Study Of Immune Pathology Of High Zinc Diet Long-term Exposure On Effects Of Weanling Piglets

An experiment was conducted to study the effects of high zinc diets long-term exposure (from 3000mg.kg^-1 ZnO or 500 mg.kg^-1Zn-Met) on the growth performance,cell-mediated immunity(CMI),humoral immunity,small intestinal mucosa immunity and the contents of copper,iron,zinc in sera and immune organ of weanling piglets,in order to study the immune security of high zinc diets application.Sixty "Duroc×Landrace×Yorkshire" crossbred weanling stress piglets(23 d±2 of age) were randomly divided into 3 Groups of 20 piglets each by weight and sex.The piglets in Group I were fed a basal diet;the piglets in GroupⅡwere fed the diet plus 3000 mg·kg^-1 Zinc(from ZnO);The piglets in GroupⅢwere fed the diet plus 500 mg·kg^-1 Zinc(from Zn-Met).The trial was conducted for 70 d.Body absolute weights(AW),average daily gain(ADG) and average daily feed intake(ADFI) in all piglets were determined on d 0,21,42,70 postweaned.Blood samples in thirty(10 per Group) of sixty piglets were taken from the anterior vena cava on d 0,21,42,70 postweaned for measuring serum lymphocyte transformation rates,white blood cell phagocytosis rates, immunoglobulin levels,and the serum contents of copper,iron and zinc were determined at the same time.At the end of the trial,15 piglets(5 per Group) were bled via the anterior vena cava.The proximal duodenum,1/4 proximal jejunum,1/4 distal jejunum and middle ileum were recllected and stored in 10%formalin solution immediately for preparing tissues section and measuring the structure of mucous epithelium,the number of intraepithelial lymphocyte and the goblet cells.The small intestines of freshly slaughtered pigs were dissected out from the beginning of the jejunum to the ileo-caecal sphincter and collect it's mucus in order to measure the level of IgA in small intestine mucous section.The results were as follows:(1)High zinc diet long-term exposure can significantly decrease growth performance in weanling pigletsCompared with the control group,AW of piglets in treatment groups enhanced significantly on d 21,42 postweaned(P＜0.01 or P＜0.05),but there was no significant difference(P＞0.05) at the end of the trial.Compared with GroupⅠ,ADG of piglets in treatmemt groups improved significantly between d 0-21 postweaned(P＜0.01).During d 21-42 postweaned,ADG of piglets in treatment groups were significantly higher than that of GroupⅠ(P＜0.05).AW and ADG in GroupⅡ,Ⅲhad no significant difference in the whole experimental period.(2)High zinc diet long-term exposure can significantly enhance the immune organ indexCompared with the control group,the spleen and thymus index of piglets in treatment groups enhanced significantly(P＜0.01).The spleen index between GroupⅡandⅢhad significant difference(P＜0.05),but no significant difference of the thymus index between the 2 groups was found(P＞0.05).(3)High zinc diet long-term exposure can significantly decrease lymphocyte transformation rate in weanling pigletsCompared with d 0,lymphocyte transformation rates of piglets in the control group decreased significantly on d 21 postweaned(P＜0.05),but enhanced significantly on d 42 and 70 postweaned(P＜0.01);lymphocyte transformation rates in the treatment groups enhanced significantly on d 21 postweaned(P＜0.05),but decreased significantly on d 70 postweaned (P＜0.01).Lymphocyte transformation rates of piglets between GroupⅡand GroupⅢhad no significantly difference(P＞0.05).(4)High zinc diet long-term exposure can significantly decrease white blood cell phagocytosis rates in weanling pigletsCompared with d 0,white blood cell phagocytosis rates of piglets in the control group decreased significantly on d 21 postweaned(P＜0.01),white blood cell phagocytosis rates on d 42 postweaned recovered to the level on d 0 postweaned.White blood cell phagocytosis rates of piglets in GroupⅡ,Ⅲon d 21 postweaned were significantly higher than those on d 0 postweaned;but decreased significantly on d 42,70 postweaned(P＜0.01).(5)High zinc diet long-term exposure can significantly decrease immunoglobulin levels in weaning pigletsCompared with weaning day,all of the serum IgG and IgA levels of piglets in 3 Groups decreased on d 21,but the rare of decline in the control group was the largest reaching a significant level.Serum IgG and IgA levels of piglets in control group on d 42 and 70 returned to the level of weaning day,and serum IgG and IgA levels of the treatment groups continued to decline.(6)High zinc diets long-term exposure can significantly influence the structure of mucosa epithelia and the numbers of IELs and GCs at all positions of small intestinal mucosa in piglets(P＜0.05 or P＜0.01)In GroupⅡ,Ⅲ,structure of mucous membrane of small intestine was evidently injured: descent villi height,deepened crypt,atrophied and shed mucosa epithelia,and thinningzed slime layer.In GroupⅡand GroupⅢ,the numbers of intraepithelial lymphocytes and the goblet cells were lower than those in the control group(P＜0.05 or P＜0.01). (7)The contents of Cu,Fe and Zn in weanling piglets were influenced by high zinc diets long-term exposure①The contents of serum Cu,Fe and Zn in weanling piglets were significantly influenced by high zinc diets long-term exposureSerum Cu and Fe levels of piglets in 3 groups had no significant difference on the prophase of trial,but serum Cu and Fe levels of piglets in GroupⅡandⅢwere significantly lower than those in control group.Throughout the trial period,serum Zn levels of piglets in GroupⅡandⅢwere significantly higher than that in control group.②The contents of Cu,Fe,Zn in immune organ were influenced by high zinc diets long-term exposureCompared with GroupⅠ,the thymus and spleen Cu levels of piglets in GroupⅡandⅢdecreased significantly(P＜0.05 or P＜0.01).The thymus Fe levels of piglets in GroupⅡand GroupⅢwere significantly higher than that in GroupⅠ(P＜0.01),but spleen Fe levels lower (P＜0.01).The thymus and spleen Zn levels of piglets in GroupⅡand GroupⅢincreased significantly(P＜0.05 or P＜0.01) compared with GroupⅠ.The sediment yield of Cu,Fe,Zn of intestinal canal of piglets in GroupⅡandⅢwere significantly higher than that in GroupⅠ(P＜0.05 or P＜0.01).These results showed that serious poisoning reactions were observed in piglets feeding high zinc diets during weeks 6 to 10 after weanling,and the immune system was severely damaged.It was confirmed that the best time of adding 500mg·kg^-1 Zn-Met and 3000 mg·kg^-1ZnO to diets in weanling piglets is 3 weeks postweaned.