More and more horses are staying active into their golden years, but just because a horse is young-at-heart doesn't mean his body is just as fresh. Fortunately, researchers are on the hunt for ways we can help keep geriatric horses' bodies up to par.

At the University of Kentucky Equine Showcase, held Jan. 23 in Lexington, Amanda Adams, PhD, assistant research professor at the University of Kentucky’s Gluck Equine Research Center, reviewed recent research on vaccinations, deworming programs, and feeding regimens for senior horses.

Adams said that recent study results have shown that 7-15% of the U.S. horse population is 20 years or older, while 29% of the United Kingdom's equine populations is 15 years or older—that's a lot of old horses. But, unfortunately, a horse’s lifespan doesn't necessarily equate to his "healthspan," she said, noting that old horses' immune responses generally start diminishing around 20 years of age.

Adams explained that as horses age, they experience a decline in immune function (known as immunosenescence) and an increase in inflammatory cytokines production resulting in chronic, low grade inflammation (known as inflamm-aging) that can contribute to age-related diseases and conditions. So, it's important for owners and veterinarians to work together to ensure senior horses age "gracefully," she said.

Three ways to counteract these challenges are through vaccination, deworming, and nutrition, Adams said. These are areas in which she's also carried out research to evaluate old horses' responses. She shared those result with attendees.

Vaccinations

Adams first described a study in which she evaluated whether aged horses with pituitary pars intermedia dysfunction (PPID, a common condition that affects about 20% of old horses) respond differently to vaccines than non-PPID senior horses.

Adams and colleagues employed 33 aged horses and blocked them into four groups: non-PPID vaccinated, non-PPID controls, PPID vaccinated, and PPID controls. The team treated the control horses with saline and vaccinated horses with a multivalent product that protected against influenza, equine herpesvirus, Eastern and Western encephalomyelitis, West Nile virus, and tetanus.

The team found no significant difference in immune response to the influenza vaccine between PPID and non-PPID horses of matched age. However, she did note some differences in how PPID compared to non-PPID horses responded immunologically to the vaccine’s herpes and West Nile antigenic components. Work on this study is still underway. In the meantime, Adams concluded that it's important that owners maintain geriatric horses on a regular vaccination program that keeps them up-to-date on all core vaccines. She also recommended that owners consult their veterinarians to consider two boosters per year of some risk-based vaccines for older horses at increased risk, including those that travel and/or compete frequently.

Dewormers

Next, Adams described a study evaluating the effects of age on strongyle fecal egg counts and whether old horses have inflammatory responses to anthelmintic (deworming) treatments.

For this study, Adams and colleagues employed 21 horses aged 20 and up and 19 horses aged 5 to 15. They separated the horses into three groups within each age group: one treated with moxidectin, one treated with pyrantel pamoate, and one untreated control group. The researchers also completed fecal egg count reduction tests (to test the dewormers' efficacy) on all the horses and evaluated their blood levels of two inflammatory markers (called IL-6 and SAA).

The team determined that, while older horses had higher fecal egg counts than younger horses, both moxidectin and pyrantel pamoate were effective at reducing the egg counts. The team also discovered that old horses treated with pyrantel pamoate had higher IL-6 and SAA levels compared to horses treated with moxidectin.

Adams stressed that while these study results suggest that old horses are more likely to be high egg shedders, it's important to let the fecal egg counts dictate how frequently you deworm your senior and with what drug class. She cautioned, however, that this study only evaluated the horses in her research herd.

She also emphasized that, while they observed less inflammation following moxidectin treatment compared to pyrantel pamoate, "that doesn't mean (moxidectin) is the only drug that should be used. Use something that works, regardless of the horse's age."

She encouraged attendees to routinely test their dewormers’ efficacy using fecal egg count reduction tests.

Nutrition

Finally, Adams shared the results of a study evaluating the effects of different dietary components on old horses' inflammation and immune responses to vaccination.

For the study, Adams and colleagues used 40 healthy horses aged 20 to 33 housed on pasture and supplemented with free-choice grass hay. Each horse received one of four diet treatments for 23 weeks:

• Oats and alfalfa pellets;
• A commercially available senior horse feed;
• A commercially available senior horse feed with added proprietary prebiotics; or
• A commercially available senior horse feed with added proprietary omega fatty acids.

The team weighed the horses and collected blood samples before the study began and monthly throughout the study. At the end of the trial, each horse was vaccinated and evaluated further.

Adams and colleagues determined that the senior horse feed with prebiotics was effective in decreasing inflammatory cytokines over time and increasing horses' body weight and condition score. The team also determined that the same feed improved horses' response to vaccination and reduced inflammation.

She concluded that proper nutrition can both reduce inflammation and improve senior horses' immune responses to vaccination. Still, she cautioned, ensure you're feeding a diet that works for your horse, even if it's not necessarily designed with a senior in mind.

Take-Home Message

While all horses age differently, they will need some help at some point to make their healthspan equal their lifespan. Adams stressed the importance of keeping geriatric horses' vaccinations up-to-date, conducting regular fecal egg counts and deworming accordingly, and feeding an appropriate diet.

Author: Erica Larson