Immunosenescence: What Owners of Old Horses Need to Know
From T-cells and IgG to INFγ and TNFα, equine immunology isn’t the easiest topic to understand. However, it holds some practical information that owners can use as they’re managing their older horses day to day.

For instance, understanding immunosenescence and how it impacts senior horses can help owners make decisions regarding multiple facets of their care. At the University of Kentucky’s (UK) senior horse care mini-symposium, held Sept. 27 in Lexington, Amanda Adams, PhD, reviewed immunosenescence and how it affects old horse care. Adams is an associate professor at UK’s Gluck Equine Research Center and focuses her research on the geriatric horse’s immune system.

Here’s a look at how immunosenescence and its effects impact the way owners and veterinarians care for senior horses.

What is it?

Immunosenescence describes a phenomenon in which as horses age, their immune systems decline. This can contribute to an increased prevalence of autoimmune and chronic diseases and increased susceptibility to common infectious diseases.

It often goes hand-in-hand with a chronic state of low-grade inflammation called inflamm-aging. Through her research Adams has confirmed that older horses have increased inflammation at both the systemic and cellular levels compared to younger horses.

How Does it Impact Vaccinations?

In addition to diminishing old horses’ immune responses, immunosenescense dampens their immune response to vaccination in comparison to younger horses. That said, Adams and colleagues were interested in determining whether various vaccine formulations (i.e., inactivated vs. live vaccines) work differently in aged horses. They found that both inactivated vaccines and the live vectored vaccine induced significant antibody responses in these old horses. Surprisingly, she said, one of the inactivated vaccines studied did a better job inducing antibody response and did the best job stimulating cell-mediated immune responses, which are important for combating viral infections. The team also tested whether giving a second dose of the vaccine 28 days after the first might further boost the horses’ immune response, which it did not, she said.

“More than likely the differences we saw in this study were due to differences in adjuvants (a substance included in inactivated vaccines to enhance the immune response) and antigen dose, all things we need to further understand,” Adams said.

Adams has also tested whether the most common endocrine disorder in senior horses—pituitary pars intermedia dysfunction (PPID)—further reduced immune responses to vaccines. She tested a multivalent (combination) vaccine containing Eastern and Western equine encephalomyelitis, tetanus, equine influenza (EIV), rhinopneumonitis (EHV-1), and West Nile virus (WNV) in senior horses with and without PPID. Her team found that all horses showed a significant increase in EIV, EHV-1, and WNV antibodies and that there was no significant difference in immune response to the EIV vaccine between PPID horses and non-PPID horses of matched age. However, they did note some differences in EHV-1 and WNV vaccine responses, she said; PPID horses had a higher response to the former product and a reduced response to the latter compared to healthy old horses.

Adams’ current vaccination-related research includes comparing multivalent and monovalent (which provide protection against many diseases vs. one disease, respectively) vaccines, studying immune responses and antibody decay post-vaccination, and determining how long immunity lasts following vaccination in older horses compared to younger adult horses.

What This Means for Owners

More research is needed to pinpoint the “ideal” vaccination program for senior horses. For now, Adams recommended owners keep their senior horses—especially those with PPID—on a regular vaccination program. Include all core vaccines (EEE/WEE, rabies, tetanus, and WNV), and work with your veterinarian to determine if senior horses need any risk-based vaccines. For example, an aged performance horse that still travels and shows should receive EIV and EHV-1 vaccines, an older broodmare might need a rotavirus vaccine, and senior horses residing in certain areas might benefit from a botulism vaccine.

How Does it Impact Deworming?

Another effect of immunosenescence is an increased susceptibility to harboring parasites and their eggs and a reduced immune response to deworming treatment.

Adams and colleagues conducted a study in which they compared old and middle-aged horses’ immune responses to two common dewormers—moxidectin and pyrantel pamoate. They left a control group untreated. They found that, in their study population, the geriatric horses were more likely to be high shedders on fecal egg counts (FECs) compared to the younger horses; Adams cautioned that this might be a farm-specific finding and encouraged owners to carry out FECs on their own horses. There was also evidence of fewer inflammatory reactions following moxidectin treatment compared to pyrantel, but “that doesn’t mean it is the only drug that should be used,” Adams said. “Use something that works, regardless of horse age.”

What This Means for Owners

Senior horses might be higher egg shedders than younger horses; however, you’ll need to conduct FECs to determine if this is true for your own horses. Keep your seniors on a regular parasite control program (which typically includes spring and fall deworming based on the American Association of Equine Practitioners’ recommendations, Adams said), but deworm based on your horse’s FEC results. Finally, carry out FEC reduction tests, which evaluate dewormer efficiency in your herd.

How Does Nutrition Impact Age-Related Immune Response Changes?

There’s currently less evidence on how immunosenescence and nutrition are related, but Adams has been conducting studies to try to learn more.

In one investigation, she found that a commercially available senior horse feed containing a proprietary prebiotic caused inflammatory cytokines associated with the inflamm-aging response to decrease over time. In a second study, Adams and colleagues found that feeding the proprietary probiotic improved senior horses’ immune response to EIV vaccination and, again, reduced inflammation.

Adams’ research group has also begun evaluating whether polyphenols (naturally occurring antioxidants found in specific fruits, vegetables, cereals, and beverages) impact inflammatory cytokine production. So far, they’ve determined that polyphenols—including curcuminoids, resveratrol, and quercetin, among others—can help reduce systemic inflammation in vitro (in the laboratory), “but we need to know more about how these compounds work systemically in the older horse,” Adams said. Their research on the topic is continuing.

Adams said she’s currently conducting additional research on how diet impacts inflammation in horses with PPID, insulin dysregulation, and both.

What This Means for Owners

The ultimate goal is to reduce inflammation and improve immune function through nutrition. At the moment, however, more research is needed before specific recommendations can be made.

Generally speaking, Adams said, “Keep the older horse on a balanced diet, and don’t hesitate to reach out to a nutritionist to help determine what nutrition is best for your older horse.”

Take-Home Message

While there’s still more to learn, researchers have made great strides in understanding how immunosenescence and inflamm-aging impact senior horses and how we can combat some of those effects so that lifespan equals health-span for the older horse.