Effect Of Threonine Supplementation On Growth Performance, Immune Function Of Weaned Piglets Challenged With PRV

Currently, there is little research on threonine as a sort of immunostimulant to enhance the anti-disease ability of livestocks, and the majority of research was based on elementary animal models. Thus, we select and use weaned piglets challenged with Pseudorabies Virus (PRV) as the animal model to evaluate the effect of threonine supplementation on growth performance, immune function of livestocks. This study could provide the experimental basis for threonine supplementary effect which could improve anti-disease ability of livestocks, alleviate immune stress and keep body health.A total of30healthy DLY weanling piglets with average body weight of (6.94±1.26) kg were randomly assigned to6treatments with5replicates per treatment and1pigs per replicate. A3x2factorial design was used. Dietary true ileal digestible threonine levels were0.74%,0.89%and1.11%respectively. Pigs were injected with live swine Pseudorabies vaccine (PRV) or sterilized saline water. The injection was carried out on day8, and the whole experiment lasted for21days. Blood samples were obtained on day8,11,15and21. Body weight of piglets was weighed the once a week. On day21, piglets were slaughtered and sampled. The results were as follows:1. The injection of PRV increased the feed:gain ratio of pigs during1th～21th day (P＜0.10) and7th～21th day (P＜0.05). Threonine supplementation, to a certain extent, could affect the feed:gain ratio during1th～21th day and7th～21th day (P<0.10). There was no significant interaction effect between dietary TIDT level and immune stress on the growth performance. 2. The injection of PRV significantly increased concentrations of serum free threonine and urea nitrogen of weanling pigs on the21th day (P<0.01). As the dietary TIDT level increased, serum free threonine significantly increased and urea nitrogen significantly decreased (P<0.01). At the same time, there was a significant interaction effect between dietary TIDT level and immune stress on serum free threonine (P<0.01). In addition, the concentrations of other serum free amino acids, to varying degrees,were influenced by dietary TIDT level and immune stress.3. The injection of PRV significantly increased serum PRV-specific antibody (P<0.01) on the21th day, which showed that stress pattern has been successfully established. In immune stress group, compared with TIDT0.74%group, the antibody of TIDT0.89%and1.11%group increased by36.74%and50.41%respectively, and the TIDT1.11%group achieved the significant degree (P<0.05). But there was no significant interaction effect between dietary TIDT level and immune stress on serum PRV-specific antibody. The injection of PRV significantly increased serum immunoglobulin A (on15th,21th d), G (on15th,21th d), M (on11th,15th d) and Complement C3(on21th d)(P<0.01).Threonine supplementation, to varying degrees, could increase the concentrations of serum IgA, IgG, IgM and C3. At the same time, there was a significant interaction effect between dietary TIDT level and immune stress on serum IgA (on21th d) and IgM (on11th d)(P<0.05).4. The injection of PRV significantly increased spleen relative weight and the ratio of CD3+(P<0.01). Threonine supplementation, to a certain extent, could affect the spleen relative weight (P<0.10). In immune stress group, compared with TIDT0.74%group, the spleen relative weight and CD3+%of TIDT0.89%and1.11%group increased a lot, but didn’t achieve the significant degree (P>0.05). There was a trend of interaction effect between dietary TIDT level and immune stress on the spleen relative weight (P<0.10). Besides, the injection of PRV decreased the average value of the ratio of CD4+to CD8+(P>0.05).5. The injection of PRV significantly increased concentrations of serum IFN-y, IL-lβ, TNF-α and IL-10of weanling pigs on the15th and21th day (P<0.01). Threonine supplementation, to varying degrees, could decrease the concentrations of serum IFN-γ, IL-1β, TNF-a and increase the concentrations of serum IL-10. At the same time, there was a significant interaction effect between dietary TIDT level and immune stress on serum IFN-γ (on15th,21th d) and IL-10(on15th d)(P＜0.05).6. The injection of PRV significantly up-regulated the relative expression of TLR3, TLR7, TLR9mRNA in lung, hilar lymph node and cerebrum tissue (P＜0.01). Threonine supplementation, to a varying degrees, could alleviate the up-regulation of TLR3(P＜0.01) and TLR9(P＜0.10) mRNA expression in lung tissue, TLR3mRNA expression in hilar lymph node (P<0.01) and cerebrum (P<0.05) which was induced by PRV challenge. The interaction effect between dietary TIDT level and immune stress, to varying degrees, had effects on relative expression of TLR3(P＜0.01), TLR7(P＜0.10), TLR9mRNA (P＜0.10) in lung, TLR3(P＜0.01), TLR7mRNA (P＜0.10) in hilar lymph node and TLR3mRNA (P＜0.05) in cerebrum.7. Under immune stress condition challenged by PRV, pathological changes occurred in duodenum, cerebrum, lung and spleen tissue, and these conditions could be alleviated in a relative high dietary TIDT level. Besides, the injection of PRV significantly increased the crypt depth (P＜0.01) in jejunum and decreased the ratio of villus height to crypt depth in jejunum of weaned piglets (P＜0.01). As the dietary TIDT level increased, the villus height in duodenum experienced an upward trends (P＜0.10). There was a significant interaction effect between dietary TIDT level and immune stress on the ratio of villus height to crypt depth in duodenum (P＜0.05).These data indicated that:1. The immune stress which was induced by PRV injection altered the immune status in weaning piglets, weakened the growth performance and impaired the target organ, which indicated the stress pattern was well-established.2. Threonine supplementation could significantly elevate the concentration of serum free threonine and decrease the concentration of serum urea nitrogen. At the same time, it could activate lymphocytes, stimulate the synthesis of immunoglobulin and elevate the spleen relative weights. In a short, it could stimulate animal’s immune system and strengthen their anti-disease ability. 3. Supplementation of dietary threonine, to a large extent, could improve growth performance and alleviate intestinal injury which is induced by PRV injection, which demonstrate the protective and regulative effect of threonine supplementation.4. In the healthy condition, optimal dietary threonine level for weaning piglets is TIDTO.74%, while for the immune stress condition induced by PRV infection, it is TIDTO.89%～1.11%.