The effect of selenium supplementation on vaccination response and immune function in adult horses.

Selenium status has been reported to affect immune function across many different species. Yet few studies have focused on the effect of Se status on the equine immune system. This study examined the effect of Se supplementation on vaccination response and immune function in mature horses. Twenty-eight horses were blocked by age and sex and were randomly allocated to 1 of 4 dietary treatment groups: low Se (LS), adequate Se (AS), Se-yeast (SP), and sodium selenite (SS). For 35 wk, horses allocated to LS, SP, and SS received a low-Se diet (0.06 mg/kg DM) with the intention to lower Se stores, whereas AS received an adequate Se diet (0.12 mg/kg DM). A 29-wk repletion phase was as follows: LS and AS were kept on the diets fed during the depletion period, whereas SP and SS received the depletion diet plus their respective Se supplements to achieve a dietary Se concentration of 0.3 mg/kg DM. The Se status of the horses was monitored using whole blood Se and glutathione peroxidase (GSH-Px) activity as indicators. At wk 22 and 25 of the repletion phase, horses were vaccinated intramuscularly with 10 mg ovalbumin (OVA). Horses were also vaccinated against equine influenza at wk 25. Blood samples were collected for 7 wk after initial vaccination for serum separation and at 0, 3, and 5 wk postvaccination for peripheral blood mononuclear cell (PBMC) isolation and whole blood cytokine mRNA evaluation. At wk 22 of the repletion phase, both Se and GSH-Px were greater for SP and SS compared with AS and LS (P < 0.001). Serum vitamin E was similar between treatments. Response to OVA vaccination, evaluated as OVA-specific IgG production, cytokine mRNA expression of PBMC stimulated with OVA in vitro, and lymphocyte proliferation, was unaffected by Se status. Similarly, memory response to the influenza vaccine was not affected by Se status. However, decreased mRNA expression of selected cytokines was observed in PBMC stimulated with phorbol 12-myristate 13-acetate for LS compared with AS, SP, and SS (P < 0.05). Whole blood mRNA expression of IL-10 was greater for SS compared with LS, AS, and SP (P = 0.043). Although the OVA and influenza vaccination responses were unaffected by Se status, other measures of immune function did indicate that low Se status could adversely affect cell-mediated immunity.

Digestive capacity in weanling and mature horses.

The ability of young and mature horses to digest DM, OM, and NDF was compared using 6 weanling colts and 6 mature (13.2 ± 3.0 yr) geldings. Each colt was paired with a gelding, and the pair was adapted to a diet containing 67% alfalfa cubes and 33% concentrate for 21 d. During the adaptation period, horses were accustomed to housing and all handling procedures. The adaptation period was also used to adjust the amount of feed offered to minimize orts and to maintain similar rates of intake within a pair. After the adaptation period, a 5-d fecal collection period using fecal collection harnesses ensued. The average age of the weanling colts at the start of the 5-d collection period was 181.8 ± 2.9 d. On the morning of the first collection day, Co-EDTA (9 mg Co/kg BW(0.75)) and ytterbium-labeled hay fiber (9 mg Yb/kg BW(0.75)) were added to the concentrate portion of the diet, and horses were closely observed for complete consumption of the markers before additional feed was offered. The fecal collection bags were emptied every 1 to 2 h, and each collection was weighed and subsampled for later measurement of Co and Yb concentrations, which were used to determine the mean retention time (MRT) of the fluid and particulate phases of digesta, respectively. The remaining feces for each horse were composited each day and then subsampled for measurement of DM digestibility (DMD), NDF digestibility (NDFD), and OM digestibility (OMD). During the fecal collection period, DMI was similar between colts and geldings (91.4 and 91.2 g/kg BW(0.75), respectively). There were no differences between colts and mature geldings for DMD, OMD, or NDFD. Across both ages, the MRT of the particulate phase was 24.9 h compared with 21.8 h for the fluid phase (P = 0.002). However, MRT for the particulate phase was not different between colts and mature geldings (24.7 and 25.2 h, respectively). There was no difference in the MRT for the fluid phase between colts and mature geldings (21.5 and 22.0 h, respectively). The results indicated that the digestibility of DM, OM, and NDF in a diet consisting of good-quality cubed forage and concentrate is similar for weanling colts and mature geldings.

Measures of antioxidant status of the horse in response to selenium depletion and repletion.

Selenium plays a role in the antioxidant mechanism via the selenoenzyme glutathione peroxidase (GSH-Px). Change in Se status because of Se depletion or supplementation is associated with a change in GSH-Px activity and could potentially affect antioxidant status. This study evaluated the impact of change in Se status on measures of antioxidant status and oxidative stress in adult horses. Twenty-eight horses were blocked by age and gender and were randomly allocated to 1 of 4 dietary treatment groups: low Se (LS), adequate Se (AS), high organic Se (SP), and high inorganic Se (SS). For 196 d, LS, SP, and SS received a low-Se diet (0.06 mg Se/kg DM) to allow for depletion of Se stores, whereas AS received an adequate Se diet (0.12 mg Se/kg DM). Then, for the next 189 d, LS and AS were maintained on the same diets, whereas SP was supplemented with Se-yeast and SS with sodium selenite to allow for a total dietary Se intake of 0.3 mg Se/kg DM. Blood samples were collected throughout the study. Variables of interest included whole blood Se and GSH-Px activity, serum vitamin E concentration, total antioxidant capacity (TAC), serum malondialdehyde (MDA), and triiodothyronine and thyroxine concentrations. Data were analyzed using ANOVA with repeated measures. Whole blood Se and GSH-Px activity decreased in LS, SP, and SS during the depletion phase and increased in SP and SS with supplementation (treatment × time, P < 0.001). At the conclusion of the supplementation period, GSH-Px activity was greater in SP and SS compared with AS and LS (P < 0.05). Vitamin E status remained adequate throughout the study, and no differences existed between treatments. Serum TAC did not change in response to Se depletion or repletion. Serum MDA was greater for AS than LS during depletion (P < 0.05) but similar across treatments after supplementation. Overall, change in Se status did not have a large impact on TAC or MDA, possibly because the horses maintained an adequate vitamin E status. However, Se supplementation at 0.3 mg/kg DM increased GSH-Px activity above that of the horses fed an adequate diet based on the 2007 NRC recommendations, indicating a potential benefit to feeding greater Se diets to horses kept in low-Se areas.