UC DAVIS EQUINE HERPES OUTBREAK SEMINAR

UC Davis School of Veterinary Medicine

I have just returned from a trip to UC Davis after attending the “Equine Herpes Outbreak Seminar” on the evening of May 27, 2011. This was an excellent seminar which sponsored six of California’s leading veterinary minds on the topic of the recent Equine Herpes Virus (EHV-1) outbreak. This disease is also referred to as Equine Herpes Myeloenchephalopathy (EHM) when the herpes virus causes neurologic disease .  Speaker topics addressed the current status of EHV-1 in North America, the regulatory and reporting issues, diagnostic challenges, treatment in both the field and in hospital settings, isolation and biosecurity procedures for affected horses and the use of vaccination during an EHV-1 outbreak.

Since the majority of you have been carefully reading and following the details
about EHV-1 and the recent west coast outbreak, I will omit a review of EHV-1 basics. There were many details about this disease revealed last Friday night that represent a new understanding for me and so may be of interest to you. I will begin with a summary  of the details that have been well researched, and include a list of the observations and questions now being discussed and investigated. 

[The following information was gathered by me during the lectures and social/dinner hour preceding the talks. The information often came in a “fast and furious” manner and I may have omitted some important details in the understanding and handling of this disease outbreak. If you have any questions about EHV-1, please seek a professional opinion for your specific situation and do not rely on this brief summary.  Much is unknown about this disease so please understand that details and opinions may change rapidly.]

“The Take-Home”

• As of May 27, the general tenor of the investigators suggests that we may be turning a corner for the better in this local EHV-1 outbreak. Each passing day will determine if this is true or not.
• The neurotropic EHV-1 virus is NOT a new virus, it has been around for decades.
• Equine Herpes Myeloencephalopathy (EHM) is a disease syndrome and is NOT always associated with a single genotype of the virus (so called neurotropic strain).  This suggests that other factors may also contribute to the onset of this neurologic disease.
• EHV-1 neurologic disease is primarily caused by a specific neurotropic   strain of  EHV-1 as identified by PCR testing,  HOWEVER,  15 to 24% of the neurologic cases can be caused by the more common “non-neurotropic” virus.
• Some type of “viral promotor” may be necessary to produce the neurotropic form of the disease.
• The neurologic form of EHV-1 infection is rare in other species. Other equids may shed the virus but usually they will be subclinical and not show signs of infection.
• PCR testing of BOTH nasal swabs and anticoagulated blood (EDTA blood) is the most effective method of making a diagnosis in the live animal.  
• Sampling of non-exposed, normal appearing  horses is NOT recommended (up to 3% of all horses in the population will test PCR positive due to the latent nature of herpes viruses in general).
• Of those horses infected with the neurotropic genotype of the virus, about 30 to 35% will develop outward signs of neurologic disease.
• Of those horses that develop clinical signs of neurologic disease there is about a  30% mortality rate. (Current outbreak in California has a 14% mortality rate, throughout the entire USA, a 39% mortality rate has been reported). 
• Those patients that go down and are recumbent have a poor prognosis, those that can remain standing have a much better prognosis.
• Those horses with neurologic damage that survive would be expected to appear clinically normal in about 6 months, all survivors have shown normal athletic ability in 12 months. In other words, nearly all of the horses that survive are expected to return to normal with time.
• Affected horses can shed the virus from 2 to 4 weeks, average is 16 days. The virus will usually survive on skin or clothing for about 4 hours.  Longest survival outside the host under perfect conditions is about 7 days.
• Although contagious, the EHV-1 virus is a large, heavy virus and CANNOT be ‘sneezed’ large distances.   30 feet is about the maximum distance it can travel in the air naturally.  (Dr. Madigan questioned the use of the phrase “highly contagious”  due to this physical property of the virus.)
• EHV-1 induces neurologic disease by causing damage to the blood vessels which induces thrombosis or blood clots that damage and causes necrosis of the nervous tissue. It does not directly attack nervous tissue
• Incubation period of the disease (the time elapsed between exposure and onset of clinical signs) is usually about 6 to 10 days, however the range is 2 to 14 days.
• The first clinical sign observed is usually fever, sometimes with edema of the legs. Some patients will show a nasal discharge. Later you may see the full-blown neurologic symptoms of weakness, lethargy,  ataxia, urinary and/or fecal incontinence. In the most severe cases, these symptoms may all occur immediately at the first indication of infection.
• EHV-1 infection in the general equine population is exceedingly common throughout the world and is assumed to have occurred in most horses during their lifetime. Almost all horses older than 2 years of age have been exposed to EHV-1.
• The neurologic form of this disease is very rare in horses during their initial infection with EHV-1. Therefore, EHV-1 neurologic disease is much more common in older horses, not younger horses.
• Treatment of this disease is primarily supportive (intravenous fluids, anti-inflammatories). Pentoxyfylline is often used to improve the impaired blood supply due to the effects of the vascular damage. Human anti-viral drugs are also being used. Good nursing care  (slings, rolling, bladder catheterization, etc.) is imperative for a successful outcome.
• The effectiveness of anti-viral drugs is being investigated. (Acyclovir, Valacyclovir and Ganciclovir are the three most common drugs utilized)  The earlier they are begun in the course of the disease, the better the expected result. Some clinicians have suggested that their helpfulness may be quite limited if begun AFTER neurologic signs are seen as the vascular damage has already occurred. Best if used on horses with a known exposure, prior to clinical signs.  In the real world, it is very tough to do that and quite expensive.
• NO VACCINE for this disease has been proven to be helpful. None have a label claim to protect against the neurologic form of the disease.
• Keep in mind that NO vaccine can block infection of ANY of the herpes viruses. They can only aid in minimizing the propagation of the virus in the host and thereby minimize the damage due to the virus.
• It was recommended that if a horse has a known, direct exposure, best to NOT vaccinate.
• In THEORY, two categories of  “rhino” vaccines are more LIKELY to be helpful if there is any benefit to be had from immunization;

    1. A modified live vaccine which induces a greater immune response. (Considered the likely “best guess” by many of the attendees. There was a lively discussion of a possible increase in adverse reaction rates in the live vaccine such as pain at the site of injection, colic etc. Several attendees opined that those reactions seemed to be associated with specific batches and not with the vaccine brand as a whole.)

OR

    2. One of the vaccines labeled for the protection of equine herpes viral ABORTION. The vaccines approved to aid in the prevention of herpes virus abortion have a higher antigenic load and MAY stimulate some cell-mediated protection. One other vaccine, Calvenza, labeled for protection against the respiratory form of EHV only, theoretically may be helpful due to its unique EHV-1 antigen and novel adjuvant. Probably best if administered I.M. and not intra-nasally.

• Other EHV vaccines labeled for protection against the respiratory form of EHV would be unlikely to provide protection against the neurologic form of this disease.
• Many disinfectants can be effectively used to control the virus in the environment including bleach (8 oz/1 gal of water). All disinfectants require that organic material be removed first in order to be effective viral killers. This is best accomplished with liquid soap and water and “elbow grease” followed by application of the disinfectant as a second step. Simply washing your hands with soap and water or hand sanitizers IS an effective method of minimizing transfer of the virus. 
• Click HERE for excellent advice about disinfecting protocols and equine biosecurity for horse owners and equine professionals. This information is easy to read and can be readily implemented by horse owners. Written by Drs. John Madigan and Rick Arthur.
• When a suspected horse is identified as likely to be infected with EHV-1 neurologic disease, that horse should be quickly and immediately REMOVED  from the facility, race track or show venue and physically isolated from all other horses. Attempting to appropriately isolate that horse on the grounds has proven to be unreliable and largely unsuccessful.
• As of May 27, 2011, the last confirmed case of EHV-1 in California was diagnosed on May 23, 2011.

Speakers included :

Drs. David Wilson,  Gary Magdesian, Nic Pusterla, and Dominic Dawson from UC Davis, and Dr. Kent Fowler from the  California Department of Food and Agriculture.