Cushings Disease in Horses

By Dr. Wendy Pearson

 

Cushings Disease is a syndrome of older horses (average age of 19 years) that is receiving increasing attention in the equine community.  Also known as hyperadrenalcorticism, or pituitary pars intermedia dysfunction (PPID), the number of diagnosed cases has increased considerably over the past few years, likely due to improvements in general husbandry practices that increase the life expectancy of horses.

The clinical signs of Cushings Disease include:

 

·          a thick, curly haircoat that does not shed out

·          an increased tendency to developing laminitis, especially in winter

·          excessive water intake and urination

·          excessive sweating

·          muscle wasting

·          poor wound healing

·          increased respiratory rate, which may or may not be associated with respiratory disease

·          reproductive problems, especially in mares, including abnormal cycling or an inability to conceive

·          immunosuppression

·          increased susceptibility to strongyle infestations

·          lethargy

 

Horses with Cushings may drink up to 80 litres of water a day, compared with the normal 20-30 litres, and they typically show a sway back and pot belly, as a result of poor collagen support.

 

The disease is similar to Parkinson’s Disease in humans, in that it is characterized by a loss of the dopamine-containing neurons in the brain. 

 
 

A typical Cushingoid horse, showing thick, curly

haircoat, muscular wasting, and the lines of laminitis

in the hooves

 

 

Pathology of Equine Cushings Disease

Cushings Disease in horses is characterized by hyperactivity of the pituitary gland.  This hyperactivity is usually caused by a benign tumor called a pars intermedia pituitary adenoma (accounting for approximately 85% of cases), but can also be caused by chronic injections of steroids. The pituitary gland is an important gland at the base of the brain that secretes several hormones regulating the proper functioning of the thyroids, gonads, adrenal cortex, and other endocrine organs.  The anterior part of the pituitary gland serves as a reservoir for hormones having antidiuretic and oxytocic actions. 

 

The loss of the normal functioning of the pituitary gland leads to excessively high levels of a glucocorticoid called adrenocorticotropin (ACTH) and endorphins.  As increasing levels of ACTH normally stimulates cortisol secretion, this, in turn, leads to excessively high levels of peripheral cortisol.

 

As the disease progresses and the tumor grows, the adjacent hypothalamus may be affected.  This part of the brain controls thermoregulation, leading to speculation that this may account for growth of the characteristic heavy hair coat.

 

Conventional Treatment of Cushings Disease

Typically, Cushings Disease is treated with one (or a combination) of three drugs; namely, pergolide mesylate, bromocriptine mesylate, or cyproheptadine.  The first two drugs are known as dopamine agonists.  The drug has an affinity for the cells normally stimulated by dopamine, and mimics its physiological activity.  Low dopamine activity leads to increased ACTH secretion.  Therefore, the drug works to indirectly suppress the secretion of ACTH.  The third is known as a serotonin antagonist, and has seen some success in treating some Cushingoid horses (approximately 60-70% of patients respond to this drug).  Essentially, the drug binds to the target cell of serotonin, prohibiting the natural neurotransmitter from binding and stimulating peptides responsible for increasing ACTH. 

 

Herbs in Treating Cushings Disease

Cushings Disease can be a very debilitating disease, and owners of Cushingoid horses are strongly discouraged from using herbs as a treatment without the knowledge of an attending veterinarian.  However, herbs which may be of help to veterinarians treating Cushingoid horses include:

 

Alfalfa (Medicago sativa):  it is noted by several veterinarians that horses with Cushings seem to do better on diets with increased alfalfa content.  There is little scientific rationale for increasing alfalfa content of the diets, though the plant has a demonstrated oestrogenic effect in ruminants.  The plant contains a number of steroids, including b-sitosterol, and isoflavones known to have oestrogenic activity.  It appears to be a safe adjunct to traditional husbandry, provided it is in accordance with veterinary advice.

 

Chasteberry (Vitex agnus castus): this herb is thought to act on the pituitary-hypothalamic axis, and has been successfully utilized in treating endocrine disorders in humans.

  

Herbs that should be avoided when feeding horses with Cushings include:

 

Aniseed (Pimpinella anisum): extract of aniseed competitively inhibits dopamine binding at receptor sites, which may further exacerbate symptoms of Cushings.