Understanding the Aging Equine Immune System
As humans and animals age, our immune systems undergo functional changes that put us at greater risk of certain health conditions as well as mortality. The effects of aging in humans are well-studied and well-documented, but what about in horses? How can we help our equine partners age slower and better?

Dianne McFarlane, DVM, PhD, MS, Dipl. ACVIM, CVM, professor of physiological sciences in Oklahoma State University’s College of Veterinary Medicine, in Stillwater, addressed this topic at the 2019 British Equine Veterinary Association Congress, held Sept. 11-14 in Birmingham, U.K.

How Our Immune Systems Age

“As we age, our bodies deteriorate and fail us,” McFarlane said in opening. “Two things happen primarily: The immune system loses its ability to respond appropriately and sufficiently when challenged (immunosenescence) and loses its ability to balance anti-inflammatory events with proinflammatory events, therefore becoming more proinflammatory (what’s called ‘inflammaging’).”

Aging can affect three levels of immune function:

  1. Physiological barriers such as the skin, respiratory tract, and gastrointestinal (GI) tract. “The microbiome on our physical barriers can change with age, and the microflora in our GI tract is intimately involved in immunes responses,” she said. “So this change in the flora may be part of what causes the change in our immune systems as we get older.”
  2. Innate immunity, which is the body’s built-in first line of defense against any infection.
  3. Adaptive immunity, which learns to attack specific pathogens.

“The most profoundly affected part of our immune system is our adaptive immune system,” McFarlane said. “Our innate system also ages but not as profoundly as our adaptive. Some of the clinical outcomes of changes in innate immunity are an increase in inflammatory conditions such as arthritis, heart disease, and neurodegenerative diseases and a lack of ability of our immune system to fight against cancer cells.”

So What Do We Know About Horses?

Studies on the aging equine immune system are some of the hardest to do, said McFarlane. This is because comorbidities (having more than one disease process simultaneously) are common, she said, and the age researchers select as “old” or “senior” isn’t standardized.

What do we know, however, is that:

  • Horses’ lymphocyte (disease-fighting white blood cells) counts decrease as they age.
  • Older horses still respond to vaccines. Researchers looking at old horses’ vaccine responses have found that this population can still mount a response similar to that of naïve horses. McFarlane said vaccine response in older horses with or without the endocrine disorder pituitary pars intermedia dysfunction isn’t significantly different.
  • As horses age, they release more pro-inflammatory cytokines (inflammatory molecules produced by the immune system) and fewer anti-inflammatory cytokines in response to bacterial toxins.
  • Cell function decreases, and the immune system is less able to proliferate cells when challenged.

Protecting Our Seniors

McFarlane reviewed two studies of senior horses that identified the gastrointestinal, musculoskeletal, and endocrine systems as the most common sources of disease.

“We can extrapolate that infectious disease is probably not the biggest cause of mortality in our horses,” she said. “Rather, they’re inflammatory-based diseases.”

Two infectious diseases that do seem to cause increased mortality in older horses compared to young ones, however, are West Nile virus (WNV) and equine herpesvirus-1. For this reason, McFarlane urged owners to vaccinate older horses and practice smart biosecurity.

“Aged horses do respond to vaccination, albeit likely less robustly,” she said. “Continue to vaccinate for preventable diseases such as West Nile virus based on risk.”

She recommended keeping aged horses away from new, traveling, or sick horses. This is particularly important for horses with PPID, an immunosuppressive disease that can be difficult to detect in its early stages.

“Treat aging PPID horses with pergolide, and monitor them closely for parasitic, bacterial, or other diseases,” McFarlane recommended. “You might need more aggressive treatment of infections in older PPID horses.”

She also emphasized the importance of good GI health and said older horses might benefit from antioxidants (molecules that protect cells from damaging free radicals). “We’re definitely seeing evidence that as horses age they have increased inflammation,” she explained. “Antioxidants may be a benefit. We don’t have good studies yet, but they certainly do no harm.”

Take-Home Message

In conclusion, said McFarlane, the immune system changes we see in our aging horses aren’t as profound as those we see in aging people, and we still have much to learn.

“We don’t really know why (immunosenescence) occurs, but there are several theories,” she said. “Most focus on wear and tear and a lifetime of exposure to oxidants, inflammatory events, viruses, and dietary changes. It might also be because you only have so many resources when you’re born, only so many naïve cells that can go out and be used to fight pathogens, and when you use up that population, then you start to show signs of aging.”