but it is not caused by dandelion, it is cause by the catsear plant - unless you know the difference you would call the plant dandelion.
Catsear has lots of little flower head and longer stems, with bigger leaves, where as dandelion has only 1 large flower head per stem and smaller leaves and shorter stem.
We have had horses get it before, the only thing you can do is keep them calm, relaxed, and put them in the big lush paddock to help flush out the toxins, a toxic binder can also help, but because it temporary deadens the nerve endings in the back legs, the more wound up the horse gets the worse the symptoms.
Quickest way to determine if its catsear poisoning, is the try and back them up. they will snatch up the hind legs, usually both, or in some causes can't back up at all.
We have had horses with this before, all have recovered some longer than other, as a general rule if they dont recover within 2 years then the chance of full recovery at slim. worst thing to do is confine them, the grass helps flush it out.
best way to get rid of catsear is to pull it out.
It most potent in the summer when there is bugger all grass (sand country is the worst, usually around the same time as facial excema dangers.) and a big downpour of rain, will cause a fungus to grow on the plant. that is what does the damage.
we have sprayed for years for catsear and it always come back, since we have been pulling the plants out, we are getting less and less, to a point that we dont have to worry about it now.
We have been about 10 yrs without incident.