mahmoud Aly

.Seventeen isolates of Aeromonas hydrophila were isolated from 250 commercial broiler chicks with an incidence of 6.8 %. Most A. hydrophila isolates (88.24 %) were positive for exotoxin assay and congo red binding test, while 52.94 % were positive for crystal violet binding activity. Most strains of A. hydrophila were sensitive to chloramphenicol, ciprofloxacin and norfloxacin followed by gentamicin and neomycin while nalidixic acid, tetracycline, streptomycin and trimethoprim sulphamethoxazole had moderate effect. On the other hand, all A. hydrophila strains were resistant to amoxicillin, cephalothin, erythromycin and penicillin G. All chicks infected with 1.5 X 109 organisms via subcutaneous and yolk sac were dead within 24 h. A. hydrophila was isolated from most organs. The lesions observed included congestion in the internal organs and few cases showed hepatic and muscular petechiae. Histopathological examination of experimentally infected chicks revealed severe necrosis in hepatic, splenic and muscular tissue in the two days old chicks. The ultrastructural study of this group showed presence of the bacilli inside the hepatocytes and macrophages with marked cellular changes. An attempt was made to determine a correlation between level of exposure and mortality. It was found that the mortality rate was relatively high (52.5 %) than in lower dose (35 %). A. hydrophila was isolated from most organs examined. Marked degenerative and necrobiotic changes in both hepatic and splenic tissue and characteristic muscular lesions manifested by muscular hemorrhage, degeneration, oedema and myositis in low dose treatment group. In the group injected with high dose, the lesions were more severe and characterized by diffuse areas of necrosis in hepatic tissue, thrombus formation in the blood vessels together with large number of bacterial colonies and bacilli in the hepatic tissue. Marked muscular necrosis and myophagia were also noticed. The ultrastructural study using transmission electron microscope for this group showed heterocells and hepatocytes contain bacilli. In other cases, the bacilli were present in the phagosomes of phagocytic cells in the splenic tissue. Cytopathological lysis was common evidence in the examined cells.