Laminitis can affect all horses and can vary from mild lameness to cases so severe that euthanasia on humane grounds is required.
Laminitis is inflammation of the lamellae contained in the hoof capsule. Lamellae are the soft tissue structures which hold the coffin bone stable within the hoof. Disruption of the bonds between the lamellae and the bone can be catastrophic for the horse.
Laminitis can occur in all four feet; however it is more commonly seen in the front feet, possibly due to the increased weight carried by the forelimbs. Within the hoof capsule, the coffin bone can either rotate or sink, depending on the degree of lamellar damage. Horses with laminitis that sink have lost a large percentage of their lamellar attachment and have a very poor outlook.
There are a number of causes of laminitis, which are loosely categorised into three groups:
Toxin induced: Many bacteria produce toxins that are released into the blood stream from a site of infection. The toxins stimulate an inflammatory cascade throughout the horse's body, including the foot. The inflammation in the horse's foot results in a breakdown of the tight bonds between the lamellae of the hoof capsule and coffin bone.
Hormone associated: There are two hormone conditions that frequently contribute to laminitis: Cushing's Disease and Equine Metabolic Syndrome. In both conditions laminitis appears to be associated with an abnormal level of the body's normal hormones and excessive soluble carbohydrates (sugars) in the horses' diet, such as is seen with lush green grass.
Excessive Mechanical Forces: This type of laminitis develops as a result of excessive force on the hoof as is seen with "Road Founder" where concussion from a hard surface causes damage to the lamellae and "Supporting Limb Laminitis", where the horse is reluctant to bear weight on fractured or injured limb and places excess weight on the opposite leg.
There are four main signs of laminitis; increased digital pulses, excess heat in the feet, pain on hoof testers and an abnormal gait. The laminitic horse is typically lame at the walk; however mild cases may only show lameness at the trot. In acute cases, the horse typically rocks back onto its hind legs when it moves, in an attempt to shift weight away from the painful front feet.
Late pregnant mares can suffer from episodes of laminitis. The specific cause is still under investigation; however it is currently proposed that the increase in body weight from the growing foetus and underlying Equine Metabolic Syndrome may predispose the mare to laminitis in the later part of pregnancy. These mares often have a history of repeated laminitis episodes. Lameness in these mares often gets worse when the soluble carbohydrate (sugar) content in the mares' diet is increased. Lush green pastures, grain and molasses contain high levels of soluble carbohydrates.
Treatment of mares with laminitis varies with the severity of lameness. It is aimed at improving the level of comfort, providing hoof support, and correcting the underlying cause. Providing soft footing such as sand yards and medical treatment such as Phenylbutazone ("Bute") for pain relief can improve the mare's comfort level.
Shoes and farriery techniques are used to help support the laminitic hoof. These include Styrofoam pads, wooden clogs, heart bar shoes, dental impression material, raised heel cuffs and foot casts.
Laminitis typically worsens in late pregnancy and and improves after foaling; therefore the aim of treatment is to maintain the mare in a comfortable state until she foals. If it is suspected that the laminitis is associated with increased soluble carbohydrates, the diet needs to be modified. It is important that the mare does not to lose body condition during the final stages of pregnancy; however other food products such as oil can be substituted to meet the correct energy requirements.
Prevention of laminitis in the pregnant mare can be difficult as there are still many aspects of the disease that are unclear. Starting the breeding season with good quality hooves, and appropriate body condition, maintaining regular trimming and exercise and reducing the percentage of soluble carbohydrates in the diet of mares with a history of laminitis are all important factors in managing these cases successfully.