Again -- it is totally a failure of the immune system. Horses with a good immune system cannot be given it -- no matter if you share blankets, brushes, etc.
It is very difficult to reach toxic levels of Vitamin A. You can feed 100,000 units a day for years and have no ill effects. As a matter of fact, the last comprehensive study I read about used that level to study horses and recommended that level for horses with immune problems, particularly skin and/or eye problems.
It is not caused by a virus. It can be caused by bacteria or fungi but is most often caused by bacteria that normally inhabit the soil. It is exacerbated by damp conditions, but can occur any time of year under any conditions if the horse is not able to fight it off. It is also seen quite often when there is no green grass and the horse has not had grass or freshly cut hay for 2 or 3 months.
Horses store it in their livers. Some store it better than others. But the bottom line is that healthy horses with good immune systems just do not get it.
The same is true of lice. Can't give them to a healthy horse. Can't kill them on one with immune system problems in late winter or early spring. An old (like 75 years old) Vet friend of mine puts it this way: "When someone comes to me in March with a bunch of calves that are being eaten up with lice -- I tell them to dust and spray them and do it 2 or 3 times and it will take about 6 weeks to get rid of them. Then I tell them that if they do nothing, there will be green grass in 6 weeks and they will clear up when they have been on grass for 1 week. It all depends on how much time and money you want to spend." Actually, you feed them a good loose mineral with 150,000 to 200,000 units of Vitamin A per pound or feed a calf feed with 10,000 until per pound in it, calves will never get lice in the winter.