LONGITUDINAL FIELD STUDIES OF AVIAN METAPNEUMOVIRUS AND TURKEY HEMORRHAGIC ENTERITIS VIRUS IN TURKEYS SUFFERING FROM COLIBACILLOSIS-ASSOCIATED MORTALITY

This study evaluated if, in field conditions, the infection with Avian Metapneumovirus (AMPV) and/or Turkey Hemorrhagic Enteritis Virus (THEV) was significant for the induction of episodes of colibacillosis in AMPV and THEV vaccinated turkeys. Colibacillosis-associated mortality was recorded and longitudinal virological studies performed in three consecutive turkey flocks reared in the same farm. AMPV and THEV diagnostic swabs and blood samples were made once a week up to 14 weeks of age. Swabs were processed by molecular techniques, and antibody titres were evaluated by ELISA. When AMPV of subtype B was detected, samples were subjected to a restriction endonuclease digestion protocol to discriminate between field and vaccine strains. Field strains were detected in all flocks at different ages of life, always associated with respiratory signs and increase of colibacillosis-associated mortality. THEV has been consistently detected in all flocks since the 9th week of age. Vaccination with a single dose of the THEV commercial inactivated vaccine available in Italy seems do not protect the birds from the infection. Sequence comparison of the hexon protein of one of the THEV strains detected, and strains isolated worldwide, revealed high similarity between them. These results are consistent with the notion that the hexon protein, being the major antigenic component of the virus, is highly conserved between the strains. Results showed that field AMPV infection is correlated to colibacillosis-associated mortality. Less clear appears the role of THEV because the endemicity of AMPV makes difficult to evaluate its role in predisposing colibacillosis in absence of AMPV.