Differentiation between virulent and avirulent strains of infectious laryngotracheitis virus by DNA:DNA hybridization using a cloned DNA marker.

The DNAs of virulent and avirulent strains of infectious laryngotracheitis virus (ILTV) showed greater than 96% homology by reciprocal DNA:DNA hybridization. Nevertheless, by use of selected restricted DNA fragments, it was possible to differentiate strains according to pathotype more readily than by the more laborious restriction enzyme analysis. Restricted DNA fragments were successfully cloned into Escherichia coli HB101 cells and could be used not only for pathotyping ILTV strains but also for their differentiation from other avian viruses.