Equine Herpesvirus (EHV) should be taken seriously. EHV can cause severe and debilitating illness; the neurological form of EHV can necessitate the euthanasia of affected horses. Prevention and vaccination should be priorities for all horse owners.

That said, EHV has been around for decades. It was first identified in the 1930s, long before your horse was born and it will be around after we are all gone. There is no evidence that EHV outbreaks are increasing in frequency. There is no evidence that the disease has become more lethal. There is no epidemic of EHV infection.

What’s more, most horses recover uneventfully from infection. Most horses do not get the neurological form of the virus. Most horse that do, recover. Many horses are already latently infected with the virus. These horses may be suffering minor, occasional recrudescence (return) of infection — now, yesterday, last month. Despite this recrudescence of infection, neither you, nor your horse, nor your vet even realized.

The point here is that it is important not to overstate the risk of EHV to our horses. The risk is small. Basic awareness and prevention are to be commended. Consulting your veterinarian about an EHV vaccination protocol is wise. But panicking, misjudging risk, or implementing overly broad prevention measures, doesn’t help anyone — not you and not your horse.

Take this example: A barn implements mandatory EHV testing for all new horses entering its facility. This barn takes a nasal swab from each new entering horse, sends the swab out for testing, and has the results back in 48 hours or less. These aren’t tests on animals showing signs of infection; there may be no fever and no respiratory distress. These are indiscriminate tests.

What’s the risk here? We know that EHV is ever present in the environment. We know that latently infected horses are commonly suffering recrudescence, shedding virus through their nasopharynx. A nasal swab taken on a horse suffering a minor infection will be positive for EHV. With the positive EHV swab, we have “EHV in the barn.” The horse or the whole barn might go under quarantine.

Never mind that, of course, there is EHV in the barn or that this sort of minor infection and viral shedding is going on constantly among our horses. We have a possible outbreak, another case to report in the news, and panic spreads. We’ve invented an “outbreak.”

Instead, the new horse could be isolated and monitored, as all new horses should, for a period of 21 days. After the isolation period ends without incident, the horse would be reintroduced to the barn without consequence. If the horse were to show signs of EHV we would know within the isolation time period, as the incubation period for both neurological and respiratory EHV is shorter than the 21 days. Isolation is a much more reasonable approach to dealing with the introduction of new horses to a barn.

I don’t know that the above scenario about testing has occurred in any given barn. I do know that there’s a lot of indiscriminate testing going on and in my opinion this represents bad policy. EHV is everywhere and there is a difference between true, serious infection and incidental infection. EHV outbreaks are real and serious. We don’t need to invent new ones.

Encouraging news for the use of Platelet-Rich Plasma (PRP) in tendon injuries. A recent journal article (J Orthop Res. 2007 Feb;25(2):230-40) examined gene expression patterns and the collagen content of equine digital flexor tendon explants cultured in media consisting of PRP and other blood products. They found that the tendons cultured in 100% PRP showed enhanced gene expression for several collagen and cartilage molecules. The findings support further study of PRP as a treatment for tendonitis.

Here’s a link to the article’s abstract. thehorse.com has several news stories on PRP: Platelet-Rich Plasma for Tendon and Ligament Problems, Platelet-Rich Plasma Treats Tendon and Ligament Problems.

According to recent news reports 21 out of the 80 horses in the University of Connecticut herd are now affected with EHV. Five of the horses are showing neurological signs. None have been euthanized and all are expected to recover.

From UConn’s recent press release regarding EHV: “Some of the horses that are ill at the UCONN facility have exhibited fever and respiratory disease and some have exhibited the neurological form of the disease. Affected horses are being tested and treated to minimize discomfort. In the meantime, the stables are being sanitized regularly and coursework directly involving the horses, polo practice and other activities has been temporarily suspended.”

As a replacement for The Connecticut Horse Symposium held annually by UConn and canceled in light of the EHV outbreak, UConn will be hosting a conference on Saturday, March 24th entitled “New England Conference on Equine Health and Management”. The conference will focus on biosecurity measures, disease prevention, and management practices for horse owners. It will take place at the Ratcliffe Hicks Arena on the University of Connecticut campus and will cost $5. Click here for more information.

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

Qustion: “I have administered the Pneumabort-K vaccine three times to my mare. The last two doses caused her to walk as though she is stiff or could not feel her lower legs. Have you ever heard of this? Do you think it will cause her to abort?”

Obviously, without being able to evaluate the mare or knowing a thorough history, I cannot definitively say anything about your mare’s condition. What caused her stiffness, etc. So take what I do write with a grain of salt. I would encourage you to seek advice and consultation from your veterinarian.

First, the stiffness and lower leg issues sound like a vaccine reaction to me. It is relatively common for horses to react in this way after being given any number of different vaccines — including the Pneumabort-K vaccine. These vaccine reactions are typically not serious and pass of themselves in a couple days with limited or no treatment.

As to whether these vaccine reactions will cause your mare to abort, the short answer is no, at least not directly. But your question and your description of your mare’s reaction to the Pneumabort-K vaccine does bring up an interesting discussion — as to whether these vaccine reactions can cause reappearance of EHV-1 in mares who are latently infected, and consequently abortion.

As you probably know, many horses are latent carriers of EHV-1. Latent carriers show no obvious effects of the virus, though they are infected and can show symptoms and spread the disease if the virus wakes from latency. This reappearance of the virus can be brought about by stress (e.g. stress induced by transport, showing, sickness due to various disease states, etc.) Horses that are carriers of EHV-1 do not produce the same immunoresponse to the Pneumabort vaccine — as previously unexposed horses. Because of this reduced or inadequate immune response, the vaccine offers them little or no additional protection against the virus. BUT, the horses still have the stress of the vaccination and of a possible vaccine reaction. This stress may be enough to cause the latent, non-vaccine strain of EHV-1 to reappear — and cause abortion in the mare. The Pneumabort-K vaccine itself does not cause abortion, but the vaccination of the mare sets off a course of events leading to abortion.

The obvious question this scenario raises is, “Is my mare a latent carrier of EHV-1?” Unfortunately, there is no way to know. No test exists to identify carriers of EHV-1. And even if there were, there is no way to predict when and if the latent virus will revert to virulence.

I want to be clear: I am not saying that this mare is experiencing the above scenario. What’s more, the above scenario is conjecture and has no data behind it. DO NOT mistake what I’ve written for fact. The question raises this interesting area of discussion surrounding the Pneumabort vaccine. But this discussion, this conjecture does not change the fact that the standard of care in veterinary medicine is to vaccinate mares with the Pneumabort vaccine at 5, 7 and 9 months.

More EHV news. The 72 horses of the University of Connecticut herd have been quarantined due to an outbreak of Equine Herpes Virus (EHV). Apparently 3 horses have already tested positive and tests are pending on 7 others. It is unclear whether the horses have the neurologic form of the disease. I’ve only seen the most basic of news stories on the outbreak. Here’s a link to a news story. If you’re a client of mine and have concerns, please email or call.

Update 1/10/07: The three horses that tested positive at UConn are showing signs of the neurologic form of EHV-1.

Update 1/13/07: Several more horses are showing clinical signs of EHV-1 at UConn. Here’s a link from thehorse.com.

Question: “What is your standard protocol for annual routine immunization & worming of healthy, non-immunocompromised, non-show circuit pleasure horses in the North East?”

First, a plug for the American Association of Equine Practitioners (AAEP). They have put together some nice vaccination guidelines on their educational web site myhorsematters.com. Their guidelines can be found here.

As to the question at hand, a standard vaccination protocol for healthy, non-immunocompromised, non-show horses in New England would include:

• Tetanus Toxoid: Annual vaccination. Additional booster if penetrating injury greater than 6 months since last tetanus vaccination.
• Equine Encephalitis: Spring annual vaccination.
• Western Encephalitis: Spring annual vaccination.
• West Nile Virus: Spring annual vaccination
• Rabies: Annual vaccination
• Potomac Horse Fever: Spring Annual vaccination. Additional fall booster if spring vaccination was prior to May 1st.

I consider the above vaccinations the MINIMUM for the type of horse you describe. If the horse were stabled in a boarding facility or similar environment I would also recommend:

• Influenza: Twice yearly vaccination. Intra-nasal vaccination appears to provide the most protection.
• Rhinopneumonitis (EHV-1 & EHV-4): Twice yearly vaccination.
• Strangles: Annual intra-nasal vaccination.

With the outbreak of a disease in a given area, the above vaccination protocol may be adjusted with additional boosters (e.g. the current EHV-1 outbreaks in Florida and elsewhere). Also note, that if your horse has never before been vaccinated for one the above diseases, or if you’ve allowed a vaccination to lapse, your horse may require additional boosters of a given vaccine to bring him up to date.

The worming regimen can be quite varied for the type of horse described. The answers depend upon herd management — especially manure management. Ideally, we advocate deworming based on the results of fecal exams. However, I often suggest a basic deworming schedule that includes:

• January: Panacur Powerpac (5day double dose of Panacur)
• March: Strongid
• May: Zimectrin Gold OR Equimax
• July: Quest
• September: Strongid
• November: Zimectrin Gold OR Equimax

OR

• Daily Dewormer with Strongid C2x and twice a year (spring and fall) with Zimectrin Gold OR Equimax

I received the following comment on my recent blog post on the EHV outbreak in Florida:

“My concern is that exposed horses could end up going from Florida to Gulfport. I know that there are horses that would show in Florida and then head to Mississippi. What is the guarantee that it will not spread into that area?”

With regional and even national and international travel becoming more common, there is absolutely no guarantee that Equine Herpes Virus (EHV) will not spread into that area or any other area. As the world gets smaller, EHV and other infectious diseases will continue to cause problems. Indeed, horses continue to become sick and test positive in Florida. And we should not allow ourselves to take the nightmare scenario of any infectious disease outbreak — uncontrolled national (and international) spread of the disease — lightly. It should not be forgotten that the most recent outbreak in Florida started with infected horses imported from Europe.

Closer to home, the highly respected Fairfield Equine Hospital has closed their doors to accepting any patients at their hospital. Due to their vigilance they recognized a recent post-surgical horse with a fever spike. The horse was quickly tested for EHV. The horse came back positive and Fairfield placed its hospital under quarantine. This horse had not show signs of the neurologic strain of EHV at the time of testing nor is the horse showing those signs at this time. (Click here for more information.)

A case so close to home highlights that even up here in New England we need to stay aware and educated. We need to take the proactive steps to keep our horses safe. That is not to say that we must shut all our horses in isolation stalls. By way of example, when a client called me recently to ask my opinion of her daughter’s performance horse traveling to another barn for lessons, we assessed the environment to which the horse will be subject and decided that there would be minimal risk — because the horse is properly vaccinated and the exposure would be minimal. I did mention that it would be important (and educational to her daughter) that the horse be “temped” once to twice a day and that any abnormal temperatures should be reported to me at once. It comes down to awareness and vigilance.

For further up to date information please refer to the aaep.org and faep.net web sites.