The effect of serial in vivo passage on the expression of virulence and DNA stability of an infectious laryngotracheitis virus strain of low virulence.

The effect of 35 serial passages in vivo on an infectious laryngotracheitis virus strain of low virulence was examined in terms of effect on virulence and DNA stability. Within 3 passages in live chickens there was evidence of increasing respiratory distress. Severe respiratory distress (with death in some cases) was observed after the 6th passage, except when there appeared to be a transient decline in pathogenicity following short term storage of the virus inoculum at -70 degrees C. Restriction endonuclease analysis of viral DNA derived from the original inoculum and the final passage did not reveal any genomic alteration. It is postulated that there is a potential for live ILTV vaccines to cause outbreaks of clinical disease in the event of inadequate or incomplete vaccination procedures.