Nearly one-third of older horses will become affected by pituitary pars intermedia dysregulation (PPID, also called Cushing's disease) in their lifetime. Horses with PPID have long, shaggy coats that don't shed out well in the spring. These horses will also have drink water excessively, have abnormal fat deposits, abscesses or laminitis and experience chronic infections, which can include dental disease, delayed wound healing, sinusitis or heavy intestinal worm loads. They will also urinate often and be lethargic from their condition.
Though some immunosuppression is expected with aging, PPID horses become even more compromised because of hormonal imbalances that come with the disease. Horses with PPID don't have the defense of antibody titers for the length of time normal horse do. PPID-affected horses have elevated levels of adrenocorticotropic hormone (ACTH) that amplifies cortisol, which is an immunosuppressant.
Researchers compared healthy and PPID-affected horses' reactions to vaccinations. They determined that there was no difference in antibody responses to the influenza part of a combination vaccine, but PPID horse's response to West Nile Virus (WNV) and equine herpesvirus (EHV) was better to single injections of each vaccine rather than a combination vaccine.
Owners of PPID horses are encouraged to ask their veterinarian for individual vaccines for diseases instead of one combination vaccine that protects against multiple diseases in one injection.
Read more at Stable Management.
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