Sero- and Molecular-Epizootiological Studies on Equine Herpesvirus Type 1(EHV-1) Infection among Race Horses: an Occurrence of Respiratory Disease with Nervous Disorders.

Abstract

Among race horses raised and trained at Ritto Training Center of the Japan Racing Association, an epizootic of respiratory disease was observed from January to February in 1989. During the epizootic, a total of 155 out of approximately 2, 000 race horses developed pyrexia. Following the respiratory symptoms, 7 Thoroughbred horses showed various degree of nervous disordersuch as incoordination of the hind limbs to recumbency with urinary incontinence. Previous study briefly reported a result of virus isolation, in which 10 EHV-1 strains were isolated from febrile horses involved in this epizootic. In this study, clinical, serological, and molecular-biological researches were conducted to elucidate an epizootiological aspect of this occurrence of EHV-1 infection. Serologically, 90 (67.7%), including 7 samples from the horses manifested nervous disorders, out of 133 paired serum samples collected from febrile horses showed significant antibody response (four-fold or greater increase in the antibody titer) against EHV-1 by complement fixation and/or serum neutralization tests. Majority (more than 85%) of the infected horses as judged by serological tests were 3 years old. By restriction enzyme and Southern blot analyses o f DNAs extracted from 10 EHV-1 isolates, including an isolate from one of the 7 horses manifested nervous disorders, the Bam HI cleavage pattern corresponded to that of the electropherotype 1 P of EHV-1 and closely resembled each other. However, minor variations were detected in the electrophoretic mobilities of certain Bam HI fragments. These results confirmed that the epizootic of respiratory disease occurred among race horses at Ritto Training Center in 1989 was caused by EHV-1 infection, and strongly suggest that the nervous disorders observed among febrile horses involved in the epizootic were caused by the paralytic form of EHV-1 infection, which had never been recognized among horses in Japan. However, no distinct difference was detected between DNAs o f the isolate from a horse with paralysis and of the isolates from horses without paralysis in the Bam HI restriction profile.