The University of Kentucky confirmed that the illness that caused the euthanasia of three horses in late April at Fox Chase Farm in Middleburg, Va., was EHV-1 (equine herpes virus type 1/rhinopneumonitis). The horses had severe neurological problems that generated quite a bit of discussion in the media and among horse owners. Unfortunately, much of that included misinformation, some half-truths and some wrong assumptions.
Myth: Neurological herpes/rhinopneumonitis is rare. This depends on what you mean by “rare.” If you compare the neurological form of the disease that claimed these horses to the level of exposure to EHV, which is virtually 100%, and the number of horses that develop respiratory disease or abort their foals as a result, it is indeed unusual. However, we wouldn’t call it rare.
In the past year, there were two other isolated outbreaks, one in northern Idaho this winter and one strongly suspected in southeast Pennsylvania last spring. In addition, it’s not a new finding that rhino can cause neurological disease.
Myth: This is a mutant virus. There are two forms, EHV-1 and EHV-4, either of which can cause respiratory, fetal or neurological disease, although it’s usually EHV-1 in the neuro cases. The EHV virus is highly variable and prone to small spontaneous genetic changes. However, it is the same virus. One Japanese study that did detailed genetic analysis on varieties isolated from horses over a 19-year period documented 42 different EHV-1 and 64 different EHV-4 viruses. The same EHV type that causes respiratory disease or abortion could produce neurological disease or no disease at all.
Myth: Available vaccines can protect against abortion and respiratory disease but not the neurological form. No rhino vaccine can guarantee protection against any form of EHV. Although vaccine challenge studies looking specifically at neurological herpes have not been done, the good news is that vaccines labeled to help labeled to help prevent respiratory disease and abortion are likely to provide the same protection against neurological forms.
Myth: Treatment of affected horses can turn them around. Horses with neurological EHV definitely need intensive support if they are going to make it, like fluids, manual removal of manure, catheterization of paralyzed bladders and frequent turnings if they are down. However, nothing your vet can give the horse can kill the virus in his system. The sad fact is that for those that are severely affected enough to become completely paralyzed and go down, the chance of recovery is slim.
Myth: Preventive treatment can stop the spread. The only preventative options are vaccination and immune stimulants, both of which take time. Horses should never be vaccinated in the face of an outbreak as this may actually be a risk factor for development of the neurological form of herpes. It’s equally important is to isolate asymptomatic horses from the sick ones and take isolation precautions.