Equine Herpesvirus i(EHV) is Number 1


Most of us are aware of the Herpes viruses that cause problems in humans.  Equine herpesvirus does not transfer from horses to humans, but humans can transfer the virus from horse to horse.  For that reason it is important that we educate our grooms as to the seriousness of EHV and implement a daily management plan that minimizes the risk of an outbreak in your barn. 


One of the most consistent characteristics of EHV is that it causes respiratory symptoms in horses.  Strangles, equine flu and equine viral arteritis(EVA) also create respiratory problems and can also be controlled to an extent by good management.  Strangles is caused by a bacteria (Streptococcus equi) so treatment with antibiotics can help reduce the length of time a horse is affected.  The other three are caused by viruses.  EVA is a venereal disease and is transferred by skin to skin contact, especially during the live cover of a mare by a stallion.  Flu is caused by a virus that usually runs its course in one to two weeks and is gone from the horse’s body.  EHV, however, is DN”A-type virus that is remains in a horse’s body after infection.  As with strangles, flu and EVA, EHV has been around for a long time.  The   sudden interest in EHV was brought on by a higher incidence of horses developing neurological symptoms than observed in past 10 years.  This article will take a brief look at EHV, ways we can prevent an outbreak and what to do if one of the horses in your stable does get sick. 


It is interesting that the “herpes” name comes from the Greek word “herpein” which means “to creep” and is the root word of serpent (thus, herpetology).  A snake may creep into a tent, curl up and go to sleep.  It does no harm unless it is “disturbed” and then it  strikes” and causes harm.  Once a horse is infected with a herpes virus that virus stays in the horse’s system for life.   As long as the virus stays “quiet”, it does not cause illness.  But if it is disturbed (by stress or other illness) it can cause a recurrence of its disease. 


Eight forms of equine herpesvirus have been identified; four that affect equids other than horses (zebras, donkeys, etc.).  The five that affect domestic horses are named EHV-1 through EHV-5.  The viruses numbered 2 and 5 are “gamma” type viruses that spread slowly and don’t seem to cause a specific disease.  Blood tests of racehorses, show horses and pleasure horses have shown that over 80% of all horses and 90% of racehorses are positive for EHV-2 and 5.  They may weaken a horse’s immune system thereby allowing other diseases to affect that horse. 


Alpha Type Equine Herpesviruses

EHV 1, 3 and 4 are “alpha” viruses meaning they are fast growing and invade a horse’s body quickly once that horse has been exposed. 


EHV-3 causes a venereal disease in horses and is transmitted primarily by skin to skin contact.  It does NOT affect the respiratory system and is not spread by airborne particles or nasal secretions.  It can be transmitted by shared equipment and by people not washing properly after handling an infected horse.  If you notice blister type lesions when you are cleaning a colt’s or horse’s sheath, or you notice blisters around the muzzle of a foal, notify your trainer and/or veterinarian.  This could indicate EVA infection.


EHV-4 primarily causes equine rhinopneumonitis (“rhino” or “the snots”).  It does not infect white blood cells like EHV-1 does and therefore does not generally cause abortion or the neurological form of the disease.  It is hard to distinguish between EHV-4 and EHV-1 based on symptoms as they both produce the same respiratory symptoms. 


EHV-1 is “number one” on the charts.  It invades the lining of the lungs usually within 2 days, though it can take as long as 10 days.  This form of the virus can infect white blood cells that then carry the virus to other places in the body such as the uterus (causing abortion in pregnant mares) and the central nervous system (causing the neurological form that is fatal up to 50% of the time). 


Original classifications of these viruses suggested that EHV-4 caused ONLY respiratory symptoms and that EHV-1 primarily caused abortions. It was determined recently that the distinction between these two sub-types has been blurred and that both of these viruses can cause respiratory, reproductive and neurological forms of the disease.


EHV is a highly contagious disease that is easily transmitted by airborne virus particles or contact with infected fluids.  The highest incidence of infection occurs in winter through early summer, most frequently in young horses.  Once the virus has invaded a horses system, it remains in affected horse for life and can be re-activated by severe stress. 


Once a horse has been exposed to the virus, the incubation period is 2-10 days as the virus invades the lining of the lungs and begins to replicate, normally causing a mild to moderate respiratory disease.  Most horses make full, uneventful recovery from the respiratory disease in two to three weeks. 

Some horse may not show any respiratory symptoms but will be contagious as long as the virus is actively multiplying and is present in the horse’s blood stream (described by the term viremia).  A horse is contagious as soon as it spikes a fever and may continue to be infectious for 2 to 3 weeks while looking perfectly healthy.


The neurological form of the disease results when the virus attaches itself to white blood cells that then damage the central nervous system. Research is currently being conducted to determine if the virus causing the neurological symptoms is a mutation of EHV-1.  The neurological form of this disease progresses rapidly, going from no obvious symptoms to obvious neurological symptoms in hours.  If an infected horse does not become “recumbent” (lying down – unable to get up) it will generally make a full recovery.  Nearly half of horses that become recumbent do not survive. 


The virus can live outside of the body for up to 48 hrs, is stable in temperature up to 130° and is susceptible to UV light, detergents and disinfectants.  These characteristics enable us to develop a plan to prevent occurrence and minimize affects of an infection. 


Symptoms of and equine herpesvirus infection are similar to all respiratory infections.  Depression and poor performance may be the first symptom followed by lack of appetite (going “off feed”).  Coughing (sometimes starting as a dry, hacking cough) followed by nasal discharge and diarrhea may be noticed.  Neurological symptoms such as stubbing of the toe, staggering, drooping ears or difficultly in rising should be considered red alert warning signs. 


A consistent and readily observable symptom is FEVER.  Any estimate of body temperature in excess of 102 degrees is a warning sign of a possible infection.  With the increased occurrence of EHV neurological infections, veterinarians are recommending taking and recording temperatures daily – twice a day in areas where an EHV diagnosis has been confirmed.  Knowing “normal” values for each horse can help provide early warning of a possible infection as an increase of 2 degrees may be considered a warning sign even if the actual temperature is not over 102 degrees.


Equine herpesvirus disease is transmitted when a non-infected horse comes into direct contact with the virus, either by inhalation or contact with fluid that contains the virus.  A coughing/sneezing horse may transmit the virus 35 ft and a fan can move it farther down the Shedrow.  A closed up barn may be warmer for the human handlers, but poor ventilation increases the likelihood of EHV virus staying in the area and infecting other horses. 


Nose-to-nose contact with a horse that is shedding the virus is one way a non-infected horse acquires the virus.  Handlers should be aware that fluids from an infected horse can get on their clothes or equipment (especially a rub-rag used to clean noses) and be transferred from stall to stall. 


Infected horse starts shedding the virus 3-8 days after first symptoms and will shed the virus for up to 3 weeks.  For that reason horses that have had respiratory disease should be isolated from non-infected horses for at least 21 days after they have not shown symptoms


Biosecurity is a term being used to describe management practices designed to prevent the spread of diseases.  Bio-surveillance is a term used to describe preventative measures as well as monitoring of biosecurity practices.  Standard-medium level bio-surveillance and security measures can minimize the incidence of new infections.  High-level and quarantine levels are necessary when a disease has been confirmed. In the sidebar are some guidelines that can help you implement procedures that may help keep your operation from being impacted by a contagious respiratory disease


Good biosecurity starts with good hygiene.  One of the most effective and inexpensive biosecurity measures is washing hands between horses.  Simplistic, but cleaning, sanitizing and disinfecting are the key elements of any bio security plan. 




A generally accepted statement regarding spread of viruses is that “90% of virus spread can be prevented by good cleaning procedures with the other 10% being taken care of by a consistent disinfecting program”.  Although used interchangeably in common usage, we will distinguish between cleaning, sanitizing and disinfecting so as to focus attention on importance and contribution of each. 


Cleaning – Using soap or detergent and water or a “dry” chemical, etc. to remove dirt, organic matter and other debris.  “Clean” does NOT imply “free from bacteria”.


Sanitizing – Using antibacterial cleaner and/or heat (boiling water, steam, microwaves) to make free from all bacteria and other microorganisms.  (You can have “sanitary” dirt and manure.)


Disinfecting – Cleaning with product that kills all bacteria and microorganisms and leaves residue that PREVENTS immediate re-growth of disease-causing organisms.


Standard and Medium BioSecurity


Cleaning feed tubs and water buckets with dishwashing detergent to remove organic matter has the added benefit of disrupting the shell of some viruses and providing a partial sanitizing benefit.  Chlorine bleach (Clorox) is an inexpensive and good sanitizer, but it is deactivated by organic matter.  For maximum benefit, buckets and feed tubs should be cleaned with detergent and rinsed completely before being left to soak in bleach water. 


All personnel should wash hands between working with new horses (in barn less than 14 days), horses exhibiting signs of respiratory illness (fever, snots) and handling any horse’s mouth or genitals (tongue tying, cleaning sheath, etc.)


Clean tack with saddle soap and/or wipe down with a “disinfecting wipe” such as those used by food service personnel.  Baby wipes can be used to wipe off bit after dipping in “dip” bucket in which you have a chlorine bleach dip (1:10) or a Virkon®  disinfecting dip (1:100).  Chlorine dip should be made daily, Virkon dip can last up to a week or when the pink color fades whichever happens first. 


Wipe down webbing daily with a fresh batch of dip. Weekly wash brushes with mild detergent, dip in Virkon® dip, shake off and place in sun to dry


High Level Biosecurity


Follow the standard-medium procedures using Virkon® as dip and wipe.  Grooms should wash hands between each horse and use disposable wipes for wiping noses and cleaning under tails.  Feed tubs, water buckets and grooming tools should be washed and disinfected DAILY!


Post Quarantine BioSecurity

Once quarantine is lifted the former “Hot Zone” should be cleaned, sanitized and disinfected.  Pay particular attention to EVERYTHING that came into contact with a horse that was in quarantined. Strip the stall making sure to remove ALL loose organic matter (manure, urine soaked dirt, feed, etc.) and wash dried organic matter from walls using water and detergent.  Remember that cleaning is a most important step, as proper cleaning will kill 90% of bacteria and other microorganisms.  If no horses are within 50 feet, a low-pressure power washer can be used. It may be best to avoid using a high-pressure washer as that could aerosol viruses more than 50 feet.  Wash feed tubs, buckets, bits, etc. using detergent and hot water and then soak in Virkon dip for at least 10 minutes.  Rinse well and allow to dry in sunshine. 


Equine herpesvirus infections have been around for as long as we have had horses.  Recently, an increase in the number of horses that developed neurological symptoms from EHV-1 (or EHV-4) infections has focused new attention on equine respiratory illnesses.  Horsemen can minimize the impact of such respiratory diseases by following basic common sense when it comes to cleanliness and horse handling techniques. Providing training for those that spend the most time each day with our racehorses, the grooms, is an increasingly important way to implement these practices and keep the horses racing.