Avian infectious laryngotracheitis

Incubation is about one to two weeks. In the peracute form spread is rapid: birds stretch the neck and gasp to breathe, make gurgling or rattling sounds, cough and may expel bloody mucus that stains walls and floors, and some die in good condition before signs appear. The subacute form develops over days with lower mortality. Mild or chronic cases show coughing, conjunctivitis, nasal, ocular and oral discharge and reduced egg production. At necropsy lesions are confined to the upper respiratory tract: haemorrhagic tracheitis with blood clots, mucoid rhinitis, or yellow caseous diphtheritic membranes on the larynx and upper trachea.

Infectious laryngotracheitis is caused by gallid alphaherpesvirus 1 (ILT virus), genus Iltovirus, subfamily Alphaherpesvirinae. The virus enters through the upper respiratory tract and eye and replicates mainly in the tracheal epithelium, destroying it and causing haemorrhagic, then fibrinous or caseous, tracheitis; the resulting plugs of blood and exudate can suffocate birds. Infected epithelial cells form syncytia and carry intranuclear inclusion bodies. Strains differ in their preference for trachea or conjunctiva. As in other alphaherpesviruses, the virus establishes latency, chiefly in the trigeminal ganglion and also in tracheal tissue, and is re-excreted later without clinical signs. Protection depends on cellular rather than antibody responses.

The disease is principally one of chickens; pheasants, partridges, peafowl and turkeys are also reported infected. Birds of all ages are susceptible, though most outbreaks are in chickens over three weeks old. Transmission is by close contact and respiratory droplets through the eye and upper airway, most readily from acutely ill birds but also from recovered latent carriers; there is no egg transmission. Spread is slower in cage houses than in loose-housed flocks. Live vaccine strains can regain virulence or recombine into new virulent variants. In Egypt, outbreaks since 2007 have involved strains related to chicken-embryo-origin vaccine virus, and a 2018-2019 survey of 30 farms found vaccine-like and recombinant strains dominant. There is no known human infection risk; ILT is WOAH-listed.

Newcastle disease can also cause severe tracheitis and must be excluded by laboratory testing. Mild ILT cannot be separated clinically from other mild respiratory diseases of chickens. Diphtheritic lesions in the mouth and larynx must be distinguished from the wet form of fowlpox. Tracheal histopathology showing intranuclear inclusions and syncytia, with real-time PCR, confirms ILT; PCR with sequencing separates vaccine from field strains.

Larynx, trachea, nasal cavity, conjunctiva, trigeminal ganglion (latency)

Live attenuated (egg- or tissue-culture origin) or recombinant vectored vaccines, weighing reversion, recombination and latency risks since vaccinated birds can spread virus to naive flocks; biosafety measures; rapid diagnosis and culling in outbreaks.

Peracute form: high morbidity, mortality can exceed 50% (WOAH); subacute 10-30%; chronic cases 1-2% of a flock. A review gives 5-70% by strain. Survivors can remain latent carriers that shed virus again.