IN OVO AND EARLY-LIFE PROBIOTIC ADMINISTRATION

Early probiotic administration has been recognized as a strategy to optimize the establishment of the gut microbiota and immune development in poultry. Recent advances (2024–2025) have substantially expanded the evidence base for _in _ovo probiotic delivery, particularly injection at embryonic day 18 (ED18) into the amniotic cavity [235, 266]. Several studies using Lactobacillus-based single strains or multi-strain cocktails have demonstrated improved hatchability, enhanced early chick viability, and reduced colonization by opportunistic bacteria, including Klebsiella spp. and Enterococcus spp. during the first week post-hatch [267, 268]. These findings suggest that microbiota modulation can begin prior to hatch, during a critical window of immune ontogeny.

Mechanistically, _in _ovo administration has been associated with modulation of local immune responses in GALTs, particularly the cecal tonsils [269]. Reported effects include altered expression of pro- and anti-inflammatory cytokines such as IFN-γ, IL-1β, and IL-8, indicating early immune priming [270]. While some cytokine modulation appears transient during the immediate post-hatch phase, emerging evidence suggests that early microbial exposure may shape longer-term immune responsiveness and mucosal barrier function [114].

Comparative studies indicate that _in _ovo delivery can influence early microbial succession patterns in a manner comparable to repeated oral dosing post-hatch. Chicks receiving _in _ovo Lactobacillus cocktails often show accelerated establishment of beneficial lactic acid bacteria and reduced relative abundance of Enterobacteriaceae during the first two weeks of life [271]. In some trials, these early shifts were associated with sustained improvements in feed efficiency, body weight gain, gut morphology (e.g., villus height), and reduced pathogen load later in the production cycle [272].

Despite these promising findings, several factors influence the success of _in _ovo probiotic strategies. Critical variables include strain selection, inoculum concentration, injection site accuracy, embryo viability, and compatibility with automated hatchery equipment [235]. Overdosing or inappropriate strain combinations may negatively affect hatchability or produce inconsistent immune responses. Therefore, precise standardization of dose and formulation is essential for safe large-scale implementation [273].

From a practical perspective, _in _ovo delivery offers the advantage of uniform administration at the hatchery level, potentially ensuring consistent early-life exposure across large flocks [234]. However, integration into commercial hatchery workflows requires validation of biosafety, equipment calibration, and cost–benefit feasibility. Moreover, the long-term persistence of immunomodulatory effects compared with conventional post-hatch oral supplementation remains an area requiring further longitudinal investigation [274].