Adrenocorticotropin-containing neoplastic cells in a pars intermedia adenoma in a horse.

The research presents a case of a horse suffering from hyperadrenocorticism, a condition caused by increased levels of adrenocorticotropin (ACTH) in the body, as the result of a tumor in the pituitary gland.

Clinical Presentation and Diagnosis

• This particular case involves a 14-year-old Arabian mare presenting with symptoms such as chronic weight loss, hirsutism (excessive hair growth), polyuria (excessive urination), and polydipsia (excessive thirst). These symptoms are indicative of pituitary-dependent hyperadrenocorticism (PDH), commonly known as Cushing’s disease in horses.
• The horse also exhibited a stress leukogram, high glucose levels in the urine (glucosuria), and consistently high blood sugar levels (hyperglycemia) which were not suppressed by insulin.
• The horse’s plasma cortisol concentrations were within normal ranges but did not respond to dexamethasone suppression and displayed an exaggerated response to ACTH stimulation. This lack of response to dexamethasone and excessive response to ACTH is another hallmark of hyperadrenocorticism, signaling a dysfunction in the regulation of cortisol production.

Postmortem Findings and Analysis

• After the horse’s death, a necropsy identified a chromophobe adenoma, a type of benign tumor, in the pars intermedia of the pituitary gland. The presence of this tumor in the pituitary gland supports the earlier diagnosis of PDH.
• Hypertrophy (enlargement) of the zona fasciculata of the adrenal cortex and the pancreatic islets of Langerhans were also observed. This suggests that these glands may have been overactive or that they may have been responding to excessive stimulation by hypothalamic hormones.
• An immunohistologic staining technique was used postmortem to confirm that the pituitary tumor contained ACTH-producing cells, reinforcing the diagnosis of ACTH-dependent hyperadrenocorticism. These cells were releasing ACTH and other peptides that initiated the chain of endocrinological events leading to the observed clinical symptoms.

Significance and Conclusion

• This research provides evidence that ACTH-producing tumors can cause PDH in horses, presenting as various physical symptoms and confirmed through laboratory tests and postmortem examination. The documentation and analysis of such cases contribute to veterinary knowledge, aiding future diagnosis and treatment of this disease in horses.