The effect of vitamin D3 on resistance to stress-related infection in an experimental model of turkey osteomyelitis complex.

Male turkeys immunosuppressed by injection with dexamethasone (DEX) were given supplemental vitamin D3 in their drinking water in two experiments. In Experiment 1, vitamin D3 was supplemented at a dosage of either 2,064 IU/kg (low level) or 4,128 IU/kg (high level) in drinking water provided ad libitum only from Days 1 through 5 after hatch. In Experiment 2, vitamin D3 was provided at the low dosage for the first 5 d after hatch, followed by treatment with the high dosage for 12 h before and 12 h after each stressful event, which included weekly weighings and two DEX treatments. In both experiments, at 5 wk of age half of the birds were given intramuscular injections of 2 mg/kg DEX on 3 alternating d. In Experiment 1, 100 cfu of Escherichia coli was inoculated into the left thoracic airsac at the time of the third DEX injection. All mortalities were examined, and 10 birds per pen were necropsied 2 wk after treatment and examined for lesions of airsacculitis and turkey osteomyelitis complex (TOC). Four birds per pen were bled before necropsy, and white blood cell total counts, differential white blood cell counts, and clinical chemistry values were determined. In Experiment 2, healthy surviving birds were grown for an additional 5-wk period, after which the DEX-treated birds were given a second series of DEX injections and were bled and necropsied 2 wk later. There were no significant effects of vitamin D3 treatment in combined general linear models analysis of Experiment 1; however, when birds not treated with DEX or E. coli were compared with those treated with both DEX and E. coli, supplementation with the low level of vitamin D3 significantly decreased TOC incidence. There were no significant effects of vitamin D3 treatment in birds treated with DEX at 5 wk of age in Experiment 2. However, when surviving birds were given a second DEX treatment at 12 wk, vitamin D3 treatment resulted in significantly lower incidence of mortality, TOC, green liver, isolation of bacteria from tissues, and lower airsacculitis scores and heterophil-to-lymphocyte ratios than controls. Vitamin D3 also improved BW, relative weights of the liver and heart, and serum levels of glucose and alanine aminotransferase (ALT) of birds receiving two treatments with DEX. The ability of vitamin D3 supplementation to protect turkeys from the immunosuppressive effects of multiple DEX treatments emphasizes the role of vitamin D3 as a prohormone that affects health and disease resistance in turkeys.