In a recently completed research project, Dr. Calvin Keeler at the University of Delaware studied the virus subpopulations in an infectious laryngotracheitis (ILT) chicken embryo-origin (CEO) vaccine to try to understand why these types of vaccines appear to cause disease in some vaccinated flocks.

The U.S. Poultry & Egg Assn. (USPOULTRY) and USPOULTRY Foundation recently announced the completion of the project.

According to USPOULTRY, Keeler found that the non-pathogenic subpopulations located within a CEO ILT vaccine spread poorly in chickens, which likely allows the more virulent subpopulations in the vaccine to spread and cause signs of the disease.

In a summary, Keeler noted that ILT -- an acute respiratory disease caused by an avian alpha herpesvirus, ILT virus (ILTV) -- is of great concern to the poultry industry.

According to Keeler, the poultry industry does not universally embrace the use of live-attenuated CEO vaccines to control this disease, because CEO ILT vaccines can exhibit significant pathogenicity in young birds, with an associated economic cost.

He said the observed pathogenicity of CEO ILT vaccines is inconsistent in the field, leading to a widely held belief that the illness seen in chickens following the use of CEO ILT vaccines results from improper administration and subsequent vaccine spread and back-passage. Vaccine back-passage is the enhancement of vaccine virulence as it spreads from bird to bird. Therefore, better understanding the cause of severe vaccine reactions following the use of CEO ILT vaccines is an important industry need, Keeler said.

CEO ILT vaccines have been shown to be a mixture of viruses with differing biological properties. In this study, Keeler said two subpopulations of virus from a commercial CEO ILT vaccine -- designated UDCEOD2 and UDCEOD3 -- were found to be non-pathogenic in broiler chickens. The specific objective of this research was to characterize these ILTV strains exhibiting reduced pathogenicity.

Keeler reported that the viruses were found to lose viability upon passage in birds, in embryonated eggs and in primary liver cell cultures. Two attempts were made to evaluate the dose and route of inoculation for vaccine studies. In both cases, Keeler said the titer of the virus was found to be too low to provide protection. Without the ability to establish immunity, experiments to evaluate the onset and duration of immunity were not possible.

These highly attenuated strains of ILTV were passaged 20 times in birds, he noted, but they did not increase in pathogenicity and demonstrated a decreased ability to replicate. It was not possible to passage these viruses multiple times in either embryonated eggs or in liver cell tissue culture.

According to Keeler, the results support the hypothesis that ILTV CEO vaccines do not revert to virulence upon passage in birds. Highly attenuated strains of ILTV appear to have a limited ability to propagate in birds.

Therefore, since vaccine stocks have been shown to be composed of a mixture of genetically related viruses exhibiting differences in pathogenicity, Keeler concluded that "reversion to virulence" may represent a competitive situation where more infectious (virulent) subpopulations in the CEO ILT vaccine outcompete those that are non-pathogenic and less able to replicate when improper vaccine administration allows vaccine spread and back-passage in the field.