Abstract
Background:
There is an obvious lack of information about the effects of ractopamine, a ß-adrenergic agonist, on the growth performance and immune responses of rabbits, particularly in those receiving the viral rabbit hemorrhagic disease (RHD) vaccine.
Aim:
The current study was undertaken to study the effects of ractopamine on growth performances and immunological parameters in rabbits inoculated with the viral RHD vaccine.
Methods:
Experimental rabbits were grouped into four groups, the first acted as a control and received distilled water, the second received ractopamine, the third received inactivated RHD vaccine, and the fourth received both ractopamine, and inactivated RHD vaccine. Then Blood analysis, histopathological, histomorphometric, and immunohistochemistry examinations were followed.
Results:
The obtained results demonstrated that ractopamine induced significant increases in body weight gain, neutrophils, monocytes, nitric oxide, lysosome, and improved feed conversion rate. A significant decrease in lymphocytes with insignificant decreases in eosinophils, phagocytic % and index, serum total protein, α, β, and γ globulin were observed. Vaccinated rabbits only showed a marked rise in WBCs, neutrophils, monocytes, eosinophils, basophils, phagocytic index and activity, nitric oxide, lysosome activity, total protein, albumin, γ globulin, and a decrease in lymphocytes. Rabbits that received ractopamine and then vaccinated had insignificant increases in body weight, weight gain, WBCs, neutrophils, monocyte, eosinophils, basophils, phagocytic activity, and index, globulins beside a significant decrease in lymphocytes. Pathologically, rabbits that received ractopamine alone, with a vaccine or vaccinated only showed an increase in villus length, villus width, and absorption surface area. Immunohistochemistry of rabbits' liver and kidneys of the control and vaccinated group showed negative expression for caspase-3, but rabbits received ractopamine only or rabbits vaccinated and received ractopamine showed diffuse positive moderate expression for caspase-3.
Conclusion:
Ractopamine induced several adverse effects on the immune responses of the rabbits inoculated with the viral HRD vaccine.

Key words: Rabbits, Ractopamine, Rabbit hemorrhagic virus vaccine, Immunity, Viral diseases