Introduction

Gut: The First Line of Immune Defence
The intestinal defence system consists of several interconnected components, including the mucus layer, intestinal epithelial cells, tight junctions, antimicrobial substances, gut microbiota and gut-associated lymphoid tissue (GALT).

The epithelial cells form a physical barrier between the intestinal contents and the internal tissues. Tight-junction proteins such as occludin, claudins and ZO-1 regulate the movement of substances between epithelial cells. The mucus layer provides an additional protective environment, while immune cells located within and beneath the intestinal mucosa continuously monitor microbial and dietary antigens.
Thus, the intestine acts as both a selective absorption system and an immune surveillance system.

Gut Microbiota and Immune Development
The intestinal tract contains a complex microbial ecosystem. A balanced microbial population can contribute to pathogen exclusion, production of microbial metabolites, maintenance of intestinal integrity and modulation of host immune responses.
Beneficial microorganisms and their metabolites communicate with intestinal epithelial cells and immune cells. Among these metabolites, short-chain fatty acids (SCFAs) such as acetate, propionate and butyrate are particularly important because they can influence epithelial health, immune-cell activity and inflammatory responses. Therefore, gut microbiota and immunity have a bidirectional relationship. A healthy microbiota supports a healthy intestinal barrier and balanced immune signalling. At the same time, the immune system regulates microbial populations and helps maintain intestinal homeostasis.
What Happens When Gut Health Is Compromised?
Commercial poultry are exposed to several factors capable of disturbing gut integrity, including coccidiosis, bacterial pathogens, mycotoxins, heat stress, poor water quality, nutritional imbalance and dysbiosis.

A compromised gut can result in epithelial damage, disruption of tight junctions, increased intestinal permeability, inflammation and oxidative stress, altered immune response and reduced performance. This process is sometimes described as increased “intestinal permeability” or “leaky gut.”

When intestinal integrity is compromised, nutrients may not be utilized efficiently and microbial products can interact more extensively with host immune mechanisms. The bird may consequently spend additional nutrients and metabolic resources on inflammation and tissue repair rather than growth or egg production.

Coccidiosis, Mycotoxins and Heat Stress

Coccidiosis is a classic example of the gut–immune relationship. Eimeria infection damages intestinal epithelial cells, alters intestinal morphology and stimulates an inflammatory and immune response. Consequently, coccidiosis control is not only a parasite-management issue but also a gut health, nutrition and immune-management issue.

Similarly, mycotoxins can affect intestinal integrity, oxidative status, microbial balance and immune function. Their impact may therefore extend beyond the feed itself to the overall gut–immune axis.

Heat stress is particularly important in tropical poultry production. Heat stress can alter feed intake, intestinal physiology, microbial balance and barrier integrity, potentially increasing inflammatory and oxidative stress. Maintaining gut integrity during heat stress is therefore an important component of maintaining flock resilience.

Gut Health and Vaccine Response
An effective vaccination program depends on the bird’s ability to recognize antigens and develop appropriate humoral and cellular immune responses. This requires adequate availability of energy, amino acids, vitamins, minerals and antioxidants. Therefore, gut health indirectly supports the nutritional foundation required for immune responses. A healthy intestine facilitates nutrient digestion and absorption, while intestinal inflammation or damage can increase nutrient requirements for tissue repair and immune activity. It is important, however, to recognize that good gut health alone does not guarantee vaccine success. Vaccine quality, administration, maternal immunity, biosecurity, disease pressure, nutrition and environmental conditions also influence the final immune response.

Nutritional Support for the Gut–Immune Axis
Nutrition is one of the most important tools for supporting the gut–immune relationship. Zinc contributes to epithelial integrity, cellular proliferation, enzyme activity and immune-cell function. Selenium is an important component of antioxidant defence systems and supports cellular protection and immune function. Vitamins A and E also contribute to mucosal integrity, antioxidant protection and immune responses.
Nutritional interventions such as probiotics, prebiotics, postbiotics, organic acids, enzymes, yeast-derived products, MOS and β-glucans may be used as part of integrated gut-health programs. MOS can support gut health through microbial interactions and pathogen-exclusion mechanisms, while β-glucans can interact with innate immune pathways and influence immune-cell activity. However, responses depend on product source, structure, dose, bird age, diet and challenge conditions; therefore, these products should be evaluated based on scientific evidence and field performance rather than assuming that all products have identical effects.

Practical Approach for Commercial Poultry
A successful gut-health program should not depend on a single feed additive. It should combine:
– High-quality and properly stored feed
– Regular mycotoxin monitoring and management
– Good drinking-water quality and sanitation
– Effective coccidiosis control
– Appropriate vaccination and biosecurity
– Heat-stress management
– Balanced amino acid, vitamin and mineral nutrition
– Appropriate use of probiotics, prebiotics/postbiotics and other gut-health products
– Monitoring of litter quality, feed intake, FCR, mortality and flock uniformity

Conclusion
The gut and immune system should be viewed as two interconnected components of the same biological defence network. A healthy intestinal microbiota, intact epithelial barrier and properly functioning GALT provide the foundation for balanced immune development and response. For commercial poultry, the concept is simple: “Protect the gut to support immunity, and support immunity to maintain flock resilience.”
Gut health is therefore not only about digestion and FCR. It is closely connected with nutrient utilization, immune development, disease resistance, vaccine responsiveness and ultimately flock productivity.
The future of poultry nutrition will increasingly focus on managing the gut–microbiota–immune axis through integrated nutrition, health and farm-management strategies rather than addressing individual problems in isolation.

Poultry excreta comprises undigested feed protein and uric acid, which microbial enzymes convert to ammonia (NH3). Several litter characteristics, including pH, temperature, oxygen, moisture concentrations, and substrate availability, influence this conversion. The recommended limit for ammonia in a chicken shed is less than 10 ppm, however, up to 25 ppm is not detrimental. Ideally, the relative humidity should range between 50 and 70%. The rainy season, defective foggers, insufficient ventilation, water leaks, and other factors all contribute to increased humidity inside the shed.

Ammonia levels and humidity in poultry houses are interconnected. High relative humidity can exacerbate the adverse effects of high blood ammonia levels in poultry. In humid environments, more NH3 may be dissolved in the air droplets and inhaled into the blood during respiration by birds, consequently increasing the blood ammonia content. When ammonia gas is exposed to moisture, it reacts and forms a corrosive solution called ammonium which causes harm to birds. Additionally, high humidity can hinder the evaporation of moisture from the litter, causing it to retain more ammonia.

Deleterious Effects on Poultry:
1. Respiratory Issues: High levels of ammonia in the poultry house air can cause respiratory problems for the birds. Ammonia gas affects the trachea’s mucosal surface, causing paralysis of cilia, sometimes deciliation of epithelial cells, and causes necrosis of the mucosal epithelium.
2. Foot Lesions: The constant exposure of poultry to ammonia can cause severe foot lesions by causing chemical burns on the foot pads of birds, leading to painful and debilitating footpad dermatitis.
3. Eye Lesions: High concentrations of atmospheric ammonia for a prolonged duration causes irritation, conjunctivitis, and damage to the cornea of the eyes. Swelling and reddening of the eyelids, irritation, reddening of the conjunctiva and nictitating membrane, and partial or complete closure of the eyes are common clinical signs.
4. Reduced performance.

How to prevent it:
Along with farming management like dietary management, stocking density, proper ventilation, house temperature, litter management, etc., other supplements like Phytogenic Feed Additives can be supplemented in a poultry diet. A phytogenic feed additive increases the digestibility of nutrients within the gastrointestinal tract and reduces the gut inflammation caused by stressors.

Thereby may considerably increase the gut integrity of the birds. Phytogenic feed additives also alter gut microflora, minimizing the adverse effect of harmful bacteria on the gut. Less undigested and unabsorbed nutrients will be excreted through faeces from a healthy gut, which means less nitrogen excretion.

STODI, a Standardized Botanical Powder, is crafted with scientifically selected herbs improving the efficiency of feed utilization and overall performance of the birds. In various studies, it has been found that STODI supplementation has significantly reduced litter nitrogen (g/100g of litter) as compared to group without supplementation. STODI maintains the gut integrity and peristaltic movement of the gut which increases time for the protein and other nutrient utilization by the birds. This increased protein utilization leads to reduced excretion reduced excretion of nitrogen which in turn decreases the production of ammonia level in litter. Along with this STODI has shown to improve the gut microbiota level and gut immunity of the birds.

In conclusion, the combined impact of ammonia and humidity in the world of poultry farming underscores the critical importance of maintaining a balanced and controlled environment for the well-being and productivity of the birds. High levels of ammonia in poultry houses can lead to a range of deleterious effects. STODI, a polyherbal formulation has shown to reduce the ammonia level in litter with improved nutrient utilization and gut microbiota balance.