It is a slow, laterally spreading respiratory disease of chickens, pheasants, peafowl and turkeys caused by a herpes virus with high morbidity but low to medium mortality. There is a marked variation in the pathogenicity of various strains of the virus. Three major forms - the peracute, the subacute and the mild or chronic forms are known. Infection is usually spread by aerosol. The route of infection is through the upper respiratory tract, where the virus replicates in the trachea and the larynx. Other portals of entry include contact of the virus with the eyes and ingestion of the virus. Mechanical transmission can occur via contaminated equipment, clothing, footwear and litter. Recovered and vaccinated birds are long-term carriers. Transmission between farms can occur by airborne particles or fomites. The virus is highly resistant outside host but is susceptible to disinfectants.
Symptoms and lesions: Once infected, a chicken will usually become sick within 2 weeks. Signs of the disease are nasal and eye discharge, moist-sounding breathing, coughing and gasping. In severe cases, heavy breathing and coughing up of bloody mucus are seen. Sometimes blood is apparent on the walls where affected birds have been coughing. Classical signs are gasping, coughing and sticking the neck forwards and upwards with each breath in an effort to clear mucus which builds up in the trachea. The percentage of birds affected can range up to 100% while death occurs usually in
5-30% of the flock. Birds may recover from the illness within 2 weeks but can remain carriers of the virus for long periods of time afterwards. These carrier birds become a threat for other poultry owners. Stress also helps to bring out the disease as it makes carrier birds shed the virus. Mixing new and old birds together, poor ventilation, inadequate space or food, or changes in temperature may all help in perpetuating the disease.
Diagnosis: Laboratory diagnosis will always be necessary to determine the presence of ILT virus. Primary cell cultures of chicken embryo kidney and chicken embryo lungs can also be used for cultivation. On the CAM of developing chicken embryo, the virus produces pock lesions, the size of which varies according to the virulence of the virus. Unlike the pocks of pox virus, ILT pocks have a depressed centre and raised periphery. In cell culture, the virus produces CPE characterized by syncytium formation with intranuclear inclusion bodies. Detection by FAT or PCR would be confirmatory. Sera may be examined by ELISA.
Prevention and control: Strict biosecurity is the only method of prevention. Although the serological evidence has been shown by some workers in India, there is no proof of clinical disease in the country.