Isolation and characterization of encephalitic bovine herpesvirus type 1 isolates from cattle in North America.

Nine CNS bovine herpesvirus type 1 (BHV-1) isolates, recovered from bovine brain samples submitted to the Texas Veterinary Medical Diagnostic Laboratories from 1974-1989, were compared by analyzing their DNA restriction endonuclease (RE) fragment migration pattern. Seven had pattern similar to that of the respiratory BHV-1 Cooper strain. The remaining 2 isolates, however, had variant patterns, similar to that of each other, but completely different from patients for the other 7. The RE patterns of these 2 variants were similar to published RE patterns for 2 encephalitic or neuropathogenic BHV-1 strains--the Australian N-569 strain and the Argentine A-663 strain. One of the Texas encephalitic variants (No. 30326) was isolated from the CNS of a calf that died during an epizootic of encephalitis in 1974. The other, designated TX-89, was isolated in 1989 from the CNS of a 7-month-old feedlot steer with acute fatal encephalitis. Microscopic lesions of encephalitis with neuronal degeneration and intranuclear inclusions were observed for 3 of the 9 isolates, the 2 variant isolates (No. 30326 and TX-89), and a respiratory isolate. The remaining 6 CNS isolates, all respiratory subtypes, were recovered from cattle that did not have clinical CNS disease or gross or microscopic CNS lesions; in 5 of these cattle, virus was recovered from at least 1 other organ (lungs) besides the CNS. We conclude that the CNS of calves can be naturally infected with 2 distinct BHV-1 subtypes, the respiratory and the encephalitic, and that the encephalitic subtype (subtype 3 or BHV-1.3) has been present in Texas cattle since at least 1974.