Genetic relatedness of epizootic hemorrhagic disease virus serotype 2 (EHDV-2) from the 2012 outbreak in the United States

During summer and early fall of 2012, the United States experienced the largest outbreak of hemorrhagic disease (HD) on record; deer (both Odocoileus virginianus and Odocoileus hemionus) in 35 states were affected including... [ view full abstract ]

During summer and early fall of 2012, the United States experienced the largest outbreak of hemorrhagic disease (HD) on record; deer (both Odocoileus virginianus and Odocoileus hemionus) in 35 states were affected including many northern states where HD typically does not occur. Epizootic hemorrhagic disease virus (EHDV) was the predominant virus isolated, with serotype 2 (EHDV-2) representing 66% (135/205) of all isolated viruses. The large number of EHDV-2 isolates from a single widespread outbreak provided an ideal opportunity to genetically determine if this outbreak resulted from multiple or a single genotype of EHDV-2. It is well known that viruses within the EHDV serogroup are genetically similar, but we hypothesized that subtle genetic distinctions between viruses would exist across the geographic range of the outbreak if multiple EHDV-2 strains were responsible. By sequencing the mammalian binding protein (VP2) gene and the insect vector binding protein (VP7) gene of 34 2012 EHDV-2 isolates from 21 states, viral relatedness and molecular epidemiology of the outbreak were examined. VP2 nucleotide sequences had 99.0% pairwise identity; VP7 nucleotide sequences had 99.1% pairwise identity. Very few changes were observed in either protein at the amino acid level. Despite the high genetic similarity between isolates, subtle nucleotide differences existed. The VP7 genes separated into two distinct clades based on two independent single nucleotide polymorphisms (SNPs). The clades were also divided geographically into Mid-Atlantic and Western clades. The VP2 genes exhibited more variation, but genotypes defined by subtle nucleotide differences contributed to a greater resolution on the phylogenetic tree. Based on these data, we conclude that multiple EHDV-2 strains contributed to this outbreak.