Phylogenetic and pathogenic characterization of current fowl adenoviruses in China

Avian adenoviruses are a very diverse group of pathogens causing a variety of problems for poultry production. For a long time, the diagnosis of an adenovirus infection was restricted to the isolation of the respective virus followed by various serological typing methods, such as immunofluorescence assay, neutralization test or haemagglutination-inhibition test. In addition, restriction enzyme analysis has been reported for differentiation of avian adenoviruses. Besides summarizing the classical diagnostic methods, this review is mainly focused on the challenges that occurred recently in the field of avian adenovirus diagnosis at the molecular level. Several polymerase chain reactions (PCRs) have been published to diagnose all three groups of avian adenoviruses. Most PCRs were established to detect fowl adenovirus (FAV) DNA. Some of them were combined with restriction enzyme analysis to investigate whether FAV reference strains and field isolates could be typed according to the restriction profiles of the PCR products. The great advantage of direct detection of the viral DNA in tissue samples was demonstrated for the haemorrhagic enteritis virus and egg drop syndrome virus. Other PCRs were developed with the aim of detecting viral DNA from all three groups of avian adenoviruses partially in target cells. Whereas the value of these PCRs for avian adenovirus diagnostics is not disputable, the problems and open questions that are still present are also discussed.

The stand-alone pathogenicity of fowl adenoviruses (FAdVs) had long been disputed, given the ubiquity of the viruses versus sporadic outbreaks, and variation between experimental studies. However, a globally emerging trend of FAdV-associated diseases has marked the past two decades, with hepatitis-hydropericardium syndrome mainly in Asia besides Arabian and Latin American countries, and geographically more disseminated outbreaks of inclusion body hepatitis. Finally, the appearance of FAdV-induced gizzard erosion (AGE) in Asia and Europe completed the range of diseases. Epidemiological studies confirmed serotype FAdV-4 as agent of hepatitis-hydropericardium syndrome, whereas inclusion body hepatitis is related to FAdV-2, -8a, -8b and -11. Members of the biologically more distant serotype FAdV-1 induce AGE. Urged by increasing problems in the field, numerous pathogenicity studies with FAdVs from outbreaks substantiated the primary aetiologic role of particular strains for distinct clinical conditions. Developments in the poultry industry towards highly specialized genetic breeds and rigorous biosecurity additionally contribute to the growing incidence of FAdV-related diseases. Confirming field observations, recent studies connected a higher susceptibility of broilers with their distinct physiology, implying the choice of bird type as a factor to be considered in infection studies. Furthermore, elevated biosecurity standards have generated immunologically naïve breeding stocks, putting broilers at risk in face of vertical FAdV transmission. Therefore, future prevention strategies should include adequate antibodies in breeders prior to production and - if necessary - vaccination, in order to protect progenies. This review aims to deliver a detailed overview on the current global situation about FAdV-induced diseases, their reproduction in vivo and vaccination strategies.

This review provides an update of the genetic content, phylogeny and evolution of the family Adenoviridae. An appraisal of the condition of adenovirus genomics highlights the need to ensure that public sequence information is interpreted accurately. To this end, all complete genome sequences available have been reannotated. Adenoviruses fall into four recognized genera, plus possibly a fifth, which have apparently evolved with their vertebrate hosts, but have also engaged in a number of interspecies transmission events. Genes inherited by all modern adenoviruses from their common ancestor are located centrally in the genome and are involved in replication and packaging of viral DNA and formation and structure of the virion. Additional niche-specific genes have accumulated in each lineage, mostly near the genome termini. Capture and duplication of genes in the setting of a 'leader-exon structure', which results from widespread use of splicing, appear to have been central to adenovirus evolution. The antiquity of the pre-vertebrate lineages that ultimately gave rise to the Adenoviridae is illustrated by morphological similarities between adenoviruses and bacteriophages, and by use of a protein-primed DNA replication strategy by adenoviruses, certain bacteria and bacteriophages, and linear plasmids of fungi and plants.