Download Infectious Laryngotracheitis in Poultry Prof.Dr. Salah M. Hassan

Infectious Laryngotracheitis
in Poultry
Infectious laryngotracheitis (ILT) is an acute, highly
contagious, herpesvirus infection of chickens and
pheasants characterized by severe dyspnea,
coughing, and rales.
It can also be a subacute disease with nasal and
ocular discharge, tracheitis, conjunctivitis, and mild
rales.
The disease is caused by Gallid herpesvirus I,
commonly known as infectious laryngotracheitis
virus (ILTV).
It has been reported from most areas in which
poultry are intensively reared, as well as from
many other countries.
Clinical Findings:
In the acute form, gasping, coughing, rattling, and
extension of the neck during inspiration are seen 5–12
days after natural exposure.
Reduced productivity is a varying factor in laying flocks.
Affected birds are anorectic and inactive. The mouth
and beak may be bloodstained from the tracheal
exudate.
Mortality varies but may reach 50% in adults and is
usually due to occlusion of the trachea by hemorrhage
or exudate.
Signs usually subside after ~2 wk, although some birds
may show signs for longer periods.
Strains of low virulence produce little or no mortality
with mild respiratory signs and a slight decrease in
egg production.
After recovery, birds remain carriers for life and
become a source of infection for susceptible
birds.
The latent virus can be reactivated under stressful
conditions.
Infection also may be spread mechanically.
Several epidemics have been traced to the
transport of birds in contaminated crates, and
the practice of litter spread in pastures is
believed to be related to epidemics of the
disease.
Diagnosis:
The acute disease is characterized by the presence of
blood, mucus, yellow caseous exudates, or a hollow
caseous cast in the trachea.
Microscopically, a desquamative, necrotizing tracheitis
is characteristic of acute disease.
In the subacute form, punctiform hemorrhagic areas in
the trachea and larynx and mild conjunctivitis with
lacrimation may be detected.
A rapid diagnosis can be achieved by detection of
intranuclear inclusion bodies in the tracheal
epithelium early in the course of the disease;
results of the microscopic examination can be
rapidly confirmed by detection of viral DNA using
virus-specific PCR assays.
Isolation and identification of the virus is done in
chicken embryos or tissue culture from embryo
liver or kidney cells or from kidney cells from
adult chickens. Chicken embryos are preferred for
virus isolation.
Chorioallantoic membrane of developing chicken
embryos (9–12 days old) is inoculated with the
specimen.
Microscopic examination of the chorioallantoic
membrane lesion shows intranuclear inclusions.
On microscopic examination of the trachea,
intranuclear inclusion lesions produced by ILTV
infection must be differentiated from the
diphtheritic form of fowlpox infection that
produces intracytoplasmic inclusions.
Field isolates and vaccine strains of ILTV are
routinely differentiated by PCR amplification
of single or multiple ILTV genome areas,
followed by sequencing of the PCR products
and comparative analysis of the sequences
obtained.
More recently, field isolates and vaccines strains
have been differentiated more accurately by
full genome sequencing analysis.
Control:
In endemic areas and on farms where a specific diagnosis
is made, the disease is controlled by implementation of
biosecurity measures and vaccination.
Vaccination is done with live attenuated vaccines and
viral vector recombinant vaccines.
Live vaccines originated from virulent isolates that were
attenuated by consecutive passages in embryos or
tissue culture. These are applied via eye drop or
through mass vaccination by water or spray.
Viral vector recombinant vaccines in fowlpox and
herpesvirus of turkeys have been designed to express
ILTV immunogenic proteins and are administered to
individual birds by in ovo, SC, or wing-web vaccination.