Infectious Laryngotracheitis Virus: Molecular Biology, Pathobiology, and Control Strategies

Infectious laryngotracheitis (ILT) is an economically important respiratory disease of chickens that is prevalent throughout the world. It is caused by an infectious laryngotracheitis virus (ILTV), also named Gallid alphaherpesvirus 1 (GaHV-1). It is a member of the genus Iltovirus, subfamily Alphaherpesvirinae, family Herpesviridae and order Herpesvirales. The ILTV genome is a linear double-stranded DNA molecule with an average genome length of 151,607 nt. Twelve herpes simplex virus -1 homologue genes have been identified in the ILTV genome, with seven of them, glycoproteins B, C, H, K, L, M and N, present in the UL region, while glycoproteins D, E, G, I and J are present in the US region of the ILTV genome. Although chicken is the natural host of ILTV, infections have also been reported in pheasants, pheasant-chicken crosses, peafowls, turkeys, and ducks. An incubation period of 3–12 days is followed by an acute phase of the infection which lasts 1–2 weeks. During this phase, the virus replication occurs in the conjunctiva, trachea, and larynx, resulting in conjunctivitis, gasping, coughing, and expectoration of blood-mixed mucus. ILTV infection results in decreased weight gain and egg production. It causes 0 to 80% mortality depending on the virulence of the strain involved. Like other herpesviruses, ILTV establishes latent infection in trigeminal ganglia and virus reactivation and shedding occur following various stress factors. ILTV infections are generally diagnosed by the typical clinical signs and detection of intranuclear inclusion bodies in the affected tissues. Furthermore, the detection of virus-specific antigen by fluorescent antibody, immunohistochemical staining of smears and tissues, detection of DNA by a polymerase chain reaction, and virus isolation by inoculating embryonated chicken eggs or cell cultures can also be performed. Virus neutralization assays and different types of ELISAs have also been established. To control ILTV infections, a combined effort is required encompassing prompt disease diagnosis, the use of geographic information system technology, biosecurity, vaccination, differentiation of infected from vaccinated (DIVA), and eradication of reservoir hosts.