Abstract
The antiviral and immunostimulant effect of dietary supplementation of Spirulina platensis (SP) against Infectious bursal disease (IBD) was assessed as follows; 126 one-day-old cobb broiler chicks were randomly distributed into six groups (21 birds/group with three replicates, 7 birds each): negative control (non-vaccinated, and non-challenged ; NC), positive control (non-vaccinated, and challenged; PC), vaccinated control (vaccinated, and challenged; VC), Spirulina 0.1% (vaccinated, 0.1% SP-supplemented, and challenged; VSP-0.1), Spirulina 0.3% (vaccinated, 0.3% SP-supplemented, and challenged; VSP-0.3), and Spirulina 0.5% (vaccinated, 0.5% SP-supplemented, and challenged; VSP-0.5). The chicks in NC, PC, and VC groups were not supplemented with SP. All vaccinated groups, regardless SP supplementation, showed significant improvement of body weight gain (19% in 0.3% SP) and FCR during IBDv challenge (10 dpc) (P˂0.05). On the 10th dpc, IBD ELISA titer in SP-supplemented groups were decreased (P>0.05) than that of G3, suggesting an antiviral efficacy of SP. This antiviral activity was evident by the viral RNA load in the cloacal swabs as well. The viral shedding peak was on the 5th day pc which was significantly decreased on G4-6 (P˂0.05) in comparison to G2 and G3. Also, the bursal ratio was improved in SP supplemented groups comparable to G3. Moreover, in unchallenged broilers, the gene expression of INF-γ and IL-18 were upregulated while that of IL-10 was downregulated in spleen of SP-supplemented chicks. Finally, severe histopathological lesions of kidneys, bursa, thymus, and spleen were observed in G2 which moderately minimized in G3 and were more alleviated in SP-supplemented groups. Collectively, SP supplementation improved body weight gain and decreasing viral shedding with antiviral activity. The mechanisms of SP against IBDV challenge may include cell-mediated immunity promoting effect, antiviral activity, and/or anti-inflammatory effect that should be further studied

Key words: Spirulina platensis, Infectious Bursal Disease, immune response, viral shedding, broiler chickens, ELISA, challenge, efficacy