Table 1.

Effect of vitamin A on chicken immune system and microbial pathogens.

Effect on the immune system and microbial pathogens	Dose	Host	Reference
Increase anti-NDV antibody	20,000 IU/kg	Broiler breeder	Yuan et al., (2014)
Decrease anti-NDV antibody	35,000 IU/kg	Broiler breeder	Yuan et al., (2014)
Lower CD4+:CD8+ ratios, reduction of intraepithelial lymphocytes, reduced lymphocyte proliferation, impair serum, IFN-γ quantity	Deficiency: Not supplemented with vitamin A	Broiler chickens	Dallul et al., (2002)
Lower CD4:CD8 ratios, increase anti-NDV antibody, developed Th-2 immune response	Deficiency: 400 IU/kg	Broiler chickens	Lessrad et al., (1997)
Developed Th-1 immune response	15,000 IU/kg	Broiler chickens	Lessrad et al., (1997)
Lower levels of serum anti-E. coli IgG antibodies, decreased T cell proliferation Higher mortality due to E. coli infection	Excess vitamin A (1,000 mg/kg) or depleted Vitamin A	Broiler chickens	Friedman et al., (1991)
Higher mortality due to coccidiosis infection	Deficiency: 800 IU/kg	Broiler chickens	Erasmus et al., (1960)
Increased mortality and damage to mucus membrane, such as metaplasia when challenged with Pasturella multocida.	Deficient diet	Turkey	Aye et al., (2000)
Increased mortality when challenged with NDV (Lasota strain)	Deficiency: 120 RE/kg	Broiler chickens	Sijtsma et al., (1989)

Abbreviations: 1IU, 0.3 mcg retinol; IFN-γ, interferon-γ; NDV, Newcastle Disease Virus; RE, retinoic equivalent; Th2, T helper 2.