Laminitis (or founder) is caused by inflammation of the sensitive tissues or lamina, within the hoof. This swells, but cannot expand due to the hoof surrounding it and so causes pain and unwillingness to walk. The horse has an appearance of leaning back when standing - the horse's way of taking pressure off the toe. It can affect all 4 feet, but one may be worse than others, or the front feet may be worse than the back feet. If the laminitis progresses, the pressure can cause the last bone in the leg (pedal bone) to rotate so that the tip of the pedal bone presses on the sole causing even more pain and eventual penetration of the sole in the most extreme cases. These extreme cases result in euthanasia. Obviously it is better to prevent this in the first instance!
 

The following text is from the laminitis trust where more information is available. Also see the condition "Cushing's"

You can help prevent laminitis by avoiding high risk situations. The following is a list of "causes" or circumstances which we know commonly precede the onset of laminitis.

1. Obesity from general over-eating (=over-feeding!)

2. Overeating on foods rich in carbohydrate or rapidly fermentable fibre i.e. cereals, coarse mixes, rapidly growing or fertilised grass - especially in the Spring and Autumn but in a wet year there is potential for laminitis during all the growing season. This is the most common cause.

3. Any illness which involves a toxaemia. This may be a bacterial infection or following the ingestion of plant or chemical toxins.

4. Cushing's Disease. This is a condition which follows an abnormality affecting the pituitary gland in the horse's head. It results in the horse failing to shed its winter coat. The coat becomes long and matted and eventually curly. The horse drinks and eats increased amounts of food while sweating excessively and losing weight. All Cushing's cases suffer from chronic laminitis and can have acute episodes.

5. Weight-bearing laminitis. When the horse is severely lame on one leg and has to put all his weight on the contra-lateral limb they often suffer from founder in the weight-bearing limb. This is particularly common in hind feet.

6. Concussive laminitis (road founder). When horses are subjected to fast or prolonged work on hard surfaces they may develop laminitis as a result of trauma to the laminae, particularly if their horn quality is poor.

7. Hormonal problems. Animals which are "good doers" may be hypothyroid or have an abnormal peripheral cortisol enzyme system. The latter condition, recently described has been called obesity related laminitis or peripheral Cushing's disease. Others develop laminitis when they are in season.

8. Cold weather. A few horses show laminitis during cold weather, fitting warm leg wraps during cold snaps prevents the problem in most cases.

9. Stress. Worming, vaccination, travelling or separation from a "friend" can trigger an attack of laminitis in a few horses.

10. Drug induced laminitis. Although some wormers can precipitate laminitis, the most common group of drugs which cause laminitis are the corticosteroids. Even injecting short acting corticosteroids into joints can cause severe laminitis. This is why we never routinely use these in horses at Parkside. 

Overeating / Obesity are the most common high risk situations which lead to laminitis. The secret to avoiding laminitis in this situation is not to turn the horse out whilst he is fatter than condition score 3. This means he should not have a fat deposit along his crest or at the tail head, around the sheath or udder or over the loins. You should be able to feel his ribs easily by running your hand along his side yet you should not be able to see his ribs easily. If you are unsure about how to check your horse’s body condition, ask your vet surgeon to show you while they are out giving vaccines, as we all check body condition when we do our vaccination exams.

Limiting the grass intake can also be accomplished by using a grazing mask or muzzle or (perhaps more easily) by restricting the area available for grazing, and paying careful attention to grass growth in the spring and fall, and avoiding turnout during the period of the highest sugars (dawn and dusk, particularly in spring and autumn). There are also now low-sugar feed supplements that make it easier to administer medication (such as Danilon) in feed without adding sugars and unnecessary calories to your horse’s diet. If you are concerned about essential vitamins and nutrients, Parkside offer a low-sugar, highly palatable supplement (sometimes called a balancer) that can be used instead of feeding high sugar, high calorie feeds.