ABSTRACT: Horses have an increased susceptibility
to infection because of a decline in immune function
with advancing age. Vitamin E has been found to play
a key role in normal immune system function. The purpose
of the study was to examine the effect of vitamin
E supplementation on immune function and response to
vaccination in older horses. Predominantly older horses
(18.9 ± 1.3 yr, range 7 to 26 yr; 523 ± 38 kg of BW)
were supplemented orally once daily for 16 wk with
either all-rac-α-tocopheryl acetate (15 IU/kg of BW; n
= 8) or a placebo (n = 8). One horse from each group
was removed from the study for reasons not related to
the study. Serum α-tocopherol concentration, neutrophil
and monocyte bacterial killing ability, lysozyme
activity, immunoglobulin concentration (IgGa, IgGb,
IgGT, and IgM), and neutralizing antibody production
to West Nile virus vaccination were determined.
The overall serum α-tocopherol concentration of the
vitamin E-supplemented horses was greater than that
of placebo-supplemented horses (P < 0.001). Bacterial
killing capacity of monocytes and neutrophils increased
in the vitamin E-supplemented horses (P < 0.05). Vitamin
E-supplemented horses had greater serum IgGa
(P < 0.001) and IgGT (P = 0.003) concentrations but
produced less serum IgGb (P = 0.023) than placebosupplemented
horses. There was no effect of vitamin
E supplementation on IgM production. The neutralizing
antibody response to vaccination against West Nile
virus was unaffected by vitamin E supplementation.
There was a continuous increase in serum lysozyme
concentration in placebo-supplemented horses, whereas
serum lysozyme concentration did not increase until wk
12 in vitamin E-supplemented horses. In conclusion, vitamin
E supplementation of predominantly older horses
differentially modulated general cell-mediated and humoral
immune function. Further research is needed to
fully understand the effect of vitamin E on the immune
function of horses.