Effects of stocking density on the homeostasis of uric acid and related liver and kidney functions in ducks

Abstract Background: Avian gout is a common metabolic disease characterized by hyperuricemia and urate deposits in the joints, and it is caused by the overproduction of uric acid by hepatic metabolism or renal underexcretion. However, the effects of stocking density on the metabolism of uric acid are unknown. In the present study, the effects of stocking density on the metabolism and homeostasis of uric acid as well as the related functions of the liver and kidney in ducks were examined. A total of 180 healthy 56-day-old Shan-ma ducks were randomly divided into the low stocking density (LSD; n = 30, density = 5 birds/m 2 ), medium stocking density (MSD; n = 60, density = 10 birds/m 2 ) and high stocking density groups (HSD; n = 90, density = 15 birds/m 2 ). Samples were collected in the 3rd, 6th and 9th weeks of the experiment for analysis. Results: The results showed that the serum levels of uric acid, LPS and inflammatory cytokines (IL-1β, IL-8 and TNF-α) were increased significantly in the HSD group. Serious histopathological lesions, including acute cellular swelling and vacuolar degeneration of hepatocytes in the liver, swelling of glomeruli, necrosis and abscission of renal tubular epithelial cells, and infiltration of inflammatory cells, could be seen in both the livers and kidneys in the HSD group in the 9th week. In addition, the mRNA expression levels of inflammatory cytokines (IL-8 and TNF-α) and related pathway components (TLR-4, MyD88 and NF-κB) were increased significantly in both the livers and kidneys in the HSD group. Moreover, the mRNA expression levels of enzymes (ADA, XOD, PRPPAT and PRPS1) related to the synthesis of uric acid increased significantly in the livers in the HSD group. However, the mRNA expression level of SLC2A9, which is responsible for the excretion of uric acid by the kidney, was decreased significantly in the kidneys in the HSD group. The effects of stocking density on the functions of the liver and kidney in the HSD group as described above might result in hyperuricemia, which might further induce gout. Conclusions: In conclusion, these results indicated that a higher stocking density could cause tissue inflammatory lesions in the liver and kidney and subsequently affect the metabolism and homeostasis of uric acid, and is helpful for guiding decisions related to the breeding and production of ducks.