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Dr. Alison Moore's Blog


Equine Herpesvirus Myeloencephalopathy: Dealing with an Outbreak

Published: March 10, 2014 10:57 am ET

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The neurological form of Equine Herpesvirus-1 (EHV-1) called Equine Herpesvirus Myeloencephalopathy (EHM) reared its ugly head in outbreaks involving both the Standardbred and Thoroughbred racing industries last May/June.

Racing opportunities were denied for those housed at the Cambpellviille Training Centre and Woodbine’s backstretch was quarantined. Sadly two horses lost their lives, three others remained neurological for days to weeks and many were afflicted with fever and respiratory disease. It will soon be that time of year when trainers, staff and horses will be making their long spring migration back to Ontario from warmer climates and so I thought a primer on EHM outbreak management would be timely.

What is Equine Herpes Myeloencephalopathy (EHM)?

  • Equine herpesvirus-1 (EHV-1) is a common virus infecting horses.

  • At least 70% of the equine population has been infected by EHV-1 and are carriers.

  • EHV-1 causes three main clinical syndromes;

    • respiratory disease in foals and young horses

    • late-term abortion, and

    • neurological disease called Equine Herpes Myeloencephalopathy (myelo = spinal cord, enceph = brain, pathy = disease) or EHM.

  • Two different strains of the virus cause neurological disease;

  • neuropathogenic strain -Horses infected with this strain are more likely to develop EHM (this strain was found in the horses at Woodbine last year) and

  • non-neuropathogenic strain (also called the wildtype) which also causes neurological disease but is considered less likely to do so (a horse at Campbellville TC had this strain).

  • Neuropathogenic strain is “immediately notifiable” by the commercial laboratories under the Ontario Animal Health Act resulting in a rapid disease response. Initiating an appropriate response to stop the spread of the non-neuropathogenic strain requires owners and veterinarians to self-report to the Ministry to seek help in infection control. Both strains can be equally devastating to the industry, however.

  • EHM is often fatal, and may cause significant economic losses.
    An outbreak may restrict movement of horses and affect the running of live racing.

  • EHM is an emerging disease. It is not a new disease (in fact the first isolate of EHV-1 was discovered in 1941) but the number of outbreaks is felt to be on the increase worldwide.

How do outbreaks of EHM occur?

  • Most horses are infected with EHV-1 as foals by their dams and current vaccines and management practices cannot prevent this.

  • EHV-1 produces a latent infection. These foals usually show no clinical signs at the time and the virus “hibernates” in the lymph nodes and in a group of nerve cells in the head called the trigeminal ganglion where it remains inactive, or latent, setting up a carrier state that is life-long.

  • Carrier horses do not show clinical signs. Carriers comprise an estimated 70% of the equine population, and there is no laboratory test available to determine if a horse is a carrier.

  • Stress and immunosuppression causes carrier horses to start shedding the virus. Stressful situations such as shipping (especially long distance) overcrowding, mixing, illness or pregnancy, can cause the virus to become active and shed by the horse. It is thought that most outbreaks of EHV-1 are caused by reactivation of the virus from a carrier state.

  • Outbreaks occur due to several factors. Breed, age, sex and immune status of the animals involved along with the stress related factors initiate an outbreak. Ponies seem to be more resistant and older horses (>3yrs), Thoroughbreds, Quarter Horses, Warmbloods and female horses are more susceptible.

The ability of a virus to hibernate in the body and remain inactive for a period of time is not unique to EHV-1. The human virus, Varicella-Zoster Virus (VZV), that causes Chicken pox (aka Varicella), is also a member of the same herpesvirus family as EHV-1. After infecting children, the chicken pox virus remains latent within the nervous system and may be reactivated later in life as Shingles (aka Zoster). Although the clinical signs of disease in people with VZV are different than in horses with EHV-1, in both species the immune system determines whether the virus becomes reactivated and the severity of disease.

What happens in an EHM Outbreak?

  • EHV-1 is transmitted by respiratory secretions. Horses become infected by inhaling the virus shed by another horse, from nose-to-nose contact or from contact with infectious viral particles in the environment (tack, grooming supplies, stalls, trailers, buckets, peoples’ clothing etc.)

  • EHV-1 infects the lymph nodes and blood. After inhalation, the virus infects the cells lining the nasal passage, entering the respiratory lymph nodes within 24-48 hours. The virus, now inside white blood cells, enters the blood stream. Virus within the blood is called viremia and the inflammation it causes triggers a high fever.

  • Fever is an important clinical sign. Fever occurs days before the onset of neurological signs and is often absent by the time neurological signs are noticed. It is, therefore, very important to take temperatures twice daily on all horses in the barn after a case has been diagnosed, and on new horses arriving at your stable as a fever may be the only indication that an active virus is present.

  • Respiratory disease may occur. Because the virus enters through the nose and infects the respiratory lymph nodes, there may be a concurrent outbreak of respiratory disease indicated by abnormal nasal discharge along with fever.

  • EHV-1 infects the central nervous system (CNS). Once in the blood stream, the virus is delivered to the central nervous system (CNS) but only in certain horses will the virus infect the cells lining these vessels. This infection then damages the vessels with the resulting inflammation leading to blood clots and subsequent dysfunction of the spinal cord.

  • EHM is characterized by decreased coordination (ataxia) and hind limb weakness. Loss of balance and recumbency (an inability to rise) may then ensue. Horses that become recumbent have a lower likelihood of survival than those that remain standing. Sometimes the nerves controlling the bladder are affected and abnormal urination is seen.

  • Only 10% of infected horses develop neurological signs during an EHV-1 outbreak. The reason why the virus attacks the vessels of the CNS in only certain horses is not completely known, although there is a strong relationship between the dominance of specific immune system cells and the susceptibility to and recovery from EHM.

  • The horse will shed the virus for 10-21 days or longer after initially infected.

How is EHM diagnosed?

  • EHV-1 infection is confirmed by your veterinarian via polymerase chain reaction (PCR) testing on nasal swabs and plasma (white blood cells). Testing is done on horses showing neurological disease and on suspect horses displaying fever.

  • EHM must be differentiated from other causes of neurological disease in horses such as equine protozoal myelitis (EPM), rabies, West Nile Virus and Equine Eastern Encephalitis where appropriate.

How is EHM treated?

  • Fluid therapy and anti-inflammatory medications are the mainstay of treatment for EHM. Specific management for recumbent horses includes frequent rolling, deep bedding or slinging and horses with urinary issues may require catheterization. Antiviral medication may be used in certain cases.

How is EHM Prevented?

  • Prevent the Spread of EHM. One of the best ways to control disease is to prevent its spread. Veterinarians can help institute some of these management decisions in your barn after the diagnosis of EHM has been made.

    1. Stop movement of horses on and off the property. Only horses going to an isolation unit under veterinary supervision should be allowed to leave. A quarantine (regulated or self-imposed) may seem like a nuisance and economic burden but perpetuating the disease in the local industry will have a greater economic impact on you and your colleagues. Depending on the situation, most quarantines will last 21-28 days. When appropriate to the situation, earlier release after 14 days is possible if there have been no new fevers or cases and PCR testing is done on ALL horses for 2-4 consecutive days. This latter approach relies on appropriate clinical monitoring and incurs greater costs and therefore may be used more judiciously.

    2. Isolate sick horses, if possible, or use infection control procedures at the stall. Latex or nitrile gloves should be worn, coveralls should be used when entering the stall and removed when exiting, and overboots used or a foot bath placed outside the stall. Entering the stall should be kept to a minimum and manure removed and feed/water buckets disinfected daily and after all other healthy horses have been attended to. Because the virus is shed from the nose, aerosolized droplets can land on anything nearby i.e. barn cats/dogs, forks, shovels, ties, towels, brushes, blankets, harness/bags so keep things away from the front of the stall. Remove stall guards and put up stall gates, or preferably doors, to limit the horse’s reach into the alleyway. Infected horses should remain in their stall until the vet says otherwise. Stalls that do NOT have solid sides or walls that are NOT high enough to prevent nose-to-nose contact will make it challenging to control spread through the barn.

    3. Take temperatures twice daily on all horses in the barn. Once a horse has been diagnosed with EHV-1 one should assume that all horses in that shed row have potentially been exposed especially the ones in the neighbouring stalls and across the alleyway. If you only take a temperature once a day you may miss a fever spike. A fever will be the first, and possibly only, sign you will have before a horse develops EHM. Once a fever occurs you need to implement infection control (as above), call your vet and test.

    4. Clean and disinfect the environment. If the horse had recently raced or shipped, clean and disinfect the harness/tack, the ties, and, most importantly, the trailer. All organic material (dirt and manure), should be removed prior to disinfecting with bleach (1:9 bleach: water) or Virkon (or other approved disinfectants). Boots also need to be cleaned of dirt /manure/shavings before being dipped in a footbath. Footbaths should be changed daily.

  • No vaccines are labelled for the prevention of EHM. Although vaccines exist to prevent respiratory disease and abortion due to EHV-1, at present, there is no vaccine licensed to prevent the neurological form of the disease. Some veterinarians promote the use of the respiratory/abortion vaccines to reduce the shedding of the virus during an outbreak and limit the spread of infection.

Much like our kids are with chicken pox, our young horses are infected with EHV-1. And much like the reactivation of the virus and development of Shingles in adult people, there is no way to predict which horse will develop EHM after infection. Fortunately, EHM is still a pretty rare occurrence considering the number of horses infected with EHV-1. Hopefully, the racing industry will not be faced with another outbreak in the near future, but if it is, being aware and understanding EHV-1 and how EHM occurs may help to minimize the impact of this disease on your horse’s health, your own business and the racing industry at large. Remember your veterinarian in an important source of information and support. Consult him/her regularly and involve him/her in all decision making with respect to disease control.

The views presented in Trot Blogs are those of the author and do not necessarily represent those of Standardbred Canada.

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