Abstract
Adenoviral gizzard erosion is an emerging disease with negative impact on health and production of chickens. In this study, we compared in vitro and in vivo characteristics of a fowl adenovirus serotype 1 (FAdV-1), attenuated by 53 consecutive passages in primary chicken embryo liver (CEL) cell cultures (11/7127-AT), with the virulent strain (11/7127-VT). Whole genome analysis revealed near-complete sequence identity between the strains. However, a length polymorphism in a non-coding adenine repeat sequence (11/7127-AT: 11 instead of 9) immediately downstream of the hexon open reading frame was revealed. One-step growth kinetics showed delayed multiplication of 11/7127-AT together with significantly lower titers in cell culture (up to 4 log10 difference), indicating reduced replication efficiency in vitro. In vivo pathogenicity and immunogenicity were determined in day-old specific pathogen-free layer chicks inoculated orally with the respective viruses. In contrast to birds infected with 11/7127-VT, birds infected with 11/7127-AT did not exhibit body weight loss or severe pathological lesions in the gizzard. Virus detection rates, viral load in organs and virus excretion were significantly lower in birds inoculated with 11/7127-AT. Throughout the experimental period, these birds did not develop measurable neutralizing antibodies, prevalent in birds in response to 11/7127-VT infection. Differences in pathogenicity between the virulent FAdV-1 and the attenuated strain could not be correlated to prominently discriminate genomic features. We conclude that differential in vitro growth profiles indicate that attenuation is linked to modulation of viral replication during interaction of the virus with the host cells. Thus, hosts would be unable to prevent the rapid replication of virulent FAdV leading to severe tissue damage, a phenomenon broadly applicable to further FAdV serotypes, considering the substantial intra-serotype virulence differences of FAdVs and the variation of diseases.
Highlights
Fowl adenoviruses (FAdVs) are non-enveloped, dsDNA viruses classified into the family Adenoviridae, genus Aviadenovirus [1]
With 11/7127-VT, a cytopathic effect (CPE) characterized by focal swelling and detaching of cells was detectable from 30 h post infection (PI) onwards, while the CPE following infection of cells with 11/7127-AT appeared slightly delayed at 36 h PI
The virulent fowl adenovirus serotype 1 (FAdV-1) strain (11/7127-VT), isolated from 10 day-old broilers during an outbreak of adenoviral gizzard erosion (AGE) in Germany [13], served as a model to determine the consequences of long-term in vitro passaging in chicken embryo liver (CEL) cells on the genome and the virulence of FAdV-1
Summary
Fowl adenoviruses (FAdVs) are non-enveloped, dsDNA viruses classified into the family Adenoviridae, genus Aviadenovirus [1]. Thereby, comprehensive epidemiological investigations together with experimental studies to reproduce lesions have defined certain FAdV species/serotypes as primary pathogens of the following diseases: adenoviral gizzard erosion (AGE), hepatitis-hydropericardium syndrome (HHS) and inclusion body hepatitis (IBH) [10,11]. Pullets and laying hens reduced performance (e.g., growth retardation, decreased egg production) and/or higher mortality rates have been documented during clinical outbreaks of the disease [12,13,14,15,16,17]. In the majority of reports, FAdV-1 was defined as the etiological agent of AGE and clinicopathological signs of AGE were successfully reproduced in specific-pathogenfree (SPF) layers and broilers using virulent FAdV-1 field isolates [10,21]. The widespread nature of the disease and its negative impact on poultry health, welfare and production indicate the need for safe and efficacious protection strategies
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