egypt in some provinces at the period from 2009 to 2011 to determine the most potent vaccine when properly used can prevent the disease, mortalities and increase resistance to infection.
This study was done on a total number of 800 swabs (tracheal and cloacal swab) representing 40 poultry flock of chickens (broiler breeders, layers and broilers) for screening the presence of HPATV subtype H5N1 using real- time
PCR.
The results showed that twenty sex flocks were positive which classified according to the vaccination into three types, un vaccinated, vaccinated with H5Nlvaccine and vaccinated with H5N2 vaccine naturally infected chickens.
Clinical signs of Naturally infected unvaccinated chickens and Naturally infected chickens previously vaccinated with H5N2 vaccine: sudden death without any signs of disease being apparent especially in broilers and young age chickens, high morbidities and mortalities reached to 100% within two days. In some affected flocks especially breeders and layers, chickens may survive few days later and suffering from off food, severe cyanosis and excessive edema in head, comb and wattle, diffuse hemorrhage on feet and shank, bloody tiged discharge from nostrils, greenish diarrhea, congestion and hemorrhage of un feathered skin, sudden cessation in egg production and nervous signs (paralysis).
Naturally infected chickens previously vaccinated with H5N1 vaccine:
The affected chickens showed low mortalities with depression, decrease in appetite , diarrhea, fever, ruffled feathers nasal discharges and edema in the head , no change in egg production and absence of nervous signs.
Gross findings naturally infected unvaccinated chickens and Naturally infected chickens previously vaccinated with H5N2 vaccine: the gross lesions may be absent or in apparent especially in sudden death chickens, however, within two days it progressed and characterized by diffuse hemorrhage in trachea, skin, subcutanous tissue and along the breast bone, gelatenous fluids under skin of the head, hemorrhage on brain surface, lung appear black in color, petechial hemorrhage on coronary fat, and small intestine, enlargement and congestion of liver with numerous variable sized hemorrhagic foci, spleen, kidneys and pancrease . Hemorrhage in proventriulus in junction with gizzard and under lining of gizzard. Rupture of egg yolk in abdominal cavity.
Immunoperoxidase test showed wide distribution of viral antigen in the organs of both un vaccinated and vaccinated naturally infected chickens, which improved that the virus is highly pathogenic and pantropic .moreover, AT vaccines do not prevent infection but can reduce the pathogenesis of disease and the pathogenicity of the virus.
The histopathological examination revealed severe alterations in both un vaccinated and vaccinated chickens with H5N2. There were congestion and edema of vascular adventitia of the lung, bronchiolitis and parabronchial necrosis. Also, tracheitis, hemorrhage in kidneys tubulointerstitial nephritis, tubular necrosis and glomerular hypercellularity. Brain had perivascular cuffing, neuronal pyknosis and gliosis. Liver showed congestion, hemorrhage, portal inflammatory cells infiltration and hepatocytic necrosis. Spleen exhibited hemorrhage, necrosis and histocytosis. As well as, there were enteritis, congestion and necrosis of pancreas. However, chickens vaccinated With H5N1
showed mild or absent alterations.
V—... Comparative pathological evaluation were done according to clinical signs, mortalities , pathological alterations and immunoperoxidase test in both un vaccinated and vaccinated naturally infected chickens with H5N1 and H5N2 vaccines.