Page 701 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

676 CHAPTER 3

VetBooks.ir EQUINE HERPESVIRUS INFECTIONS endothelial and lymphoid cells in the lamina pro-
pria and establishes a cell-associated (CD+ T lym-
Definition/overview
Equine herpesviruses (EHVs) are ubiquitous in horse phocyte) viraemia that distributes virus body-wide.
During viraemia there is a leucopenia, principally
populations worldwide and have major economic and a lymphopenia, with a leucocytosis, mainly a lym-
welfare impacts on all sectors of the horse industry. phocytosis, on recovery. EHV-2, 4 and 5 generally
EHV-1 causes respiratory disease, abortion, neu- do not cause viraemia. Viraemia disseminates virus
rological disease and ocular disease, while EHV-4 throughout the body. In the uterine endothelium,
is generally (but not exclusively) associated with central nervous system and eye, viral infection of
respiratory disease only. EHV-2 and EHV-5 have vascular endothelium results in thrombosis and
uncertain clinical significance. Morbidity is high. ischaemia, causing abortion, paresis/paralysis and
Mortality is generally low but may be high in some chorioretinal disease. Virus may cross the chorio-
outbreaks of neurological disease. Serologically, the allantois to reach the fetus, or placental ischaemia
prevalence of EHVs in Thoroughbred populations may be sufficiently extensive for sudden abortion of
approaches 100%, but estimates of the prevalence a virus-negative fetus.
of EHV respiratory disease vary from 10% to 60%, Life-long latency is established following primary
mainly associated with EHV-4. Natural immunity infection in circulating T lymphocytes and also tri-
is short-lived. EHV-1 and EHV-4 establish life-long geminal ganglion neurons. Latent virus forms a res-
latent infections, which re-activate periodically and ervoir of virus that, through periodic re- activation
spread virus to new, susceptible horses. Control is by from latently-infected horses, maintains EHV
management precautions and vaccination. EHV-1 is in the horse population. Re-activation occurs in
an OIE-listed disease and equine herpesvirus myelo- response to stress (e.g. transport) and can be induced
encephalopathy (EHM) is considered an emerging by administration of high doses of corticosteroids.
disease in the USA. Re-activation results in shedding of infectious virus
in nasal secretions and, although usually asymptom-
Aetiology/pathophysiology atic, may result in clinical disease, including abor-
There are nine equine herpesviruses (EHV-1 to tion and/or neurological disease.
EHV-9); EHVs 1 to 5 are associated principally
with horses and EHVs 6 to 8 (also known as asi- Clinical presentation
nine herpesviruses 1 to 3) are associated principally The incubation period following experimental infec-
with donkeys. The alpha herpes-viruses EHV-1 and tion with EHV-1 and EHV-4 is short (<48 hours),
EHV-4 are respiratory viruses and are regarded as although longer incubation periods of up to 10 days
the two most important EHVs that infect horses. have been suspected in the field. The severity of
EHV-3 causes venereal disease (see p. 493). EHV-2 URT disease varies and is mild or subclinical in
and EHV-5 are gamma herpesviruses with uncer- horses previously exposed to the virus, but it is more
tain clinical significance. obvious in naïve horses and is usually more clini-
The source of EHV infection is either through cally apparent with EHV-4. In naïve horses there
direct or indirect contact or through aerosolised is depression and biphasic pyrexia of 10 days’ dura-
virus in respiratory secretions from infected or tion. Nasal discharge is initially serous but becomes
horses with re-activating virus. Uterine fluids from mucopurulent. There is some coughing and lymph-
aborted mares are highly contagious. EHM cases adenopathy. In some horses an ill-defined poor per-
shed virus via the respiratory route and are conta- formance syndrome develops. A chronic post-viral
gious to other horses. EHV-1 has tropism for a range fatigue syndrome is suspected to develop in some
of cells including epithelial, endothelial, lymphoid recovered horses, but such disease is poorly defined
and neuronal cells. EHV-4 usually establishes pro- and difficult to diagnose. EHV-1 viraemia persists
ductive infection in epithelial cells only. Following for up to 20 days and animals usually recover within
infection of the URT epithelium, EHV-1 infects 21 days of infection.