Mannan oligosaccharide improves immune responses and growth efficiency of nursery pigs experimentally infected with porcine reproductive and respiratory syndrome virus

This study was conducted to determine whether the ingestion of mannan oligosaccharide (MOS, Bio-Mos) alters the immune response of nursery pigs challenged with porcine reproductive and respiratory syndrome virus (PRRSV). A total of 64 pigs (3 wk old), free of PRRSV, were used in 2 separate but similar experiments conducted sequentially. Pigs were blocked by initial BW. Sex and ancestry were equalized across treatments. Pigs were randomly assigned from within blocks to 1 of 4 treatments in a 2 x 2 factorial arrangement [2 types of diet: control (0%) and MOS addition (0.2%); 2 levels of PRRSV: with and without]. There were 8 replicate chambers of 2 pigs each. After 2 wk of a 4-wk period of feeding the treatments, pigs were intranasally inoculated with PRRSV or a sterile medium at 5 wk of age. The PRRSV challenge decreased ADG, ADFI, and G: F throughout the experiment (P < 0.001). Feeding MOS improved G: F of the pigs during d 7 to 14 (P = 0.041) postinfection (PI). Serum concentrations of tumor necrosis factor (TNF)-alpha, C-reactive protein, and haptoglobin were increased by PRRSV (P < 0.001). The MOS x PRRSV interaction was significant for TNF-alpha at d 14 PI (P = 0.028), suggesting that infected pigs fed MOS had less TNF-alpha than those fed the control. Dietary MOS increased serum IL-10 at d 14 PI (P = 0.036). Further, MOS-fed pigs had greater numbers of white blood cells (WBC) at d 3 (P = 0.048) and 7 PI (P = 0.042) and lymphocytes at d 7 PI (P = 0.023) than control-fed pigs. In contrast, PRRSV decreased (P < 0.01) WBC numbers until d 14 PI. Dietary MOS appeared (P = 0.060) to increase the neutrophils in PRRSV-infected pigs at d 3 PI, but no (P = 0.202) MOS x PRRSV interaction was found. Infection with PRRSV increased rectal temperature (RT) of pigs at d 3 PI (P < 0.001) and continued to affect the infected pigs fed the control diet until d 14 PI. The MOS x PRRSV interaction for RT was found at d 7 (P < 0.01) and 10 (P = 0.098) PI, indicating that the infected pigs fed MOS had a decreased RT compared with those fed the control. This could explain why feed efficiency was improved by MOS. No effect (P > 0.05) of treatments on viremia or PRRSV-specific antibody was observed. These results suggest that MOS is associated with rapidly increased numbers of WBC at the early stage of infection and alleviates PRRSV-induced effects on G: F and fever. The results also indicate that the reduced intensity of inflammation by MOS may be related to changes in inflammatory mediator levels at the end of the acute phase.