Thursday, May 19th, 2011 | Filed under Handy Tips

I’m sure that most of you have been inundated with email blasts regarding the current outbreak of EHV-1 virus.

If not, this is important information to know.

DO NOT PANIC – educate yourselves and make a game plan.  I’m sure the next few weeks will reveal much more information.  But for now, learn about this and make informed decisions.

First, an email I received from my vet:


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May 17, 2011: EVI-1/ EHM Outbreak and Best Practices

Many of you have called with concerns about an outbreak of equine herpes virus myeloencephalopathy(EHM) affecting an unconfirmed number of horses in the U.S. and Canada. As of this morning, there are 6 confirmed cases in California. None are known in Nevada County.  EHM is caused by the EHV-1 neurologic virus and is easily spread. The incubation period of EHV-1 infection is typically 1-2-days, with clinical signs of fever occurring, often in a biphasic fever, over the following 10 days. A biphasic fever would be constant for several days, spontaneously reduce for 1 or 2 days, and then increase again. When neurological disease occurs it is typically 8-12 days after the primary infection, often starting after the second fever spike. In horses infected with the neurologic strain of EHV-1, clinical signs may include: nasal discharge, incoordination, hind end weakness, recumbency, lethargy, urine dribbling and diminished tail tone. Fatality is high. This recent outbreak appears related to initial cases at a cutting horse show in Ogden, Utah, which was held from April 29 – May 8. Horses at that event may have been exposed to this virus and subsequently spread the infection to other horses. While the true extent of this disease outbreak is uncertain, we must all be concerned with this latest outbreak and respond appropriately.

What should the horse owner do?

Biosecurity (containment and isolation of the disease) is the most important initial response. Horse-to-horse contact, aerosol transmission, and contaminated hands, equipment, tack, and feed all play a role in disease spread. Like the human cold, the EHV-1 virus spreads easily. Therefore, we are advising owners to keep their horses at home for the next two to three weeks minimizing exposure. Prevention (in this case lack of exposure) is the best cure. If your horse shows any signs of illness, be sure to take their temperature, separate them and call us.

Should you vaccinate?

Currently, there is no equine vaccine that has a label claim for protection/prevention (100%) against the neurological strain of the virus. However, there are several vaccines which have shown some benefit, as have immunomodulators (Zylexsis). These may be appropriate for stables or barns with horses coming and going. Vaccines and immunomodulators may also be beneficial for horses that must travel or show (not recommended). If you have a horse that may have been exposed or is showing ANY of the signs of EHM, DO NOT vaccinate.  Contact us and we will review the best options for your situation. We have an excellent EHV-1 vaccine and Zylexsis, if this is appropriate for your situation.

The following are excellent resources/links for better understanding the current outbreak along with suggestions for best practice. We will be updating our website with more information in the days to come.

UCD, Center for Equine Health

Vaccines and EHV-1


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EHV-1 Outbreak: State Veterinarians Monitoring Closely

by: Erica Larson, News Editor
May 18 2011, Article # 18262

The number of horses affected by the neurologic equine herpesvirus-1 (EHV-1) outbreak in the western United States and Canada continues to rise. Since health officials identified the virus as stemming from involvement with the National Cutting Horse Association (NCHA) Western National Championship competition, held April 28-May 8 in Ogden, Utah, the number of confirmed cases has reached the double digits.

Equine herpesvirus is highly contagious and can cause a variety of ailments in horses, including rhinopneumonitis (a respiratory disease mostly of young horses), abortion in broodmares, and myeloencephalopathy (evident in the neurologic form). The virus is not transmissible to humans. Clinical signs of the neurologic EHV-1 form include fever, ataxia (incoordination), weakness or paralysis of the hind limbs, and incontinence. The virus is generally passed from horse to horse via aerosol transmission (when affected animals sneeze/cough) and contact with nasal secretions.

State and provincial veterinarians are keeping close tabs on the number of confirmed and suspected EHV-1 cases. Since there is no centralized data reporting system, case counts are based on reports from individual states (although the Veterinary Infection Control Society has been collecting information regarding suspected and confirmed cases).

Utah State Veterinarian, Bruce L. King, DVM, said, “At this point we have had no confirmed cases of EHV-1 in Utah since the NCHA show that was held in Ogden. We do have some suspect horses that were at the show that laboratory confirmation is pending.”

California now has 10 confirmed cases of EHV-1, according to a news release from the California Department of Food and Agriculture. A total of 54 horses from California competed at the NCHA championships.

Canada veterinarians also are on the lookout for cases. “Our office has been notified of one case of (neurologic) EHV-1 in the province of Alberta,” said Chief Provincial Veterinarian Gerald Hauer, DVM. “The horse has been attended by a veterinarian, is isolated, and is recovering.”

Idaho agriculture released a statement indicating that two horses were euthanized that exhibited clinical signs consistent with neurologic EHV-1 and “several others” are under veterinary care, but there have been no confirmed cases.

Washington saw its first EHV-1-positive case discharged from Washington State University’s (WSU) Veterinary Teaching Hospital on May 13. Today Debra C. Sellon, DVM, PhD, Dipl. ACVIM, a professor of equine medicine at WSU, reported that since the discharge all PCR diagnostic tests run on hospital patients have been negative, and the university hospital remains under quarantine.

Today (May 18) a second case of EHV-1 was confirmed. The horse presented with neurologic signs and was taken to Pilchuck Veterinary Hospital (PVH) in Snohomish. Attending veterinarian Wendy Mollat, DVM, Dipl. ACVIM, said in a press release issued by the hospital that upon presentation to PVH, “the horse was immediately admitted to our designated equine isolation facility and has not been allowed direct or indirect contact with our general equine population. The horse is clinically stable and will remain hospitalized until confirmed to be no longer shedding the virus.”

Oregon confirmed its first case of EHV-1 today when one horse that attended the NCHA competition produced a positive test. A press release issued by the state Department of Agriculture indicated there are no travel restrictions on non-exposed horses at this time.

New Mexico Livestock Board officials issued a press release from indicating that there are two suspected EHV-1 cases in that state, however both horses are currently under quarantine and veterinary treatment. There were no confirmed cases at press time. Additionally, all New Mexico horses that attended the NCHA show are under voluntary quarantine at their regular barns and are being monitored for signs of disease.

Currently Unaffected States on High Alert

Fortunately, authorities in some Western states have not received reports of any EHV-1 positive horses. Nonetheless, veterinarians and horse owners remain on high alert and are taking precautionary steps to prevent the spread of any possible disease.

“At this time there are no EHV-1 positive or suspect horses in Wyoming that we are aware of,” said State Veterinarian Jim Logan, DVM. “We do know that there were some horses that attended the Ogden event and returned to Wyoming. All of these animals have been accounted for, are isolated, and under movement restriction and observation by veterinarians. None have shown any clinical signs to date.”

The Texas Animal Health Commission (TAHC) issued a statement yesterday (May 17) indicating that there are no EHV-1 positive horses in the state currently, and the commission “has no plans to change entry requirements for equine animals or to cancel any equine events at the present time.”

Both North and South Dakota state veterinarians–Susan J. Keller, DVM, and Dustin Oedekoven, DVM, respectively–said that EHV-1 has not been confirmed in their states. Oedekoven added that none of the horses that attended the Ogden show from South Dakota were showing any clinical signs of the disease.

Likewise, State Veterinarian Phil LaRussa, DVM, said Nevada is free of EHV-1 at present.

Shifting Transportation Requirements

The Colorado Department of Agriculture issued a press release yesterday, introducing new travel requirements for any horses entering the state.

Horse owners must obtain a permit to enter the state in addition to presenting the standard health certificate (issued within 30 days prior to entry) and a proof of a current negative Coggins test.

“Horse owners who wish to bring their horse into Colorado must first call their veterinarian,” the department press release stated. “That veterinarian can then contact the Colorado Department of Agriculture’s State Veterinarian’s Office at 303/239-4161 and request a permit number. That number would then be included on the health certificate.”

Understanding Equine Medications is your A-Z guide to learning more about generic and brand-name pharmaceuticals, possible side effects and precautions, and proper dosage.

“We are considering all of our options for protecting Colorado’s horse industry,” State Veterinarian Keith Roehr, DVM, said in the press release. “At this point, we do not believe it’s necessary to stop horses from entering the state but we need to be able to know where those horses are coming from and where they are going; trace-back is a vital part of disease control.”

Yesterday the NCHA released a statement on its website indicating that while the organization was not requiring show producers to cancel or reschedule shows, it urged these individuals to make horse health the No. 1 priority: “While the NCHA is at present not mandating cancellation of all shows nationwide, we do strongly urge all show producer(s) to consider the possible horse health risks of conducting an event until the extent of the virus can be determined and contained.”

Additionally, the American Paint Horse Association released a list of shows canceled due to the EHV-1 outbreak on the organization’s website.



EHV-1 Vaccination

While there are several vaccines available for protection against both respiratory and abortogenic forms of EHV-1, at this time there is no equine vaccine that has a label claim for protection against the neurological strain of the virus. Consult your veterinarian for further guidance if you are considering the use of EHV-1 vaccines.

Discussion of EHV-1 Vaccination for Veterinary Professionals

Vaccination for Equine Herpesvirus-1 Myeloencephalopathy: The Dilemma
By W. David Wilson BVMS, MS
The goal of vaccination is to induce resistance to infection and disease by eliciting a strong and durable immune response without inducing clinical signs of disease in the vaccinated animal. Traditionally, this has been accomplished by altering disease-causing infectious organisms so that they no longer cause disease but retain the antigens necessary for inducing a protective immune response.

Until recently, only two types of vaccines have been available for use in horses and other animals. These include inactivated (killed) vaccines containing “dead” organisms, and attenuated or modified live vaccines (MLV) containing living organisms that have been attenuated in virulence so that they multiply in the host after administration but do not cause disease. While the introduction of West Nile virus infection into North America in 1999 has resulted in devastating consequences to the horse, human and bird populations of many states, it has also spurred the development and licensing of advanced vaccine technologies-including recombinant vectored, chimera, and DNA vaccines-that will greatly benefit equine health in the future.

The ideal vaccine would completely block infection by inducing so-called sterile immunity. In this instance, not only is clinical disease prevented when the vaccinated animal is exposed to the infectious agent, but shedding of that virus or bacterium by the vaccinated animal is also prevented, and the animal’s potential to act as a source of contagion to infect other animals is eliminated.

In reality, few vaccines accomplish this goal. They may prevent development of clinical disease or at least reduce the severity of signs associated with infection by limiting multiplication of the infectious agent in the vaccinated animal. In turn, shedding of the infectious agent by the vaccinated animal is reduced but not eliminated so that the potential to transmit infection to other animals remains.

For most diseases, the best vaccines stimulate immune responses that closely mimic those that result from recovery from natural infection. Thus, the maximum possible degree and duration of protection induced by a particular vaccine can usually be predicted based on the effectiveness of the immune response resulting from natural infection. It is unreasonable to expect traditional MLV or inactivated vaccines to induce immunity that is stronger than that resulting from recovery from field infection.

In the case of EHV-1 and many other herpesviruses, resistance to re-infection resulting from recovery from field infection is short-lived, lasting only a few weeks to a few months. EHV-1 infects the horse through the respiratory tract and rapidly becomes internalized by cells, including circulating lymphocytes. It is then passed directly from cell to cell without an extracellular phase during which the virus could otherwise be exposed to neutralizing antibodies and other immune effectors. As such, EHV-1 vaccines would need to satisfy a challenging set of demands in order to be highly effective.

The ideal EHV-1 vaccine would not only be safe and lend itself to efficient delivery, it would also invoke strong and persistent local humoral (virus-neutralizing antibody) and cellular (cytotoxic T-lymphocyte, CTL) responses at the level of the mucosal lining of the respiratory tract in order to block infection. In addition, it would induce durable systemic humoral and CTL responses to rapidly clear free virus and destroy virus-infected cells in the event that the mucosal response was not successful in blocking infection. Beyond that, the ideal vaccine would be capable of inducing this broad spectrum of immune responses in foals at a young age to protect them against the field challenge that inevitably occurs during the first year or two of life and leads to a chronic latent-carrier state.

Currently Available EHV-1 Vaccines

Commercially available vaccines currently include two single-component inactivated vaccines (Pneumabort K and Prodigy) marketed for the prevention of EHV-1-induced abortion in pregnant mares; several multicomponent inactivated vaccines (Prestige, Calvenza, Innovator); and one MLV vaccine (Rhinomune) marketed for prevention of respiratory disease induced by EHV-1 and EHV-4. All are administered by intramuscular injection. Each of these vaccines induce some but not all of the desired components of the immune response. None induces sterile immunity or complete protection from clinical disease. The best that can be hoped for is a reduction in the severity of clinical signs and in the amount of EHV-1 shed by vaccinated horses that do become infected, which in turn may reduce the incidence of disease within the herd or group.

Prospects of Vaccinating to Prevent EHV-1 Myeloencephalopathy

Frequent revaccination of mature horses to prevent EHV-1 myeloencephalopathy is not clearly justified in most circumstances for the following reasons:

  • Most mature horses have been infected previously with EHV-1 and are latent carriers of the virus.
  • EHV-1 myeloencephalopathy is a relatively rare disease from a population standpoint.
  • Currently available vaccines do not reliably block infection or claim to prevent myeloencephalopathy.
  • EHV-1 myeloencephalopathy has been observed in horses vaccinated against EHV-1 regularly at 3- to 4-month intervals.
  • Vaccination has been cited by some as a potential risk factor for development of myeloencephalopathy.

On the other hand, regular revaccination of pregnant mares and other horses on breeding farms to reduce the risk of EHV-1-induced abortion is strongly recommended.

An Issue with Difficult Pros and Cons

In the wake of recent outbreaks of EHV-1 myeloencephalopathy in diverse populations of horses in several regions of North America, many racing jurisdictions and managers of equine facilities and events have imposed EHV-1 vaccination requirements for incoming and resident horses in the hope that manifestations of EHV-1 infection, particularly EHV-1 myeloencephalopathy and the febrile phase that typically precedes the onset of neurological signs, can be prevented.

The efficacy of this approach remains to be proven. However, there is little doubt that enforcement of strict biosecurity measures and hygiene practices are likely to be more effective than widespread vaccination in reducing the risk of acquiring infection. Nevertheless, recent research demonstrates that viral shedding is much reduced in horses with high circulating titers of virus-neutralizing (VN) antibody, as well as in horses that have been vaccinated recently with the Rhinomune MLV vaccine. Of the available inactivated vaccines, Calvenza and both vaccines marketed for prevention of abortion (Pneumabort K and Prodigy) contain the highest amounts of antigen and stimulate the highest levels of VN antibody.

On premises with confirmed clinical EHV-1 infection (myeloencephalopathy, fever, respiratory disease, or abortion), booster vaccination of horses that are likely to have been exposed already is not recommended. However, it seems rational to booster-vaccinate nonexposed horses and horses that must enter the premises with one of the four vaccines listed above (i.e., Rhinomune, Calvenza, Pneumabort K, or Prodigy) if the horses have not been vaccinated against EHV-1 within the previous 90 days. This approach relies on the reasonable assumption that the immune system of most mature horses has already been “primed” by prior exposure to EHV-1 antigens through field infection or vaccination and can, therefore, be “boosted” within 7 to 10 days of administration of a single dose of EHV-1 vaccine.

While this approach by no means guarantees protection of individual horses against the potentially fatal neurological consequences of EHV-1 infection, the hope is that reduced nasal shedding of infectious EHV-1 by recently vaccinated horses will indirectly help protect other horses by reducing the dose of virus to which they are exposed.

A recent publication described a study in which 5 Rhinomune-vaccinated horses, 5 horses vaccinated with an inactivated multicomponent respiratory EHV-1 vaccine, and 5 nonvaccinated control horses were all challenged with the Findlay ’03 neuropathogenic strain of EHV-1 by nasal spray. The results of this study provided preliminary evidence that recent vaccination with Rhinomune may provide some protection against EHV-1 myeloencephalopathy. Similarly, a study published in 1978 by the manufacturers of Pneumabort K to test the effectiveness of this vaccine in preventing abortion provided tentative evidence for induction of at least partial protection against EHV-1 myeloencephalopathy.

While results of these studies are encouraging, they should be interpreted with caution because of the relatively small number of horses used, the difficulty that researchers have experienced in the past in designing a challenge model that produces consistent results, and the knowledge that in the field, high morbidity outbreaks of EHM have been encountered in regularly vaccinated horses. Undoubtedly, further research is needed before definitive conclusions can be drawn.

Summary of EHV-1 Vaccination Dilemma

  • Current EHV-1 vaccines do not prevent primary infection or establishment of the latent carrier state.
  • Maternally derived antibodies interfere with the response of most foals to vaccination with inactivated and MLV vaccines until foals are 5 months of age, and likely older in some cases. If we wait until 5 months or older to start vaccinating foals, many (or most) will encounter field challenge, become infected with or without clinical signs (nasal discharge, cough, fever), and fail to clear the virus, thereby establishing a chronic latent carrier state. This problem applies to both EHV-1 and EHV-4. The introduction and widespread use of EHV vaccines on stud farms in Australia in recent years has failed to reduce the incidence of EHV-1 and EHV-4 infection and disease in foals, weanlings and yearlings.
  • Available vaccines do not reliably prevent infection, even when they induce high antibody titers. They do, however, variously accomplish (a) a reduction in (non-neurologic) clinical signs after challenge in the experimental setting, at least when challenge is performed within a few weeks following vaccination, and (b) reduced viral shedding from the respiratory tract after challenge, thereby helping reduce the magnitude of challenge experienced by other horses and potentially helping reduce spread to minimize outbreaks. The latter point makes sense from a clinical standpoint but has not been proven in either the field or experimental setting.
  • Most mature horses are latent carriers of EHV-1 and EHV-4. Therefore, most of the horses to which vaccines are administered are already latently infected.
  • Available vaccines do not claim to prevent EHV-1 myeloencephalopathy.
  • The incidence of EHV-1 myeloencephalopathy seems to be increasing in both North America and Europe, in countries that vaccinate against EHV-1/4 regularly, as well as in countries such as Holland, where EHV vaccines are rarely used.
  • EHV-1 myeloencephalopathy has been encountered in horses vaccinated regularly at 3- to 5-month intervals.
  • Pregnant mares appear to be at increased risk of developing EHV-1 myeloencephalopathy, even though this is the population of horses that is most intensively vaccinated against EHV-1.
  • Vaccination has been cited as a risk factor for development of EHV-1 myeloencephalopathy in one (unpublished) paper written to report on an outbreak in 1984 involving many horses on at least five premises in Southern California.
  • The paper reporting the 2003 outbreak at the University of Findlay showed a strong trend toward increased risk of developing EHV-1 myeloencephalopathy in frequently vaccinated horses. However, this result was likely confounded by the finding that horses more than 5 years of age were at increased risk of developing neurological disease compared with younger horses. The older horses in the Findlay outbreak were also the ones that were vaccinated most frequently against EHV-1. This age-associated risk has been documented in other studies.


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HORSE AND MAN is a blog in growth... if you like this, please pass it around!

3 comments have been posted...

  1. Paul Atteberry

    All options should be explored before deciding on euthanasia. We have just developed a new homeopathic formula that has a great success rate with this disease,it retails for $17.99 per spray vial and can be found at:

  2. Linda Horn

    Thanks for the info. Your research is always so complete. We can never be too careful with the heath and lives of our families and beloved companions. Let’s hope everyone is watchful and responsible so more horses (llamas and alpacas as well?) aren’t infected.

  3. Sharon

    Thank you so much for all of the useful information. What a frightening disease!!

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