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Figure 13.10 Stages of replication of pox viruses (Reproduced from Hruby and Byrd, 2006: Microbe 1(2):70–75, courtesy of American
Society of Microbiology). ER, Endoplasmic reticulum; TGN, Trans–Golgi network; IV, Immature virus; IMV, Intracellular mature virus; IEV,
Intracellular enveloped virus; CEV, Cell-associated virus; EEV, extracellular enveloped virus.

ATP-GTP binding protein A32L, which likely functions in virion Ubiquitin-proteasome
assembly and DNA packaging. Three genes that encode proteins Ubiquitin-proteasome system regulates many cellular processes.
potentially associated with EEVs are present in FWPV. Homo- FWPV and CNPV each contain two distinct p28-like ubiquitin
logues of five VV genes representing two conserved poxvirus gene ligases. FWPV150 and FWPV157 are both ubiquitinated during
families with putative structural functions, some of them thought infection. While FWPV150 was actively transcribed early,
to be essential are present in FWPV (Afonso et al., 2000). FWPV157 was expressed late, suggesting different temporal roles
for FWPV150 and FWPV157 (Bareiss and Barry, 2014). Since an
ubiquitin gene has been identified and sequenced in the genome
Virus survival strategies of an avian pox virus isolated from Hawaiian forest birds, it is
likely that it is conserved in all avian pox viruses.
A-type inclusions
As mentioned earlier, cytoplasmic inclusion bodies produced by
the avian pox viruses are diagnostic of infection. In this regard, two Host-related functions
genes homologues of poxvirus A-type inclusion (ATI) proteins, FWPV contains a significant number of putative host range genes
insoluble proteins that constitute the protein matrix of ATIs are that exhibit similarity to cellular genes and to other known pox-
present in FWPV. Cytoplasmic ATIs are thought to protect mature virus genes. This diverse complement of host range genes, some
virions from environmental insults, and they may be of significance of which are novel, is suggestive of significant adaptation to the
for FWPV transmission in nature (Afonso et al., 2000). All avian avian host. These genes may function in host immune evasion,
pox viruses produce these cytoplasmic inclusions. This gene does immune modulation, and aspects of cell and/or tissue tropism or
not appear to be essential for virus replication. perform other cellular functions. Most of these genes are found in
terminal regions of the FWPV genome, although several group-
Photolyase ings of them are more centrally located (Afonso et al., 2000)
A homologue of class II cyclobutane pyrimidine dimer (CPD)
photolyase is a photoreactive enzyme that efficiently repairs UV-
induced CPD lesions in DNA, using visible light as an energy Mixed infections
source. Presence of this enzyme helps viral survival in the external Birds often get mixed infection which can lead to evolution of
lesions in poultry and virus containing scabs in the environment a pathogen either with increased or decreased virulence. The
(Srinivasan et al., 2001; Srinivasan and Tripathy, 2005). This gene outcome of such interactions depends on specific aspects of
is non-essential for virus replication. A photolyase deficient virus pathogen activity and host response to infection. Thus, even an
is as immunogenic as the parent virus. The gene encoding for avirulent pathogen can play a significant role in influencing the
CPD-photolyase in FWPV is also present in the CNPV and other outcome of coupled host–pathogen systems (Thomas et al.,
avian pox virus genomes. 2003). In a case report (Ogasawara et al., 2016) on concurrent