Wednesday, May 25, 2011

Outbreak of Neuro form Herpes

I realize I am a little late weighing in on this, but I wanted to make sure you are aware. There is currently an on ongoing outbreak of Equine Herpesvirus Myeloencephalopathy (EHM) affecting horses throughout the United States and Canada. The initial cases were identified at a cutting horse show in Ogden, UT. So far, most of the cases are centered around the western United States. There are, as of yet, no identified cases in Michigan. The challenge with these outbreaks occurring at large shows like this, is that horses from all of the United States can come, be exposed and return home before the first diagnosis is made. This lends to transmission of the virus across wide geographical areas.

Symptoms include fever, decreased coordination, nasal discharge, urine dribbling, loss of tail tone, hind limb weakness, leaning against a wall or fence to maintain balance, lethargy and the inability to rise. First symptoms of a fever and runny nose are usually seen about 2 days after initial exposure. Neurologic deficits do not usually appear until 10-12 days after exposure. The severity of neurologic deficits this virus can cause is what really makes this infection so vicious.

We do not yet fully understand this disease. It is caused by equine herpes virus-1 (EHV-1), more commonly referred to as a rhino virus. This virus is a common cause of upper respiratory infections. However, on seemingly rare occasion, it can mutate and then cause fairly severe neurologic symptoms. There is a point mutation of the virus that has been linked to the neurologic form, but not all horses who become infected with the neurologic form go on to develop neurologic deficits. There is a piece to this puzzle we have not yet found.

This complexity of the virus mutation and variation in symptoms lends to a lot of confusion about what actually constitutes a case of EHM. EHM implies neurologic symptoms, but most agencies define an EHM case based on identification of the mutation in the virus. The problem with this is that a lot of horses that become infected with the mutated form never go on to develop any neurologic symptoms. So a lot of the stats you may be hearing in the press are likely very misleading as to how many horses were ever actually infected with the neurologic form of EHV-1.

Another frustrating point about the neurologic form of EHV-1 is that vaccination is not effective. We do believe that vaccination, with a high antigen vaccine, will lessen the amount of virus circulating and thus lessen the potential for the development of the neurologic form to develop, but we also know that vaccination does not prevent the neurologic form. Vaccination does protect against the respiratory and abortion forms of the disease.

Not all Rhino vaccines are created the same. The high antigen vaccines do a better job a limiting virus shedding. Calvenza is the high antigen EHV vaccine that we use and recommend. There is also a modified live vaccine for EHV, which may be better still at limiting spread. However, there are additional considerations with the use of the modified live vaccine that go beyond the scope of the discussion here. It is best to talk directly with your veterinarian about which vaccine may be best in a given situation. The long and short of it is that over the counter Rhino vaccines are not very good at limiting transmission.
EHV-1 is one of those viruses that can develop into latent infections. Cold sores in humans do this same thing. The virus is always there, but at times of stress (or some other reason) the virus all of the sudden becomes active again. For this reason, seemingly healthy horses potentially could be latently infected and then when placed in a stressful environment, like a horse show, start shedding virus again.

This, is really intended to only be a brief summary about some of the complexities of this virus and the challenges we face in dealing with such outbreaks. The neurologic form of EHV-1 is not anything to mess with. It is a truly nasty disease. Our best strategy for control at this point is good biosecurity practices (see the links below). Vaccination may be of some benefit in limiting the spread of virus, but does not currently protect against the neurologic form. As you can image, there is a ton of more information about this. Here are a couple of good resources if you want to learn more.

USDA info and updates
FAQ’s, provided by AAEP
USDA Equine Biosecurity brochure
AAEP Biosecurity Guidelines
AAEP EHV Control Guidelines

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