CUSHINGS DISEASE: In the classical sense, this refers to a condition characterized by excess production of the hormone cortisol as a result of a tumor in the pituitary gland of the brain which produces high levels of the hormone ACTH. In other species, Cushings Disease just means high production of cortisol, whether due to a brain tumor or not. In other species, pituitary tumors may also produce other hormones, such as Prolactin. Studies are currently underway to determine if this is the case in horses as well.
ACTH: A hormone produced by the pituitary gland that triggers production of cortisol from the adrenal gland.
CORTISOL: A hormone produced by the adrenal gland which is released naturally during times of stress (e.g. excitement, transport, exercise, infections, trauma, pain, fear). When production is abnormally high and sustained, it can result in impaired immune function, resistance to insulin, electrolyte abnormalities and impaired ability to utilize protein in the diet.
INSULIN: A hormone produced by the pancreas which is needed for cells to be able to take in glucose derived from dietary carbohydrates, and to take in amino acids (proteins) to use in manufacture of enzymes, cell structures and muscle.
INSULIN RESISTANCE: A condition where the cells of the body do not respond to insulin like they should. As a result, the pancreas needs to produce more insulin than normal to get glucose and protein into the cells. This is similar to early type 2 diabetes in people the type that does not require insulin. People with insulin resistance commonly also have an elevated blood sugar (blood glucose) level, which we dont see in horses except in very severe cases. Insulin resistant horses will have abnormally elevated insulin though. Insulin resistance may be seen as a result of the high cortisol production in classical Cushings Disease. Might also be found in association with hypothyroidism.
Ponies and easy keeper breeds of horses appear to be genetically programmed to be insulin resistant. In that case, it probably should not be considered a disease or disorder per se but does mean you have to be careful with their diet.
PROLACTIN: A hormone produced by the pituitary gland that causes mammary development and production of milk. In people and experimental animals, some pituitary tumors produce high levels of Prolactin.
LAMINITIS: Inflammation of the laminae tissues that connect the coffin bone to the hoof wall inside the foot.
FOUNDER: Often used synonomously with laminitis. Some people use the term founder to indicate horses whose coffin bone has come loose from its hoof wall attachments and is displaced.
ROTATION (of the coffin bone): Movement of the coffin bone away from a parallel position with the hoof wall, so that its tip is pointed down toward the sole.
SINKER: A form of laminitis where the attachments of the coffin bone are weakened/lost around most of its circumference so that the whole bone sinks down inside the hoof and is putting pressure on the sole.
T4 The largely inactive form of thyroid hormone. Most commonly done thyroid function test.
T3 The metabolically active form of thyroid hormone.
PRECUSHINGS: A term sometimes used to describe horses that may have some symptoms of Cushings disease but cannot be confirmed to have it on laboratory testing.
SYMPTOMS OF LAMINITIS
These vary greatly depending on the severity of the laminitis. From least severe to most they include:
- Less spontaneous activity
- Less spontaneous trotting/cantering
- Reluctance to turn (puts more weight on one foot)
- Reluctance to move forward when lead
- Lying down more than normal (when pain severe, stay down most of the time)
- Standing with the front feet further in front of the body than normal and the hind feet further under the body than normal
- Stiffness in the shoulder muscles
- Buckling at the knee
- Refusal to move
- Hind end muscles tightly bunched up (shifting most of their weight to the hindquarters)
Examination of the feet may show (usually worst in front feet):
- Feet feel warmer/hotter than usual
- Pulses in the arteries running over the sesamoid bones at the back of the ankle are very strong and pounding
- Puffiness or redness at the coronary band
- Pain on sole pressure about ¼ to ½ inch in front of the point of the frog
- A bruised appearance to the sole
- Red or black discoloration of the white line
- Widening of the white line
- Appearance of rings on the feet that are close together at the toe but get progressively wider over the quarters and heels
SYMPTOMS OF INSULIN RESISTANCE:
- Easy weight gain
- Abnormal fat desposits such as a cresty neck or lumpy, cellulite-like fat at the tail base. These fat deposits will usually persist even if the horse loses weight elsewhere on his body
- Puffiness (fat) in the hollows above the eyes
- History of laminitis commonly induced by grass
- Advanced symptoms include increased thirst and urination, loss of body condition, especially muscle, weakness, low energy levels
SYMPTOMS OF HYPOTHYROIDISM
VERY nonspecific and many overlap considerably with those of insulin resistance. Slow shedding/longer than normal coat may be seen. Energy levels poor, exercise tolerance poor. Horse may be irritable and sensitive to touch. Cannot be diagnosed with any certainty by symptoms alone. Need to do blood tests.
SYMPTOMS OF CUSHINGS DISEASE
- Abnormal hair coat (longer than normal, often curly) that does not shed out normally in the spring
- Mammary enlargement and/or milk production in some mares
- Fatty sheath and/or heavy sheath secretions in some geldings
- Any or all of the symptoms listed above for insulin resistance and/or hypothyroidism
BLOOD ACTH: Used to diagnose classical Cushings disease, where ACTH will be greatly elevated. Higher than normal levels may also be found in stressed horses, and possibly in aged horses, but not to the same degree.
CORTISOL RHYTHM TEST: Screening test for Cushings disease. May be both false positive and false negative results. False positives can be as high as 35%. Involves sampling twice on one day (AM and PM)
DEXAMETHASONE SUPPRESSION TEST: Screening test for Cushings disease. Involves injection of the corticosteroid drug dexamethasone. Risk of causing or worsening laminitis. Felt to be diagnostic by some veterinarians but potential for false positives and negatives well described in other species and may be the same in the horse. Involves collection of samples over 8 to 12 hour period
TRH STIMULATION TEST: Test for Cushings disease that is probably highly diagnostic. Involves injection of TRH, which will cause a cortisol peak in a horse with a pituitary tumor. Postinjection sample drawn at 15 to 30 minutes after TRH.
COMBINED DEXAMETHASONE SUPPRESSION/TRH STIMULATION: Test for Cushings disease involves both tests as above.
URINARY CORTISOL:CREATININE RATIO: Test done on urine as a screen for Cushings disease. Not 100% diagnostic but highly suggestive. More accurate than blood cortisol levels in detecting increased production of cortisol.
BLOOD INSULIN: Measurement of blood insulin to test for insulin resistance.
T4 AND T3: Thyroid function tests.
CHEMISTRY SCREEN: Checks for organ function, dehydration, blood sugar, electrolyte abnormalities. With Cushings/insulin resistance, should also request testing for blood cholesterol and triglyceride levels.
CUSHINGS (PITUITARY TUMOR)
- Cyproheptadine a drug which helps correct chemical imbalance in the brain by suppressing serotonin levels, which is may help control hormone output from the tumor.
- Pergolide (brand name Permax, generics available) drug which helps correct chemical imbalance in the brain by boosting activity of dopamine, low with pituitary tumors. Most effective treatment but some risk of side effects.
- Vitex agnus-castus (aka Chasteberry). Herb with pharmacological activity similar to that of pergolide. Much less clinical experience with this than with pergolide but works well for some horses.
- Effective treatment also involves other measures to directly address insulin resistance, hypothyroidism and laminitis if these are present. See below.
INSULIN RESISTANCE (also see the File BASIC DIET for further details)
- Most important is control of the starch and sugar level in the diet. This means eliminating ALL grains, pelleted feeds made with grains (read the ingredients on the label), supplements with a grain base, carrots, apples and access to grass. (Some stages of grass growth and/or specific grass species may be OK for some horses. Need to determine on an individual basis.)
- Feeding of grass hay only, or grass hay with some soaked beet pulp to replace grain. Some horses are OK with small amounts of rice bran as well (2 oz/lb of beet pulp).
- Analysis of hay to make sure NSC (nonstructural carbohydrate levels primarily sugars in hays) is below 10% for severely insulin resistant horses, no more than 15% for any insulin resistant horse
- Analysis of mineral levels in hay to allow selection or mixing of a supplement that specifically provides what the horse needs (see other files)
- Provision of a thyroid supplement such as Thyro-L (levothyroxine sodium), at least initially
- Provision of adequate amounts of minerals in the diet to support thyroid function
- Treat Cushings Disease, insulin resistance, hypothyroidism as above to eliminate those triggers of laminitis
- Expert hoof care is CRITICAL, involves trimming to realign any malposition of the coffin bone as a result of damage to the laminae and trimmings at frequent enough intervals to maintain this correction