Equine Herpes Virus has a number of variants that cause disease ranging from mild
respiratory signs to abortions and severe neurologic disease. It should be noted that
most horses are exposed to EHV many times in their lives and show little to no outward
signs. Clinical signs typically are identifiable within 7-14 days. The present neurologic
strain EHV-1n causes a significant debilitating disease that can be fatal.
All equine herpes viruses are able to lay quiet in the white blood cell with no outward
clinical signs. During periods of stress the virus replicates causing disease and
shedding. The phenomenon is called recrudescence. EHV-1n has been documented
to lay quiet up to 4 years in mandibular lymph nodes. EHV-1n has the ability to replicate
10x faster than other EHV strains. This factor is why it can create an epidemic. The
replication takes place in the respiratory tract, blood leukocytes and vascular epithelium.
A high viral load can lead to rapid progression of the disease; the heavier the viral load
the more likely the neurologic problems will occur.
There are a number of risk factors for EHV-1n. Groups such as aged horses,
aggressively vaccinated horses, horses with no natural immunity and those chronically
exposed are especially susceptible. Older horses and those with extensive vaccination
histories may have a greater immune response which can cause deposits of immune
complexes affecting the vessels that supply nervous tissue. The cells that line the
vessels to the central nervous system are some of the first effected. This increases the
effect on white matter. When clots form in the vessel the nervous tissue dies. Horses
that are stressed or have no vaccination history can be overwhelmed by viral exposure.
Testing the blood, leukocytes and nasal secretions with a PCR test allows for rapid and
accurate diagnosis. The clinical signs often include fever which precedes the
neurological symptoms. Taking rectal temperatures at feeding time and writing it down
will help identify trends if your horse or group is becoming febrile.
Treatment with antiviral drugs such Acyclovir have had some success in the
rehabilitation. It works by inhibiting the replication of the virus. The early identification
of disease and supportive care are essential to a recovery.
Prevention of this disease depends on good husbandry and stewardship practices.
Concentration on everyday practices like clean water, good ventilation, and stress
reduction are all important to prevent infection or recrudescence Healthy horses
require regular veterinary maintenance including vaccinations. If the horse has been previously vaccinated in less than 60 days prior to today, a Killed Virus vaccine should be readministered. If the time period has been greater than 60 days, a Modified Live vaccine should be administered.
Simple things likefeeding horses in a pan on the ground will help clear upper airways improving immunedefenses. This is particularly important to horses hauled in trailers with mangers. The
vaccination will not specifically prevent EHV-1n but promoting health will decrease
chances of contracting the disease. Immune stimulants in the not yet exposed horses
also help promote the overall health of your horse. It is very important to clean fomites
such as bits, feed troughs, and buckets.
In closing keeping our horses healthy revolves around taking personal responsibility for
their environment. We need to be diligent in keeping our feed and water buckets clean
and not using a community approach. Reducing stress by reducing the number of runs
we would have in a day or a weekend. Just because we're there doesn't mean we have
to keep entering. We need to tie our horses were there is good ventilation and not
where they are likely to have nose to nose contact. We are responsible to our horses
and with some dedication this will be behind us.