Elephant Endotheliotropic Herpesviruses EEHV1A, EEHV1B and EEHV2 from Cases of Hemorrhagic Disease are Highly Diverged from Other Mammalian Herpesviruses and May Form a New Subfamily

Abstract

A family of novel endotheliotropic herpesviruses (EEHV) assigned to the genus Proboscivirus have been identified as the cause of fatal hemorrhagic disease in 70 young Asian elephants worldwide. Although EEHV cannot be grown in cell culture, we have determined a total of 378-kb of viral genomic DNA sequence directly from clinical tissue of six lethal cases and two survivors. Overall, the data obtained encompasses 57 genes, including orthologues of 32 core genes common to all herpesviruses, 14 genes found in some other herpesviruses, plus ten novel genes including a single-large putative transcriptional regulatory protein (ORF-L). Based on differences in gene content and organization plus phylogenetic analyses of conserved core proteins that have just 20% to 50% or less identity to orthologues in other herpesviruses, we propose that EEHV1A, EEHV1B and EEHV2 could be considered a new Deltaherpesvirinae subfamily of mammalian herpesviruses that evolved as an intermediate branch between the Betaherpesvirinae and Gammaherpesvirinae. Unlike cytomegaloviruses, EEHV genomes encode RRB, TK and UL9-like OBP proteins and have an alphaherpesvirus-like dyad-symmetry ori-Lyt domain. They also differ from all known betaherpesviruses by having a 40-kb large-scale inversion of core gene blocks I, II and III. EEHV1 and EEHV2 DNA differ uniformly by more than 25%, but EEHV1 clusters into two major sub-groups designated EEHV1A and EEHV1B with ancient partially chimeric features. Whereas large segments are nearly identical, three non-adjacent loci totalling 15-kb diverge by between 21 and 37%. One strain of EEHV1B analyzed is interpreted to be a modern partial recombinant with EEHV1A. Importance: Asian elephants are an endangered species whose survival is under extreme pressure in wild range countries and whose captive breeding populations in zoos are not self-sustaining. In 1999, a novel class of herpesviruses called EEHVs was discovered that have caused a rapidly lethal hemorrhagic disease in 20% of all captive Asian elephant calves born in zoos in the USA and Europe since 1980. The disease is increasingly being recognized in Asian range countries as well. These viruses cannot be grown in cell culture, but by direct PCR DNA sequence analysis from segments totalling 15-30% of the genomes from blood or necropsy tissue of eight different cases, we have determined that they not only fall into multiple types and chimeric subtypes of a novel Proboscivirus genus, but propose that they should also be classified as the first examples of a new mammalian herpesvirus subfamily named the Deltaherpesvirinae.