Articles - Equine Herpes Virus-1 - FAQ

Why is there no vaccine?

This is a question of semantics.  There are several vaccines “approved” for use in prevention of Rhinopeneumonitis (EHV-1 thru EHV-4) on the market today.  These vaccines have been around for years, as has the EHV disease, otherwise known as Rhinopneumonitis.  These are 2 key immunization types:  killed and modified live virus vaccines.  Both vaccines impart antibody production “humeral immunity” within the horse to protect against viral insult by EHV; the modified live vaccine has the added benefit of promoting “cell mediated immunity” which acts as an additional armament to the body’s antibody defense system to ward off exposure to the virus.  NO vaccine to date specifically carries an FDA label-efficacy against the “neurogenic strain of EHV-1” which has emerged sporadically in recent months.  This is in part due to the difficulty and risks vaccine companies must take to get FDA licensing against this rare and quite deadly strain of the “normal EHV-1” virus. 

However, research lab tests involving “EHV-1 naive” horses (ie. horses without any antibody or cell mediated immunity on board) who were challenged with millions of viral EHV-1 neurologic viral particles showed 2 key advantages of the modified live vaccine (Rhinommune®).  First the vaccinated horses had either decreased or minimal signs of disease when inoculated with massive levels of virus nasally (compared to non-vaccinates who get sick or died) and secondly (and perhaps more importantly to barn or multiple horse scenarios) the vaccinated horses did not shed the EHV-1 neuro virus nasally in secretions.  This lack of “shedding” is important because the key spread of the virus occurs in the nasal secretions of adjacent sick horses that may not yet show any other marked signs of sickness of neurological manifestation during the first 8 days of infection.  So, if vaccination of horses helps stimulated antibody (humeral) and cell mediated protection against either individual infection OR helps to prevent the spread of this aggressive EHV-1 neuro strain from horse to horse, then maintaining current immunization should be the first basic preventive measure horse owners can take.  “An owner ounce of prevention is worth a pound of cure.” 

The ultimate determinant of whether a horse gets sick is dependent upon the amount of virus encountered (“viral load”).  Therefore, decreasing shedding and increasing immunity at oral-pharyngeal areas of uptake will best help prevent the overwhelming of the body’s antibody levels against these virus particles that might find their way into your horse!

Why are some horses sick and others not?

Simply, the level of “viral load” balanced against the individual horse’s neural & cell mediated protection (ie. the competency of their immune system) will determine whether a horse gets sick or not.  If you get exposed to massive volumes of influenza virus while traveling on a plane, such that your body’s levels of immune resistance are exceeded, you will get the ‘flu’.  The EHV-1 scenario is not much different, except that the normal prevalence of EHV-1 in the horse population, while is yet undefined and the means of exposure (e.g. sick horse vs. asymptomatic carrier) is still under investigation.

How does the infection spread?

This is uncertain still.  The Nov ’06 “outbreak” came into the US from Europe via a sick horse that showed minimal symptoms but was “shedding” the virus.  Those horses in close contact with it during shipment helped to disseminate the virus throughout CA/FLA/CONN/VA.  However, in years prior to ’06 we would see sporadic EHV-1 ‘neuro outbreak’ in ‘vaccine naive’ horses here in the US.  These occurrences may have been a result of ‘spontaneous mutations in the normal non-neurogenic strain (aka upper respiratory form of EHV-1), OR there may be a true vector pattern due to “asymptomatic carriers” (like “typhoid-mary” in human history involvement with typhoid fever spread).  There is some thought that mules or other “asymptomatic carriers” may shed the disease but NOT show clinical signs.  The key concept here is that the horse does NOT seem to be the true end-host target for EHV-1 and therefore is likely NOT a chronic “shedder” over long term.  During the next months, more info will emerge on the epidemiology of the EHV-1 neuro strain for sure!

What is the current status at Moss Beach?

As of 4/15, two barns (Moon Valley and Renegade Ranch) remain under “voluntary quarantine” and one barn (Ember Ridge) remain under “voluntary movement restriction” due to its proximity to the quarantined two.  The guidelines for quarantine are for a 21-day period beginning after the last febrile horse showing clinical signs is detected.  I am under the understanding that this quarantine is still under effect.  These guidelines and the specifics of release and subsequent testing will be determined between attending veterinarians, UC Davis, and State of California vets; and we will post info as it becomes available.  *THE QUARANTINE RESTRICTIONS UPON MOVEMENT HAVE ALL BEEN RELEASED AS OF 5/1/07 AND NO NEW CASES HAVE BEEN REPORTED TO DATE*

Can horses move freely and go to events?

Horses at the two quarantined barns cannot be moved until termination of the quarantine.  All other horses in the Peninsula Region are free to move, attend shows and events, and live life routinely.  Fortunately the affected barns are quite geographically isolated AND owners of horses at these facilities are being very contentious and attentive to the quarantine conditions.  There have been no new cases since the original quarantine group.  Horses are being monitored closely throughout the quarantine period to identify any new cases should they occur.  We are optimistic that the quarantine measures will successfully prevent further spread.

What is the different between rhinopnuemonitis (ehv-1) as we know it and and the ehv-1 nuero strain?

The respiratory form of the EHV-1 virus has been in the US for many years.  Mutation of the EHV-1 virus at specific locations results in the much more potent “neurogenic” strain.
This neurogenic strain replicates much more rapidly within the body and the markedly increased viral load results in both more affects upon neurologic tissues AND greater shedding of the virus in the nasal secretions (infectivity). There are numerous references in previous literature regarding chronic or recurrent shedding of EHV-1 in affected horses subsequent to recovery.  This does not appear to be the case with the neuro strain to date.  Dr. John Madigan and other researchers across the country have begun to monitor horses recovered from the neurogenic strain and have not detected subsequent recurrence of shedding or disease.  Indeed, recovered affected horses have returned to normal exercise and performance in uses ranging from racing to show.

What are 'immuno modulators' used for in the treatment of ehv-1?

Immunomodulators such as Zylexus®  and Eqstim® are drugs which are utilizable in prevention rather than treatment.  They are frequently used in horses that are at high risk of exposure to EHV-1 and other viruses.  Their action is to increase the body’s production of substances such as Interferon, which is a key component in the body’s defense mechanism against viruses.  Significant increases in interferon are stimulated after two treatments with Zylexus®.  Veterinarians utilize these immunomodulators in unaffected horses who are at high risk due to anticipated contact with increased viral loads.  Their use should be performed only by directive of your horse’s veterinarian who is familiar with their application.

Can you review what I should do to best protect my horse from ehv-1?

The best protection is achieved through maintaining a current immunization program, apply the principles of basic “biosecurity” measures in your daily horse management and husbandry procedures, and by continuing your attentiveness to your horse’s health and well being.  We have learned that prevention coupled with early detection of cell diseases including EHV-1 will result in the best prognosis should your horse become sick AND will best insure the health of all horses in our equestrian community.

Peninsula Equine Medical Center
(650) 854-3162 | peneq@yahoo.com
100 Ansel Lane, Menlo Park, CA 94028

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