A Single-Nucleotide Polymorphism in a Herpesvirus DNA Polymerase Is Sufficient to Cause Lethal Neurological Disease

Epidemiological studies have shown that a single-nucleotide polymorphism in the equid herpesvirus type 1 DNA polymerase gene is associated with outbreaks of highly lethal neurological disease in horses. Reverse genetics experiments further demonstrated that a G(2254) A(2254) nucleotide mutation introduced in neurovirulent strain Ab4, which resulted in an asparagine for aspartic acid substitution ( D-752 N-752), rendered the virus nonneurovirulent in the equine. Here, we report that the nonneurovirulent strain equid herpesvirus type 1 strain NY03 caused lethal neurological disease in horses after mutation of A(2254) G(2254) (N-752 D-752), thereby providing final proof that the D-752 allele in the viral DNA polymerase is necessary and sufficient for expression of the lethal neurovirulent phenotype in the natural host. Although virus shedding was comparable between the N-752 and D-752 variants, infection with the latter was accompanied by efficient establishment of prolonged cell-associated viremia in peripheral blood mononuclear cells and neurological disease in 2 of 6 animals.