Cushing's (PPID) and equine metabolic syndrome (EMS)
TESTING FOR CUSHING'S DISEASES or PARS INTERMEDIA DISFUNCTION (PPID)
What is the best test for diagnosis of PPID?
Cushing's has a characteristic clinical picture and diagnosis could be made on occasions based on clinical signs.
Laboratory tests could confirm and add valuable information for treatment and management.
Plasma ACTH measurement is the gold standard test. It is easy, practical and only requires a single blood sample. Horses with Cushing's are expected to produce higher levels of ACTH by the pituitary gland.
The ACTH molecule is fragile and could deteriorate rapidly; therefore a sample needs to be chilled as soon as possible and sent to our lab promptly to obtain the most reliable result. Sampling can only be done at certain times to fit in with this.
There are others test for Cushing's that could improve the sensitivity of the diagnosis, however these are not as simple and require a stimulation test, which involves the acquisition of several samples.
When is it best to blood sample for Cushing's? Do I need to do further follow up samples?
Blood samples could be taken at any time of the year to be diagnostic. However there is a natural variation within the normal range of ACTH throughout the year. Such variation is more pronounced in affected horses during the autumn period and this is a good time to blood sample borderline cases. Alternatively samples could be taken any time of the year and by adjusting the reference range of the result accordingly we obtain a valid diagnosis.
Following initial diagnosis follow up samples are taken around 2 to 3 weeks following the start of medical treatment to adjust treatment doses.
Cushing's is a degenerative disease and horses will deteriorate with time hence the necessity to take blood samples at regular intervals and keep the treatment adjusted according to the horses condition at that time .
As results of the sample could vary throughout the year, we recommend that follow up samples be taken always at the same time of the year for a reliable comparison.
It may be advisable to obtain two follow samples a year, in which case obtaining them in spring and autumn are appropriate. Alternatively a single follow up sample could be taken in which case July is an appropriate time.
EQUINE METABOLIC SYNDROME (EMS)
Measurement of the degree of insulin resistance is the basis for EMS diagnosis. In particular insulin concentration is a potential indicator of the susceptibility to horses prone to laminitis and is a valuable measure to gauge the response to a program of diet and exercise.
A single blood sample could be obtained with high resting values suggestive of the presence of insulin resistance, however a rise in insulin levels can also occur under other confounding factors such us pain, stress, etc.
We generally suggest two protocols for obtaining an insulin reading:
1. Blood sample for insulin following fasting for 6 hours.
High insulin levels would indicate the presence of insulin resistance, however low insulin doesn't completely rule out the presence of insulin resistance.
2. A more reliable test, and still practical to be performed at stables, would be a stimulation test, which involves the following protocol:
- Fast horse/pony overnight
- Administer wet chaff (low calorie content) and add 1g/kg body weight of glucose or dextrose powder.
- Arrange for us to take a blood sample two hours later.
Horses insulin resistance will have a clearly high above normal post-feeding reference range.