Understanding Immunosenescence in Horses

With more horses living to a ripe old age than in the past, veterinarians have become incredibly well-versed in managing senior equids. But there are still some points that researchers are working to understand. For instance, exactly what impact does aging have on the immune system?

At the 2013 American Association of Equine Practitioners' Convention, held Dec. 7-11 in Nashville, Tenn., Dianne McFarlane, DVM, PhD, Dipl. ACVIM, ABVP, associate professor in physiological sciences at Oklahoma State University Center for Veterinary Heath Science, reviewed what veterinarians know and what they're still working to learn about immunosenescence in horses.

"Immunosenescence is the effects of aging on the immune system," McFarlane explained, but cautioned that there's still not much published data on equine immunosenescence.

Challenges for Senior Horse Studies

One of the challenges veterinarians face when studying immunosenescence is horses' age itself: "What chronological age is 'aged'?" she asked. "Age-induced changes may be missed if the study group is too young. Less intuitively, a study population that is too old may cause age-related immune deficits to be missed because the selected population may have survived to extreme old age as the result of exceptional immune function."

Also, in evaluating seniors horses, "it is difficult to know if changes that occur are due to age or a subclinical disease," she said. "In an aged population, this can be difficult because of the high prevalence of co-morbidities that contribute to chronic, low-grade inflammation."

Ideally, McFarlane said, scientists should select study horses carefully based on history, a clinical examination, and a variety of other factors, including hormonal status: "In studies of aged horses it is important to consider hormonal status during the study period because several of the pituitary and adrenal hormones are strong modifiers of immune function," McFarlane explained.

What We Know

Despite the challenges associated with studying senior horses, veterinarians have accrued some important knowledge about the aging equid's immune function.

Infectious Disease—While there's not a lot of evidence of age's effect on disease in horses, aged equines could be more susceptible to contracting West Nile virus, McFarlane said. Also in experimental settings, aged mares contracted neurologic equine herpesvirus-1 more frequently than young animals.

But overall, "we just don't have any solid data suggesting old horses are significantly more susceptible to infectious disease than younger horses," McFarlane said.

Parasites—Researchers have determined that age has no effect on fecal egg counts before or up to 12 weeks after dewormer administration, meaning healthy old horses were just as capable of controlling fecal shedding as younger horses. However, McFarlane said, old horses with pituitary pars intermedia dysfunction (PPID, or equine Cushing's disease) have higher fecal egg counts both before and after anthelmintic administration, meaning these horses (about 15-30% of senior horses) shed more eggs than healthy aged horses.

Immune System—Researchers have also studied cytokine (an inflammatory mediator) and neutrophil (a type of white blood cell capable of engulfing and destroying bacteria and other disease agents) function in middle-aged, old, and PPID-affected horses, McFarlane said. Key findings from that study:

  • Interleukin (IL)-6 and interferon-γ concentrations were higher in aged horses without PPID;
  • IL-8 concentrations were higher in old horses and those with PPID;
  • When evaluating the balance between pro- and anti-inflammatory cytokines, aged horses had an increased pro-inflammatory to anti-inflammatory ratio;
  • Aged horses had increased chemotaxis levels compared to middle-age and PPID horses; and
  • PPID horses had a lower oxidative burst (bacteria or viruses are being attacked and destroyed by the free radicals) than middle-age and old horses, and the oxidative burst was negatively correlated to melanocyte-stimulating hormone and insulin concentrations.

Vaccine Response—Finally, McFarlane turned to vaccine responses in aged horses. While there's little research on the topic, in one study researchers found no difference in horses' response to rabies vaccines, even when given as a naive vaccination, McFarlane said. She also described a study showing that influenza titers appear "less robust" in aged horses compared to younger horses. She noted that there are no studies evaluating PPID horses' response to vaccination.

Recommendations for Aged Horse Care

Despite the fact that research is scarce, McFarlane recommended a few tips for maintaining aged horses (those 20 years of age or older) to minimize disease risk:

  • Employ the same biosecurity techniques when managing old horses as for immature horses;
  • Follow the same vaccination protocol for aged horses as for mature horses;
  • Perform routine fecal egg counts and deworm based on the results;
  • Monitor aged horses' weight, body condition score, and hair coat regularly for changes;
  • Test for endocrine disorders at least annually; and
  • Be vigilant in monitoring for disease.

"With optimal care and good genetics horses can remain actively performing into their late 20s and later," McFarlane concluded. "In addition, many horses will maintain a good quality of life into their 30s and 40s when attention is paid to early recognition and intervention of health issues."

Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.