Infectious laryngotracheitis: Assessing protection & vaccination programs

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Laryngotracheitis (LT) is an acute respiratory disease that affects poultry production worldwide. The disease is caused by Gallid alphaherpesvirus type I (GaHV-1), better known as infectious laryngotracheitis virus (ILT).
The main characteristics of the disease in its most acute form are severe respiratory distress accompanied by hemorrhagic tracheitis, mucoid rhinitis, and conjunctivitis. The disease is controlled through vaccination and biosecurity measures.
GENERAL CHARACTERISTICS OF VACCINES AGAINST LT
Live Attenuated Vaccines
Chicken embryo vaccines (CEO)
The vaccines manufactured from chicken embryo origin (CEO) were the first to be developed and used to control the disease.
Despite its high potential to revert to a virulent state (Fig. 1 and 2) and cause post-vaccination reactions and losses in production, embryo vaccines continue to be in force due to the excellent protection they offer. In the United States, in high-challenge and high-density poultry areas during severe disease outbreaks, CEO vaccines are an important tool in disease control.
Tissue Culture Vaccines (TCO)
Vaccines manufactured from tissue culture origin (TCO), were the second ones to emerge to control this disease.
This vaccine was attenuated by means of successive passages in tissue culture. Unlike embryo vaccines, the potential of the TCO to reverse its level of virulence and to generate post-vaccination reactions is much more limited (Fig. 1 and 2) .
For better protection, the cell culture vaccine should be administered by eye drop and not by massive methods (drinking water, spraying).
Recombinant Vaccines
Recombinant Herpesvirus Vaccine   Turkey – Laryngotracheitis (rHVT-LT)
Recombinant vaccines in turkey herpesvirus vector with laryngotracheitis or laryngotracheitis antigens plus Newcastle virus (rHVT-LT & rHVT-ND-LT) constitute another alternative for disease control. Unlike live attenuated vaccines, recombinant HVT vector vaccines do not gain virulence, do not induce respiratory reactions, and are not transmitted from bird to bird. Therefore, they are low-risk vaccines and very safe to use.  Also, recombinant LT vaccines in HVT vector induce long-lasting immunity and have the advantage of being able to be supplied in the hatchery, avoiding additional handling in the field.
Recombinant Smallpox Vaccine   avian – Laryngotracheitis (rFPV-LT)
Recombinant smallpox vector vaccines (FPV) were approved for commercial use ...