I had not heard of this until my horse began fussing with his mouth and head while I ride.  It started last fall and has gotten a little worse each time, until today when it was outright dangerous to ride.  I sat down and realized it started last fall after he had all his shots.  I had his teeth floated last week, plus a flu and rhino vaccination, not thinking there would be a problem.  Now after reading about the head shaking issues, it sure sounds like my boy is having allergic reactions to the vaccines.  His nose is insanely itchy.  He acts like a bug has flown up his nose every 30 seconds, and he tries to itch his nose on everything we pass, even dragging his nose through the bushes.  Last week he drug his nose through the snow we were riding in.  He also has started to strike at his nose while we're riding - not fun.  He will even do it while we're trotting or cantering (stop to itch his nose on his leg or drag it across the ground)   Today when I put him away he continued these behaviours while I was leading him to the pasture, acting like a bee landed on his nose and striking at his face, and then did it when I put him in his stall too, while eating his grain. 

I have never seen him this way and am freaked out by it.  I'm calling local vets to find out who is most experienced with this and getting them out asap!   I hope it is allergies and can be treated.  I ordered a leather fringed nose band to try and may also try a fly net over his nose while riding.  It is so sad to see this beautiful horse behaving like he has lost his mind. Does anybody out there have any similar experiences they can share?  I am so worried about my horse. He just started this last fall. 

Well, I went and checked on him last night and this morning, and he is showing no symptoms.  I will work him in the roundpen this afternoon and see if after work the symptoms show up.  I will try the mesh nosenet I made and see if that helps if he begins to show symptoms today after excercise.  I have read a lot about this now; it is often aggravated by excercise and light.  Monday morning I will start calling all the local vets to see who has the most experience with this.  I am happy his symptoms subside when it's cool, no excercise and no sunshine though because it will hopefully make the diagnosis easier.  I read the following article and noted the paragraph about the rhino vaccine...  I am so bummed about this.  I don't think I'll get my horse vaccinated for a while, if ever.  He seems hyper-sensitive to it.   He does have a tendency to be OC but this does not seem to be the same - he is obviously uncomfortable when he does it.

Headshaking is a mysterious problem in horses.  Recent research suggests that it can be caused by a variety of factors:  it can be behavioral or biological (caused by physical discomfort arising from tack, teeth or ear problems, allergies, nerve stimulation, sensitivity to light, and a variety of other possible causes). 

I'm sure your vets looked into his inner ears for fluid buildup?  I've seen horses get like this when sweat rolled into the ears during workouts.  Simple cure to that problem was cotton rolled-up and packed in the ears during riding and removed after the post workout rinse off.

Good luck with whatever buggs your gelding.

Well now, after I roundpenned the horse for 5 minutes the symptoms began to appear, getting worse and worse.  When he got the the point of dragging his face along the roundpen panels, banging his nose into them (yikes!!)  I stopped him and put the mesh nose cover on, making sure it was snug around his nose and mouth.  Then I got him going again around the roundpen.  You won't believe this but he showed no symptoms at all... NONE.  I was shocked.  No head shaking, no bobbing or dragging his nose on the ground or fence and no striking.  Can you believe that?   I had a couple friends with me that could not believe the instant change.   It was like night and day. I had read about this happening and people saying it was like a switch got turned off, and that is exactly what happened.  It is the contact with the nose that stops the itching he feels.  Similar to if you have to sneeze and pinch your nose it stops the feeling. I will try it again tomorrow and the next day I'll try to ride in the roundpen with the mesh nose net on.  I am still calling the vet but in the meantime, this is really hopeful. I ride endurance and planned to do 50 miles every ride this year... hard to do with this kind of issue.  I can't ride endurance with antihistamines in him , or any other type of meds.

I will let you know what the vet's diagnosis is and what all the tests reveal.  I'll have a full physical done, including ears, eyes, nose, throat, bloodwork, etc. 

That's good advice about the bridle.  His bridle has a browband and a noseband.  The browband is kind of snug now that I think about it.  But, he does it in the roundpen when he gets warmed up, with nothing on his head.  I take him to the vet tomorrow to start the process of elimination and diagnosis, if one is ever found.  They will check his eyes, ears, nose, throat, do bloodwork, etc... the whole enchilada.  I hope it is something obvious but I fear it will be unkown and the treatment will be trial and error at best.  I worked him 1/2 hour today and the nosenet did not work as well when he got warmed up.  I tried it yesterday for a few minutes and it was good but with longer work it did not do as well.  I will ask the vet about accupunture for sure.  I'll report back once the vet exam has been done.  This is so worrisome.

Our horse did the same. It ended up the his nose and face were really sensitive. So we would put vaseline on the inside of his nostriles and on his nose and face. Never had a bit of trouble after that.

Hope this helps.

I have a head shaking gelding. While I tried everything to  figure out what his problem was. Tried changing all my tack, took him to a chiropractor many times, had his teeth done by dentist, had him scoped by the Vet, had him tested for allergies(he has a ton and is now on the serum shots) that didn't help either. I believe the rhino/flu shot is what did this to my gelding as well  But my guy is mostly set off into his head shaking if I ride him in a group. I can ride him alone and only have a few "ticks" as they are called. He will sometimes do it in the pasture when he gets excited. But it mostly when I ride him in a group so I just don't ride him in a group. This is painful for horses and well aggravating to them as well. So I bought another horse for trail rides and just ride this guy at home alone every now and then.  There is a Yahoo Group just for headshaking horses. These people are a great help and wealth of info on different things to try etc

http://pets.groups.yahoo.com/group/headshakers/

Hope this helps. Sorry you have a head shaker. Its very heart breaking :(