Isolation of equine herpesvirus 3 (EHV-3) from equine coital exanthema of two stallions and sero-epidemiology of EHV-3 infection in Japan

In the spring of 2015, two stallions reared in Farms A and B in Hokkaido in Japan showed symptoms of equine coital exanthema. Equine herpesvirus 3 (EHV-3) was isolated from penis swab samples of both stallions, and the isolates from each stallion in Farms A and B were designated as SS-1 and YS-1 strains, respectively. BamHI restriction profiles of SS-1 and Japanese reference strain Iwate-1 were indistinguishable, but the BamHI-A fragment of YS-1 was larger than those of SS-1 and Iwate-1 by 1.9 kbp because of the lack of two BamHI sites. Nucleotide sequence analyses of glycoprotein G (gG), gB, gC and VP13/14 coding regions revealed that SS-1 and YS-1 had 99.77% to 100% identities to each other. These results suggested that the origins of SS-1 and YS-1 were different. For a sero-epidemiological survey, serum neutralizing tests using SS-1 against 319 sera of horses from eight farms in Hokkaido were conducted. Six of the eight farms were EHV-3 antibody-positive, and positive rates ranged from 2.6% to 17.6%. To determine the infection time of four EHV-3 antibody-positive horses, a retrospective study was conducted. Infection time of the four horses was in the breeding season, and re-infection or reactivation of latently infected EHV-3 might have occurred in one horse. However, these four horses had never shown any clinical symptoms. The results suggested that several EHV-3 strains are distributed in Japan and that infection is maintained widely in horses without clinical symptoms.