Laminitis refers to a condition that impairs the circulation in the horn-producing tissues of the foot (corium). The disease results in the hoof producing poor-quality horn, which leads to increased injury from concussion and other causes. There can be several degrees of laminitis:
- Acute or subacute laminitis is rare and is usually due to a single, gross metabolic insult such as a grain overload or diseases like retained placenta or mastitis. It develops rapidly and causes severe signs of acute pain, but does not produce lesions that are visible in the hoof.
- Subclinical laminitis is a multifactorial disease, meaning several factors work together to cause it, with lesions first visible about six to eight weeks after distinct insults.
- Chronic laminitis develops from continuous or repeated insults that cause lesions affecting the shape and function of the feet and eventually locomotion.
The precise, initiating cause of laminitis in cattle is unknown. Researchers think that substances released into the circulation may trigger changes in the corium’s blood vessels. The vessels dilate and their walls become paralyzed, making blood stagnate. The blood vessels in the corium receive insufficient oxygen and become inflamed. Vessel walls become permeable and allow blood to seep out, which leads to fluid accumulation, swelling and hemorrhage. Large, dark red patches develop in the normally pink corium.
Blood vessels supply the corium’s horn-producing cells with nutrients and oxygen. When the blood flow to the tissue decreases, the corium starts producing inferior-quality horn. This is especially critical at the white line where the horn and the corium connect the pedal bone to the wall, because this area suspends a considerable part of the cow’s weight. When this junction fails and the connection between corium and bone separates, the pedal bone sinks in the claw, putting even more pressure on the already compromised corium.
As the laminitis progresses, the white line becomes disrupted and starts to separate. The pedal bone sinks inside the horn capsule and puts more pressure on the sole and heel corium. Once the pedal bone sinks, the situation is irreversible. The early stages of phase two have no obvious signs although several weeks may have passed since onset. Prolonged compression of sole and heel corium lead to more capillary damage, hemorrhage, blood coagulating in vessels, inflammation, and tissue dying from a loss of blood. Lesions are generally focal, very painful and can cause lameness.
A sign of a sinking pedal bone is a groove in the corium along the edge of the sole and a corresponding ridge in the inner horn along the junction between the sole and wall of the horn capsule.
Several weeks pass from onset of laminitis until clinical signs and lesions become visible. This is one reason why, historically, the many lesions were considered to be separate diseases. Some of the changes seen in the chronically deformed hoof are from repeated episodes of laminitis.
The following laminitis lesions all stem from the inferior horn tissue that the corium produced, which takes time to appear on the surface of the hoof. Also, most of these lesions will not be visible until sole horn is pared away.
- Yellow, soft horn. Serum escapes from the damaged corium and incorporates into the horn, producing a yellowish colour. This inferior horn is soft and easily damaged and is one of the primary signs of chronic, subclinical laminitis.
- Hemorrhages usually appear on the surface of the sole and heel approximately six weeks later. A white-line hemorrhage might take longer to appear if it develops farther up the wall.
- Double sole. Accumulation of cellular debris on the surface of newly produced horn can result in a double sole or sole separation. Separation of the layers in the heel can provide an entrance for infection.
- White-line disease. Fissures can form in the softer tissue of the white line. If they are deep enough, infection can occur, which may eventually work its way up to the coronary band and form a white-line abscess. The abscess breaks through at the coronary band whereas the white-line lesion found when trimming the sole might be very small.
- Sole or toe ulcer. Damage to the sole corium may be severe enough that the lesion extends through the whole thickness of the sole or toe. The most common site for a sole ulcer is on the rear, outside claw beneath the part of the pedal bone that connects to the flexor tendon.
- Deformed claws are the result of chronic laminitis, which causes horizontal (hardship) grooves, concave walls, and a bulged or dropped sole. Many of these cows would not be described as clinically lame, but if you watch them walk and were to assign a locomotion score, many of them would score 3 on a 1-5 scale. By the time the whole claw capsule is deformed, it is unlikely that hoof trimming can return it to normal shape and function.
source: Steven L. Berry, University of California