## 4 Interactions between Vaccine Administration and Gut Microbiome Status
### Gut Dysbiosis and Its Influence on Vaccine Efficacy in Poultry
Since dysbiosis associated with antibiotic treatments may result in dampened immune responses to vaccinations, the positive modulation of the gut microbiota through probiotic supplementation has been investigated in poultry. Commensal bacteria and probiotics have been referred to as \"endogenous vaccine adjuvants\" that aid in developing antigen-specific adaptive immune responses to pathogens present in the gut mucosa [76]. For example, [PERSON] et al. [65] observed that broiler chickens provided ad libitum access to diets formulated with limomycin had diminished microbial metabolic activities essential for nutrient utilization, such as vitamin biosynthesis and nitrogen metabolism, when compared to chicks fed the same diet but supplemented with _Lactobacillus plantarum_ P-8 probiotic in the water. Additionally, the Simpson index (evenness of microbial species populations) of fecal samples collected from broilers supplemented only with limomycin in feed was higher (\(p\) = 0.02) than those receiving limomycin in feed and _L. plantarum_ P-8 in water, indicating lower microbial diversity [65]. Similarly, [PERSON] et al. [69] vaccinated Mahuang broilers with a commercially available coccidiosis vaccine and investigated the influence of _Clostridium butylticum_ probiotic inclusion on live performance and gut microbiota status after a coccidiosis challenge. In summary, _C. butylticum_ inclusion increased the richness and evenness of cecal pouch microbiota, regardless of vaccination status prior to challenge. Prior to coccidiosis challenge (15 days of age), birds only vaccinated for coccidiosis had at least a four-fold lower abundance of the _Bacteroides_, _Barnesiella_, and _Megamonas_ genera and a four-fold greater abundance of the _Enterococcus_ genus when compared to those that were vaccinated for coccidiosis and supplemented with _C. butylticum_[69]. \begin{table}
\begin{tabular}{l l l l l l} \hline \hline \multicolumn{1}{c}{**Model**} & \multicolumn{1}{c}{**Source of Symbols**} & \multicolumn{1}{c}{**Vaccine-Challenge Treatment**} & \multicolumn{1}{c}{**Vaccine-Manage Response**} & \multicolumn{1}{c}{**Ref. 1**} \\ \hline  & & & & \\  & & & & & \\  & & & & & \\  & & & & & \\  & & & & & \\  & & & & & \\  & & & & & \\  & & & & & \\  & & & & & \\  & & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Influences of gut microbiota dysbiosis on vaccine efficacy in avian and mouse models. These alterations to gut microbiota may be associated with improved mucosal immune responses. For example, White Roman chickens orally vaccinated with Newcastle disease virus vaccine and supplemented with a yeast cell wall product had an elevated serum hemagglutination titer, greater presence of IgA\({}^{+}\) cells in the mucosa of the duodenum, and greater fecal shedding of Newcastle disease virus-specific secretory IgA when compared to birds that were vaccinated but not supplemented with a yeast cell wall product [77]. Additionally, supplementing with a yeast cell wall product in the feed of vaccinated birds produced greater fecal pouch microbiota richness than that of non-supplemented, vaccinated birds [77]. These results indicate that the beneficial modulation of the gut microbiota could effectively increase protective immunity at the surface of the mucosa. Apart from in-feed probiotic and prebiotic supplementation, another method of probiotic supplementation that has advanced in recent years is in ovo, or in-egg, administration of beneficial microbes into the amniotic sac of 18-day-old chicken embryos. It has been shown that these in ovo-supplemented microbes colonize in the gastrointestinal tract of the embryo [78] and that some species of probiotics could decrease TLR4- and iNOS-mRNA expression in the fecal tonsils [79] as well as increasing cytokine mRNA expression (IL-1\(\beta\), IFN-\(\gamma\)) in the spleen [80] in the days and weeks post-hatch when compared to age-matched chicks that were not supplemented with in ovo probiotics.