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Abstract
Subtherapeutic and prophylactic doses of virginiamycin are capable of altering the
intestinal microbiota as well as increasing several growth parameters in chickens.
In spite of the fact that the microbiota plays a role in shaping the host's immune
system, little information is available on the effects of in-feed antibiotics on the
chicken immune system. The objective of this study was to examine the effects of an
antibiotic, virginiamycin, on the development of antibody responses. Chickens were
fed diets containing no antibiotics, along with either subtherapeutic (11 ppm) or
prophylactic (22 ppm) doses of virginiamycin. Chickens were then immunized with keyhole
limpet hemocyanin (KLH) and sheep red blood cells systemically, and with BSA and KLH
orally. Although antibodies were detected against BSA in the intestinal contents of
birds that were orally immunized, there was no difference among different treatment
groups. Systemic IgG, and to a lesser extent IgM, antibody responses to KLH were greater
(P < 0.05) in birds fed a diet containing 11 or 22 ppm of virginiamycin compared with
control birds fed no antibiotic. No treatment effect was found in the sheep red blood
cell-immunized birds. Results of the present study implicate virginiamycin in enhancing
antibody responses to some antigens in chickens. Further studies are required to determine
to what extent these effects on antibody response are mediated through changes in
the composition of the microbiota.