With the recent outbreaks of EHV1 at Dixie Stampede and the cutting horse show in Ogden, Utah earlier this spring, we thought it might be a good idea to give you the latest school of thought on the subject. This recent outbreak at Dixie has been limited and confined to cases at the Pigeon Forge facility, which fortunately for us, is a relatively closed herd (little traffic in and out). There was one of the Dixie horses that was transported to UT Vet School and subsequently euthanized; this led to a self-imposed quarantine of the hospital horses, and luckily no other horses there (at the Vet School) have contracted the disease. The vet school did a great job in isolating the patient and getting a quick diagnosis for the case presented to them; they figured with neurological symptoms and a fever that it had to be EHV1, one of the encephalitides (West Nile, EEE, WEE), bacterial meningitis, or possibly rabies (not as likely at an indoor facility like Dixie). So far there has been no spread (at the vet school) and likely won't be this far out; this is good practice on their part since the Findley outbreak back in 2000 resulted in the worst cases coming into the Ohio State Vet School for care, whereupon others in the hospital became infected and subsequently spread it into other northeastern states upon the discharge of non-related patients. Dixie has had a half dozen or so horses testing positive and running fevers (out of 30 to 40 horses at their facility) with 2 or 3 of these showing neurological signs, but so far these horses are doing better and will hopefully recover.

So, hey... what does this mean to you and me as horse owners? Hermit city? The end of the pitiful social life that horse peeps are so privy to? There should be some thoughtful consideration of this pestilence; here is some info for you to ruminate on as you pack for New Zealand: EHV1 is an acronym for equine herpesvirus type 1, the causative agent of the recent outbreak; it is also called "rhino" or "rhinopneumonitis" (someone came in the office one day and asked me for one of those "elephant" shots for their pregnant mare) or herpes virus myelitis ("myelo" refers to the spinal cord, "encephalo" refers to the brain). There are at least 5 or 6 different herpes viruses that affect horses, with EHV4 being the most common cause of respiratory disease today; it used to be pretty nasty, but now is not quite as severe. There is a genital herpes in horses (EHV3), and there is an EHV5 that may cause some lung problems (multi-nodular fibrosis), but it is currently not thought to be too significant, although some researchers are uncomfortable about how many crime scene photos it seems to be innocently caught in. There aren't any other species of animals that we're concerned with that carry the horse bug or trade it back and forth with the horse... people don't give it to horses without physically carrying the snot or other bodily horse fluid from one horse to another, and while it is very important to guard against this type of transmission, it isn't quite as likely as the horse to horse route (I did see a research paper where someone had found it in some sea lions of all things, but I think the closest ones are in Del Rio). There is a "plain old EHV1" strain and then there is a super-dooper "neurotrophic EHV1" which has 2 different sequences in its viral genome; hence it is thought to be a mutated form of the "plain old version". One neurological expert I spoke with said the mutation was not a recent change and that it probably happened around the time of the Spaniards first bringing horses to this continent. This neurotrophic version is more potent and seems to be able to more readily overwhelm an unsuspecting immune system and cause neurologic disease. EHV4 and the other EHV1 strain can also cause neurologic disease, even though respiratory disease is usually associated with EHV4; EHV1 can also cause abortions in mares.

The most interesting thing about herpes viruses in general is their ability to lie dormant in an animal or person once they have become infected and then reappear at a later date; think of the reoccurring cold sore (human herpes simplex I) or getting shingles later in life after having chicken pox (yes, a herpesvirus, too) as a kid. The horse is or can be infected in the same way (in addition to horizontal or horse to horse transmission). Stress or who knows what can have the EHV1 virus rise up or recrudesce out of latent state within the own body of an individual horse, so that the horse "catches it" from itself. An estimated twenty per cent of the horses in the country have the neurotrophic version of the EHV1 virus lying dormant in their lymph nodes, while almost every horse in the country has EHV4, this from Dr. Stephen Reed at Rood and Riddle Hospital in KY; he told me that they routinely survey lymph nodes on necropsied horses there at the diagnostic lab and Gluck Research Center and 20% is what they have found. So... duck, duck, duck, duck, goose... you do the math. Once it arises out the dormant state, the virus replicates (the horse will be running a fever) and can then be shed to other horses in the barn, show, or facility. Because of the size of viral particles, it seems to travel much quicker and easier through a barn or showgrounds than a bacterial disease such as strep equi (strangles). The incubation period is 1 to 2 days (the time from which a horse comes in contact with the virus and then gets sick or a fever) and viral shedding can variably be over 1 to 2 weeks.

The most obvious question to me is if 20% of the horses do in fact have the virus, then why don't we see EHV1 disease more than we do? I suspect that there may in fact be some protection conferred by the current EHV4 vaccines which coincidentally have an EHV1 strain in them. The EHV4 component seems to be quite effective (in suppressing EHV4 respiratory disease), while the EHV1 protection from neurologic disease is variable. Unfortunately, measurable titers or antibody levels resulting from herpes vaccinations in horses last only 60 to 90 days (don't fret as there are other components of the immune system which are not so easily measurable and probably still working under the radar), while the flu vaccine will confer about 6 months of protection. From a practical aspect, we have only advocated flu-rhino vaccinations on a 6 month basis in what we call the high-risk or high-traffic barns, and this has seemed to work well in the past. The problem is that we don't legitimately know if any clientele' horses have in fact been EHV1-challenged (how would one prove or disprove that a vaccine suppressed a recrudescing EHV1 flare up; interestingly, though, we do believe that the EHV4 vaccine is effective in suppressing recrudescence or flare-ups of EHV4 respiratory disease). Since there is a modified-live EHV1 vaccine available, we have been adding it to the rotation where there has been client concern; Dr. Reed seems to think, and I would agree, that we may get some additional cell mediated immunity from the mlv (modified-live virus) EHV1 vaccine on top of the antibodies produced by traditional vaccine. The problem with solely EHV1 strain vaccine is that there is no cross-protection against EHV4 or influenza; therefore, we have been administering the mlv EHV1 vaccine (Rhinomune) and then waiting 10 to 14 days before giving the traditional (killed-virus) EHV4/EHV1/Flu vaccine as we typically do for a lot of horses in the fall. This way there there would be an additional boost of EHV1 immunity with the killed virus component of the 2nd vaccine, although the order given is probably not important.

It will be interesting to see if this quietens down as quickly as it arose, but the uneasy aspect is that the last outbreak I remember before the Utah one this spring was at Findley College back around 2000. So with Utah and then Dixie, which are as far as we can tell two totally separate incidents, it may be a sort of wake-up call as to what's around the corner. From a practical perspective, if you are back-woods/backyard/low exposure in a more isolated type of situation, then keep doing what you've been doing. If you have been doing flu/rhino vaccinations once a year, then either continue as before or consider adding a mlv EHV1 vaccine maybe soon initially, but then spacing it out to where you do one in the fall and the other in the spring (I would eventually have it where you would give the mlv EHV1 in the spring with the EWT, etc and then do the flu/rhino EHV1/4 in the fall). Those of you who do the respiratory vaccinations in the spring and fall, I would probably add the mlv EHV1 immediately, and then down the road space it out to where it (the mlv EHV1 vaccine) falls equidistant between the two; that way you would be doing some form of EHV1 quarterly. Call us if any questions. We'll keep you posted. Whoops, sorry... just checked... looks like New Zealand has EHV1 as well.