Quantity of virulent fowl adenovirus serotype 1 correlates with clinical signs, macroscopical and pathohistological lesions in gizzards following experimental induction of gizzard erosion in broilers

In the present study day-old specific-pathogen-free (SPF) and commercial broilers with maternally derived fowl adenovirus serotype 1 (FAdV-1) antibodies were orally infected with a European "pathogenic" FAdV-1, isolated from broilers showing signs of gizzard erosion. During the experiment, broilers were observed and weighed daily up to 17 days post infection (dpi). Clinically, both infected groups showed significant decrease of weight compared to respective negative control groups. Birds were examined by necropsy at 3, 7, 10, 14 and 17 dpi. Pathological changes in the gizzards were noticed in both experimentally infected groups from 7 dpi onwards. Macroscopically, erosion of the koilin layer and inflammation or ulceration of the gizzard mucosa were observed. Histologically, presence of FAdV-1 in intranuclear inclusion bodies of degenerated glandular epithelial cells was demonstrated by in-situ hybridization and inflammatory cell infiltration of the lamina propria, submucosa and muscle layer was detected. Tissue samples were investigated by a recently developed real-time PCR and the viral DNA load was calculated from gizzard, liver, spleen and cloacal swabs with the highest amounts of FAdV-1 DNA found in the gizzard. For the first time, successful reproduction of clinical signs in broilers as well as pathological lesions in the gizzard were achieved with a European FAdV-1 isolate displaying some genetic differences to so far reported virulent FAdV-1 from Japan. Furthermore, highest viral load in gizzards could be linked with macroscopical and histological lesions. Therefore, the conducted analyses provide important insights into the pathogenesis of adenoviral gizzard erosion.