Laminitis, also known as founder, is inflammation of the sensitive lamina of the foot in a horse, the complications of which often result in the horse having to be euthanized. The front hooves are most commonly affected, although it's not uncommon for a horse to have laminitis in one or both hind legs as well. (Note: a few times in this article, a horse is referred to as "foundering." This is simply another way of saying that a horse is afflicted with laminitis.)

Causes of Laminitis

1.) Carbohydrate Overload
2.) Sudden drop in body temperature due to water
3.) Hard ground
4.) Lush pastures
5.) Exhaustion
6.) Untreated Infections
7.) Colic
8.) Lameness
9.) Allergic reactions
10.) Heat stroke

Carbohydrate Overload
If a horse is given grain in excess, this results in the horse being unable to digest all of the grain in the foregut. The grain moves on to the hindgut, and ferments in the Cecum. The fermentation of grain in the Cecum kills "good" bacteria that live in the horse's hindgut that are needed for the removal of toxins from matter passing through the digestive tract. When these bacteria are killed, the toxins are then absorbed into the horse's bloodstream. Circulation is restricted, particularly in the horse's feet. This results in laminitis.

Sudden drop in body temperature due to water
Whenever a horse is undergoing prolonged or strenuous work, it is imperative that the horse be given sufficient time to cool down afterwards BEFORE he or she is allowed to drink. The horse may be allowed to take a few sips of water during exercise, but if you do this, give them several small sips of water throughout the entire time of their work, not just once or twice. Also, DO NOT hose off a horse that is extremely hot, or that has just got done exercising. The horse must be allowed to do some "cool down" work BEFORE being hosed off or being allowed to drink water. If these precautions are not adhered to, the water can result in a sudden drop in body temperature which is a shock to the horse's system. This, again, results in restricted blood flow to the horse's foot, resulting in laminitis.

Hard ground
Whenever it's possible, avoid working horses on hard ground. This includes concrete or gravel roads, and any other type of hard surface. This is also why it is imperative that, if you are riding a horse in an indoor or outdoor arena, the arena be periodically dragged so that the dirt can be loosened and broken up, resulting in softer ground for the horse to walk on. If a horse is worked on hard ground, there is increased concussion on their feet. The greater the concussion, and the longer a horse exercises while enduring that concussion, the more likely they are to contract laminitis.

Lush pastures
It is important that in the spring time, when reintroducing horses back into a pasture situation after being stalled all winter, that they are reintroduced gradually. Feed horses before turning them out, and don't turn them out for more than 45 minutes to an hour at first, gradually increasing the amount of time you turn them out and decreasing the amount you feed them beforehand as the season progresses. If a horse consumes too much lush pasture after being used to eating dry hay all winter, the excess moisture can be a shock to their system. If the horse is fed beforehand, they won't eat as much when turned out, and will be less likely to founder.

Exhaustion
This can never be emphasized enough: DO NOT WORK A HORSE TO THE POINT OF OVEREXHAUSTION. THERE ARE SERIOUS MEDICAL CONSEQUENCES OF DOING SO. THIS IS ALSO CONSIDERED CRUELTY TO ANIMALS AND IS A PROSECUTABLE OFFENSE IN MANY STATES AND COUNTRIES. We ask our horses to do a lot. We've taken them from their natural environment, in which they roamed many miles a day, and grazed 16 hours a day, and confined them to stalls and paddocks, and reduced them to 2-3 meals a day. On top of that, we challenge them mentally and physically by asking them to work for us. The least we can do in return is not work them too hard, and allow them plenty of turnout time whenever possible. Laminitis is only one of MANY possible side affects of exhaustion and overwork.

Untreated infections
Any time a horse contracts an illness, or has any medical problems, it's important they are seen by a veterinarian. Untreated illnesses caused by infectious diseases, and untreated wounds that become infected and are left untreated, cause toxins to build up in the horse's blood stream.

Colic
Any time a horse colics, it is important to monitor them carefully while they are recovering and after they recover. Laminitis can sometimes develop after a serious case of colic. (See horse colic)

Lameness
Whenever a horse develops a lameness in one leg, their weight is often redistributed unevenly onto the adjacent leg due to the pain of the horse bearing weight in the lame leg. This results in more stress and concussion on the adjacent leg, and can result in laminitis in one or both legs.

Allergic reactions
Whenever a horse has an allergic reaction to an injection or an insect bite, it is believed by veterinarians that the histamine released into a horse's blood stream restricts blood flow in the hooves, and can lead to laminitis in one or all feet.

Heat stroke
If a horse suffers heat stroke, this is another shock to their system. Laminitis can develop shortly afterwards.

Symptoms of laminitis

1.) Increased temperature of the wall, sole, and/or coronary band of the foot.
2.) A pounding pulse in the digital palmar artery. (The pulse is very faint or undetectable in a normal hoof)
3.) Anxiety
4.) Visible trembling
5.) Increased vital signs and body temperature
6.) Sweating
7.) Flared Nostrils
8.) Walking very tenderly, as if walking on egg shells
9.) The horse standing in a "founder stance" (the horse will attempt to decrease the weight they're bearing on the affected feet. If they have laminitis in the front hooves, they'll bring their back legs further underneath them and put their front legs further out in front of them.)

Complications of laminitis

1.) Separation of the hoof wall.
The destruction of the sensitive laminae results in the hoof wall becoming separated from the rest of the hoof. Pus may leak out at the white line or at the coronary band.

2.) Rotation of the third phalanx.
The third phalanx, also known as the coffin bone, rotates downward. Normally, the third phalanx should be parallel wto the hoof wall, but in laminitis the tension of the deep digital flexor tendon allows the coffin bone to rotate. The degree of rotation is often determined by when laminitis is first detected in the horse, and how soon actions are taken to treat the horse.

3.) Penetration of the third phalanx through the sole.
If rotation of the third phalanx continues, it can eventually penetrate the sole of the foot. This results in the horse going into shock, and the horse must be euthanized.

Treatment

There is no cure for laminitis. A horse can live for a few years after being diagnosed with laminitis, with proper care and corrective shoeing, but will never fully recover. Don't work a horse with laminitis, as it will worsen the situation. Plenty of rest and corrective shoeing by a competent farrier are the only things that can be done to help a horse with this condition.

Suggested Reading
The Illustrated Veterinary Encyclopedia for Horsemen Equine Research Inc.
Veterinary Medications and Treatments for Horsemen Equine Research Inc.
Horse Owner's Veterinary Handbook James M. Giffin, M.D. and Tom Gore, D.V.M.