Infectious laryngotracheitis (ILT) is an economically important respiratory disease of poultry. This highly contagious disease is caused by Gallid alpha herpesvirus type 1 (GaHV-1), commonly known as infectious laryngotracheitis virus (ILTV). The virus can be easily transmitted by infected birds and fomites. Lax biosecurity, transportation of infected birds, and spread of contaminated litter facilitates spread of the virus. Clinical signs of respiratory disease are not pathognomonic. Diagnosis is by real-time PCR and histopathology . Implementation of biosecurity is necessary for prevention, but vaccination is commonly used for control of the disease in endemic regions worldwide.
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 of the USA in which poultry are intensively reared, as well as from many other countries.
In the acute form of infectious laryngotracheitis virus, gasping, coughing bloody mucoid exudate, 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. 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 weeks, 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 infected birds or contaminated equipment and litter.
The acute form of infectious laryngotracheitis virus is characterized by the presence of blood, mucus, yellow caseous exudates, or a hollow caseous cast in the trachea. Microscopically, the acute phase of the severe form of the disease is characterized by a desquamative, necrotizing tracheitis and conjunctivitis. The mild forms of the disease are characterized by discrete hemorrhagic areas in the upper trachea and larynx and mild conjunctivitis. A rapid diagnosis of the disease can be achieved by the detection of lesions that are pathognomonic of the infection, such as syncytial formation and intranuclear inclusion bodies in the trachea and conjunctiva mucosal epithelium. This diagnosis can be rapidly confirmed by detection of viral DNA using virus-specific PCR assays.
Rapid and accurate diagnosis of the disease is central for the establishment of swift control measures. Although not pathognomonic, the diagnosis is initiated by the recognition of clinical signs and gross lesions of the disease. Laboratory diagnosis includes detection of microscopic lesions characteristic of ILTV replication, detection of viral DNA or viral antigen from upper respiratory tissues, and ultimately, virus isolation.
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 analysis of the sequences obtained. More recently, field isolates and vaccines strains have been differentiated more accurately by full genome sequencing analysis. Genotyping of the virus is optional in the diagnosis of ILT. Genotyping analysis answers whether the virus originated from previously used live attenuated vaccines, if it is related to previous outbreak strains, or if it is a new field strain.
In endemic areas and on farms where a specific diagnosis is made, infectious laryngotracheitis virus 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, subcutaneous, or wing-web vaccination.