Laminitis – Causes and Consequences

Question 1

Why is laminitis such an important disease?

Answer 1

• Very common problem in First-opinion practice.
  -Causes huge amount of equine suffering and welfare issues.
• A considerable drain owner emotions/finance/time
• A common cause of euthanasia
• Often (badly) dealt with by owners without (enough) veterinary involvement

Question 2

Define laminitis

Answer 2

Inflammation of the lamellae
“An acute onset lameness of variable severity involving one or more feet”
“the failure of the attachment between the distal phalanx (coffin bone) and the inner hoof wall”

Question 3

Describe the anatomy of the hoof

Answer 3

• Underneath the horn is the dermis
• Horn produced by the coronary band
• Junction between the sensitive and insensitive tissue (area that goes wrong in laminitis)
• Lamellae = “Thin sheet”
• About 600 Primary lamellae per hoof
• About 150 Secondary lamellae per primary lamellae

Question 4

Describe the anatomy of the sole and describe weight bearing in the hoof

Answer 4

• No lamellae on sole of foot
• Weight bearing mostly done through lamellae, frog and walls of hoof.
• So the sole isn’t designed for weight bearing
• Excessive pressure on solar corium (dermis) tissue = painful

Question 5

Describe the blood supply in the hoof

Answer 5

Foot – very well vascularised.
Aterio-venous shunts present in laminae

Question 6

Why does the foot need to be well vascularised?

Answer 6

Constant supply of glucose needed to keep basement membrane structures intact.
- A single horses’ hoof uses more glucose per day than its brain.
- Glucose uptake in the foot not insulin mediated.

Question 7

Name some of the modern theories for laminitis

Answer 7

1. Vascular Theory
2. Endotoxaemic\sepsis\inflammatory theory
3. Carbohydrate overload theory
4. Supporting limb laminitis
5. Mechanical laminitis
6. Glucose deprivation theory

Question 8

Name a real and common cause of laminitis

Answer 8

Endocrinopathic laminitis

Question 9

Name 5 real and uncommon causes of laminitis

Answer 9

Endotoxic/septic/inflammatory laminitis
Mechanical laminitis
Supporting limb laminitis
Glucocorticosteroid induced laminitis
Carbohydrate overload

Question 10

What is the vascular theory of laminitis

Answer 10

Laminitis caused by alterations in blood flow to digit – leading to tissue hypoxia, ischemia, tissue necrosis, inflammation and ultimately laminar failure

Question 11

Compare the evidence for and against the vascular laminitis theory

Answer 11

For: Vascular changes well proven - both vasoconstriction and vasodilation
Against:
- No consistent pattern of change found
- Histopathological signs of inflammation not always present in pasture associated laminitis.
- What initiates the vascular changes?

Question 12

What is the endotoxic/sepsis/inflammatory theory of laminitis

Answer 12

Laminitis caused by the effects of endotoxins and other inflammatory substances on the foot.

Question 13

Compare the evidence for and against the endotoxic/sepsis/inflammatory theory of laminitis

Answer 13

For: Laminitis well recognised as a potential complication of SIRS/Endotoxemia in horses e.g. retained foetal membranes, colitis, post colic surgery, pleuropneumonia
Against:
- Not every endotoxic horse gets laminitis.
- Most laminitics not endotoxic.
- Injection of endotoxins to horses does not reliably induce laminitis

Question 14

Describe the supporting limb laminitis theory

Answer 14

Horses which are non-weight bearing on one leg often develop laminitis in the contralateral limb

Question 15

Compare the evidence for and against the supporting limb laminitis theory

Answer 15

For: Definitely does occur – in very severe lameness (fractures etc.)
Against:
- Limited to severe lameness: not every lame horse gets laminitis
- Not representative of pasture associated laminitis

Question 16

Describe the mechanical laminitis theory

Answer 16

Any force which physically tears hoof from the laminae is technically laminitis.
Either occurs as a (very) traumatic one-off incident or as a result of chronic forces on an already weakened digit.

Question 17

Compare the evidence for and against the mechanical theory

Answer 17

For: Logically can occur. Probably quite common in severe chronic laminitis
Against: Little researched evidence, mostly anecdotal, difficult to prove condition exists

Question 18

Describe the glucocorticoid induced laminitis theory

Answer 18

Glucocorticoid administration to horses can cause laminitis.
Long held belief that glucocorticoids practically contra-indicated in horses

Question 19

Compare the evidence for and against the glucocorticoid induced theory

Answer 19

For: Well documented evidence that this can occur - even after one-off intra-articular doses.
Against:
- Modern epidemiological studies shown risk is genuine, but actually quite low.
- Deliberately trying to induce laminitis by giving steroids (for research purposes) rarely works
- Using high doses of steroids or using them in animals already predisposed to laminitis likely to increase the risk.
- Oral prednisolone shown to be quite safe.

Question 20

Describe the fructans and carbohydrate overload theory for laminitis

Answer 20

• Mammalian digestive tracts lack the enzymes to digest fructans therefore they spill into the large bowel intact.
• Bacteria can digest fructans.
• Fructan overload therefore implicated as the source of pasture-associated laminitis.

Question 21

Fructan level in plants depends on which factors?

Answer 21

• Type of plant, part of plant, growing season, ambient temperature, sunlight levels, “stress” levels
• Highly variable, can change from hour-to-hour – can’t predict pasture levels.

Question 22

Compare the evidence for and against the the fructans and carbohydrate overload theory for laminitis

Answer 22

For:
- Large quantities of oral fructans will reliably induce laminitis in experimental studies.
- Could easily explain pasture-associated laminitis
Against:
- Can horses physically consume enough fructans in a day to cause laminitis ?
Controversial – most experts consider it unlikely, although some disagree.

Question 23

Describe the practical relevance of the fructans and carbohydrate overload theory for laminitis

Answer 23

• Carbohydrate overload is just a form of endotoxic/septic/inflammatory laminitis (since a form of colitis is induced) and does not reflect pasture-associated laminitis.
• Concept still widespread with horse owners.
• Lead to use of in-feed antibiotics to prevent hindgut fermentation

Question 24

Name 3 underlying endocrinopathies that can lead to laminitis

Answer 24

• Equine metabolic syndrome
• Equine pars pituitary intermedia dysfunction
• EMS and PPID together

Question 25

The chance of an individual suffering from laminitis if it does not have EMS or PPID is …

Answer 25

Low

Question 26

Describe equine metabolic syndrome

Answer 26

Syndrome obesity, insulin dysregulation and laminitis

Question 27

What is the trigger cause of equine metabolic syndrome related laminitis?

Answer 27

Hyperinsulinaemia due to insulin dysregulation

Question 28

How does hyperinsulinaemia lead to laminitis?

Answer 28

Sustained hyperinsulinaemia will reliably induce laminitis in normal (non-EMS) horses.
Hyperinsulinaemia damages cytoskeleton of laminar cells and also has vasoactive properties (remember foot glucose metabolism insulin independent).

Question 29

Describe the features of Pituitary Pars Intermedia Dysfunction

Answer 29

• Equine Cushing’s Syndrome.
• Equines 15 years or older.
• Extremely common in UK aged horse populations.
• Testing for PPID and monitoring treatment large part of modern leisure horse practice - good for preventing laminitis
• Exact mechanism by which PPID initiates laminitis unknown

Question 30

What are the consequences of laminar failure in the hoof?

Answer 30

The pedal bone is free to move within the foot.
- Where it goes and how far depend on the forces acting on the bone and the extent and severity of laminar failure

Question 31

Describe how a normal hoof should appear on radiography

Answer 31

• Depth from coronary band to tip of extensor process is variable but approx. 10 mm in normal horses
• Distance form dorsal surface PIII to hoof should be constant.
• Dorsal surface of PIII and PII in line

Question 32

Describe cases of laminitis when there is no movement of the pedal bone

Answer 32

Mild cases might not have any pedal bone movement (or it is so slight not to be visible on radiographs).
Important to explain to owner that this does not imply the animal has not got laminitis

Question 33

Describe capsular rotation of the pedal bone

Answer 33

The hoof capsule diverges form the dorsal surface of PIII, but PIII itself remains aligned with PII. Common.

Question 34

Describe bony rotation of the pedal bone

Answer 34

The hoof capsule diverges form the dorsal surface of PIII, and PIII itself is rotated palamarly around the DIP. Implies significant pull of DDF and usually a sign of severe damage. Less common.

Question 35

Describe the appearance of a ‘sinker’ laminitis on radiography

Answer 35

• Vertical displacement of PIII downwards
• Sole depth thin
• Distance from tip of extensor process to coronary band increased
• Usually implies very serious pathology, horses usually very painful

Question 36

Describe the consequences of pedal bone movement

Answer 36

• In the acute stage: resultant gap becomes a haematoma/seroma /necrotic mess
• If this gets infected a foot abscess will result.
• Foot abscesses are a common complication of laminitis: usually 2-3 weeks post the acute stage – warn the client.
• Forces on the foot stretch the white line junction, providing an easy ingress route for abscess causing bacteria
• Eventually the haematoma is replaced by poor quality, dysplastic horn – a lamellar “wedge”

Question 37

How does pedal bone movement affect the blood supply?

Answer 37

Downward the pedal bone drags the coronary band with it leading to a compromised (crushed) blood supply to the coronary band.

Question 38

How can the visual conformation of the hoof change due to pedal bone movement?

Answer 38

• Abnormal horn growth results: the toe goes slower than the heels leading to diverging hoof rings (parallel growth rings just indicate a change in growth rate)
• The hoof attempts to remodel itself around the new position of the pedal bone leading to abnormal hoof shapes (which may lead to chronic lameness).