PATHWAYS TO LAMINITIS

Over the past two decades, researchers have made great strides in understanding the causes of laminitis. Three primary pathways leading to the disease have been identified:

• The inflammatory pathway is associated with fever and inflammation from infections commonly associated with diarrhea or any systemic disease, such as Potomac horse fever. This form is also associated with concussion to the feet, exposure to black walnut shavings or ingestion of toxic plants. The inflammatory form, referred to as “acute laminitis,” tends to cause intense pain and massive separation of the laminae, causing the coffin bone to separate from the hoof wall and descend downward in the foot, which can develop within days of the onset. In the most severe cases the bone descends through the sole to the ground.

• The supporting-limb pathway develops under mechanical stresses, such as when a horse has injured one leg and is required to support all of his weight on the opposite limb. The mechanical stress on the supporting limb results in laminitis occurring in the foot.

• The endocrine pathway is by far the most common form of laminitis seen today. Also called “pastureassociated” or “grass laminitis,” this form of the disease results from elevated levels of insulin in the blood. It is seen in horses who have EMS or PPID. It tends to develop slowly, is sometimes called “chronic laminitis” and causes soreness in the feet, but not necessarily pain as severe as in the acute form. Separation and displacement of the coffin bone from the hoof wall still occurs but is likely to be gradual. Repeated bouts result in permanent deformity of the feet, crippling the horse. Eating grass in the spring and fall are associated with increasing levels of insulin that initiate a bout of painful laminitis. In this form of the disease excess insulin causes the laminae to lengthen and weaken, resulting in abnormal growth rings on the hoof. Eventually, the bone descends downward, producing laminitis.