The complexity of infectious laryngotracheitis virus: a focus on vaccination and challenge viruses
Infectious laryngotracheitis virus (ILTV) causes respiratory distress, decreased egg production, conjunctivitis, and death in chickens. ILT can be prevented by vaccination. Traditionally, that vaccination is with a chicken embryo origin (CEO) vaccine. However, CEO vaccines can revert to virulent ILTV and cause outbreaks. One of the successes of the recombinant vaccines with gene inserts for ILT is that they do not revert to virulence, but do prevent ILT symptoms. However, these same recombinant vaccines do not necessarily prevent replication of ILTV in the trachea of chickens, which then can lead to ILT outbreaks.
One of the regulations for ILT vaccines is that they must protect chickens against inoculation with an ILT challenge virus. All licensed vaccines in the U.S. are licensed by the United States Department of Agriculture’s (USDA) Center for Veterinary Biologics (CVB). The CVB provides ILT challenge virus to biologics companies for efficacy testing of vaccines. The current USDA challenge virus strain was generated through egg passage from a 1960s strain of ILTV. Recently, the CVB was able to acquire an ILT field isolate from an outbreak in a laying hen flock that had been vaccinated with a recombinant ILT vaccine.
Mortality, clinical signs and pathogenicity of this field isolate were compared to the current USDA challenge virus. An interesting aspect of the research was that several asymptomatic chickens still had tracheal lesions, both from the field isolate and current challenge virus, indicating viral replication without clinical signs. This is not entirely surprising due to research indicating similar results with some recombinant ILT vaccines, but it does reinforce that prevention of symptoms does not equal prevention of viral replication.
The ILT field isolate produced clinical signs and tracheal lesions similar to the USDA challenge virus, and the field isolate was able to cause symptoms at a lower infectious douse then the USDA challenge virus to achieve the same results. These results indicate that the ILT field isolate is a viable candidate for use in vaccination challenge studies. Future work will need to be undertaken to perform genetic analysis of the ILT field strain isolated by the USDA and other field strains to determine genetic relationship to vaccine strains.