Page 382 - Avian Virology: Current Research and Future Trends

Avian Pox Virus | 373

fowlpox and candidiasis in backyard chickens with unusual pox highly pathogenic fowlpox virus with high mortality. The authors
lesions in the bursa of Fabricius (BF) confirmed by PCR amplifi- believed that the virulence was associated with the integration of
cation of 4b and TK genes. Pox lesions were not observed in the REV sequences in the FWPV genome. The flock had not received
internal organs except for the BF. any vaccination. When immune responses of fowlpox vaccine
strains and field isolates were compared (Wang et al., 2006),
CMI responses were significantly depressed in chickens inocu-
Role of REV in the fowlpox virus genome lated with field isolates which appeared in the first 3 weeks post
The FWPV genome contains an ORF that encodes for a putative infection. The field strains of FWPV contained integrated REV
protein with similarity to the protein encoded by Marek’s disease provirus while vaccine strains carry only REV LTR sequences.
virus and fowl adenovirus, the presence of this homologue in The results of this study further suggest that virulent field isolates
three different DNA viruses of birds suggests its important role FWPV with full-length REV have immunosuppressive effects
in avian host range function. Interestingly, a natural dual viral when inoculated into young chickens.
infection of trachea by FWPV and herpesvirus has been reported
previously (Tripathy et al., 1975). In this case presence of herpes
virus in the nucleus and poxvirus in cytoplasm of the same cell Prevention
was demonstrated by histopathology and electron microscopy. As Live virus vaccines of FWPV and pigeonpox virus origin are
homologues of FWPV open reading frames were detected in the used in domestic poultry in areas where the disease is endemic.
genome of Marek’s disease virus (Brunovskis and Velicer, 1995), Chicken embryos or avian cell cultures are used for propagation
the likelihood of exchange of genetic material from one virus to of vaccine viruses.
another and emergence of a genetically and antigenically differ- The vaccines are administered by wing-web stab method. Vac-
ent virus is possible. In this regard, integration of full-length REV cinated birds are examined after 7–10 days, for swelling of the skin
in majority of the field isolates of the FWPV genome indicates ‘takes’. Attenuated cell-culture vaccine can be used on chicks as
an event of ‘natural genetic engineering’ in viruses. Both experi- early as day old. Although the vaccine is administered to 4-week-
mental and field observation (Tripathy et al., 1998; Singh et al., old chickens, and again at 1–2 months prior to egg production,
2000) indicate fowlpox as an emerging/re-emerging disease. Kim the practice may vary depending upon the epizootiology of the
and Tripathy (2001) observed that an isolate of fowlpox virus disease in a particular operation and area. The chickens may be
lyophilized in 1949 had full-length REV insertion in its genome revaccinated if held for more than one laying cycle. Experimental
indicating that this REV insertion happened long ago. While the studies show that FWPV vaccines given in ovo provide protection
information on the origin and date of isolation of fowlpox virus against the disease. A cell-culture FWPV vaccine provides protec-
strain were not available, it is assumed that the event occurred tion against FWPV in day-old turkeys (Sarma et al., 2015).
sometime in 1930–1940 or earlier. Phylogenetic and historical
evidences (Niewiadomska and Gifford, 2013) suggest that REVs Recombinant FWPV and CNPV vaccines
originated as mammalian retroviruses which were accidentally A large variety of genes encoding antigenic proteins of specific
introduced into avian hosts in the late 1930s, during experimen- pathogens have been inserted into the genome of FWPV and
tal studies, and subsequently integrated into the FWPV genomes, CNPV. Because of its large genome size with several non-essential
generating recombinant DNA viruses that now circulate in wild loci, genes from more than one pathogen can be inserted into its
birds and poultry. REV has been associated with immunosup- genome to create a polyvalent vaccine (Boyle and Heine, 1993;
pression and tumour formation. Boyle et al., 2004).
Only variable-length, REV LTR remnants are present in the Several non-essential regions, including some in the terminal
genome of all FWPV vaccine strains (Moore et al., 2000). These inverted repeats, have been identified in the FWPV genome and
remnants are also retained, presumably after the loss of the REV have been used in the generation of recombinant viruses. Thymi-
provirus, by a minor proportion of each field strain population. dine Kinase (TK) gene has been one of the common loci where
Isolation of majority of FWPV strains with intact provirus foreign genes have been inserted. Since TK is associated with
copy of REV genome from chicken flocks which had been vac- virulence and is non-essential for virus replication, the recom-
cinated with fowlpox and pigeonpox virus vaccines is indicative binant FWPV are less virulent than parent FWPV (Beard et al.,
of emergence of strains which are antigenically, and biologically 1991). Pox virus promoters are relatively conserved and, thus, are
different and current vaccines do not provide adequate protec- recognized by heterologous pox viruses. Initially, therefore, vac-
tion against them. cinia virus promoters were used in lieu of FWPV transcription
Both immunosuppression and tumour formation by REV regulatory elements in creating recombinant FWPV. Although
containing FWPV are supported by field observations. For exam- homologous FWPV promoters have since been identified
ple, visceral lymphomas were observed in chickens infected with (Srinivasan et al., 2006) and a synthetic, early late transcriptional
fowlpox virus containing REV by Koo et al. (2013). Since sera of regulatory element has been used, 2 vaccinia virus promoters, the
affected birds were positive for REV specific antibodies but not early-late P7.5 and the late P11, have been used for the construc-
for avian leukosis virus, the authors concluded that the lympho- tion of recombinant avian pox viruses (Beard et al., 1991). Several
mas were caused by the REV containing FWPV field strain. In homologous FWPV promoters, including a bidirectional one,
another observation, Zhao et al. (2014) reported an outbreak of have been evaluated. Because some of these promoters appear