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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. It has been reported from most areas of the USA in which poultry are intensively reared, as well as from many other countries.
Clinical Findings
In the acute form, gasping, coughing, 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. The mouth and beak may be bloodstained from the tracheal exudate. 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 wk, although birds may cough for 1 mo. Strains of low virulence produce little or no mortality with mild respiratory signs and lesions and a slight decrease in egg production.
After recovery, some birds remain carriers for extended periods 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 birds in contaminated crates, and the practice of litter spread in pastures is believed to be related to epidemics of the disease.
Diagnosis
The acute disease is characterized by the clinical signs and by finding blood, mucus, and yellow caseous exudate or a hollow caseous cast in the trachea. Microscopically, a desquamative, necrotizing tracheitis is characteristic. In the subacute form, punctiform hemorrhagic areas in the trachea and larynx, and conjunctivitis with lacrimation permit a presumptive diagnosis. In uncomplicated cases, the air sacs usually are not involved. The diagnosis may be confirmed by demonstrating intranuclear inclusion bodies in the tracheal epithelium early in the course of the disease; by isolating and identifying the specific virus in chick embryos, tissue culture, or chickens; or by inoculating the infraorbital sinus or vent of known immune and susceptible birds. Chicken embryos are preferred for virus isolation. Chorioallantoic membrane of developing chicken embryos (9–12 days old) is inoculated with the specimen. Microscopic examination of the chorioallantoic membrane lesion shows intranuclear inclusions. ILT must be differentiated from the diphtheritic form of fowlpox (see Fowlpox), especially with tracheal lesions. Fowlpox virus produces intracytoplasmic inclusions.
Field isolates and vaccine strains of ILT virus can be compared by restriction endonuclease analysis of viral genomes. This method is useful for comparing closely related DNA genomes and in the epidemiology of the disease. However, significant differences may not be apparent between field and vaccine strains analyzed by this method.
PCR, which can amplify ILT virus genomic DNA sequences of various sizes using specific primers, is useful when an extremely small amount of viral DNA is present with cellular DNA in the sample. Restriction fragment length polymorphism of PCR products with restriction enzymes or sequencing analysis of PCR products is frequently used to differentiate strains.
Prevention and Treatment
Some relief from signs is obtained by keeping the birds quiet, lowering the dust level, and using mild expectorants, being careful that they do not contaminate feed or water. Vaccination should be practiced in endemic areas and on farms where a specific diagnosis is made.
Immediate vaccination of adults in the face of an outbreak shortens the course of the disease. Vaccination is best done with modified live attenuated vaccines. After consecutive passages in chicken embryos or chicken cells, these low virulence strains are applied to the conjunctiva by eye drop. Results are less consistent with mass methods of vaccination such as spray or drinking water administration. Broiler flocks in some areas where the disease is endemic must be vaccinated when young, but this is unlikely to be effective if done at <4 wk of age. Some vaccine producers recommend revaccination when birds are to be held to maturity. A new generation of ILT recombinant vaccines, including recombinant fowlpox and herpesvirus of turkey viral-vectored vaccines expressing genes from ILT virus, are commercially available for in ovo, subcutaneous, and wing-web vaccination.
Last full review/revision March 2012 by Maricarmen Garcia, BS, MS, PhD
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