The specific effects of Se source (organic vs. inorganic) on Se status and immune function have not been adequately examined in the horse. We compared measures of Se status and innate and adaptive immunity in horses receiving inorganic, organic or no supplementary Se.
Fifteen Standardbred horses, previously unvaccinated for rabies, were assigned randomly to 3 groups: the Control group received no supplementary Se, while the Inorganic and Organic groups received sodium selenate or a commercial Se yeast product, respectively, at a rate calculated to deliver 0.3 ppm of diet dry matter. Supplementary Se was mixed with barley and fed once daily with the maintenance diet of hay and oats. The basal diet contained < 0.05 ppm Se throughout the study. Blood samples were obtained at the beginning of the study and monthly thereafter for the experimental period of four months. Serum, and blood cell Se concentrations, blood glutathione peroxidase activity, neutrophil phagocytosis, lymphocyte proliferation in response to mitogen ConA, and antibody production in response to rabies vaccination were measured.
Measures of Se status were increased in all horses receiving supplementary Se when compared to horses in the Control group. Further, horses in the Organic group had higher serum and blood cell Se concentrations when compared to horses in the Inorganic group, with the greatest difference in blood cell Se. Mean Se concentrations at the end of the study for the Control, Inorganic and Organic groups were 0.030±0.003, 0.144±0.009 and 0.166±0.017 ppm Se in serum, and 0.085±0.024, 0.299±0.018 and 0.442±0.044 ppm Se in blood cells, respectively (all group comparisons differ P<0.05). Glutathione peroxidase activity was higher in the Inorganic and Organic groups when compared to the Control group (P<0.001), but there was no significant difference between the Inorganic and Organic groups. Phagocytosis activity increased in all groups during the course of the study but tended to increase more in the horses in the Organic group. Mean increase in phagocytosis index for the Control, Inorganic and Organic groups were 15±9, 16±10 and 26±21 percent, respectively (P<0.10 for the AVOVA contrast Organic vs. Inorganic and Control). Other measures of immune function did not differ significantly between groups.
To conclude, supplementation of adult horses with organic Se resulted in significantly higher serum and blood cell Se concentrations when compared to supplementation with inorganic Se, but this difference was not reflected to the same degree in blood glutathione peroxidase activity. This is the first study looking at the effect of Se supplementation and Se source on phagocytosis activity in adult horses. When compared to unsupplemented controls, phagocytosis activity tended to be enhanced in horses supplemented with organic Se, but not in horses supplemented with inorganic Se. This latter finding is the first to suggest an effect of Se source on phagocytosis activity in horses and is the subject of further investigations in mares and their foals.