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Research on ILT


USPOULTRY recently circulated the results of Project 692 Modified Live Vaccines of Infectious Laryngotracheitis Virus.  The research was conducted by Dr. Calvin L. Keeler, Jr. of the University of Delaware.  It is accepted that chick-embryo-origin (CEO) vaccines against infectious laryngotracheitis are effective in suppressing clinical manifestations of infection. In contrast CEO vaccine is generally regarded as being responsible for field outbreaks of clinical ILT allegedly due to reversion to virulence by back-passage.  Studies conducted at the University of Georgia have demonstrated conclusively that most field “breaks” of ILT are attributed to CEO vaccine.


Dr. Keeler and his team evaluated a CEO ILT vaccine to determine the characteristics of the live virus component.  Two sub-populations of virus were identified, designated University of Delaware CEO D2 and D3.  Both were nonpathogenic in broiler chickens.  On successive passage in SPF chickens, embryonated eggs and liver cell cultures, viruses showed a decline in viability.  The attenuated strains of ILT virus were passaged twenty times in chicks without inducing heightened pathogenicity.  Passage in embryonated eggs or liver cell tissue culture could not be achieved.


On the basis of the laboratory experiments Dr. Keeler maintains that CEO vaccines do not revert to virulence under field conditions by back-passage and these strains have limited ability to propagate in avian hosts.


Since ILT virus vaccines represent “a mixture of genetically related viruses exhibiting differences in pathogenicity” a competitive situation may exist in which the virulent sub-populations in the vaccine dominate the apathogenic strains contributing to vaccine spread and increased virulence consistent with field experience.