THE EFFECT OF PROBIOTICS ON DISEASE RESISTANCE IN POULTRY

Improving disease resistance is a key focus in modern poultry production, particularly in efforts to reduce antibiotic use [203]. Probiotics have been shown to play a crucial role in strengthening poultry resistance to viral and bacterial infections through two distinct but complementary mechanisms: immune priming and direct antimicrobial activity [16].

Resistance to viral infections is primarily mediated through enhancement of the host adaptive immune response rather than direct antiviral activity. In this context, probiotics primarily function as immune-priming agents, enhancing baseline immune readiness prior to pathogen exposure [22]. Several studies have shown that probiotic administration can increase the effectiveness of vaccines against major pathogens in poultry, such as NDV, IBDV, and AI [204–206]. This antiviral protection is associated with stimulation of mucosal and systemic immunity, increased production of virus-specific antibodies (mucosal IgA and circulating IgY), enhanced antigen presentation, and activation of CD4⁺ and CD8⁺ T lymphocytes, resulting in a faster and more robust immune response upon viral exposure [13]. Importantly, these effects are preventive rather than therapeutic, as probiotics do not directly inactivate viruses but instead enhance the host’s immunological preparedness prior to or during vaccination.

In contrast, resistance to bacterial infections involves both immune-mediated and direct microbiological mechanisms. Unlike immune priming against viruses, probiotic effects against bacteria often include direct antimicrobial actions within the gastrointestinal tract [37]. In addition to their effects on viral infections, probiotics are effective in reducing colonization by pathogenic bacteria such as Salmonella spp. and Campylobacter spp., which are common causes of enteric diseases and food contamination [207]. Unlike viral protection, which depends largely on adaptive immunity, antibacterial effects are strongly linked to competitive exclusion within the gastrointestinal tract. Probiotics suppress pathogen growth through competition for space and nutrients, production of antimicrobial metabolites such as organic acids and bacteriocins, and stimulation of mucosal immune responses that limit pathogen colonization in the gastrointestinal tract [208].

From a preventive perspective, continuous probiotic supplementation can reduce initial pathogen colonization and intestinal shedding [150]. From a therapeutic perspective, when administered during or after bacterial challenge, probiotics may help mitigate disease severity by restoring microbiota balance and modulating inflammatory responses; however, therapeutic effects are generally supportive rather than curative and do not replace antimicrobial treatment in severe infections [12].

The reduction of Campylobacter spp. colonization is particularly relevant for food safety. Lower intestinal loads and fecal shedding may translate into reduced carcass contamination at slaughter, thereby decreasing the risk of zoonotic transmission to humans [209]. However, reported reductions in Campylobacter counts vary widely among studies, and complete eradication is rarely achieved, highlighting the strain-specific and context-dependent nature of this effect [210].

It is important to note that the magnitude of probiotic effects may vary considerably under field conditions compared to controlled experimental settings. In commercial production systems, factors such as housing density, litter management, environmental stress (temperature and humidity fluctuations), feed formulation, water quality, vaccination programs, biosecurity standards, and existing gut microbiota composition can influence probiotic efficacy [211]. Consequently, responses observed in research trials may not always be directly replicated at the farm level. This field-level variability underscores the need for context-specific evaluation and optimization of probiotic strains, dosages, and administration strategies within different production environments [212].

The combined effects of immune priming, direct antimicrobial activity, and reduced pathogen shedding may positively impact poultry production performance, such as increased body weight, feed conversion efficiency, and meat and egg quality [3]. Thus, probiotics not only enhance poultry health but also naturally and sustainably enhance disease resistance, making them an effective nutritional strategy in modern poultry production [17].