Equine
Herpesvirus Induced Abortion
The Disease
The events leading to EHV-1 induced abortion may be considered
in three phases. Primary EHV-1 infection is acquired via
the respiratory route, whereupon the virus infects the epithelial
cells lining the naso-pharynx and respiratory tract. Initial
infection is rapidly followed by virus transmission to underlying
tissues and infection of endothelial cells (vascular and
lymphatic) and leucocytes. The infection then enters a 'viraemic'
phase, where the virus is carried around the body via infected
blood cells (monocytes and T lymphocytes). In this way, the
virus can be spread to other sites, including the endothelium
of small blood vessels in the endometrial lining of the uterus.
Subsequent thrombosis and/or vasculitis of the affected blood
vessels compromises the microcirculation of the placenta
and allows leakage of virus and/or infected cells into the
placental circulation. Where virally-induced thrombosis is
particularly extensive this can lead to abrupt placental
separation before the fetus has become infected; but the
more usual situation in the field is that viral infection
spreads to the fetus via the umbilical cord and that further
cycles of replication occur in the fetal tissues before abortion
occurs. The abortion process is therefore likely to reflect
an interplay between vascular compromise in the uterus, and
stress following viral infection in the fetus.
As for other herpesviruses, EHV-1 is able to persist lifelong
within an infected animal, utilising a quiescent state known
as latency. It is believed that up to 75% of the British
horse population are carriers of latent EHV-1. Examples of
familiar human herpesviruses that exhibit latency are herpes
simplex virus (HSV-1 - cold sores) and varicella-zoster virus
(VZV - chicken pox). Reactivation from latency occurs when
the virus goes from the quiescent state to a state of active
replication, thereby spreading to other cells, frequently
to sites of virus shedding (enabling the virus to infect
other hosts). In the case of HSV-1 and VZV, the virus goes
latent in cells associated with nerve ganglia; HSV-1 reactivation
results in cold-sores (which can recur periodically) while
VZV reactivation results in Shingles (which tends to occur
only once). EHV-1 is unusual, with latently infected cells
reported in both nerve ganglia and lymphocytes. The relative
importance of virus reactivation from latency versus primary
infection as a cause for EHV-1 induced abortion is uncertain.
In addition to abortion following infection via the respiratory
route, it is also possible that infection developed following
reactivation of EHV-1 from latency may lead to abortion.
Prevention of abortion
Currently, there is no effective drug treatment of EHV-1
infection. Good stud management and hygiene are therefore
the most useful tools in disease control.
The Horse Race Betting Levy Board produces Codes of Practice
for the prevention and control of important equine diseases
such as Equine Herpesvirus. These codes outline the minimum
measures which should be implemented by horse owners, in
conjunction with their veterinary surgeons, as a means of
limiting and resolving disease outbreaks. Copies of the Codes
of Practice are available on the HBLB website and from:
Horserace Betting Levy Board
52 Grosvenor Gardens
London
SW1 0AU
Tel: 0171 333 0043
Fax:0171 333 0041
Email: hblb@hblb.org.uk
http://www.hblb.org.uk
The main aims of the control measures outlined in the Codes
of Practice are to avoid exposure of susceptible horses to
infection, and to minimise reactivation of latent virus.
Stress is an important factor in the reactivation of latent
virus in carriers. Unnecessary stresses such as prolonged
transport should be avoided during late pregnancy.
Any horse or pony can be a source of virus, even if apparently
healthy. Pregnant mares should therefore be kept separately
from all other stock.
Any mare that aborts or produces a dead or sickly foal must
be isolated immediately and veterinary advice should be sought.
Other pregnant mares must be prevented from coming into contact
with the aborted fetus, placenta and fluids, as these are
a major source of infectious virus. Movement on and off the
stud should be stopped until EHV-1 has been excluded as the
cause of disease.
Thorough cleaning and disinfection of the site is required,
as the virus may otherwise survive in the environment for
several weeks.
Diagnosis of Herpesvirus
Following immediate control measures, a specific diagnosis
is required as quickly as possible. The presence of EHV-1
can only be diagnosed in a laboratory. It is important that
appropriate samples are provided for this purpose. A blood
sample from the mare is not useful, as serology results following
abortion can be misleading.
Diagnosis is currently made by Polymerase Chain Reaction
(PCR) amplification of viral DNA from fetal tissues and by
the demonstration of characteristic herpesviral lesions in
the affected organs by routine histology and immunostaining.
Appropriate samples for submission are fetal liver, lung,
spleen and thymus in neutral buffered formalin for histopathology,
or frozen or pooled in viral transport medium for PCR. AHT
staff are happy to advise on sample collection. Ideally,
the entire fetus and its membranes may be submitted. PCR
has recently replaced virus isolation as a means of EHV-1
abortion diagnosis at the AHT. This means that positive diagnoses
can now be made on the same day that the aborted fetus and
placenta are examined.
Vaccination
Vaccination against EHV-1 is available and provides a means
of restricting the spread of the virus between mares and
reducing the chances of abortion in infected mares. It is
recommended as part of an overall disease control strategy.
However, vaccination does not provide total protection and
is not an adequate substitute for good management practices.
Once a programme of EHV vaccination has been started it
should be maintained wherever possible. The primary course
often involves 2 injections 4-6 weeks apart and booster injections
every 6 months thereafter. However, pregnant mares, as a
high risk group, are usually vaccinated at 5, 7 and 9 months
of gestation.
As with all vaccines, horses should be in good health at
the time of vaccination in order that maximum immunological
response is achieved. Where there is the possibility of concurrent
EHV-related or other disease, it is advisable to delay vaccination
until the disease has been investigated. All animals should
be considered for inclusion in the programme, including youngstock
which are often implicated as the source of infection.
Vaccination should be particularly beneficial in limiting
disease within groups of horses, especially where there is
movement and mixing within groups (eg competition horses).
Vaccination will not necessarily guarantee protection against
EHV-1 abortion but will reduce the amount of virus shed,
therefore limiting the spread of infection to in-contact
animals and subsequently helping to prevent abortion storms.
Researchers at the AHT are actively involved in projects
investigating the development of improved vaccines against
EHV-1 induced abortion.
To find out more about the AHT why not visit their website
www.aht.org.uk
