**Abstract**

It is important to consider the health and well-being of birds in various production methods. The microbial makeup and function of a bird's gastrointestinal (GIT) system may vary based on the bird's food, breed, age, and other environmental conditions. Gut flora play a critical role in maintaining intestinal homeostasis. Environmental exposure to contaminants such as heavy metals (HMs) has been linked to a wide range of disorders, including the development of dysbiosis in the gut, according to many studies. Changes in the gut microbiota caused by HMs are a major factor in the onset and progression of these illnesses. The microbiota in the gut is thought to be the first line of defense against HMs. Thus, HMs exposure modifies the gut microbiota composition and metabolic profile, affecting HMs uptake and metabolism by altering pH, oxidative balance, and concentrations of detoxifying enzymes or proteins involved in HM metabolism. This chapter will focus on the exposure of chicken to HMs from their feed or water and how these HMs affect the immune system resulting in various diseases.

**Keywords:** poultry, chicken, broiler, GIT, heavy metals, microbiota, diseases

### **1. Introduction**

Among the most popular types of poultry raised for human consumption are domestic chickens. At 35–40 days of age, a typical broiler chicken will weigh around two kilograms [1]. During this period, they require approximately 3–4 kilograms of feed per day because of their rapid growth. While raising chickens in close proximity is necessary to meet the demand for chicken meat, this practice puts the birds at greater risk of infection and speeds disease transmission [2].

There is a wide range of microorganisms that colonize an animal's digestive system as soon as it is born or hatches, and these microorganisms change over time [3]. The gut microbiota of an animal, a human of the same species, and the location of the host's body all differ [4]. In the gut microbiota, which is a complex, interconnected community of organisms, the actions of all microbial components have a direct effect on its functions [5]. When the host and microbes interact in a way that benefits both of them, an ecological system is created [6]. As with humans, animals' gut microbiome serves many of the same functions: scavenging energy from undigested feed

components through fermentation, creating an immune barrier to keep harmful bacteria out of the digestive tract, and aiding in the absorption of vitamins and amino acids by animals [5]. This is largely true for both species. Farm animals must fulfill environmental and dietary responsibilities, as well as economic ones, in order to be productive [7]. The GIT microbiota has a significant impact on animal performance, particularly in young animals who are exposed to a wide range of stressful situations [8]. Dietary fiber, vitamins, and minerals are all provided by the microbiota that inhabit the GIT. The GIT microbiota may also play an important role in hen health and immunity, according to some evidence [9].

Data shows that heavy metal (HM) exposure may play a role in the etiology of metabolic disease by altering the GIT microbiota [10]. It's important to remember that the gut microbiota protects the body from harmful microbes. Furthermore, HM exposure alters the composition and metabolic profile of the gut microbiota, which in turn affects the uptake and metabolism of these HMs by altering pH, oxygenation, and the concentrations of enzymes or proteins that are involved in the detoxification process [11]. As the intestinal barrier is influenced by gut flora, HM absorption can also be affected.
