Equine Herpesvirus: Background, Current Vaccines and Prevention
From the January 2007 Newsletter:
With the recent news of Equine Herpesvirus infections at the University of Connecticut and Fairfield Equine Hospital, on top of the well-publicized outbreaks in Florida and elsewhere, I’d like to offer a little perspective. There are a lot of rumors going around and I’ve seen some misinformation on web sites and other information sources. I’ve even seen a little bit of panic.
If you haven’t heard the news regarding Equine Herpesvirus at the University of Connecticut, Fairfield Equine Hospital, or in Florida, you can get some basic background on my blog. Click these links for each story: Outbreak in Wellington Florida, Fairfield Equine Hospital quarantine, Equine Herpes Virus at UConn.
Background
Equine Herpesvirus (EHV-1) is a virus of a ubiquitous nature. EHV-1 causes respiratory disease, abortion in pregnant mares, neo-natal mortality and neurological disease — sometimes severe and paralytic in its effect. As many as 70 to 80% of horses are latently infected with the virus. In latently infected horses, the virus lies dormant (in the lemphocytes and the cranial nerves) sometimes to resume activity during times of stress. EHV-1 is spread by direct contact with an infected animal or by contact with an “infected” object or substance (tack, water buckets, blankets, etc.)
The respiratory form of EHV-1 primarily affects young horses. Clinical signs generally include a snotty nose, loss of appetite, and a nagging cough. This presentation of the virus has what is known as a high morbidity but low mortality rate. That is, it is highly contagious and many horses may get sick but most young horses recover uneventfully.
The neurological from of the disease as well as the abortions caused by the virus are of greater concern. As mentioned above, the neurological form of EHV-1 can be severe in its effect, sometimes leading to the euthanasia of the infected horse.
EHV-1 generally first infects the upper respiratory tract of the horse. After infecting the upper respiratory tract, the virus quickly becomes intracellular in nature and spreads rapidly throughout the body, sometimes leading to areas of secondary infection. It is thought that when these areas of secondary infection include the reproductive organs, we see abortion in pregnant mares. Though less well established, it is thought that when the area of secondary infection includes the spinal chord, we see the neurological effects.
The Vaccines
Generally, there are two types of vaccines against EHV-1 currently on the market. There are those that used a killed form of the virus to stimulate immunity in the horse and those that use a modified live form of the virus. The modified live virus vaccines use a form of the virus that has been altered in such a way to make it less pathogenic, meaning it is less virulent and more easily combatted by the horse’s immune system.
Killed Virus Vaccine:
Killed virus vaccines elicit an antibody response in the horse, especially in the upper respiratory tract — in the mucosal tissue. This mucosal antibody response provides a first line of defense at the site of primary EHV-1 infection.
Despite this antibody response, killed virus vaccines may not prevent infection in a given horse. The protective antibody response may be too short lived or too weak, the exposure to EHV-1 may be too great, and the particular EHV-1 virus may be too virulent, that killed virus vaccination is inadequate to prevent infection. When EHV-1 becomes intracellular in nature, moving beyond the upper respiratory tract, the immune response stimulated by killed virus vaccines offer little or no protection. In a given horse, killed virus vaccines do not offer protection against the neurological form of the virus.
Killed virus vaccines do reduce the duration and amount of viral shedding during times of infection, meaning the horse is less infectious towards other animals. Killed virus vaccines may also reduce the severity of clinical signs in an infected horse. This reduction in viral shedding, and possible reduction in severity of clinical signs, may be the primary benefits of killed virus EHV-1 vaccines.
To the extent that killed virus vaccines reduce the spread of EHV-1, by reducing viral shedding, they do protect horse populations from the clinical effects of EHV-1 infection: abortions, the neurological effects and respiratory disease. In my opinion, though, because killed virus EHV-1 vaccines elicit a relatively poor immune response, they should be used only in low disease pressure environments.
Modified Live Virus Vaccine:
Modified live EHV-1 vaccines stimulate, in addition to an antibody response, a cellular immune response. Modified live virus (MLV) vaccines have been shown, in challenge studies, to stop or reduce cell associated viremia. In other words, they prevent the virus from getting into the blood stream. Like killed virus vaccines, MLV vaccines also reduce the amount and duration of viral shedding as well as the severity of clinical signs in an infected horse.
Protection against EHV-1 induced abortion and neurological disease is thought to be correlated with a cellular immune response (specifically a cytotoxic T lymphocyte response) — as opposed to antibody production. Because of this correlation and because MLV vaccines produce a better cellular immune response, it is thought that EHV-1 MLV vaccines may offer some protection against neurological disease and against abortion. I emphasize the word may here and it is important to state that no vaccine exists labeled or proven for protection against the neurological form of EHV-1.
The immune response elicited by MLV vaccines is not perfect and we are still waiting for the day when there is a vaccine that produces a superior cell mediated immune response. It is also important to note that just because a horse is infected with the neurological strain of the virus it does not mean that horse will have neurological effects. There appear to be other factors that may contribute to manifestations of EHV-1 including season, breed and gender. It isn’t clear exactly the combination of factors that produce the neurological effects and so it isn’t clear how vaccination may combine with these factors to provide protection — or fail to do so.
There are down sides to the MLV vaccines. MLV vaccines cause immunosuppresion in the horse for a short period of time after vaccination — possibly making the horse vulnerable to secondary infection. Vaccination may actually cause cell associated viremia, meaning the horse is actually infected with EHV-1, though the vaccine virus has low pathogenicity. For these reasons, it is recommended that horses not be transported, shown, or stressed after vaccination for a period of at least 7 days. It should be said, though, that this recommendation goes for any vaccination, modified live or not.
MLV vaccines offer short-lived protection and require frequent revaccination. The current MLV vaccines on the market are only labeled for 3 months of protection. Horses vaccinated with the modified live virus may also shed the vaccine virus. The shed vaccine virus may pose a hazard to other horses, though most researchers believe the shed vaccine virus is of limited concern.
Despite the down side to current MLV vaccines, I recommend them for horses in high disease pressure environments — such as for show and performance horses. Generally, the risks associated with MLV vaccines are small. MLV vaccines can be given safely and they appear to elicit a superior immune response versus killed virus EHV-1 vaccines.
EHV-1 Immune Boosters
Zylexis is the brand name of the primary EHV-1 immune booster currently on the market. Zylexis stimulates a horse’s immune system in the face of the virus. Though it does not prevent EHV-1 infection, Zylexis does significantly reduce the clinical signs associated with the disease. Zylexis does not offer durable immunity in the face of EHV-1.
Most of the data surrounding Zylexis is in weanlings and yearlings and not in adult horses. The effects of Zylexis appear to be quite short lived and indeed in the studies that show the effectiveness of Zylexis, the product was given day 0, 2 and 9 pre EHV-1 challenge as well as 2 and 5 days post challenge. These results suggest that Zylexis should be used strategically to help limit the disease in times when a horse is going to be predictably under stress and when a horse is predictably in a high disease pressure environment.
Zylexis is not a miracle drug but can be used strategically in a larger plan to combat EHV-1.
As a side note, vaccinating an EHV-1 infected horse (with either a killed virus or modified live virus vaccine) should not be done. Zylexis may provide some benefit in the face of a recent infection.
General Prevention
While vaccinating your horse for EHV-1 is important, none of the vaccines offer great or perfect protection against EHV-1. It is important to take other precautions to help prevent the spread of the virus and of other infectious disease. While a few of these recommendations relate specifically to EHV-1, most the recommendations represent good and basic management practice — always.
Don’t share stuff. This includes medications like Bute and Banamine, trailers, stalls, tack, blankets, water buckets, brushes, and most anything else that comes in contact with other horses. If you must share items, disinfect them before use. EHV-1 can be easily killed with common disinfectants.
Isolate new horses. When a horse comes into you barn, isolate him away from the other horses for a period of 21 days. Isolate horses for this time period so that you can watch for evidence of infection. Take the isolated horse’s temperature twice daily. Don’t allow traffic from the general barn horse population to the isolation area. Only introduce the horse to your barn after the isolation period passes with a symptom free horse.
Minimize traffic in your barn. Dogs, cats, your sister-in-law, your friend or your neighbor, your farrier, equine dentist or trainer, if they don’t have to be in the barn then they shouldn’t.
When your horse is being exposed to new horses, monitor him closely. Take your horse’s temperature; look for respiratory signs and other signs of distress. Continue to monitor your horse for several days after the initial exposure to new horses.
Wash your hands in between the handling of horses.
For the more worried among us, implement foot baths and other means of disinfecting at all entrances to your barn.
During times of travel and other stress, monitor your horse closely for signs that a possibly latent EHV-1 infection is reappearing.
Take the above suggestions and set rules for your boarders and for anyone entering and exiting your barn. Educate those people about your barn rules and then hold them to account for following them.
For more detailed prevention guidelines see the link titled “Prevention and Control of Equine Herpes Virus” at the end of this post.
What to do NOW in face of infections at the University of Connecticut
First, don’t panic. The risk to our horses is small, even with cases so close to home. The cases at UConn and Fairfield Equine Hospital appear to be isolated and contained.
If your horse doesn’t travel, isn’t in a boarding facility, isn’t being exposed to new horses, then the risk of EHV-1 infection in your horse is minimal. I would not recommend revaccinating horses in this low risk category.
If your horse is stabled at a boarding facility, is traveling to Florida, other shows, or for training, then the recommendations would be different. First, in the case of travel, consider if it is necessary. When introducing your horse to new or larger horse populations, take precautions and make sure your horse’s vaccinations are up to date. Second, take the above recommendations regarding general prevention and implement them. If you’re in this category of showing, traveling, stabling at a boarding facility, and you are not taking infectious disease prevention measures, you are asking for a problem — be it EHV-1 or another infectious disease. Third, call your veterinarian and discuss your travel plans and stabling arrangements. Discuss your vaccination history and current vaccination protocol in light of recent news. It may be appropriate to update your horse’s EHV-1 vaccination if his last vaccination fell outside the last 3 months
It is important that we all stay educated and vigilant in the face of infectious diseases like EHV-1. But also know that EHV is an endemic disease in horse populations. It has been around from before your horse was born and will be around after we are all gone. Generally horses, if infected, recover. There is currently no EHV epidemic or outbreak of a wide scale. Keep a level head.
This post reflects far from the whole story regarding EHV-1. There are some real points of subtlety that I’ve glossed over and there are some areas of EHV-1 transmission, diagnosis and treatment that are not talked about at all above. As always, you should consult your veterinarian to get complete and current information and recommendations regarding EHV-1.
A few links:
Fairfield Equie Hospital client education on EHV-1
UC Davis School of Veterinary Medicine: Discussion of EHV-1
Prevention and Control of Equine Herpes Virus by George P. Allen, PhD at the Gluck Equine Research Center, University of Kentucky
Again, this article does not substitute for an EHV-1 vaccination and prevention program recommended by your veterinarian — who knows the medical history of your horses and the details of your exact situation. Clients of Eggleston Equine, contact me at dr@egglestonequine.com or at (860) 942-3365 if you have concerns about EHV-1 and your horses
Aimee M. Eggleston, DVM