Today is going to be a double path round day, since we have two related diseases to discuss! Both were a common request. This post is on ๐ฉ๐ข๐ญ๐ฎ๐ข๐ญ๐๐ซ๐ฒ ๐ฉ๐๐ซ๐ฌ ๐ข๐ง๐ญ๐๐ซ๐ฆ๐๐๐ข๐ ๐๐ฒ๐ฌ๐๐ฎ๐ง๐๐ญ๐ข๐จ๐ง (PPID), also known as ๐๐ช๐ฎ๐ข๐ง๐ ๐๐ฎ๐ฌ๐ก๐ข๐ง๐ โ๐ฌ.
๐๐ก๐๐ญ ๐ข๐ฌ ๐ข๐ญ?
๐๐๐๐ is an endocrine dysfunction caused by an ๐๐๐๐ง๐จ๐ฆ๐ (a benign tumour) in the pituitary gland of the horse. The pituitary glandโs main job is to produce hormones that control the other hormone-secreting organs, such as the adrenal glands or thyroid gland. So when it develops an adenoma, the tumour can interfere with the normal hormone production of the pituitary gland, causing issues like PPID.
๐๐ก๐จ ๐ ๐๐ญ๐ฌ ๐ข๐ญ?
This is primarily an old horse disease, usually affecting horses over 18 years of age.
๐๐ก๐๐ญ ๐๐๐ฎ๐ฌ๐๐ฌ ๐ข๐ญ?
As mentioned above, the primary lesion in PPID is an adenoma of the pituitary gland, specifically the ๐ฉ๐๐ซ๐ฌ ๐ข๐ง๐ญ๐๐ซ๐ฆ๐๐๐ข๐. The pars intermediaโs main job is to take a compound called ๐๐๐๐ and convert it into compounds including ๐๐๐ซ๐๐ง๐จ๐๐จ๐ซ๐ญ๐ข๐๐จ๐ญ๐ซ๐จ๐ฉ๐ข๐ ๐ก๐จ๐ซ๐ฆ๐จ๐ง๐ (ACTH), which stimulates the adrenal cortex. The pars intermedia is regulated by nerves that respond to ๐๐จ๐ฉ๐๐ฆ๐ข๐ง๐, so when there are high levels of dopamine, the pars intermedia reduces processing of POMC. This is called ๐ง๐๐ ๐๐ญ๐ข๐ฏ๐ ๐๐๐๐๐๐๐๐ค. In PPID, the adenoma causes degeneration of these regulating nerves, leading to uncontrolled conversion of POMC, and extremely high levels of ACTH. When ACTH acts on the adrenal glands, they produce ๐๐จ๐ซ๐ญ๐ข๐ฌ๐จ๐ฅ (the main stress hormone), which leads to the clinical signs seen in the horse.
๐๐ก๐ฒ ๐ข๐ฌ ๐ญ๐ก๐ข๐ฌ ๐ ๐ฉ๐ซ๐จ๐๐ฅ๐๐ฆ?
The main outcomes of PPID are ๐ฉ๐จ๐ฅ๐ฒ๐ฎ๐ซ๐ข๐ (excessive urination), ๐ฉ๐จ๐ฅ๐ฒ๐๐ข๐ฉ๐ฌ๐ข๐ (excessive drinking), loss of muscle tone, ๐ฅ๐๐ฆ๐ข๐ง๐ข๐ญ๐ข๐ฌ and ๐ก๐ฒ๐ฉ๐๐ซ๐ญ๐ซ๐ข๐๐ก๐จ๐ฌ๐ข๐ฌ (excessive hair coat). These are all in some way related to the production of excessive POMC-derived compounds. For example, one of the compounds helps regulate the growth of a winter coat, so when it is produced in excess, affected horses think it is winter all the time, leading to hypertrichosis. These horses wonโt shed in the spring, which is often the first sign of PPID for many owners. Some compounds can also contribute to increased insulin secretion, leading to ๐ข๐ง๐ฌ๐ฎ๐ฅ๐ข๐ง ๐๐ฒ๐ฌ๐ซ๐๐ ๐ฎ๐ฅ๐๐ญ๐ข๐จ๐ง, similar to what is seen in Equine Metabolic Syndrome (todayโs other post!). Insulin dysregulation can cause chronic laminitis, which can have serious consequences for the horseโs welfare.
๐๐จ๐ฐ ๐ข๐ฌ ๐ข๐ญ ๐๐ข๐๐ ๐ง๐จ๐ฌ๐๐?
PPID is diagnosed by measuring the levels of ACTH in the blood. To do this, a drug called ๐๐๐ฑ๐๐ฆ๐๐ญ๐ก๐๐ฌ๐จ๐ง๐ that stimulates the body to reduce ACTH secretion is given to the horse. Then, blood samples are taken to measure ACTH levels. If the ACTH level goes down, then the horse likely does not have PPID. If they stay high, then there is something producing ACTH when it shouldnโt, indicating that the horse has an adenoma causing PPID.
๐๐จ๐ฐ ๐ข๐ฌ ๐ข๐ญ ๐ญ๐ซ๐๐๐ญ๐๐?
PPID must be treated medically, as there is no real surgical option for removing these tumours deep within the skull. To treat them, a drug called ๐ฉ๐๐ซ๐ ๐จ๐ฅ๐ข๐๐ is used, which is a ๐๐จ๐ฉ๐๐ฆ๐ข๐ง๐ ๐๐ ๐จ๐ง๐ข๐ฌ๐ญ (a dopamine stimulant). Remember how dopamine provides ๐ง๐๐ ๐๐ญ๐ข๐ฏ๐ ๐๐๐๐๐๐๐๐ค on POMC conversion? By stimulating more dopamine in the body, pergolide reduces the amount of POMC that is converted, thus reducing the amount of ACTH and other compounds, and getting the bodyโs endocrine function back under control. Isnโt science cool?
๐๐จ ๐ฐ๐ก๐ฒ ๐๐ข๐ ๐ฒ๐จ๐ฎ ๐ฉ๐จ๐ฌ๐ญ ๐๐จ๐ญ๐ก ๐จ๐ ๐ญ๐ก๐๐ฌ๐ ๐๐ข๐ฌ๐๐๐ฌ๐๐ฌ ๐ญ๐จ๐๐๐ฒ? ๐๐ก๐๐ฒ ๐๐จ๐งโ๐ญ ๐ฌ๐๐๐ฆ ๐ฌ๐ข๐ฆ๐ข๐ฅ๐๐ซ ๐๐ญ ๐๐ฅ๐ฅโฆ
Good question! Because both diseases can cause laminitis and insulin dysregulation, and can actually occur at the same time in the same horse, some people think they are the same thing. However, they do have very different root causes and treatment plans. This is why itโs important to speak to your veterinarian about any changes you see in your horsesโฆ sometimes Dr. Google can lie to you!
๐๐ก๐จ๐ญ๐จ๐ฌ
1) A horse with muscle wasting and hypertrichosis, characteristic of PPID.
2-3) An enlarged pituitary gland with a pituitary adenoma. This is the tumour that would cause all the issues! In these photos, you can see it bulging out of the ๐ฌ๐๐ฅ๐ฅ๐ ๐ญ๐ฎ๐ซ๐๐ข๐๐, a boney cavern within the skull that the pituitary gland sits in.
4) A cross-section of a brain showing a large pituitary adenoma.
๐๐จ๐ฎ๐ซ๐๐๐ฌ
Maxie, G. Jubb, Kennedy and Palmerโs Pathology of Domestic Animals, Volume 3. Sixth Edition.
Photo 1 courtesy of Wikimedia Commons.
Photos 2-3 courtesy of University of Calgary Diagnostic Services Unit.
Photo 4 courtesy of Noahโs Arkive.
