|| Home || Search || Contact || Publications Eshop || Privacy Statement ||
Summary of full report
Latent equine herpesviruses -
Role of latent equine herpesviruses in respiratory disease
October 2004 RIRDC
Web-only Publication No W04/157 RIRDC Project No UMU-28A
Executive Summary
The objective of this project
was to further understanding of the role of equine gammaherpesviruses,
equine herpesvirus types 2 and 5 (EHV2 and EHV5), in respiratory disease
in horses, an important cause of wastage in the horse racing industry.
The clinical implications of EHV2 and 5, are poorly understood: EHV2 has
been implicated as a cause of respiratory disease, although the evidence
for this is limited; EHV5 has been recognised only recently and its potential
contribution to respiratory disease in horses has not been evaluated. Our
investigation was designed to investigate when horses became infected with
these viruses, if the virus infections persisted in the horses, whether
the virus detected by PCR represented latent or active virus replication,
if the virus was latent whether it was reactivated by stressors such as
weaning, and if this occurred, whether the reactivations was associated
with the development of respiratory disease.
Methods of differentiating active and latent herpesvirus infection were investigated. A method was developed for the differentiation of active and latent virus infections of EHV2 and EHV5 by combination of PCR for the detection of DNA to the gB gene and RT-PCR for the detection of RNA transcripts of the gB gene. The results obtained indicated that the high prevalence of EHV5 which had been detected in horses represented, in most cases, latent virus infections and not active virus infection. To confirm this result, an alternative method of determining latency by the detection of latency-associated transcripts (LATs) of EHV2 has been investigated. Three potential genes have been tentatively identified as encoding LATs and investigation of the possibility of utilising transcripts of these genes as an indication of latency is continuing.
In an attempt to develop an EHV2-specific serological test which would be invaluable in epidemiological investigations of this virus, truncated recombinant proteins of EHV2 were produced.
One of these proteins appears to be type-specific and does not cross-react antigenically with EHV5 proteins. Further studies using purified protein as a potential antigen for type-specific serological tests are continuing.
In an epidemiological study of EHV2 and EHV5 infection in cohorts of foals on small commercial studs, the viruses were commonly detected by PCR in foals in commercial studs. Infection with EHV2 and EHV5 occurred in the pre-weaning period in 2 of 3 studs, presumably acquired from latent virus infection in mares, but were not detected in any foal before 2 months of age and most foals were not infected until 4-5 months of age, suggesting maternally-acquired immunity was able to suppress infection by these viruses. There was a marked increase in the prevalence of EHV2 and EHV5 infection after weaning; this increase was not associated with the development of clinical respiratory disease. EHV5 infection tended to persist in foals once it was detected, in peripheral blood leukocytes, but infection with EHV2 was more transient and seemed to be associated with an acute but subclinical infection of the respiratory tract.
In contrast to previous Australian studies, no evidence of EHV1 infection was detected in foals in the period from birth to 1-2 months after weaning in any of the three commercial studs where samples were obtained.
