Of the 5.32 million horses and ponies in the United States, 400,000 (over 7.5%) are 20 years or older and considered geriatric, according to Frank Andrews, DVM, MS, Dipl. ACVIM. About 50 horse owners convened at the University of Tennessee’s (UT) College of Veterinary Medicine on Jan. 31, 2004, to attend a seminar highlighting techniques for managing older horses.

Loss of muscle mass, change in body condition, swayback, and a potbelly are signs of age in horses, but internal changes also occur, including decreased immune response, antibody production, and immune response to vaccines, said Andrews.

The vaccine failure rate in older horses is as high as 50%. In addition, the rhinopneumonitis vaccine is not recommended for non-pregnant older horses because it can reactivate a latent or hidden virus, leading to neurologic signs. He added that older horses have already been exposed to this virus and have some natural immunity. Owners should assess the relative risk of exposure and if possible use attenuated or killed vaccines, which are safer in older horses than modified-live versions.

More than 70% of horses living beyond the age of 20 will develop Cushing’s disease, said Nicholas Frank, PhD, DVM, Dipl. ACVIM. (See page 83 for more on Cushing’s.) Measuring levels of the hormone plasma adrenocorticotropin (ACTH) and performing a dexamethasone suppression test–where the horse’s natural steroid level is measured before and after giving this man-made steroid to gauge pituitary function–are useful diagnostic tools for detecting Cushing’s, but the commonly used cortisol rhythm test cannot be relied upon. In the summer of 2003, Frank and his colleagues at UT’s Center for Endocrine and Metabolism Research examined the cortisol rhythm test by measuring morning and evening cortisol concentrations in four healthy horses every two weeks for eight weeks. Both normal and abnormal rhythms were detected at different times in the same animals, indicating that healthy horses would have been incorrectly diagnosed with Cushing’s