Equine Science Update
Reports from the world of  equine research.
Cushing’s disease:
Chaste berry ineffective.
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Chaste berry extract  (Vitex agnus castus) does not appear to be as useful in the management of Cushing`s disease as was first thought. A recent trial, reported at the annual convention of the American Association of Equine Practitioners, found that the herbal medicine did not improve the signs of Cushing`s disease in affected horses. These findings contradict earlier work that suggested that the herbal medicine was beneficial for horses with the condition.

Cushing`s disease is a condition of older horses and ponies. Most are about 20 years old before they show signs. A common sign is a long curly coat ( hirsutism ) which does not shed in the summer. It is due to a functional tumour of the intermediate lobe (pars intermedia) of the pituitary gland. The cells involved  (known as chromophobe) produce various hormones including ACTH (adrenocorticotrophin) and pro-opiomelanocortins (POMC) which act on the adrenal gland to release cortisol. It is the cortisol that is responsible for many of the signs of Cushing`s disease.

In the normal horse an increase in the blood cortisol levels would lead to a reflex inhibition of further ACTH activity. Unfortunately in the horses with Cushing`s disease the normal feedback mechanism does not work

Affected animals frequently develop diabetes. They have high blood glucose levels and drink and urinate more than normal. They are also prone to laminitis, which is usually the eventual cause of them being put down.

Drugs can be used to suppress the over activity of the pituitary gland.  Pergolide, is probably the most widely used medication for Cushing`s disease. It is a dopaminergic agonist, and has been shown to reduce the blood levels of POMCs and ACTH in the horse. However, the cost of long-term treatment with pergolide, and a growing interest in  natural medicine , has prompted the search for alternatives. Previous work has shown that extracts of Vitex agnus castus (chaste berry) stimulate dopaminergic receptors in rats.  It has been suggested that the extract might be useful for treating equine Cushing`s disease. *

Jill Beech, Mark Donaldson and Sue Lindberg working at the University of Pennsylvania's New Bolton Center have been investigating the use of Vitex agnus castus extract as an alternative to pergolide for the treatment of Cushing`s disease.

They treated fourteen horses with a commercial preparation of Vitex agnus castus extract, and compared the effect with pergolide. Horses were selected for the study because they showed typical signs of Cushing`s disease - including curly coat, failure to shed the coat in summer, weight loss, and laminitis. All but three of the horses were over 20 years old.

The researchers confirmed the diagnosis of Cushing`s disease using a combination of laboratory tests. Twelve of the horses had raised plasma ACTH levels. A dexamethasone suppression test was carried out on four horses, including the two with normal ACTH concentrations. All showed an abnormal response, supporting the diagnosis of Cushing`s disease.

The Vitex agnus castus extract was administered by the owners according to the manufacturer's recommended dose. It had no beneficial effect on horses with Cushing`s disease. The condition of some horses deteriorated while they were being treated with the Vitex extract. The ACTH concentration in the blood decreased in only one horse, a mare that was treated for two months. In that horse, the ACTH level fell over two months, and remained low or only slightly elevated for the next three months during which no further treatment was given.   “We had originally planned to treat the horses for 6 months,”  reports Dr Beech.  “But some horses showed worsening clinical signs or a deteriorating response to hormone tests.”

In comparison, in nine horses treated with pergolide, the response was better.  With the exception of one horse, pergolide had a beneficial effect - although some horses needed higher doses than are commonly used . Three horses that originally showed an abnormal response to the dexamethasone suppression test returned to normal. One of them required a higher than normal dose for three months before responding. The researchers checked the ACTH levels of six horses one month after starting the pergolide. Four horses had levels less than half the pre-treatment levels.  In one horse the pergolide was discontinued because it had no effect and the horse appeared to be dull. Its demeanour improved after the drug was stopped.

The results confirmed the value of pergolide but failed to show any benefit in using Vitex in the management of Cushing`s disease. Dr Beech concludes  “Until more information is available, we do not advise using Vitex agnus castus for treating equine Cushing`s disease.”

*Herbal hope for Cushing`s syndrome.
Kellon EM
Horse J (2000) 7, 3 - 7.

For more details see:
Comparison of Vitex agnus castus extract and Pergolide in the treatment of equine Cushing`s disease. Jill Beech, Mark T Donaldson, Sue Lindborg.
Proc AAEP (2002) 48, p 175- 177
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