Headshaking appears to be light-stimulated in approximately 60% of horses. The condition is seasonal and recurring in the majority of horses. Trials show treatment with cyproheptadine produced improvement of symptoms in 76% of cases. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC548606/#:~:text=Cyproheptadine%2C%20an%20H%2D1%20receptor,properties%20(1%2C4).
Cyproheptadine (Periactin) is an oral antihistamine used for treating allergic reactions. It further blocks the neurotransmitter acetylcholine in the central and the peripheral nervous system. Inhibiting nerve impulses by selectively blocking the binding of the neurotransmitter acetylcholine to its receptor in nerve cells. These nerve fibres are responsible for the involuntary movement of smooth muscles present in the gastrointestinal tract, urinary tract, lungs, and many other parts of the body. In head shaking case its the involuntary head movement.
At further trials there have been great success at treating head shaking with sulphonamides and pyrimethamine. Sulphonamide is used as an antibacterial and antimicrobial drug. Pyrimethamine is used to treat the toxoplasma gondii parasite, one of the world’s most common parasites and is used also to treat an intestinal disease caused by the parasite Cystoisospora Belli. https://pubmed.ncbi.nlm.nih.gov/9316239/
It seems likely from these successful outcomes, that head shaking is likely caused by either bacteria, pathogens or parasites or a combination of these. As head shaking symptoms increase with rain and sunlight, this makes sense too, pathogens, bacteria and parasites thrive under these conditions.
As a first line of treatment, you should ask your vet for their recommendations to treat head shaking. Or a holistic alternative, feed something like Gut Centric –an extremely potent probiotic blend with matched pre biotics to feed the new microbes beginning to colonize. Plus, it has the soothing properties of marshmallow root, and some toxin binders to help flush the newly killed off pathogens. Its a 3-week course (2kg), best you are not required to feed ongoing. It might be worth feeding a bag each time there is a flush of green grass. Once you rebuild a healthy gut microbial population the body triggers the immune system to be strong enough to self-treat pathogens or parasites.
Some companies produce gut formulas with aloe vera, a word of caution, aloe vera opens up the villi of the gut wall, allowing toxic pathogens to enter the blood stream and further exasperate many health conditions. There are many companies making claims that aloe vera cures ulcers and many other conditions, but significant studies disagree strongly. A good read is on the website https://thenudehorse.com.au/guthealth/aloe-vera-v-marshmallow-root/. You will see marshmallow root is a safer choice for gut health issues.
Headshaking while working
A theory behind head shaking has to do with a horses natural reaction to a sensation (like a fly) at the follicle level. If this anti-fly headshaking is observed carefully, it will be seen that it is an automatic action over which the horse, that is the brain of the horse, has no control. This reaction can be triggered when working. How? A horse does not normally have high blood pressure at rest, although we have seen a few rare cases. Once the horse starts to exercise, if obstructed circulation causes pressure on the hair bulbs of the neck or head, then the head will shake. This is why in most cases, horses only headshake when they are asked for work. How can you reduce this kind of headshaking? After their daily exercise and when the horse is warmed up, hosing with cold water over the head and neck, and along the length of the spine. The reason for the cold water is there is a small muscle attached to the hair bulb which pulls the hair upright in response to cold. By exercising this muscle we can help good circulation to the hair bulb and free up the nerves.
Other considerations to head shaking can be mineral toxicities such as manganese. The symptoms of manganese toxicity may appear slowly over months and years. Manganese toxicity can result in a permanent neurological disorder known as manganism with symptoms that include tremors, difficulty walking and facial muscle spasms. Manganese also produces damage to the extrapyramidal system (this is responsible for the involuntary and automatic control of all musculature).
Over supplementing with magnesium compromises the benefits of calcium, this can result to in muscle twitching. Magnesium imbalances can cause problems related to muscles, cramps and neurological dysfunction. Higher than desired Lithium can cause fine tremors and cognitive dysfunction. Molybdenum deficiency is linked with involuntary muscle contraction that affect the head and neck (fasciculations). Of interest too Vitamin E and Selenium together act to maintain normal muscle function. Magnesium and Boron are synergistic and when one is too high, too low, the other will be compromised. Both of these nutrients at imbalanced levels have been associated with head shaking in horses.