A team of researchers from the University of California, Davis (UC Davis), recently tested if idiopathic headshaking in horses could be similar to a condition in humans--trigeminal nerve pain caused by the reactivation of a latent virus.
In horses the trigeminal nerve provides sensation to the face and muzzle. Similarly, in humans, the nerve is responsible for facial sensations and some motor functions including chewing and swallowing. Humans suffering from trigeminal neuropathic pain often complain of burning, itching, shock-like or tingling sensations, and shingles, a reactivation of a latent herpesvirus (varicella-zoster virus, more commonly known as chickenpox), has been implicated as a potential cause.
In horses, reactivated latent equine herpesvirus-1 (EHV-1) in the trigeminal ganglion could cause neuropathic pain similar to pain sensations experienced by people with shingles. This pain in horses could potentially result in unexplained headshaking.
Monica Aleman, DVM, PhD, Dipl. ACVIM, explained that this study investigated headshaking with no discernible physical cause, such as tooth, ear, or eye problems. "These are the ones we call idiopathic head shakers," she said. "It has been proposed that EHV-1 infection is involved in or is the cause of headshaking in horses."
To that end, the team set out to determine if the presence of EHV-1 latency in the trigeminal ganglia in horses was associated with idiopathic headshaking using 19 horses (11 control animals and nine head shakers). The head shakers, the team noted, displayed severe clinical signs to the point they were unsafe to ride and handle, and had previously received multiple therapies, to which they remained unresponsive.
All the horses underwent genomic DNA (gDNA) extraction from whole trigeminal ganglia. All control-group samples and seven out of eight in the headshaking group tested negative for EHV-1; the one horse that tested positive for EHV-1 possessed a single copy of EHV-1gene. Thus, researchers concluded latent EHV-1 infection is unlikely to be the cause of idiopathic headshaking.
"We checked for DNA of the herpesvirus and found that all of the horses we tested were negative, except for one in which we saw just a single copy of DNA; a single copy in such a very specific test does not support the theory of EHV-1 as the cause of the disorder," notes Aleman. "So headshaking doesn't seem to be related to or caused by herpes virus in the horse."
Further research into the cause of headshaking in horses is under way at UC Davis, with preliminary findings in electrophysiology studies indicating that at least part of problem likely lies in the trigeminal nerve.
"In our studies, we have found that by testing trigeminal nerve conduction, we're finding differences between control and headshakers, although we still don't know why," Aleman explained. "Based on these electrophysiology studies, we're actually promoting the horse as an animal model in human disease, because humans also suffer from something very similar. For example, some people that are exposed to sunlight start sneezing or having a tingling sensation in the nose."
This study, "Latent equine herpesvirus-1 in trigeminal ganglia and equine idiopathic headshaking," was published in the Journal of Veterinary Internal Medicine. The abstract is available online.
Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.