Page 326 - Avian Virology: Current Research and Future Trends

Infectious Laryngotracheitis Virus

Mauricio J.C. Coppo , Amir H. Noormohammadi and Joanne M. Devlin * 11
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1 Melbourne Veterinary School, University of Melbourne, Parkville, Australia.
2 Melbourne Veterinary School, University of Melbourne, Werribee, Australia.
*Correspondence: devlinj@unimelb.edu.au
https://doi.org/10.21775/9781912530106.11

Abstract alphaherpesvirus 1 (PsHV-1). These two viruses form a distinct
Infectious laryngotracheitis (ILTV) is an alphaherpesvirus that clade within the subfamily Alphaherpesvirinae (King et al., 2012).
causes economically significant respiratory disease in poultry
industries worldwide. The disease was reported as early as the Physical properties of the virion
mid-1920s. A strong research focus on control measures, includ- The morphology of ILTV is typical of a herpesvirus. The virion
ing the development of vaccines and diagnostic tests, has helped comprises DNA, a capsid, a tegument and an envelope. The DNA
to limit the impact of this disease; however, ILTV infection is linear, double-stranded DNA with a buoyant density of 1.704 g/
nevertheless remains prevalent in many poultry producing areas. ml (Plummer et al., 1969). Full genome sequencing has shown that
Recent advancements in whole genome sequencing and bioin- ILTV genome is approximately 153 kbp in length, with a guano-
formatic analyses of genome sequences have contributed much sine plus cytosine (G + C) content of 48% (Lee et al., 2011a). The
to our understanding of ILTV, including the epizootiology of the structure of the capsid surrounding the DNA is well conserved in
disease. Furthermore, recent advances in vaccine development, herpesviruses and is composed of 162 capsomeres (150 hexam-
including the availability of vectored vaccines, have provided ers and 12 pentamers) in an icosadeltahedral arrangement and is
additional tools to help control ILTV in the field. A number of approximately 100 nm in diameter (Roizman and Pellet, 2001).
key gaps in our understanding of ILTV still remain and these can The tegument is situated between the envelope and the capsid. The
limit our ability to effectively control this disease. diameter of the ILTV virion varies between 200–250 nm, depend-
ing on the thickness of the tegument. The capsid frequently has an
eccentric location within the virion (Granzow et al., 2001). The
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Introduction molecular weight of the DNA is approximately 10 g/mol (Kotiw
et al., 1982). The envelope renders the virus sensitive to the envi-
History ronment. It is readily inactivated by organic solvents (Meulemans
Infectious laryngotracheitis (ILT) was reported in poultry as and Halen, 1978). Despite this sensitivity to the environment,
early as the mid-1920s in North America (May and Titsler, the virus can maintain infectivity in tracheal tissues and exudates
1925) but the term ILT was not adopted until 1931 by a spe- for extended periods (weeks or months) if protected from light
cial committee on poultry diseases of the American Veterinary (Jordan et al., 1967).
Medical Association. Reports of the disease followed in Aus- The processes of virus attachment, entry, replication, assembly
tralia and Britain in 1935, and in other European countries from and egress have been comprehensively characterized for many
the 1940s (Cover, 1996). Early reports of disease commonly herpesviruses but are less highly studied for ILTV. Studies that
described coughing, gasping and expectoration of blood and have specifically investigated these processes in ILTV infection
mucus (Cover, 1996). The disease has now been identified in have indicated that, unlike other herpesviruses, attachment to the
most countries. The causative agent, infectious laryngotracheitis cellular membrane appears to occur in a heparin-independent and
virus (ILTV), was identified as a herpesvirus in 1963 (Cruick- chondroitin-independent manner (Kingsley and Keeler, 1999).
shank et al., 1963). Electron microscopy studies have been used to demonstrate empty
virus capsids and pro-capsids in the nucleus of ILTV infected cells,
Taxonomy and then the migration of DNA-filled capsids through the nuclear
Infectious laryngotracheitis virus, taxonomically recognized membrane into the cytoplasm (Guo et al., 1993). The capsids
as Gallid alphaherpesvirus 1, is classified within the subfamily obtain an envelope at the inner membrane of the nucleus but
Alphaherpesvirinae of the family Herpesviridae, one of three fami- appear to lose this envelope through fusion with the outer leaflet of
lies within the order Herpesvirales. This virus is the type species the nuclear membrane. ILTV capsids have been shown to gain an
for the genus Iltovirus. The only other virus in this genus is Psttacid unusually large but variable amount of tegument in the cytoplasm