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Gizzard erosion in broilers due to an infection with virulent fowl adenovirus serotype 1 (FAdV-1) is an emerging disease. Although experimental studies were performed, a possible prevention strategy was not reported so far. The present study was set up to determine (i) a possible influence of birds' age at time of inoculation on the pathogenicity of a European FAdV-1 field strain (PA7127), (ii) the virulence of a apathogenic FAdV-1 strain (CELO), and (iii) its capability to protect SPF broilers from adenoviral gizzard erosion caused by the field virus. Oral infection of birds with PA7127 at 1-, 10- and 21-days of life, resulted in reduced weight gain compared to non-infected birds, with significance for birds infected at day-old. Independent of the birds' age at time of inoculation, clinical signs appearing approximately one week after challenge coincided with gizzard lesions. Birds infected exclusively with CELO at the first day of life did not show any clinical signs or pathological changes in the gizzard, confirming the apathogenicity of this European FAdV-1. A similar result was obtained for birds orally infected at the first day of life with CELO and challenged three weeks later with the pathogenic PA7127 strain. Therefore, complete protection of adenoviral gizzard erosion in broilers by vaccination of day-old birds could be demonstrated for the first time, although virus excretion was detected post challenge. Establishment of an amplification refractory mutation system quantitative PCR (ARMS-qPCR) facilitated the identification of the FAdV-1 strain and presence of challenges virus was confirmed in one sample. [ABSTRACT FROM AUTHOR]