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of infected cells and then to gain their final envelope at membranes regions, with inverted repeat (IR) sequences flanking the US
of the trans-Golgi network before egress of virus progeny by exocy- region (Leib et al., 1987; Johnson et al., 1991). Recent advances
tosis of virus-containing vesicles (Granzow et al., 2001). in next generation sequencing have contributed greatly to charac-
terizing and describing the ILTV genome, with the first complete
Virus propagation ILTV genome published in 2011 (Lee et al., 2011b). A schematic
In the laboratory ILTV can be grown in a variety of avian cell of the ILTV genome is shown in Fig. 11.1.
cultures, or in embryonated hen eggs where it replicates on the The complete genome of ILTV encodes 80 predicted open
chorioallantoic membrane (CAM), and in the allantoic cavity. reading frames (ORFs) (Lee et al., 2011a). Most of these ORFs
Many different primary cell cultures are able to support ILTV share significant homology to ORFs in herpes simplex virus 1
propagation. These include chick embryo liver (CEL), lung and (HSV-1, the prototype virus in the subfamily Alphaherpesvirinae)
kidney (CEK) cell cultures, as well as chicken kidney (CK) cell in regard to their position within the genome and the predicted
(Hughes and Jones, 1988). The virus can also be propagated sequence of their translation products (Thureen and Keeler, 2006;
in the continuous liver male hepatoma (LMH) cell line, which Lee et al., 2011a). Unique features of the ILTV genome include
was derived from a chemically induced chicken liver tumour a large internal inversion in the UL region that extends from the

(Kawaguchi et al., 1987). However, some adaptation of ILTV is UL22 gene (glycoprotein H) to the UL44 gene (glycoprotein C)
required for growth in LMH cells and so they are not suitable for (Ziemann et al., 1998a) and also translocation of the UL47 gene
diagnosis via primary isolation (Schnitzlein et al., 1994). In cell to the US region, upstream to the US4 ORF (Kongsuwan et al.,
culture ILTV rapidly produces a cytopathic effect. Typically, the 1995; Wild et al., 1996; Ziemann et al., 1998a). The ILTV genome
resultant plaques have characteristic multinucleated giant cells also encodes an additional ORF (UL3.5) that lies between UL2
that form on the plaque margin as a result of cytoplasmic fusion and UL3. This gene is not present in HSV genomes but is pre-
(Schnitzlein et al., 1994; García et al., 2013a). sent in many other alphaherpesviruses (Fuchs and Mettenleiter,
1996). The ILTV genome lacks a UL16 homologue, which is
otherwise conserved among alphaherpesviruses (Fuchs and
Molecular biology and viral genetics Mettenleiter, 1999). The ILTV genome encodes eight ORFs that
are unique to the genus Iltovirus (ILTV and PsHV-1) (Johnson et
Genome arrangement al., 1997; Ziemann et al., 1998b; Veits et al., 2003c; Thureen and
The ILTV genome has a type D herpesvirus genome arrangement Keeler, 2006). Five of these unique genes are in a cluster (ORF
consisting of unique long (UL) and unique short (US) genome A–E) in the UL region, between the conserved UL45 and UL22

Figure 11.1 Arrangement of the ILTV genome. The unique short (US) region is flanked by inverted internal and terminal repeat regions (IRs
and TRs, respectively). Based on the complete genome sequence of Australian class 10 ILTV isolate (NCBI accession number KR822401.1)
Image provided by Paola Vaz, The University of Melbourne.