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Avian Pathology
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Avian infectious laryngotracheitis: Virus‐host
interactions in relation to prospects for
eradication
Trevor J. Bagust & Michael A. Johnson
To cite this article: Trevor J. Bagust & Michael A. Johnson (1995) Avian infectious
laryngotracheitis: Virus‐host interactions in relation to prospects for eradication, Avian Pathology,
24:3, 373-391, DOI: 10.1080/03079459508419079
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Avian Pathology (1995) 24, 373-391
REVIEW ARTICLE
Avian infectious laryngotracheitis: virus-host
interactions in relation to prospects for
eradication
TREVOR J. BAGUST & MICHAEL A. JOHNSON
Commonwealth Scientific and Industrial Research Organisation, Animal Health
Research Laboratory, Parkville, Melbourne, Victoria, 3052, Australia
SUMMARY
This review examines the virology, immunology and molecular biology of infectious
laryngotracheitis virus (ILTV) and its interactions with the chicken, in the context of
assessing the feasibility of eradication. Establishment of the latent phase during
infection of the host, its central role in biological survival of ILTV and the host-viral
events that are associated with reactivation of infection, are considered.
In counterpoint there are several features of the biology of ILTV in its natural mode
of infection which can be exploited in eradicating this pathogen from intensive
poultry production sites. These include the high degree of host-specificity of ILTV,
dependence on contact for spread, the short-lived infectivity outside the chicken and
the stability of the genome and lack of significant antigenic variation. Further, ILTV
cannot replicate productively in its main target organ, the trachea, in the face of local
specific cell-mediated immunity.
Genetically-engineered vaccines that are capable of generating immunity, but without the ILTV latent infections induced by conventional modified-live ILT vaccine
strains, are now well into development. This paper postulates that, used in conjunction with specific site quarantine and hygiene measures, such vaccines can provide
the technological tools required to eradicate ILTV from production sites, and then
regionally, in developed poultry industries from around the year 2000.
INTRODUCTION
Whether the European-Australian acronym "ILT" or the American "LT" is used
for infectious laryngotracheitisj this viral respiratory pathogen of chickens (now
known also as Gallid herpesvirus 1, Roizman, 1982), has proved to be of enduring
interest since being the first major avian viral disease for which an effective
vaccine was developed (Hudson & Beaudette, 1932). Application of virulent ILT
virus (ILTV) to the mucous membrane of the vent of young layers resulted in
rapid development of immunity in the respiratory tract. This was the first method
to control previously widespread and severe ILT outbreaks.
In the 6 decades since the pioneering observations on ILTV were made in the
Received 21 March 1995; Accepted 21 March 1995.
0307-9457/95/030373-19 © 1995 Houghton Trust Ltd
374
T. J. BAGUST & M. A. JOHNSON
1930s (see Hitchner, 1975), some hundreds of scientific papers on ILTV have
been published each decade. Whilst the great majority of these have been
vaccines-related, very few have been directed to examining the feasibility of
eradication of ILTV on production sites or regionally. Reviewing this aspect of
prevention of ILT disease now however could well prove timely for two reasons.
Firstly, scientific understanding of the pathogenesis, immunology and viral ecology of ILT in both the acute and latent phases of infection has reached an
advanced stage. Secondly, there are major developments occurring in molecular
biological technology. These can be applied to control avian infectious diseases
using a new generation of genetically-engineered vaccines and delivery systems.
This paper aims, therefore, to focus on the knowledge that is currently available
for assessing the feasibility of excluding ILT infection from flocks of chickens on
intensive production sites. For detailed information on historical or other aspects
of ILT disease and virus, other reviews which might be consulted include Jordan
(1966, 1981), Hitchner (1975) or the current ILT chapter in Diseases of Poultry
(Hanson & Bagust, 1991).
ILT DISEASE AND VIRUS IN MODERN PERSPECTIVE
Forms of disease in differing production systems
Epizootic forms of ILT were often described in earlier years associated with
haemorrhagic tracheitis, expectoration of blood-stained mucus and mortalities of
20% or higher. Benign forms of ILT with low morbidity and very low mortalities
(0.1-2%) are the more regular features of modern ILT in the developed intensive
poultry industries of Europe, the USA or Australasia (Jordan, 1966; Hanson &
Bagust, 1991).
In countries with modern intensive poultry industries, ILT in layer flocks is no
longer recognised as a cause of significant economic losses and can be prevented
or readily controlled if adequate vaccination regimens are maintained. Outbreaks
on single or several sites may occasionally occur, usually originating in pullets for
which vaccination has been overlooked or immunity has waned prematurely.
These cases are the exception and are usually quickly rectified. For intensive
broiler production systems, the high degrees of site isolation and quarantine tend
to combine with the short growing periods of some 6 to 7 weeks so as to usually
exclude the risk of exposure to ILT. Broilers may therefore no longer be routinely
vaccinated in some regions.
The second major ILT infection and disease situation occurs in the same
countries but outside the intensive industry, and is in backyard or fancier chicken
flocks, where a somewhat laissez-faire approach to ILT control can apply. The
birds on these sites may be of various ages and generations, be vaccinated or not
and immune or otherwise depending on natural exposure. On such sites ILTV
strains of varying virulence can co-exist for generations as endemic infections,
with sporadic natural spread producing immunity but little evidence of disease
unless direct or indirect contact occurs with susceptible chickens elsewhere.
AVIAN INFECTIOUS LARYNGOTRACHEITIS
375
However, these sites can remain as major reservoirs of ILTV and some of these
strains may be moderately pathogenic. Hence, any planning for regional ILT
eradication must also take such small sites into consideration.
The third scenario for ILTV infection and disease remains in the village poultry
of many of the world's economically-developing ("industrialising") countries.
Periodic outbreaks of ILT disease among fully-susceptible village poultry in
African or Asian villages can be of a severity sufficient to decimate village chicken
populations (Biggs, 1982). ILT vaccines may be unavailable through constraints
such as isolation of rural regions or a lack of financial resources or Government
extension assistance to these areas. This permits ILTV to persist throughout these
regions in its classical forms as a fearsome poultry pathogen. Resolution, even if
practicable, will require an altered and integrated developmentally-based approach to upgrading poultry health in production within these countries (Bagust,
1994).
Acute ILT infection: pathogenesis and transmission
The target organ system for ILTV infection and disease is the respiratory tract.
The epithelium of the trachea and larynx is always affected whilst other mucus
membranes, respiratory sinuses, air sacs and lung tissue may periodically also
become infected. This depends on the route, challenge level and sequelae of
infection. Whether chickens are exposed to ILTV by nasal, oral, conjunctival or
other routes, e.g., via the infraorbital sinus, the most active replication of ILT
occurs within the tissues of the trachea (see Hanson & Bagust, 1991).
Numerous experimental infection studies have examined the pathogenesis of
ILTV infection, using chickens of different ages, routes of infection, strains of
virus and various laboratory technologies for detection of the virus or its antigens.
To indicate the major points, the comprehensive studies of Hitchner et al,
(1977), Bagust et al, (1986) and Williams et al, (1992) can be considered
together, and show the features which are described below.
Involvement of respiratory tissues
Active virus replication occurs only during the 1st week of infection, although low
levels of ILTV can sporadically be detected up to 10 days post-infection (p.i.).
Bagust et al, (1986) quantified virus replication occurring in the trachea and
found titres in trachéal exúdate and tissues from days 2 to 5 of 102 to 107
plaque-forming units per gram or ml of trachéal washings or tissues. Subsequent
virus replication was rarely detectable by infectivity or fluorescent antibody (FA)
beyond day 7 p.i. Williams et al, (1992) recorded virus yields in trachéal swabs
as a mean daily titre of 2.7 logio TCID 50 /ml between days 3 to 7, then falling to
the order of 1 logio or less. Of the three major studies denoted however, only
Williams et al, (1992) have reported being able to detect ILTV infectivity in the
trachea other than sporadically at 8 to 10 days p.i., although antigen detection by
376
T. J. BAGUST & M. A. JOHNSON
FA testing has been described over somewhat longer periods (Wilks & Kogan,
1979).
Clinical signs of respiratory disease occur 6 to 12 days after natural exposure of
a flock but in experimental infections, where the virus inoculum is routinely
delivered directly to the trachea, between 2 to 4 days p.i. Chickens recover from
primary ILT disease within 7 to 10 days or so of showing clinical signs, the
appearance of these reflecting the phase of active virus replication which is
occurring in the trachea. From 10 days through to 4 weeks or so after trachéal
infection, whilst shedding of ILTV infectivity may have ceased, the latency phase
of ILTV infection is being established (see later).
Involvement of non-respiratory tissues
Infection of the conjunctiva regularly occurs in both natural and experimental
infections, via spread of contaminated exudates from adjacent clinically-affected
birds. After local replication, the trachéal tissues are then involved. There is little
evidence for the occurrence of a viraemic phase, and then only sporadically in
young chickens (Bagust et al, 1986), although avian macrophages are known to
be able to support the multiplication of ILTV strains in vitro (Von Bülow &
Klasen, 1983; Calnek et ai, 1986).
ILTV invasion of the trigeminal ganglion (TRG) was discovered to occur
regularly from days 3 to 6 during the acute phase of ILT infections by Bagust et
al, (1986). The exact route of infection of the TRG is not known but neural
migration can be strongly inferred as this ganglion is known to provide the main
sensory innervation to the tissues of the upper respiratory tract, the mouth and
the eyes whilst the distal ganglia are also involved in the sensory innervation of the
trachea (Bubien-Waluszewska, 1981). Williams et al, (1992), using polymerase
chain reaction technology, have subsequently confirmed that the trachéal ganglion is the main site of ILTV latency (see later).
Recently, ILT viruses have been detected in two samples of proximal femora
that were obtained from lame broiler chickens (Jones et al, 1993). The
significance of this observation is not yet clear, and further investigations are
merited.
Transmission of ILTV by direct contact occurs naturally via the upper respiratory and ocular routes, mainly when infectious respiratory exudates are aerosolised
or expectorated. However, there is no evidence for egg-transmission of ILTV or
for shedding of ILTV on shells of eggs laid by infected hens. The chicken is the
only significant primary host species for ILTV, and the existence of another
reservoir host species is unknown, although some other avian species such as
pheasants can sometimes be naturally infected through contact with chickens (see
Hanson & Bagust, 1991).
ILT latency: sites of establishment and reactivation
Latency establishment circumvents the usual immune clearance mechanisms of
AVIAN INFECTIOUS LARYNGOTRACHEITIS
377
the host and allows persistence of ILTV infection in host flocks even of low
numbers of chickens.
Integration of the viral genomic material into the host DNA occurs
during latency (see later). A functional virological definition of latency requires
that when putative latently-infected host tissues are placed in susceptible
cell cultures, they must remain free of viral infection for several days before
replication commences and infectious virus particles are produced. When organ
cultures are established from the tracheas of chickens previously infected with
either virulent wild-strain or vaccine-strains of ILTV, virus is not released
until after some days in culture (Turner, 1972; Bagust, 1986; Hughes et al,
1989).
The most detailed studies with ILT in trachéal organ cultures (TOC) systems
have shown that wild-strain ILTV could be detected in 38% of chickens examined 3 to 15 months after trachéal exposure, while a similar proportion of
chickens infected with an Australian ILT vaccine-strain were detected from 2 to
10 months p.i. (Bagust, 1986). Also reported in this study was the finding that the
sites of establishment of latent infection were highly focal in that only 8.3% of the
TOC preparations, though obtained from locations throughout entire tracheas,
were positive. It may be deduced from these results, along with those of Hughes
et al, (1987), that all field—and vaccine—strains of ILTV will establish latent
phases of infection.
Onset of the latent ("hidden") phase of ILTV infection appears to commence
during the immediate post-acute phase of infection, i.e., 7 to 10 days after
trachéal exposure. Latent ILTV infections are not readily demonstrable during
the first few months after infection (Bagust et al, 1986; Hughes et al, 1987,
1991), and this finding probably reflects the host chicken initially exerting a high
level of immune control and surveillance.
From around 3 months p.i. and then throughout the life of the host chicken,
reactivation of latent ILTV infection with shedding of infectious virus into the
trachea will occur sporadically. Hughes et al, (1989), in a landmark study which
went far towards explaining how apparently spontaneous outbreaks of ILT occur
in field situations, demonstrated that rates of shedding of ILTV into the trachea
were significantly increased ( P < 0.001) by the stresses of either the onset of lay
or the mixing with unfamiliar birds.
A number of studies have been directed towards elucidating the site(s) of ILTV
latency establishment from which, after reactivation, infectious virus can eventually manifest in the trachea! epithelium. Whilst the TRG is known to be a site of
latent infection for alphaherpesviridae of several other animal species (Ackermann
et al, 1982; Feldman, 1991), and to be associated with nervous supply to the
trachea and larynx (see Pathogenesis), it is only quite recently that firm evidence
was obtained for the association of ILTV with the TRG. Two observations have
strongly implicated the TRG in ILT latency—first, evidence from Australia of the
involvement of the TRG regularly during the pathogenesis of acute trachéal
infection with ILTV (Bagust, 1985) and then a German report (Kaleta et al,
1986) of isolation of ILTV from the TRGs of two field chickens some 15 months
378
T. J. BAGUST & M. A. JOHNSON
after vaccination. Confirmation that the TRG is the major site of ILTV latency
was finally obtained through polymerase chain reaction studies in the UK by
Williams et al, (1992).
Spreading of ILTV between production sites
From the prece*ding two sections, it can be seen that the sole source of infection
on a production site will be the chickens. Whilst these may have been adequately
vaccinated, ILTV production will be continually occurring through reactivation of
latent infections in a small proportion of these chickens. Stressing of these birds
through coming into lay or mixing of flocks will cause a higher proportion of the
hens in a flock to shed virus. Further, after exposure of a flock to "live" ILT
vaccine, or after an outbreak, it cannot be assumed for practical purposes that
active ILTV shedding has ceased until some 2 weeks after exposure or the onset
of the last clinical cases in a naturally-infected flock.
ILTV transfer between sites also occurs regularly by indirect contact, through
infectious trachéal exudates from chickens and surface contamination of personnel, service vehicles and poultry equipment (reviewed by Hanson & Bagust,
1991). Hence risk management measures will be required in production sites
which adjoin one which has undergone a recent outbreak of ILT. Whilst ILTV is
known to be readily susceptible to various disinfectants and heat treatments
(Hanson & Bagust, 1991), the ability of ILTV infectivity to persist in trachéal
exudates and in carcasses for periods between 21 to 80 days with ambient
environmental temperatures up to 20°C (see review by Jordan, 1966) appears no
longer to be common knowledge. Hence high priority must be accorded to
disposal of carcasses and measures to control potential predatory and other
migratory animals (rodents, birds) on production sites.
THE IMMUNOBIOLOGY OF ILTV
Host.responses following infection
A variety of responses are generated by the immune system following infection by
ILTV (reviewed by Jordan, 1981; Hanson & Bagust, 1991). Best known are
virus-neutralising antibodies which become detectable in the serum within 5 to 7
days of trachéal exposure, peak around 21 days, and then wane over the next
several months to low levels at which they can persist for a year or more. Mucosal
antibodies (IgG and IgA) which are capable of binding to ILTV antigen and of
low levels of virus-neutralising and ELISA antibody activity become detectable in
trachéal secretions and washings from approximately 7 days p.i. (Bagust, 1986;
York et al, 1989) and plateau at days 10 to 28. York et al, (1989) observed the
numbers of IgA- and IgG-synthesising cells in the trachea to be substantially
increased by days 3 through to 7 p.i.
Cell-mediated immune (CMI) responses to ILTV infection have generally only
been studied indirectly (e.g., using bursectomised chickens) because of the
complexity of requirements in conducting CMI experimentation, but DTH
AVIAN INFECTIOUS LARYNGOTRACHEITIS
379
responses to ILTV have been detected (York & Fahey, 1990). The duration of
CMI responses can only be intuited from observations that levels of ILT vaccineinduced protection are highest for 15 to 20 weeks (see Hanson & Bagust, 1991),
after which vaccine breakdowns in the field become more common.
ILT immune protection
Primary ILT vaccination confers protection against challenge which is partial by
3 to 4 days post-exposure, and complete from 1 week (see reviews by Hitchner,
1975; Jordan, 1981). High levels of protection then ensue for 15 to 20 weeks
post-vaccination, with variable degrees of protection occurring within a flock over
the next year. Revaccination with infectious ("live") vaccines may or may not
assist in maintaining protection levels (Jordan, 1981), as the infectivity of the new
vaccine virus may well just be inactivated at the portal of entry to the host
chicken.
Numerous laboratory and field studies have independently confirmed that
immune protection to ILTV challenge is neither indicated by nor conferrable
through the presence of serum antibody. Transfer of immunity to ILTV challenge, using spleen cells and peripheral blood leukocytes obtained from congenie
immune donors, has been reported by Fahey et al., (1984). However, the onset
of ILT immunity in the recipient chickens could not be demonstrated until some
7 days after transfer of the immune cells, and possible reasons for this delay in
onset require to be examined.
Studies by Fahey & York (1990) using vaccinated-bursectomised chickens have
demonstrated that mucosal antibody is not essential in preventing the replication
of virus in vaccinated chickens. These authors concluded that the effector mechanism of protection from ILT is the local cell-mediated immune response in the
trachea. Effector mechanism(s) of cytotoxic lymphocytes or lymphokines released
by activated T-cells, or both, have been surmised. Of most importance however
within the present context of assessing the feasibility of eradication of ILTV has
been the decisive demonstration in two studies (York et al., 1989; Fahey & York,
1990), that when chickens are challenged with virulent ILTV 28 to 30 days after
vaccination, viral replication in the trachéal epithelium is completely prevented.
Both the effector mechanism and duration of this protection post-vaccination
urgently need to be resolved.
The latent phase of an ILTV infection may be induced as well as maintained
by immune-mediated mechanisms. It is known that ILTV latency is established
in the face of development of detectable host immune responses 1 to 2 weeks after
trachéal exposure. Latency establishment is the biological survival mechanism
which permits ILTV to evade host immune surveillance and clearance by the
host. As immunity begins to wane, or effectiveness is reduced due to stress factors
(Hughes et al., 1989), reactivation to produce infectious ILTV can occur. For
ILTV, in contrast to latent infections by the alphaherpesviridae of other domestic
animals, experimental treatments using regimens of immunosuppressive drugs
such as corticosteroids or cyclophosphamide (Bagust, 1986; Hughes et al, 1989)
380
T. J. BAGUST & M. A. JOHNSON
have not been shown to induce reactivation of ILTV infectivity in chickens known
to be latently infected.
MOLECULAR BIOLOGY OF ILTV
The viral genome and antigens
ILTV is a member of the alphaherpesviridae, with a genome consisting of a linear
155 kilobase double-stranded molecule, consisting of unique long (UO and
unique short (Us) segments flanked by inverted repeats (IRs) (Leib et al, 1987;
Johnson et al, 1991). From random DNA sequencing of ILTV DNA, Griffin
(1989) was able to identify 21 genes. Of these, 20 were identified by comparison
to varicella-zoster virus (VZV) and 19 by comparison to herpes simplex virus type
1 (HSV-1); only 12 genes were found by comparison with the gammaherpesvirus
Epstein-Barr virus (EBV). Sequence data of the thymidine kinase gene and the
upstream overlapping genes of ILTV also provided evidence that homology exists
at the DNA level between ILTV and other alphaherpesviruses (Griffin &
Boursnell, 1990; Keeler et al, 1991). A number of ILTV genes which have HSV
homologues have now been sequenced including glycoprotein B (gB) (Kongsuwan et al, 1991; Griffin, 1991; Poulsen et al, 1991), glycoprotein C (gC)
(Kingsley et al, 1994), glycoprotein D (gD) (Johnson et al, 1995a), ICP4
(Johnson et al, 1995b), ICP27, glycoprotein K (gK) and DNA helicase complex
and a gene with homology to glycoprotein X (gX) of pseudorabies virus (PRV)
(Kongsuwan et al, 1993a). All of these genes were located in colinear positions
with respect to the prototype genome HSV-1. The level of amino acid identity of
these ILTV genes compared to homologues of other members of the alphaherpesviridae would indicate that ILTV is an ancient member of this family of
viruses.
The study of the protective antigens of ILTV has focused on the glycoproteins,
as these molecules are responsible for stimulating both humoral and cell-mediated
responses to other herpesviruses (Spear, 1985). The characterization of ILTV
glycoproteins is still in an early stage, although a number have now been
characterized by sequencing including gB, gC, gD, gX, gK (see above references)
and a unique glycoprotein, gp60 (Kongsuwan et al, 1993b, 1995).
York et al, (1987) showed that chicken antisera to ILT vaccine strain (SA-2)
and to a virulent isolate immunoprecipitated five major viral glycoproteins
of 205, 160, 115, 90 and 60 Kilodaltons (kDa), respectively. Additional
glycoprotein bands were recognized by immune chicken and rabbit sera in
Western blotting using a glycoprotein fraction purified from extracts of virus
infected cells. Further work using monoclonal antibodies defined the five major
glycoproteins into two groups; the 205 complex (205, 160, 115 and 90 kDa
glycoproteins) and the 60 kDa glycoproteins (York et al, 1990). Monoclonal
antibodies which reacted with the 205 kDa complex were used to screen a Agtl 1
expression library to identify and characterize glycoprotein B (Kongsuwan et al,
1991). This approach was also used to characterize the 60 kDa glycoprotein
AVIAN INFECTIOUS LARYNGOTRACHEITIS
•• 3 8 1
(gp60); Kongsuwan etal, 1993b, 1995). This glycoprotein, which maps in the U s
region, has no homologue with other alphaherpesviruses and is therefore unique
to ILTV. Although gp60 is one of the primary proteins recognized by sera from
ILTV-infected chickens, its precise role in immunity or on the pathogenesis of
ILTV has not been determined. The depletion of gp60 from total glycoprotein
preparations using monoclonal antibody did not reduce the efficacy of preparations as a vaccine (York, 1989) and infected birds have been found to produce
both humoral and cell-mediated responses to gp60 (York & Fahey, 1990).
Finally, a glycoprotein with homology to PRV gX, a member of the gG family
of glycoproteins, has also been characterized. PRV gX is not incorporated into
PRV virions (Ben-Porat & Kaplan, 1970), is the predominant protein in the
culture medium of PRV-infected cells (Rea et al, 1985) and is immunogenic.
ILTV gX, like PRV gX, is secreted into tissue culture medium and hence is not
a structural component of the virion, whilst being immunogenic (Kongsuwan et
al, 1993a). Attempts at vaccination with GST-gX fusion protein, followed by
challenge, showed that birds were not protected from clinical disease nor from
replication of the challenge virus in the trachea (C. Prideaux and K. Kongsuwan,
personal communication 1994).
Mechanisms of latency and reactivation in host cells
Latency is a property of all herpesviruses, and this mechanism is marked by two
key features, being (1) the maintenance of multiple copies of circular extrachromosomal viral DNA in nuclei of one or two unique cell types, and (2) limited
transcription from these viral genomes. At some time after the establishment of
latency, conditions may be presented which trigger small numbers of latently
infected cells to enter into the productive stage of infection, and infectious virus
re-emerges. This infection may be clinically distinct from the primary infections,
with no apparent lesions. However, in some cases it can produce recrudescence
of clinical disease. Most of the knowledge surrounding latency mechanisms has
been derived from studies on HSV and EBV, and excellent current reviews are
available, e.g., Wagner (1994). Advances in knowledge have centred around
detailed investigations on the genes involved in latency, latency associated transcripts (LATs), the cells involved in latency and application of genetic engineering
techniques.
There have been a number of studies with HSV in animal models which have
shown that the establishment of latency in the cells of a target tissue by the
original viral strain, precluded colonization of the same tissue by another strain
(Klein et al, 1978; Centifanto-Fitzgerald et al, 1982; Meignier et al, 1983;
Thomas et al, 1985). Other studies however have indicated that different strains
can establish latency in the same tissue of the host animal (Lewis et al, 1984;
Whetstone & Miller, 1989). A general premise of vaccination strategies used has
been that attenuated strains should reduce the incidence of establishment of
latency by wild type (WT) virus. Vaccination regimes in Australia show for ILTV
that widespread use of tissue-culture adapted strains can reduce the incidence of
382
T. J. BAGUST & M. A. JOHNSON
virulent outbreaks, presumably by a combination of immunity and early colonisation by the SA-2 strain inhibiting latency establishment by other ILT field
viruses.
The most recent studies on PRV infection suggest that there is indeed a
significant inverse correlation between the level of latency established by attenuated strains and the level of latency established by a superinfecting WT virus
(Schang et al., 1994). This study indicates that vaccination with attenuated virus,
using routes of inoculation that can maximize the establishment of latency, might
significantly decrease latency establishment by WT strains. Further, genetically
engineered viruses that have undergone deletions for the genes of both glycoprotein I (gl) and thymidine kinase (TK) are impaired from establishing latency
because of an inability to target the virus to the tissues of the central nervous
system, a function of gl, and impaired replication in neurones, a function of TK.
Therefore, attenuated strains which are gl + may be better in precolonization of
neurones of the TG and consequently have a greater capacity to interfere with
WT latency.
ILT VACCINES AND THE NEW TECHNOLOGIES
Conventional ILT vaccines
All of the current vaccines for prevention and control of outbreaks of ILT are
based upon modified-live (ML) strains of this virus. These possess varying
degrees of residual virulence, naturally or through attenuation, and are grown
using chicken embryos or tissue culture. Various vaccination routes have been
developed during the past 60 years (see Hanson & Bagust, 1991). From the initial
cloacal route, the application of ILT vaccines has evolved through feather follicle
abrasion, then intranasal or sub-conjunctival instillation, and currently via the
drinking water or using aerosols with selected strains.
The value of using ML ILT vaccines to prevent or control disease is beyond
question. It has to be borne in mind however that ML vaccines regularly retain
inherent pathogenicity (requiring careful control of the dosage and conditions of
administration), and are capable of spreading from vaccinated to non-vaccinated
birds to cause serious problems (e.g., Clarke et al, 1980). The levels of virulence
of ML ILT vaccine strains are liable to be increased by bird-bird or flock-flock
passages after vaccination, and this may result in reversion to virulence (Guy et
al, 1991). Life-long latent ILTV infections, with sporadic reactivation and
shedding of virus (Bagust 1985, 1986; Hughes et al., 1991) are a further
complication with the use of ML ILT vaccines. Successful application of recombinant DNA technology to ILTV vaccines, especially in the areas of safety and of
delivery, would therefore be a very significant advance.
Strategies for developing recombinant vaccines and vector systems
Increasing knowledge of the antigens of ILTV makes it possible to use genetic
engineering to produce sub-unit vaccines, to construct virus vectors expressing
ILT antigen(s), or to reconstruct ILTV itself as a viral vector.
AVIAN INFECTIOUS LARYNGOTRACHEITIS
383
There is now good evidence that vaccination with herpesvirus glycoproteins can
induce protective immunity (Ghiasi et al, 1994). For example, HSV gB has been
demonstrated, both with temperature sensitive mutants (Glorioso et al, 1984)
and immunopurified proteins (Chan et ai, 1985), to evoke circulating antibody
and cell-mediated responses which can protect mice against lethal challenge.
Further, HSV gB expressed either by recombinant vaccinia virus vector (Cantin
et al, 1987), adenovirus vector (McDermott et al, 1989), or baculovirus
vector (Ghiasi et al, 1994), has protected mice against lethal challenge. For
ILT, a subunit vaccine containing only glycoproteins of the 205 kDa complex
protected 100% of chickens against clinical disease and also against viral
replication (York & Fahey, 1991). Since this 205 kDa complex contains
ILTV gB (Kongsuwan et al, 1991), this glycoprotein could prove to be a major
protective immunogen of ILTV and therefore a prime candidate for inclusion in
subunit or recombinant vaccines. These might be any of fowlpox virus-ILT gB,
fowl adenovirus-ILT gB, Marek's disease virus-ILT gB, or herpesvirus of turkeysILT gB viral vectors. Such an approach might also be extended to produce
multi-valent recombinant viral vectors by including other glycoproteins that
can induce a protective response such as gD and gC. It would then be possible
to vaccinate flocks even though these chickens have not experienced ILTV
infection.
Another approach is to engineer ILTV as a vector to include other antigens
whose expression is controlled by ILT promoters. The prime site for insertion is
in the ILT gX (gG) gene. Since PRV gX can be deleted from the herpesvirus
genome without reducing its ability to kill mice (Thomsen et al, 1987a), and
foreign genes can be inserted into gX-deleted genomes (Thomsen et al, 1987b;
Mettenleiter et al, 1990; van Zijl et al, 1991), it is most likely that this site in
ILTV can also be used to construct viral vectors expressing foreign antigens. It
has, however, been difficult to produce recombinant ILTVs. The recent development of a continuous cell line that can support ILTV replication (Scholz et al,
1993) may well assist in future. Other recent reports of the construction of ILTV
recombinants have included Guo et al, (1994), who inserted the jS-gal marker
gene into an open reading frame and has claimed the isolation of a recombinant.
No evidence has been given to date however of virus isolation or characteristics
of the recombinant, although inspection of the sequence showed that the
marker gene to be inserted into ILTV ICP4. Another group (Okamura et al,
1994) has reported successfully inserting the /?-gal marker gene into the TK gene
and have isolated a stable recombinant. This would seem to have been made
possible by the use of a strain of ILTV which is capable of propagation in
chicken embryo fibroblasts, thus allowing successful transfection/recombination
experiments.
It should be noted however that one of the disadvantages attending the use of
ILTV as a vector is that vaccination is unlikely to completely prevent latency.
Further, any such recombinant virus will have to be constructed so that rearrangements in vivo cannot occur, i.e., with wild type virus in the field. This will
require the precise deletion of non-essential genes as has been demonstrated for
384
T. J. BAGUST & M. A. JOHNSON
pseudorabies virus vaccines. Nevertheless, even that highly successful vaccine
appears unable to prevent some Aujeszky's disease virus infection within swine
herds (Stegeman et al, 1994). More positively, an ILTV TK~, gX~ recombinant
could provide a most promising tool for eradication.
Alternative future approaches may involve not using ILTV at all in the field,
but rather to rely upon other vectors such as fowl adenovirus or fowlpox to deliver
the major ILTV neutralising antigen (gB). However, other glycoprotein genes
might also be required and it is yet not known what effect this type of regime
might have on preventing latency. Any validation protocols for future putative
ILT vaccines should therefore include systematic studies on this aspect.
ASSESSING PROSPECTS FOR ERADICATION OF ILT
The feasibility of site eradication?
The following features of the biology and ecology of ILTV in both virus-host and
virus-site relationships are major indicators of the potential eradicability of ILTV.
1. ILT virus is not egg-transmitted: hence the infection status of the parent
flocks is not transmitted to progeny chickens whether placed on the same or
other sites.
2. ILT infection is essentially confined to chickens: the chicken is the
primary host species, and through latency establishment, is also the reservoir
host.
3. Levels of ILTV infectivity are usually low: titres of infectivity in excess
of very low levels will only occur in the first 7 days or so after trachéal
infection of a chicken whether exposed through vaccination or by natural
infection.
4. Spread of ILTV infection between sites can be strongly circumscribed by industry precautions: relatively simple site quarantine and
hygiene procedures will prevent ILTV from entering intensive broiler production sites.
5. ILTV infectivity is readily inactivated outside the host chicken by
disinfectants and by low levels of heat. Carry-over between successive
production batches in a house can therefore be prevented by adequate shed
cleanup.
6. Immunity will absolutely protect against ILTV challenge: for 4 to 5
months post-vaccination, replication of challenge ILTV can be prevented at
the trachea! surface. Denial of the trachéal epithelium as a substrate for viral
replication will prevent latency establishment by ILTV.
7. ILTV strains are antigenically homogenous: the DNA genome is stable,
and antigenic variation is not a problem in ILT vaccines as cross-neutralization and cross-protection occurs between all known strains of ILTV.
8. Immunity to ILTV is cell-mediated: maternal antibody to ILTV does
not interfere with the vaccination of very young chickens, or even in ovo.
AVIAN INFECTIOUS LARYNGOTRACHEITIS
385
When these features are considered separately, and together, the prospects for
systematic site and then regional eradication of ILTV can be seen to be most
positive providing that the (vaccine-based) technological tools required can be
developed. These will be recombinant vaccines which contain the ILTV immunogens that can induce protection through immune responses in the trachea.
Some practical considerations
For an eradication approach to ILT the benefit-costs ratio and the likely time
scale for introduction are both major issues once feasibility is accepted.
For benefit-cost considerations, were developments needed through recombinant-DNA technology to be focused solely on ILT, the costs at all levels in a
developed poultry industry would outweigh the gains from initial eradication.
However, there are several other major avian viral pathogens (infectious bursal
disease virus, Marek's disease virus, infectious bronchitis virus, Newcastle disease
virus) for which recombinant vector vaccines are projected to be available around
year 2000. ILT recombinant vaccines should also now be included. This would
allow the costs of development to be amortised, with particular value for ILTV
because of the numerous biological and ecological features (see previous section)
that render it specially susceptible to eradication measures.
While awaiting new-generation vaccines however, any region likely to be
designated for ILT eradication could most usefully be systematically swamped
with ML ILT vaccines on all production sites whether large, small or fancier-type.
Broiler production sites which do not vaccinate now however could be excluded,
provided that their strict quarantine and hygiene measures (Hanson & Bagust,
1991; Bagust, 1992) were maintained.
Government-industry cooperation will be needed for success of such eradication programmes. Laudable results obtained from earlier attempts at exclusion
of ILT from some regions of the USA (e.g., Mallison et al., 1981) should not be
lost sight of. Whilst the eradication of ILT from Australia, a sub-continent
bounded by seas, has long been a vision for the poultry industry (Hart, 1992), its
realisation must await the commercial availability of ILT vaccines that have been
engineered for this task.
IN CONCLUSION
Demand and need will inevitably increase over the next decade for avian vaccines
which can not only prevent diseases, but can also facilitate exclusion or eradication of endemic pathogens from production sites. The ILT vaccines which have
been developed and used to date have generally served the developed poultry
industries well. In the final analysis however, they operate through controlled
exposure to infection, and they perpetuate persistence of ILTV on sites through
latent infections. Advancing technology is now presenting opportunities to develop alternative strategies as we have indicated here.
ILT control through site and then regional eradication would be a further
386
T. J. BAGUST & M. A. JOHNSON
major development in avian health. Interestingly, it is only the recombinant-DNA
ILT vaccines which will have the potential to provide the solutions needed to
meet the deep apprehensions of poultry producers concerning ILT control.
Explicitly, if vaccination is stopped, with ILTVs then being excluded from
production sites only by quarantine and hygiene, what concrete measures can be
taken to provide cover for these flocks should ILTV exposure occur? Whilst
conventional ML ILT vaccines cannot provide immune cover in the absence of
active infection (and thence latency, etc.), recombinant DNA vector-delivered
vaccines do have the clear potential to provide the immunologically-based tools
needed for such a task.
Historically, there have been relatively few significant avian pathogens for
which the eradication option can realistically be planned. For ILTV , however,
research and development, as well as preliminary strategic planning in health by
developed poultry industries, can realistically be directed towards such an approach now.
ACKNOWLEDGEMENTS
We would wish to honour the work of numerous scientists, but this review is
particularly dedicated for their pioneering research studies on ILTV to Professors
S. Hitchner and L. Hanson (USA), Professor B. Sinkovic and Dr L. Hart
(Australia) and to Dr F. Jordan and his colleagues at Liverpool University, UK.
We also wish to thank our colleagues, both past and present, at CSIRO Animal
Health, Melbourne for their efforts and support in ILT research. The Australian
Poultry Industry Research Committees are acknowledged for their financial
support of ILTV research for more than a decade as are Cyanamid-Websters Pty
Ltd during recent years. Dr J. York and Mr N. White are thanked for their
comments on the manuscript, as is Ms Jayne Anderson for her diligent preparation of this manuscript.
REFERENCES
ACKERMANN, M., PETERHANS, E. & WYLER, R. (1982). DNA of bovine herpesvirus type 1 in the trigeminal
ganglia of latently infected calves. American Journal of Veterinary Research, 43, 36-40.
BAGUST, T.J. (1985). Infectious laryngotracheitis herpesvirus (ILT): latency detection and in vitro
reactivation of haemorrhagic and vaccine strains. Proceedings of the VIIIth International Congress of the
World Veterinary Poultry Association, Jerusalem, Israel, Program and Abstracts, p. 23.
BAGUST, T.J. (1986). Laryngotracheitis (gallid-1) herpesvirus infection in chickens. 4. Latency establishment by wild and vaccine strains of ILT virus. Avian Pathology, 15, 581-595.
BAGUST, T.J. (1992). Laryngotracheitis, in: Veterinary Diagnostic Virology: a practitioner's guide, pp. 40-43
(St. Louis, Mosby Year Book).
BAGUST, T.J. (1994). Improving health for poultry production in Asia: a developmental perspective. Avian
Pathology, 23, 395-404.
BAGUST, T.J., CALNEK, B.W. & FAHEY, K.J. (1986). Gallid-1 herpesvirus in the chicken. 3. Reinvestigation
of the pathogenesis of infectious laryngotracheitis in acute and early post-acute respiratory disease.
Avian Diseases, 30, 179-190.
BEN-PORAT, T. & KAPLAN, A.S. (1970). Synthesis of proteins in cells infected with herpesvirus. V. Viral
glycoproteins. Virology, 41, 265-273.
BIGGS, P.M. (1982). The world of poultry disease. Avian Pathology, 11, 281-300.
AVIAN INFECTIOUS LARYNGOTRACHEITIS
387
BUBIEN-WALUSZEWSKA, A. (1981). The cranial nerves, in: A. S. KING & J. MCLELLAND (Eds) Form and
Function in Birds, Vol. 2, pp. 385-438, (London & New York, Academic Press).
BÜLOW, V. VON & KLASEN, A. (1983). Effects of avian viruses on cultured chicken bone-marrow-derived
macrophages. Avian Pathology, 12, 179-198.
CALNEK, B.W., FAHEY, K.J. & BAGUST, T.J. (1986). In vitro infection studies with infectious laryngotracheitis virus. Avian Diseases, 30, 327-336.
CANTIN, E.M., EBERLE, R., BALDICK, J.L., Moss, B., WILLEY, D.E., NOTKINS, A.L. & OPPENSHAW, H.
(1987). Expression of herpes simplex virus 1 glycoprotein B by a recombinant vaccinia virus and
protection of mice against lethal herpes simplex virus 1 infection. Proceedings of the National Academy
of Sciences, USA, 84, 5908-5912.
CENTIFANTO-FITZGERALD, Y.M., VARNELL, E.D. & KAUFMAN, H.E. (1982). Initial herpes simplex virus
type 1 infection prevents ganlionic superinfection by other strains. Infection and Immunity, 35,
1125-1132.
CHAN, W.L., LUKIG, M.L. & Liew, F.Y. (1985). Helper T cells induced by an immunopurified herpes
simplex virus type 1 (HSV-1) 115 kilodalton glycoprotein (gB) protect mice against HSV-1 infection.
Journal of Experimental Medicine, 162, 1304-1318.
CLARKE, J.K., ROBERTSON, G.M. & PURCELL, D.A. (1980). Spray vaccination of chickens using infectious
laryngotracheitis virus. Australian Veterinary Journal, 56, 424-428.
FAHEY, K.J. & YORK, J.J. (1990). The role of mucosal antibody in immunity to infectious laryngotracheitis
virus in chickens. Journal of General Virology, 71, 2401-2405.
FAHEY, K.J., YORK, J.J. & BAGUST, T.J. (1984). Laryngotracheitis herpesvirus infection in the chicken. II.
The adoptive transfer of resistance with immune spleen cells. Avian Pathology, 13, 265-275.
FELDMAN, L.T. (1991). The Molecular Biology of Herpes Simplex Virus Latency, in: E. K. WAGNER (Ed.)
Herpesvirus Transcription and its Regulation, pp. 233-243 (CRC Press, Boca Raton, Florida).
GHIASI, H., KAIWAR, R., NESBURN, A.B., SLANINA, S. & WECHSLER, S.L. (1994). Expression of seven
herpes simplex virus type 1 glycoproteins (gB, gC, gD, gE, gG, gH and gl): comparative protection
against lethal challenge in mice. Journal of Virology, 68, 2118-2126.
GLORIOSO, J., SCHRÖDER, C.H., KUMEL, G., SZCZESIL, M. &LEVINE, M . (1984). Immunogenicity of herpes
simplex virus glycoproteins gC and gB and their role in protective immunity. Journal of Virology, 50,
805-812.
GRIFFIN, A.M. (1989). Identification of 21 genes of infectious laryngotracheitis virus using random
sequencing of genomic DNA. Journal of General Virology, 70, 3085-3089.
GRIFFIN, A.M. (1991). The nucleotide sequence of the glycoprotein gB gene of infectious laryngotracheitis
virus: Analysis and evolutionary relationship to the homologous gene from other herpesviruses. Journal
of General Virology, 72, 393-398.
GRIFFIN, A.M. & BOURSNELL, M.E.G. (1990). Analysis of the nucleotide sequence of DNA from the
region of the thymidine kinase gene of infectious laryngotracheitis virus; potential evolutionary
relationships between the herpesvirus subfamilies. Journal of General Virology, 71, 841-850.
Guo, P., SCHOLZ, E., MALONEY, B. & WELNIAK, E. (1994). Construction of recombinant avian infectious
laryngotracheitis virus expressing the β-galactosidase gene and DNA sequencing of the insertion
region. Virology, 202, 771-781.
GUY, J.S., BARNES, H.J. & SMITH, L. (1991). Increased virulence of modified live infectious laryngotracheitis vaccine virus following bird-to-bird passage. Avian Diseases, 35, 348-355.
HANSON, L.E. & BAGUST, T.J. (1991). Infectious laryngotracheitis. In: B. W. CALNEK (Ed.) Diseases of
Poultry, 9th edn pp. 485-495 (Ames, Iowa State University Press).
HART, L.W. (1992). Australia could be free of ILT. Milne's Poultry Digest, Dec-Jan, p. 10.
HrrcHNER, S.B. (1975). Infectious laryngotracheitis: The virus and the immune response. American
Journal of Veterinary Research, 36, 518-519.
HrrcHNER, S.B., FABRICANT, J. & BAGUST, T.J. (1977). Afluorescent-antibodystudy of the pathogenesis
of infectious laryngotracheitis. Avian Diseases, 21, 185-194.
HUDSON, C.B. & BEAUDETTE, F.R. (1932). The susceptibility of cloacal tissues to the virus of infectious
bronchitis. Cornell Veterinarian, 23, 63-65.
HUGHES, C.S., GASKELL, R.M., JONES, R.C., BRADBURY, J.M. & JORDAN, F.T.W. (1989). Effects of certain
stress factors on the re-excretion of infectious laryngotracheitis virus from latently infected carrier
birds. Research in Veterinary Science, 46, 274-276.
HUGHES, C.S., JONES, R.C., GASKELL, R.M., JORDAN, F.T.W., JONES, R.C. & BRADBURY, J.M. (1987).
Demonstration in live chickens of the carrier state in infectious laryngotracheitis. Research in Veterinary
Science, 42, 407-410.
388
T. J. BAGUST & M. A. JOHNSON
HUGHES, C.S., WILLIAMS, R.A., GASKELL, R.M., JORDAN, F.T.W., BRADBURY, J.M., BENNETT, M. & R. C.
JONES (1991). Latency and reactivation of infectious laryngotracheitis vaccine virus. Archives of
Virology, 121, 213-218.
JOHNSON, M.A., PRIDEAUX, C.T., KONGSUWAN, K. & SHEPPARD, M. (1991). Gallid herpesvirus 1 (infectious laryngotracheitis virus): cloning and physical maps of the SA-2 strain. Archives of Virology, 119,
181-198.
JOHNSON, M.A., TYACK, S.G., PRIDEAUX, C.T., KONGSUWAN, K. & SHEPPARD, M. (1995a). Sequence
characteristics of a gene in infectious laryngotracheitis virus homologous to glycoprotein D of herpes
simplex virus. DNA Sequence. The Journal of DNA Sequencing and Mapping, 5, 191-194.
JOHNSON, M.A., TYACK, S.G., PRIDEAUX, C.T., KONGSUWAN, K. & SHEPPARD, M. (1995b). Complete
nucleotide sequence of infectious laryngotracheitis virus (gallid herpesvirus 1) ICP4 gene. Virus
Research. 35, 193-204.
JONES, R.C., WILLIAMS, R.A. & SAVAGE, C.E. (1993). Isolation of infectious laryngotracheitis virus from
proximal femora of lame broiler chickens. Research in Veterinary Science, 55, 377-378.
JORDAN, F.T.W. (1966). A review of the literature on infectious laryngotracheitis (ILT). Avian Diseases, 10,
1-26.
JORDAN, F.T.W. (1981). Immunity to infectious laryngotracheitis, in: Avian Immunology, British Poultry
Science Symposium Series, 16, 245-254.
KALETA, E.F., REDMANN, T., HEFFELS-REDMANN, U. & FRESE, K. (1986). Zum nachweis der latenz des
attenuierten virus der infektiösen laryngotracheitis des huhnes im trigeminus-ganglion. Deutsche
Tierärzlichte Wochenschrift, 93, 40-42.
KEELER, C.J.JR., KINGSLEY, D.H. & BURTON, C.R.A. (1991). Identification of the thymidine kinase gene
of infectious laryngotracheitis virus. Avian Diseases, 35, 920-929.
KINGSLEY, D.H., HAZEL, J.W. & KEELER, C.J. JR. (1994). Identification and characterization of the
infectious laryngotracheitis virus glycoprotein C-gene. Virology, 203, 336-343.
KLEIN, R.J., FIREDMAN-KIEN, A.E. & BRADY, E. (1978). Latent herpes simplex virus in ganglia of mice after
primary inoculation and reinoculation at a distant site. Archives of Virology, 57, 161-166.
KONGSUWAN, K., PRIDEAUX, C.T., JOHNSON, M.A. & SHEPPARD, M. (1991). Nucleotide sequence of the
gene encoding infectious laryngotracheitis virus glycoprotein B. Virology, 184, 404-410.
KONGSUWAN, K., JOHNSON, M.A., PRIDEAUX, C.T. & SHEPPARD, M. (1993a). Identification of an infectious
laryngotracheitis virus encoding an immunogenic protein with a predicted Mr of 32 kilodaltons. Virus
Research, 29, 125-140.
KONGSUWAN, K., JOHNSON, M.A., PRIDEAUX, C.T. & SHEPPARD, M. (1993b). Use of λgt11 and mono-
clonal antibodies to map the gene for the 60,000 dalton glycoprotein of infectious laryngotracheitis
virus. Virus Genes, 7, 297-303.
KONGSUWAN, K., PRIDEAUX, C.T., JOHNSON, M.A. & SHEPPARD, M. (1995). Identification and character-
ization of infectious laryngotracheitis virus glycoprotein 60. Journal of General Virology (Submitted).
LEIB, D.A., BRADBURY, J.M., HART, C.A. & MCCARTHY, K. (1987). Genome isomerism in two alphaher-
pesviruses: Herpesvirus saimiri-1 (Herpesvirus tamarinus) and avian infectious laryngotracheitis virus.
Archive of Virology 93, 287-294.
LEWIS, M.E., LEUNG, W.-C, JEFFREY, V.M. & WARREN, K.G. (1984). Detection of multiple strains of
latent herpes simplex virus type 1 within individual human hosts. Journal of Virology, 52, 300-305.
MCDERMOTT, M.R., GRAHAM, F.L., HANKE, E.T. & JOHNSON, D . C . (1989). Protection of mice against
lethal challenge with herpes simplex virus by vaccination with an adenovirus vector expressing HSV
glycoprotein B. Virology, 169, 244-247.
MALUSON, E.T., MILLER, K.F. & MURPHY, C D . (1981). Cooperative control of infectious laryngotrachei-
tis. Avian Diseases, 25, 723-729.
MEIGNIER, B., NORRILD, B. & ROIZMAN, B. (1983). Colonization of murine ganglia by a superinfecting
strain of herpes simplex virus. Infection and Immunity, 41, 702-708.
METTENLETTER, T.C., KERN, H. & RAUH, I. (1990). Isolation of a viable herpesvirus (pseudorabies virus)
mutant specifically lacking all four known nonessential glycoproteins. Virology, 179, 498-503.
OKAMURA, H., SAKAGUCHI, M., HONDA, T., TANENO, A., MATSUO, K. & YAMATA, X. (1994). Construction
of recombinant infectious laryngotracheitis virus expressing the gene of E.coli within the thymidine
kinase gene. The Journal of Veterinary Medical Science, 56, 799-801.
POULSEN, D.J., BURTON, C.R.A., O'BRIAN, J.J., RABIN, S.J. & KEELER, C . L . J R . (1991). Identification of the
infectious laryngotracheitis virus glycoprotein gB gene by polymerase chain reaction. Virus Genes, 5,
335-348.
REA, T.J., TIMMINS, J.G., LONG, G.W. & POST, L.E. (1985). Mapping and sequence of the gene for the
AVIAN INFECTIOUS LARYNGOTRACHEITIS
389
pseudorabies virus glycoprotein which accumulates in the medium of infected cells. Journal of Virology,
54, 221-29.
ROIZMAN, B. (Ed.) (1982). The Family Herpesviridae: General description, taxonomy and classification.
in: The Herpesvirus, pp. 1-23. (New York, Plenum).
SCHANG, L.M., KunsH, G.F. & OSORIO, F.A. (1994). Correlation between precolonization of trigeminal
ganglia by attenuated strains of pseudorabies virus and resistance to wild type virus latency. Journal
of Virology, 68, 8470-8476.
SCHOLZ, E., WELNIAK, E., NYHOLM, T. & Guo, P. (1993). An avian hepatoma cell line for the cultivation
of infectious laryngotracheitis virus and for the expression of foreign genes with a mammalian
promoter. Journal of Virological Methods, 43, 273-286.
SPEAR, P.G. (1985). Glycoprotein specified by herpes simplex viruses, in: B. ROIZMAN (Ed.) The Herpesviruses, Vol. 3, pp. 315-356 (New York, Plenum).
STEGEMAN, J.A., KIMMAN, T.G., VAN OIRSCHOT, J.T., TIELEN, M.J.M. & HUNNEMAN, W.A. (1994).
Spread of Aujeszky's disease virus within pig herds in an intensively vaccinated region. Veterinary
Record, 134, 327-330.
THOMAS, E., LYCKE, E. & VAHLNE, A. (1985). Retrieval of latent herpes simplex virus type 1 genetic
information from murine trigeminal ganglia by superinfection with heterotypic virus in vivo. Journal
of General Virology, 66, 1763-1770.
THOMSEN, D.R., MARCHIOLI, C.C., YANCEY, R.J. JR & POST, L.E. (1987a). Replication and virulence of
pseudorabies virus mutants lacking glycoprotein gX. Journal of Virology, 61, 229-232.
THOMSEN, D.R., MAROTTI, K.R., PALERMO, D.P. & POST, L.E. (1987b). Pseudorabies virus as a live virus
vector for expression of foreign genes. Gene, 57, 261-265.
TURNER, A.J. (1972). Persistence of virus in respiratory infections of chickens. Australian Veterinary
Journal, 48, 361-363.
WAGNER, E. (Ed.) (1994). 'Herpesvirus Latency.' Seminars in Virology 5(3).
WHETSTONE, C.A. & MILLER, J.M., (1989). Two different strains of an alphaherpesvirus can establish
latency in the same tissue of the host animal: evidence from bovine herpesvirus 1. Archives of Virology,
107, 20-34.
WILKS, C.R. & KOGAN, V.G. (1979). Immunofluorescence diagnostic test for avian infectious laryngotracheitis. Australian Veterinary Journal, 55, 385-388.
WILLIAMS, R.A., BENNETT, M., BRADBURY, J.M., GASKELL, R.M., JONES, R.C. & JORDAN, F.T.W. (1992).
Demonstration of sites of latency of infectious laryngotracheitis virus using the polymerase chain
reaction. Journal of General Virology, 73, 2415-2430.
YORK, J.J. (1989). Immunogens of Infectious Laryngotracheitis Virus, PhD Thesis. University of Melbourne, pp. 95.
YORK, J.J. & FAHEY, K.J. (1990). Humoral and cell-mediated immune responses to the glycoproteins of
infectious laryngotracheitis. Archives of Virology, 115, 289-297.
YORK, J.J. & FAHEY, K.J. (1991). Vaccination with affinity-purified glycoproteins protects chickens against
infectious laryngotracheitis herpesvirus. Avian Pathology, 20, 693-704.
YORK, J.J., SONZA, S. & FAHEY, K.J. (1987). Immunogenic glycoproteins of infectious laryngotracheitis
herpesvirus. Virology, 161, 340-347.
YORK, J.J., YOUNG, J.G. & FAHEY, K.J. (1989). The appearance of viral antigen and antibody in the trachea
of naive and vaccinated chickens infected with infectious laryngotracheitis virus. Avian Pathology, 18,
643-648.
YORK, J.J., SONZA, S., BRANDON, M.R. & FAHEY, K.J. (1990). Antigens of infectious laryngotracheitis virus
defined by monoclonal antibodies. Archives of Virology, 115, 147-162.
VAN ZIJL, M., WENSVOORT, G., DE KLUYVER, E., HÜLST, M., VAN DER GULDEN, H., GIELKENS, A., BERNS,
A. & MOORMANN, R. (1991). Live attenuated pseudorabies virus expressing envelope glycoprotein El
of hog cholera virus protects swine against both pseudorabies and hog cholera. Journal of Virology, 65,
2761-2765.
RESUME
Laryngotrachéite infectieuse aviaire: interactions virus-hôte en relation
avec les possibilités d'éradication
Cet article de synthèse traite de la virologie, de l'immunologie et de la biologie moléculaire du
virus de la laryngotrachéite infectieuse (ILTV) et de ses interactions avec le poulet dans le but
390
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d'envisager la faisabilité de son eradication. L'établissement d'une phase de latence pendant
l'infection de l'hôte, son rôle primordial dans la survie biologique de l'ILTV et les relations
virus-hôte qui sont associées avec la réactivation de l'infection sont étudiés.
Plusieurs aspects de la biologie de l'ILTV dans son mode naturel d'infection peuvent être
utilisés pour éradiquer cet agent pathogène des fermes de production intensive de volailles, à
savoir le haut degré de spécificité de l'hôte pour l'ILTV, la nécessité de contact pour sa
dissémination, son infectivité courte en dehors du poulet, la stabilité du génome et l'absence
de variation antigénique significative. De plus, l'ILTV ne peut se répliquer abondamment dans
le principal organe cible, la trachée, lorsqu'il existe une immunité à médiation cellulaire
spécifique locale.
Des vaccins préparés par génie génétique, capables de générer une immunité sans les infections
latentes de l'ILTV induites par des souches vaccinales de l'ILTV vivantes modifiées conventionnelles, sont en cours de développement. Cet article démontre qu'en conjonction avec des
mesures de quarantine spécifiques à la ferme d'élevage et des mesures hygiéniques, de tels
vaccins peuvent être des outils technologiques permettant l'éradication, à l'approche de l'an
2000, de l'ILTV des sites de production et par la suite, des zones géographiques où est
pratiquée l'industrie de la volaille.
ZUSAMMENFASSUNG
Aviäre infektiöse Laryngotracheitis: Virus-Wirt-Interaktionen in bezug
auf Aussichten für die Tilgung
Die vorliegende Übersichtsarbeit untersucht die Virologie, Immunologie und Molekularbiologie des Virus der infektiösen Laryngotracheitis (ILTV) und seine Interaktionen mit dem Huhn
im Zusammenhang mit einer Beurteilung der Möglichkeit der Tilgung. Die Etablierung der
Latenzphase während der Infektion des Wirtes, deren zentrale Rolle beim biologischen
Überleben des ILTV und die Wirt-Virus-Ereignisse, die mit der Reaktivierung der Infektion
verbunden sind, werden betrachtet.
Auf der anderen Seite gibt es verschiedene Charakteristika der Biologie von ILTV bei seiner
natürlichen Infektionsweise, die beim Tilgen dieses Erregers von Plätzen mit intensiver
Geflügelproduktion ausgenutzt werden können. Diese umfassen die hochgradige
Wirtsspezifität von ILTV, die Abhängigkeit von Kontakt für die Ausbreitung, die kurzlebige
Infektiosität außerhalb des Huhnes und die Stabilität des Genoms mit dem Ausbleiben einer
signifikanten Antigenvariation. Ferner kann sich ILTV angesichts einer lokalen spezifischen
zeilvermittelten Immunität in seinem hauptsächlichen Zielorgan, der Trachea, nicht produktiv
replizieren.
Die Entwicklung gentechnologisch hergestellter Vakzinen, die fähig sind, eine Immunität zu
bewirken, aber ohne die latenten ILTV-Infektionen, die durch die konventionellen
modifizierten ILT-Vakzinestämme verursacht werden, ist jetzt gut im Gange. Die vorliegende
Arbeit postuliert, daß mit solchen Vakzinen, wenn sie in Verbindung mit Quarantäne- und
Hygienemaßnahmen angewendet werden, das technologische Instrument geliefert wird, das
erforderlich ist, um ILTV aus Produktionsstandorten zu tilgen und dann regional, in der
entwickelten Geflügelwirtschaft etwa ab dem Jahr 2000.
RESUMEN
Laringotraqueitis infecciosa aviar: interacciones hospedador-virus en
relación a la posibilidad de erradicar la enfermedad
Esta revisión abarca la virología, inmunología y biología molecular del virus de la laringotraqueitis infecciosa (ILTV) y sus interacciones con la gallina en el contexto de observar las
AVIAN INFECTIOUS LARYNGOTRACHEITIS
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posibilidades de erradicar. Se considerarán el establecieminto de la fase latente durante la
infección del hospedador, su papel central en la supervivencia biológica del ILTV y los eventos
virus-hospedador asociados con la reactivación de la infección.
En contrapartida, hay varias aspectos de la biología de la infección natural que pueden ser
explotados en la erradicación de este agente de las zonas de producción avícola intensiva. Estas
incluyen un grado elevado de hospedador-especificidad de ILTV, dependencia del contacto
para su diseminación, infectividad reducida fuera del pollo, estabilidad del genoma de ILTV
y la ausencia de una variación antigénica significativa. Además ILTV no puede replicarse
productivamente en su órgano proncipal, la tráquea, en presencia de una inmunidad celular
local específica.
Las vacunas de ingienería genética en vías de desarrollo son capaces de generar inmunidad sin
producir infecciones latentes por ILTV al contrario de lo que sucede con las vacumas
modificadas convencionales. Este artículo postula que las vacunas deingieneria genética, al ser
empleadas en conjunción con medidas específicas de cuarentena y medidas higiénicas, pueden
proporcionar las herramientas necesarias para erradicar ILTV de las áreas de producción
avícola y posteriormente regionalmente en industrias avícolas avanzadas alrededor del año
2000.