Aging Horses' Immune Systems Reviewed

As if equine Cushing's, metabolic syndrome, laminitis, and insulin resistance weren't enough to worry about in our aging horses, we can now add "immunosenescence" and "inflammaging" to the list.

Immunosenescence refers to the changes in immunity and in particular, the decrease in both the number and function of lymphoid cells (a group of infection-fighting white blood cells produced in the bone marrow).

"It is widely thought that the function of these cells, especially T cells, is altered in aged horses," said David Horohov, PhD, the William Robert Mills Chair in Equine Immunology at the University of Kentucky's Maxwell H. Gluck Equine Research Center, who wrote a recent review of the issue. "There are dramatic effects of aging on the horse's immune response and immune competence due to the lower responsiveness of the aged lymphoid cells."

In contrast, the term, "inflammaging" refers to the increased production of inflammatory mediators such as interleukin-6, C-reactive protein, and tumor necrosis factor-α, among others in the elderly. It is a chronic inflammatory condition that can affect multiple body systems. Scientists believe that inflammaging could contribute to many age-related problems seen in elderly humans and horses.

Why are immunosenescence and inflammaging important?

"It is thought that the altered lymphoid cell responses occur following viral infections and after vaccination resulting in decreased immune responses and a susceptibility to infection," relayed Horohov.

According to Horohov, immunosenescence and inflammaging might both be important when considering vaccine efficacy in older horses.

"Multiple studies have shown that older horses vaccinated with an equine influenza vaccine mounted immune responses that were significantly less effective than the immune response that occurred in younger horses post-vaccination," he said.

The underlying mechanisms responsible for the decreased responsiveness to vaccinations in older horses remain unclear but are clinically relevant as alternative vaccination approaches could prove to be more efficacious in aged horses than the currently available methods and products. Research is ongoing.

Immunosenescence and inflammaging in horses was reviewed in detail in the article, "Immunosenescence of the Equine Immune System," scheduled to be published in an upcoming edition of the Journal of Comparative Pathology. The abstract is currently available on PubMed.

About the Author

Stacey Oke, DVM, MSc

Stacey Oke, MSc, DVM, is a practicing veterinarian and freelance medical writer and editor. She is interested in both large and small animals, as well as complementary and alternative medicine. Since 2005, she's worked as a research consultant for nutritional supplement companies, assisted physicians and veterinarians in publishing research articles and textbooks, and written for a number of educational magazines and websites.

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