She reminded horse owners that horses can test negative for EPM early in the onset of the disease, so re-testing 10-14 days after onset of clinical signs is important if a negative test result is obtained within the first 7-10 days of clinical disease.
Some horses suffer from a problem that results in headshaking. This can be from mild, easily controlled clinical signs to debilitating headshaking and nose rubbing that requires euthanasia.
Johnson said headshaking is a syndrome for which diagnosis of the underlying cause can be difficult, and clinical management can be even harder. “Recent investigation has provided more information regarding the underlying physiologic problem in many cases, and newly described treatment modalities can help improve horse comfort,” she noted.
Headshaking was first described in equine texts in 1809. The mean age of onset is 7.5 to 9 years of age, with geldings and Thoroughbreds more likely to suffer from this syndrome. In 2000, one researcher created a grading system for headshakers that is still used today.
1. Intermittent and mild clinical signs. Facial muscle twitching. Rideable.
2. Moderate clinical signs. Definable conditions under which they occur/develop. Rideable with some difficulty.
3. Rideable, but unpleasant ride, difficult to control. 4. Unrideable, uncontrollable. 5. Dangerous with bizarre behavior patterns.
There can be many causes of headshaking, including mites and structural problems (such as sinus cysts or guttural pouch infections), but one cause that is focused on is similar to trigeminal neuralgia, a debilitating condition in humans. Johnson said people describe their symptoms as intermittent to persistent, brief to prolonged, episodes of “electric shock-like” facial pain that is usually just on one side.
Recent research in horses has associated headshaking with trigeminal nerve hyperexcitability. The research showed that headshakers have a much lower threshold for nerve activation compared to clinically normal horses. This research also supports the supposition that headshaking syndrome is similar to idiopathic trigeminal neuralgia in people. Treatments can be varied and include: nose nets as a counter-stimulant (one study reported improvement in 33% of horses and another study reported complete response in 27% of horses and partial response for 34% of horses);
eye shading or blindfolding to reduce sunlight exposure in one study resulted in a “substantial difference” in 52% of horses;
various medications (some studies showed up to 76% improvement), and
surgical intervention. Some of the surgical interventions resulted in post-operative increase of clinical signs in about 60% of horses, including those that self-traumatized their noses or increased headshaking. Some horses showed long-term improvement after this initial setback. However, 7% post-surgical complications were so severe that they resulted in euthanasia. Johnson said that horse owners who elect for surgery should be prepared for a week up to one month of worsening of clinical signs before they determine if the surgery was successful long-term. Sometimes multiple surgeries were required to help clinical signs. Johnson advised that surgery is a last-ditch effort in controlling this syndrome.
A pilot study that has shown positive results, according to Johnson, is the use of percutaneous electrical nerve stimulation (PENS) therapy. The thought behind this treatment is that it might ameliorate the likely hypersensitization of the trigeminal nerve in idiopathic headshakers. Initial results were positive, with five of seven horses demonstrating remission for months. “Longerterm follow-up with a larger group of horses showed that approximately one-third of cases had useful remission,” noted Johnson.
“Interventions not shown to be beneficial include a gonadotrophin-releasing hormone vaccine, feed supplements, and pulsed high-dose dexamethasone,” she added.
Johnson said that three PENS treatments were required for maximum duration of remission, which lasted about three months. Horses can be shown in between treatments. He warned that some horses are so “people-oriented” that they won’t show pain or clinical signs as much when people are around. Sometimes these horses will need to have video observation around the clock to determine the extent of the syndrome.