CLUSTAL X v1.83. Unrooted neighbor-joining trees were constructed using MEGA5.1. The complete ORF30 was sequenced for 41 US strains and 23 European strains, as well as the modified live virus (MLV) vaccine strain (Rhinomune, Pfizer Animal Health Inc.). Sequence data analysis demonstrated that all ORF30 sequences are very closely related, sharing 96.4-99.9% and 96.6-99.9% nucleotide (nt) and amino acid (aa) sequence identity to the published sequence of V592 strain of EHV-1 (GenBank number AY464052), respectively. However, numerous nonsynonymous amino acid substitutions were observed in all six domains of the viral DNA polymerase enzyme. Based on phylogenetic analysis of ORF30, EHV-1 isolates could be segregated into 4 distinct groups, regardless of their year of isolation or geographic location. Interestingly, one strain isolated from an equine placenta had significantly different sequences and this virus grouped very closely with an EHV9 isolate from the lung of an aborted zebra fetus. Results from this study indicate that additional mutations in ORF30 or other viral genes, as well as host or environmental factors have the potential to contribute to the development of neurologic disease in horses infected with EHV-1.