Immune response to vaccination and selenium and vitamin E status in horses of varying ages

Immunosenescence refers to alternations in the immune system associated with aging that may subsequently lead to an increased susceptibility to infectious, autoimmune and neoplastic diseases. There are limited studies investigating the effects of immunosenescence on the geriatric equine immune system. The primary and secondary (anamnestic) immune responses may be different in aged horses in comparison to their younger counterparts. Therefore, this raises the question whether or not aged horses would have a protective immune response post vaccine or exposure to a pathogen.

The focus of this study was to investigate the effects of aging on the equine immune system. The first goal was to evaluate the effects of age on specific systemic antibody response to vaccination against rabies and equine influenza. Since horses selected for the test population were naïve to rabies, rabies vaccination was used to evaluate the horse's primary immune response. Healthy aged horses ≥20 years) mounted a primary immune response similar to that of younger adult horses (4-12 years). Since the test population had previously most likely been exposed to equine influenza, vaccination was used to evaluate the secondary (anamnestic) immune response. Healthy aged horses had a significantly reduced anamnestic response in comparison to the younger adult horses even though the aged horses' pre-vaccination titers were higher.

Prior to assessing the immune response of horses in this study a detailed physical examination was performed and results of a CBC and biochemistry profile were analyzed to rule out underlying disease that may impact on their immune response. Serum concentrations of Se, vitamin E, and thyroid hormones were measured to examine the effect of confounding factors on the immune response. Preliminary results showed that significant numbers of horses in PEI had sub-optimal serum Se concentrations. The second focus of this study was to access the Se and vitamin E status in representative groups of horses (aged pleasure horses, young adult pleasure horses, racehorses, broodmares and their foals) on Prince Edward Island. Resting T4 and T3 serum concentrations were measured and the relationship between the Se status and thyroid hormone concentrations was determined.

The results of this study showed that 79% of the horses tested had inadequate serum Se concentrations and a strong correlation existed between mare's and their foal's serum Se concentration. In contrast, 87% of the horses tested in PEI had adequate concentrations of vitamin E. Selenium concentration affected T4 concentration but there was no effect on T3 concentrations.

These studies demonstrated that the equine geriatric immune system differs from that of younger adult horses and that inadequate Se intake is widespread in the horse population in PEI.