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Newly hatched broiler chickens are susceptible to infections partially because their immune system is still maturating [1, 2]. After hatching, adaptive immunity takes at least two weeks to develop, and in the meantime the chicken relies on maternal passive protection as well as innate immunity [3]. However, even the innate immune system is not properly matured in the first week post-hatching [2, 4]. This vulnerability can severely affect the broilers overall health and welfare, often leading to antibiotic treatments. Given the concern about the emergence of antimicrobial resistance, alternative strategies to aid in innate resistance against infections are required [5]. While often overlooked, the embryonic period poses the opportunity to improve immune function and disease protection prior to hatching. During embryonic development the yolk sac (YS) has two crucial roles, one as the main nutrient source and another as a temporary substitute for the developing immune organs [6]. The yolk is a source of maternal antibodies, which are transferred from the hen and aid in passive immunity [7]. However, the yolk sac tissue itself also plays a part in immune defense, by producing antimicrobial molecules, and more specifically avian beta-defensins (AvBD), throughout embryonic development [6, 8]. Because of these antimicrobial properties, the YS is a promising target for improving early immune functioning, thereby potentially reducing susceptibility to infections.
A widely recognized approach to intervene in the embryonic phase is by in ovo delivery. This technique involves injection of vaccines, nutrients, or other bio-active compounds into the amniotic fluid surrounding the developing embryo [9, 10, 11–12]. Plant derived compounds have gained interest as potential alternatives for antibiotics, because of their positive effects on gut health, which is closely linked to immune function [13]. Through the activity of the gut-associated lymphoid tissue, a healthy gut directly influences immune regulation and supports overall immunity. The monoterpenoid phenol carvacrol, more specifically, is known for its antimicrobial, anti-inflammatory and antioxidant properties [14, 15–16]. Carvacrol, which is the main bioactive compound in essential oils such as oregano and thyme, also has been shown to reduce bacterial growth and downregulate inflammatory cytokines during a bacterial lipopolysaccharide challenge, thereby potentially reducing disease incidence [17, 18]. On the contrary, when applied in ovo, carvacrol has been shown to migrate and accumulate into the...