EHM in horses

A look at what is currently known about EHM – a neurologic manifestation of the EHV-1 virus – along with integrative therapies for its prevention and treatment.

Integrative therapies aren’t often considered for the treatment of infectious disease. However, many integrative modalities can integrate smoothly and beneficially into established treatment protocols. In the face of a disease outbreak, where the best that conventional medicine can do is provide physical support, many nutritional and integrative modalities such as acupuncture, herbal medicine and homeopathy could possibly assist with prevention, acute disease treatment and recovery.

One such disease facing equine practitioners is Equine Herpes Virus-1 (EHV-1), especially in its neurologic form of Equine Herpes Myeloencephalopathy (EHM). This article will provide a brief review of what is currently known about the EHV-1 virus and its neurologic manifestation, along with possible integrative intervention strategies that can be added for prevention and treatment.

What is EHM (neurologic EHV-1)?

The neurologic variant of EHV-1 was first officially identified in the US during a serious outbreak at a college in Ohio in 2003 (although potential outbreaks may have occurred in Kentucky in the 1970s). Of the 135 affected horses, 86% showed clinical signs and 39% showed neurologic signs. Twelve horses died or were euthanized.

Since then, there have been numerous outbreaks, especially in California, Florida, Connecticut and Utah. This rise in reported outbreak incidence of the formerly uncommon EHV-1 and even rarer EHM could represent a concerning change in virus virulence and host susceptibility. To date, while no increased virulence has yet been demonstrated, the genomic areas of variation (the variants are now identified as D752 and N752) have been identified. While each can cause both illness and neurologic symptoms, the D752 variant has a higher correlation with clinical EHM symptom onset. Horses can be infected and become carriers of both variants at the same time, thus a latent infection with one does not prevent infection with the other.

Most importantly, EHV-1, like all other herpes viruses, has a strong ability to lie latent in the body until stress stimulates reactivation. It tends to hide in both the lymphoreticular system and the trigeminal ganglion. Latency studies show viral presence in up to 66% of horses with an unknown exposure history, and 54% of Thoroughbred broodmares. Once activated in neurologic disease, the spinal cord gray and white matter are most commonly affected, resulting in ischemic necrosis. (The author wonders if there is a link between trigeminal neuralgia syndrome [head shakers] and latent EHV-1.)

EHV-1 is transmitted via shedding from the nose (or infected fetal membranes); via fomites such as hands, brushes, blankets, feed or water buckets; or via aerosol. The virus can survive in the environment for several days up to a couple of weeks, depending on conditions. Subclinical shedding is not considered a factor in the development of clinical outbreaks – rather, this type of exposure probably leads to latent infection in exposed animals.

Both the abortive and neurologic manifestations are thought to occur only when latent disease is activated to full clinical disease, or there is exposure to a clinically sick animal. Protection from reinfection after recovery is thought to last only three to six months.

The development of clinical neurologic disease in any individual horse seems to be linked to the level of viremia (a prerequisite for the development of both neurologic and abortive clinical signs), the degree of pyrexia, the length of time since vaccination (see more in vaccine section) and stress.

The incubation period is approximately three to seven days. The viremia can last as long as 14 days, peaking approximately five to nine days after intranasal exposure. A high fever (>103.5°F) that recurs several days after the initial onset of fever is thought to be predictive for the onset of EHM.

Risk factors for EHM appear to be different from those associated with EHV-1 respiratory illness. Older and larger horses are more susceptible (ponies, of course, seem to be particularly resistant). Mares, especially pregnant ones, are also more likely to be affected. Abortion symptoms are more likely to occur in the last trimester.

Symptoms of EHM

Initial symptoms may be vague – fever, lack of appetite and lethargy – and can progress rapidly to the neurologic form. Neurological symptoms include incoordination, leaning against walls for stability, urinary incontinence or dribbling, loss of tail tone, paralysis, and eventually an inability to stand.

To vaccinate or not to vaccinate?

In general, vaccinating in the face of an outbreak is always controversial. In some situations, when there is enough time for the occurrence of an immune response to the vaccine stimulus, vaccination might provide for enough antibody response. However, it has been demonstrated that many vaccinations can cause a transitory period of immune-suppression, so an animal could be at greater risk if exposed during that period. Currently, the standard EHV-1 vaccine for the respiratory or abortive infection is not thought to stimulate cross-protection to the EHM variant. There is some controversy as to whether or not increasing the presence of Killer T lymphocytes at the nasal mucosal boundary by performing parenteral EHV-1 vaccination may prevent the viremia that is a critical step in the development of EHM. The following excerpts were taken from a presentation given by Dr. Julie Watson (Internal Medicine) at an American Association of Equine Practitioners meeting in December of 2005:

“Conventional IM (intramuscular) vaccines usually require at least one week for measurable humoral responses to a booster or a second dose and similar time period in naive animals. This time lag has discouraged the use of vaccines in exposed animals, yet vaccination has been successful in protecting adjacent groups not yet exposed.

“In a California outbreak of neurologic EHV-1 infection, horses vaccinated with either type of vaccine within the previous year were nine to 14 times more likely to develop neurologic signs than non-vaccinated horses. Because the vasculitis associated with the neurologic form is immune-mediated, vaccination after exposure raises concerns of producing a more severe disease. Consequently, vaccination in the face of a confirmed outbreak of EHV-1 neurologic disease has been controversial.”  (Wilson, 2005)

In the 2003 Finley University (Ohio) outbreak, Dr. Stephen Reed (an equine neurologist) discovered that animals vaccinated against EHV-1/4 had both higher morbidity and mortality rates than horses that had received no parenteral vaccinations at all. To be fair to all perspectives, some argue that this is because older animals are more susceptible, but no one has argued that perhaps older animals are more susceptible because they have had more vaccinations.

Because EHV-1 viremia is mononuclear cell-associated, and EHM is associated with a significant inflammatory and immunemodulatory vasculitis, integrative practitioners, especially those who might be new to the various modalities, might want to delay vaccination in the face of exposure and resort to therapies that can boost activity of immune cells, especially lymphocytes and modulate inflammation. (Note: when dealing with competitive horses in which EHV-1 vaccination is mandatory, the author recommends using an intranasal vaccination.)

Steps to take during an EHV-1 outbreak

Whether a veterinarian primarily uses conventional or integrative therapies, one thing common to both approaches is basic infectious disease management. First and foremost, clients should avoid transporting horses to areas where the transmission of active virus could occur. If the horse has already been exposed, then he should be isolated and quarantined from contact with other horses, and strict disinfection protocols should be started (biosecurity). A clinically ill animal can continue to shed the virus for 21 days (possibly longer) after initial infection. New PCR tests (especially live time) are now available to help identify potentially exposed individuals early on, allowing for both conventional and integrative interventions that can help lower viremia and boost immune function.

Second, remember that the animal’s own immune system is still the best protection. In the face of environmental or performance stresses, immune support may delay, decrease or prevent clinical signs. There are a variety of ways an integrative veterinarian can support immune function:

  • Manage and try to avoid things that can suppress the immune system, such as stress (travel, strange places, separation from herd buddies), alterations to gut microbiome (antibiotics, NSAIDs), and suppressive medication (hormones, steroids for skin problems, sedatives or tranquilizers, over-vaccination before travel).
  • Stimulate and support immune function with nutrition, acupuncture and herbs. For example, moxibustion has been shown to increase the activity of cytotoxic T-lymphocytes and promote the production of anti-inflammatory cytokines (Takayama, 2010).

Integrative medicine for prevention and treatment

The difference in infectious disease perspective between conventional medical practitioners and more integrative ones is the emphasis on the body’s role in preventing disease. Conventional medicine often has to rely on either vaccinations (which may worsen the EHM) or a more wait-and-see approach in the face of possible exposure.

Integrative medicine has many nutritional, herbal and homeopathic options to help prevent and also treat symptoms that may appear. There are many ways to stimulate the immune system. While there are products on the market that are advertised to boost the immune system, they are very general and coarse ways of stimulating a system that is so complex and specifically fine-tuned. In the author’s experience of treating chronic immune deficiencies, these products can cause chronic health problems and immune sensitivities that can last long past the initial need. It is much like an usher getting a smoker to put out his cigarette by yelling “FIRE” in a theater. The smoker has put out the offending burning object, but there are also people screaming and causing undirected havoc all around him. In other words, significant collateral damage can occur, with hypersensitization in some areas and deficiencies in others. The best way to boost the immune system is by doing it from the ground floor, through nutrition. After addressing diet, acupuncture and herbs can be used to increase immunity.

Using nutrition to boost immunity Feeding whole, clean, quality grains is important – be cautious using overly processed feeds with ingredients made from by-products, since these actually increase oxidative stress in the body. In addition, I recommend the following supplements:

  • Vitamin C: double the dose if there has been a recent potential exposure.
  • Vitamin E: give a minimum of 4,000 IU/day. Vitamin E is a potent fat-soluble antioxidant and has been shown to act as an anti-inflammatory in nerve tissue.
  • Omega 3 fatty acids: numerous studies show the anti-inflammatory and neurologic protective effects of Omega 3s.
  • Zinc: found in supportive levels in hoof and skin supplements.
  • Caretenoids: the precursor to vitamin A, which boosts activity of virus-fighting killer T cells.
  • Probiotics: a plethora of research in just the past few years shows links between healthy gut bacteria, lymphatic cell, body cytokine (inflammatory and anti-inflammatory) production, and total body immune function. This indicates probiotics should be among the first things an integrative practitioner thinks of.

Immune-boosting herbs (Western and Chinese)

  • Ginseng (Korean or Siberian): both types (American ginseng is weaker) support the adrenal glands, which when stressed or exhausted are unable to support the body’s stress functions. For a practitioner trained in Traditional Chinese Medicine, many immune-supporting Chinese herbal formulas contain ginseng (Panax or Eleuthro) and can be custom blended at a Chinese herbal pharmacy (the author uses either Mayway or Jing Tang).
  • Echinacea: needs to be on-board at the time of exposure. Also, to be effective, it needs to be given at least three times a day. Human research has shown that Echinacea is more effective if given as a water extract (tea).
  • Goldenseal: has antiviral activity, but can only be used short-term.
  • Astragalus: this Chinese herb is extremely effective as an immune booster, especially when combined with Ligustrum. An Astragalus and Ligustrum formula has been used in Chinese Medicine for almost 1,000 years. Today, it has been shown to be very effective in treating HIV and Epstein-Barr virus.
  • Gan Mao Ling: a traditional Chinese formula that’s effective in preventing or reducing the symptoms of rhino virus in humans.
  • Garlic: has been shown in several studies to increase the activity of virus-fighting lymphocytes. If you use garlic in your horse, since it is a “warm” herb in the Chinese pharmacy, make sure you combine it with other herbs that are a bit more cooling, such as mint, elderberry or lemon balm.
  • Turmeric (curcumin): from the ginger family, this Asian herb is backed by strong data demonstrating its antimicrobial and anti-inflammatory properties (in humans it has been shown to reduce C-reactive protein).

Managing stress

Managing stressors that can affect emotional, mental and immunologic health is where integrative therapies shine. Most competitive horses have probably been exposed to natural EHV-1. If a horse is stressed by trailering, travel or leaving his/her herd, try to scale back if there has been a potential exposure or cases reported in the area. Be aware of a horse’s training schedule and counsel clients when their horses might need a break. In the world of sports medicine, rest is considered as important as exercise in the overall picture of conditioning and fitness, but with high drive clients or with a big show in the works, it can be easily overlooked or skipped. Remember that horses younger than five may be more susceptible to training stress than seasoned campaigners.

Also be cautious of overusing non-steroidal anti-inflammatories (Bute, Banamine) as well as steroidal medications such as prednisone and dexamethasone. These medications can affect immune cell function or cause additional inflammation and stress in the digestive system (the location of 80% of the body’s immune cells). Vaccinations can create a transient drop in immunity within three to ten days afterwards. The more viruses combined into one vaccine, the greater the transient drop. Many labs now run antibody titers so it’s easier to determine if an animal even needs to be vaccinated (because, isn’t the point of a vaccination to create immunization?). There are now accurate and cost-effective tests for EEE, WEE, WNV and rabies.

Homeopathy for EHV-1

While homeopathy doesn’t work well as a preventive, it might be extremely effective in treating the initial stages of disease, especially when combined with conventional supportive care. During the cholera epidemic of WWI, homeopathic hospitals in Philadelphia had a16% mortality rate versus 60% for the general hospital population.

OTC remedies can be purchased online from homeopathic pharmacies or from most health food stores. Remedy potencies are 6C or 30C.

  • Belladonna: the best remedy for sudden onset of high fevers with neurologic symptoms.
  • Aconitum: sudden, vague fevers with lethargy.
  • Gelsemium: initial stages of viral infection with neurologic symptoms and fatigue.
  • Oscillococcinum from Boiron: specifically for human flu symptoms such as lethargy and fever – symptoms very similar to the early signs of EHV-1. The author has used this OTC remedy (or had clients use it) with excellent outcomes in horses with these symptoms.

Smokejumpers have a saying: “Every fire starts small.” The best way to avoid having to treat a horse seriously ill with EHM is to prevent the disease in the first place. Many of the therapies we can use to support an animal exposed to EHV-1 can also be used in more general and preventive circumstances in competitive and traveling equine athletes. All it takes is for the integrative veterinarian to start a dialogue for educating horse caretakers.


Allen GP. “Risk factors for development of neurologic disease after experimental exposure to equine herpesvirus-1 in horses”. American Journal of Veterinary Research, 2008.

Goehring LS, Brandes K, Ashton LV, et al. “Anti-inflammatory drugs decrease infection of endothelial cells with EHV-1 in vitro”. Equine Veterinary Journal, 2016.

Henninger RW. “Proceedings of the equine herpesvirus-1 Havermeyer workshop 2004”. Veterinary Immunology & Immunopathology, 2006.

Henninger RW, Reed SM, Saville WJ, et al. “Outbreak of a neurologoic disease caused by equine herpesvirus-1 at a university equestrian center”. Journal of Veterinary Internal Medicine, 2007.

Kydd JH, Townsend HG, Hannant D. “The equine immune response to equine herpesvirus-1: the virus and its vaccines”. Veterinary Immunology and Immunopathology, 2006.

Kydd JH, Slater J, et al. “Third International Havemeyer Workshop on equine herpesvirus type 1”. Equine Veterinary Journal, 2012.

Lee MJ, Jang M, Choi J, et al. “Bee venom acupuncture alleviates experimental autoimmune encephalogmyelitis by upregulating regulatory T cells and suppressing Th1 and Th17 responses”. Molecular Neurobiology, 2016.

Lunn DP, Davis-Poynter N, Flaminion MJBF, et al. “Equine herpesvirus-1 consensus statement”. Journal of Veterinary Internal Medicine, 2009.

Lunn DP, Morley P. EHV-1 Information. Colorado State University, Dept Clinical Sciences, 2011.

Muller N. “Essay on protecting your horse for EHV-1”, Los Caballos Equine Practice (Galt, CA). ePub. Takayama Y, Itoi M, Hamahashi T, et al. “Moxibustion activates host defense against herpes simplex virus type 1 through augmentation of cytokine production”. Microbiology and Immunology, 2010.

Traub-Dargatz JL, Pelzel-McCluskey AM, et al. “Case-control study of a multistate equine herpesvirus-1 myeloencephalopathy outbreak”. Journal of Veterinary Internal Medicine, 2013.

Walter J, Seeh C, Fey K, et al. “Prevention of equine herpesvirus myeloenceophathy – is heparin a novel option?” Tierärztliche Praxis Großtiere, 2016.

Williams JE. “Review of antiviral and immunomodulating properties of plants of the Peruvian rainforest with particular emphasis on Uña de Gato & Sangre de Grado”. Alternative Medicine Review, 2001.

Reed S. “Data on Finley EHV1 Outbreak”, personal communication with Dr. Julie Wilson, 2004.

Wilson J. “Vaccine Efficacy and Controversies”. AAEP Annual Proceedings, 51: 409-420. 2005.


Dr. Kimberly Henneman is a 1986 graduate of Purdue University School of Veterinary Medicine. She is certified in veterinary chiropractic (AVCA), and was the 12th to pass AAVA’s advanced acupuncture exam. She is certified by IVAS in veterinary Chinese herbal medicine; has trained in classical veterinary homeopathy; studied Traditional Chinese and Tibetan Veterinary Medicine in China; and is working on a Master's degree in TCVM from the Chi Institute and the Southwest Veterinary College in Sichuan, China. Dr. Henneman’s practice in Park City, Utah is 50% performance equine and 50% companion animal, and uses integrative therapies and thermal imaging in sports medicine, rehabilitation and chronic disease ( She is currently the only veterinarian in the US to be board-certified in both equine and canine sports medicine and rehabilitation.