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

Currently,there is little research which consider ed 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.As a result,it could provide the experimental basis for threonine supplementation effect which could improve anti-disease ability of livestocks,alleviate immune stress and keep body healthy.A total of 30 healthy DLY weanling piglets with average body weight of（6.94±1.26）kg were randomly assigned to 6 treatments with 5 replicates per treatment and 1 pigs per replicate.A 3×2 factorial design was used.Dietary true ileal digestible threonine levels were 0.74%,0.89%and 1.11%respectively.Pigs were injected with live swine Pseudorabies vaccine（PRV）or sterilized saline water.The injection was carried out on day 8,and the whole experiment lasted for 21 days.Blood samples were obtained on day 8,11,15and 22.Every week we weighed the body weight of piglets.On the 22^rd day,piglets were slaughtered and sampled.The results were as follows:1.The injection of PRV increased the feed:gain ratio of 1^st-21^st day（P=0.09）,7^th-21^st day（P<0.05）.Threonine supplementation,to a certain extent,could affect the feed:gain ratio of 1^st-21^stday（P=0.05）and 7^th-21^st day（P=0.07）.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 the 2 2^rd 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 the 22^rd day,which showed that stress pattern has been successfully established.In immune stress group,compared with TIDT 0.74%group,the antibody of TIDT 0.89%and 1.11%group increased by 36.74%and 50.41%respectively,and the TIDT1.11%group achieved the significan t 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(15^th,22^rd d),G(15^th,22^rd d),M(11^th,15^thd)and Complement C3(22^rd d)（P<0.01）.Threonine supplementation,to varying degrees,could increase the concentrations of serum Ig A,Ig G,Ig M and C3.At the same time,there was a significant interaction effect between dietary TIDT level and immune stress on serum Ig A(22^rd d)and Ig M(11^th 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 TIDT 0.89%and 1.11%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⁺,but the result was not marked（P>0.05）.5.The injection of PRV significantly increased concentrations of seru m IFN-γ,IL-1β,TNF-αand IL-10 of weanling pigs on the 15^th and 22^rd day（P<0.01）.Threonine supplementation,to varying degrees,could decrease the concentrations of serum IFN-γ,IL-1β,TNF-αand 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(15^th,22^rd d)and IL-10(22^rd d)（P<0.05）.6.The injection of PRV significantly up-regulated the relative expression of TLR3,TLR7,TLR9 m RNA 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）m RNA expression in lung tissue,TLR3m RNA 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 m RNA expression of TLR3（P<0.01）,TLR7（P<0.10）and TLR9（P<0.10）in lung,TLR3（P<0.01）,TLR7（P<0.10）in hilar lymph node,and TLR3（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 condition s which were induced by PRV 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=0.09）.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,the supplementation level of dietary threonine for weaning piglets should be on TIDT0.74%,while in the immune stress condition induced by PRV infection,the supplementation level of dietary threonine for weaning piglets should be on TIDT0.89-1.11%level.