Infectious Langyngotracheitis is an acute or subacute highly infectious disease usually of adult poultry, characterised by respiratory distress, fits of coughing causing expulsion of blood and blood stained mucous and presence of haemorrhagic tracheitis. Sub acute form is characterized by diptheric mass adhered to larynx and trachea.
Distribution
The disease was first recognized in USA. The disease was also recorded in UK in 1925. It is present in the continent of Europe, Australia and New Zealand.
In India, Infectious Langyngotracheitis was first recorded by Singh (1964.) Serologically the disease is widely prevalent in all the states of India. The incidence of the disease is 5.1 to 70.4% in Rajasthan.
Aetiology
Infectious Langyngotracheitis is caused by a DNA virus under the family Herpes viridae and the subfamily alpha herpes viridae and currently designated as Gallid herpes virus. Hexagonal virion is measuring 80-100 micrometre in diameter. The virus is sensitive to heat, lipolytic agents and different disinfectants. The virus becomes non-infective after exposure for 24 hours to ether. The virus is destroyed in 10-15 minutes at 55° C. In dead birds, at 37° C the virus can remain viable for about 48 hours. Herpes virus strains appear to be antigenically homogenous. However, some strains are poorly neutralized by antisera. Viral strains vary greatly in their pathogenicity, some are highly pathogenic and others are mildly pathogenic or non-pathogenic.
Susceptible Hosts
The disease has a narrow host specificity. Chickens and pheasants are naturally infected. Fowls of all ages and all breeds are susceptible. The disease is mostly seen in birds of 3-9 months of age and Youngs are susceptible greatly. Heavier breeds are more susceptible than light breeds. The disease also occurs in concurrent infection with Infectious Langyngotracheitis, fowl pox, Newcastle disease, Haemophilus and Mycoplasma and deficiency of Vitamin A and excess ammonia in house may predispose he disease condition.
Mode of Transmission
The infection is transmitted in various ways:
- Aerosol transmission occurs from the exudates of nares, oropharynx, trachea and conjunctiva of the infected birds to the healthy birds through their upper respiratory tract and conjunctiva.
- The disease is also transmitted through ingestion of infected materials.
- Transmision may take place through contaminated litter, equipments, clothings, eggs or chicken handling appliances.
- Rats, crow, vulture etc., may act as mechanical carriers and thus spread the disease from one farm to others.
- Contaminated, ill ventilated poultry house may remain as source of transmission. The disease is most often noticed in autumn and winter when the ventilation remains to be inadequate.
- Birds recovered from outbreak may also act as carriers.
- Vaccinated birds may spread the disease to the non-vaccinated flocks for a number of weeks following vaccination.
- Transmission through eggs do not occur. The virus is inactivated in less than 24 hours at 37° C.
Pathogenicity
Incubation period generally ranges from 6-12 days after natural exposure. Mortality varies from 5-70% as stated by Seddon (1935).
Clinical Findings
The disease occurs in per acute, acute, mild or asymptomatic form.
In per acute form, there is sudden death of the birds without showing any signs or in some cases acute dyspnoea, coughing, expectoration of blood clots or blood stained or mucous containing exudate from nares or oropharynx. Birds usually die within 1-3 days.
In acute form, birds will show severe respiratory distress. There is sneezing, coughing and rattling sounds. Cough may appear in paroxysms. The cough may result in expulsion of bloody mucous from the trachea. Blood may stain the beak and the neck feathers. The head and neck remain extended and beak opened during each inspiration. There may be cyanosis of comb and wattles.
Conjunctivitis and ocular discharge specially from the anterior canthus are evident. There is drop in egg production in laying birds. Course of the disease is about 2-4 weeks. Death of the birds are due to asphyxiation as a result of occlusion of trachea by blood and mucous.
In mild forms, there is moist rales, slight coughing, head shaking, nasal discharge, conjunctivitis, unthriftyness and drop in egg production.
Lesions
The lesions depend upon the severity of infection. In per-acute cases there is haemorrhagic tracheitis and trachea contains blood casts and blood stained mucous throughout its whole length. Mucous membrane of larynx and trachea shows oedema, haemorrhage and necrosis.
In acute cases, caseous, diptheric exudate, mucous and some haemorrhages found in trachea and usually cause obstruction in laryngeal and syrinx regions. There is no lesion in lungs and air sac, unless secondary complication occurs. Haemorrhagic lesions may be noted in conjunctival and nasal mucosa.
Histologically, there is destruction of epithelial layer of respiratory tract leading to sloughing and extensive ulceration. Cellular infiltration and mucosal degeneration is most extensive in trachea and larynx. Intra nuclear inclusion bodies are present within 12 hours of post infection.
Diagnosis
It is based on the following considerations:
- Characteristic clinical signs.
- Post-mortem and microscopic changes.
- Isolation and propagation of the virus in chorioallantoic membrane as stated by Prasad and Malik (1968) and Sharma and Agarwal (1973).
- Gel diffusion test
- Serum virus neutralization test in chick embryo and cell culture
- Fluorescent antibody technique is considered to be most sensitive.
- Demonstration of intranuclear inclusion bodies in the tracheal epithelium or CAM.
- Reproduction of the disease in susceptible chicks by intratracheal or intrasinus route.
- Antibodies to Infectious Langyngotracheitis virus may be demonstrated by agar-gel precipitation, indirect immunofluorescence on infected cell culture, pock plague reduction test or ELISA.
- Examination of exudate with DNA probes with or without the use of polymerase chain reaction.
Treatment
There is no specific and effective treatment. Broad spectrum antibiotics may be used to check secondary bacterial invaders.
Control
- Prompt accurate diagnosis and segregation of the affected birds are necessary to prevent further spread of infection.
- Vaccination of unaffected birds may confer sufficient protection to keep the rest of the birds in the house from contracting the disease. Infectious Langyngotracheitis virus is attenuated by passage in cell culture or embryonated eggs or by feather follicle passage in chickens. In indegenous areas, birds may be vaccinated at 1-3 days of age and in other areas, it is to be done 3-18 weeks of age. Vaccination may be achieved by eye drop, coarse spray, inclusion in the drinking water or cloacal scarification. In India, no commercial vaccine is available. Srinivasan (1976) have developed an egg embryo adopted vaccine from mild Infectious Langyngotracheitis strain.
- Care should be extended to avoid mixing of vaccinated or recovered chickens with susceptible one as vaccination often give rise to carrier birds.
- Vaccination of replacement pullets should be encouraged at 6 weeks or more age.
