Cushing’s disease or Pituitary Pars Intermedia Dysfunction (PPID) is a common condition in older horses, resulting from increased activity in the intermediate lobe of the pituitary gland. This excessive activity has been attributed to nerve damage impairing its regulation, with dopamine acting as the crucial neurotransmitter. When dopamine levels are deficient, the pituitary gland becomes overactive, leading to the release of various hormones, such as adrenocorticotropic hormone (ACTH).
Clinical manifestations associated with PPID are well-documented and include hypertrichosis, laminitis, polyuria, polydipsia, lethargy, muscle wastage, and delayed wound healing.
Dopaminergic agonists, medications that mimic the action of dopamine, function by stimulating dopamine receptors in the pituitary gland. This stimulation results in the inhibition of the release of certain hormones, including ACTH, thereby mitigating clinical signs such as abnormal hair coat, muscle wasting, and laminitis.
Pergolide, a dopamine agonist, stands as the primary treatment for managing PPID and is approved for oral administration in equines. However, some horses may exhibit resistance to daily oral dosing. Cabergoline, another dopamine agonist like pergolide, currently is not licenced for use in horses.
A recent retrospective study conducted by Tania Sundra of Avon Ridge Equine Veterinary Services in Brigadoon, WA, Australia, along with colleagues, explored the efficacy of intramuscular extended-release cabergoline (ERC) injection in treating PPID. Despite being unlicensed, this treatment modality is increasingly utilized off-label in clinical practice for managing PPID. The study examined clinical records of privately-owned horses with PPID that had undergone intramuscular cabergoline treatment at Avon Ridge Equine Clinic.
The study examined the short-term (5–8 days) and longer-term (12 months) clinical and endocrinological responses to two doses of cabergoline: a low dose (0.005mg/kg) extended-release cabergoline (LDERC) and a high dose (0.01mg/kg) extended-release cabergoline (HDERC).
Results showed that although both doses of ERC led to a decrease in median ACTH concentration, the levels remained above the seasonal reference range in about half of the treated horses. However, similar responses had been noted in previous studies involving pergolide treatment for PPID.
Owners reported clinical improvement in 78.3% and 100% of horses treated with LDERC and HDERC, respectively.
Common side effects of dopamine agonists, such as decreased appetite, lethargy, gastrointestinal upset, and behavioural changes, were observed. Notably, partial, self-limiting inappetence was reported in 30.0% of LDERC cases and 60% of HDERC cases, with seven horses exhibiting lethargy.
In cases of partial anorexia, owners noted that horses preferred long-stem forage (hay or grass) over cereal-based feeds. The clinical and endocrinological responses observed were consistent with previous reports of oral pergolide treatment.
The authors suggest that weekly injection of ERC may serve as an effective alternative to pergolide. The 0.005 mg/kg dose appeared to be as effective as the 0.01 mg/kg dose but with a lower risk of inappetence.
While cabergoline shows promise as a treatment option for PPID in horses, further research is necessary to fully establish its efficacy, safety profile, and optimal usage protocols.
For more details, see:
Sundra T, Kelty E, Rossi G and Rendle D (2024)
Retrospective assessment of the use of extended-release cabergoline in the management of equine pituitary pars intermedia dysfunction.
Front. Vet. Sci. 11:1332337.
https://doi.org/10.3389/fvets.2024.1332337
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