During the last three decades, selective breeding has considerably improved performance in broiler chickens. As well as faster growth, much better feed efficiency, improved meat production and a higher proportion of breast meat have been achieved. However, this development has had negative effects on the immune system, leading to an increase in the incidence of metabolic disorders, reduced resistance to infectious disease and higher mortality. Many reports have suggested that chicks in the first week after hatch lack adequate immune responsiveness and they are therefore highly susceptible to infections. It is important to understand the development of the immune system in broilers and its capacity to respond effectively to the diversity of antigens required for protection during early life. The immune system provides the body with the ability to mount a defence against foreign organisms and antigenic substances.

Avian immune system

Because of this widespread function, the immune system is a complex system, both functionally and structurally. The immune system is widely dispersed throughout the body and it is composed of organs, cellular elements and soluble (humoral) elements.

As in mammals, immunity in birds develops through the lymphoid system. Organs of the immune system can be classified into primary (central) and secondary lymphoid organs. The bursa of Fabricius and thymus are primary lymphoid organs in which lymphocyte precursors develop into immune-competent native lymphocytes. Secondary lymphoid tissues are the spleen, bone marrow, Harderian gland, pineal gland and organised lymphoid tissues associated with mucosal surfaces (MALT), including bronchial-associated lymphoid tissues (BALT), gut-associated lymphoid tissues (GALT), conjunctival associated lymphoid tissues (CALT) and other less well organised clusters of lymphoid cells in various organs. These lymphoid tissues are located at strategic sites where foreign antigens entering the body from either the skin or a mucosal surface can be trapped and concentrated. Lymphocytes, antigen transporting and presenting cells and other regulatory cells are also located in these anatomically defined tissues and are thought to be organised into structures that optimise cellular interactions that support the efficient removal of unwanted pathogens.

Ontogeny of immune system

The development of the immune system of embryonic and neonatal chickens is a dynamic process involving the migration of lymphocyte stem cells to primary immune glands, the rapid and extensive proliferation of lymphocytes in the absence of antigen within the bursa and thymus, the concurrent differentiation of bursal and thymic lymphocytes, and the eventual seeding of differentiated lymphocytes to the peripheral lymphoid organs such as the spleen and caecal tonsils. Stem cell migration to the chick thymus occurs in three waves at 6.5, 12 and 18 days of embryogenesis, and in the bursa between 7.5 and 14 days of embryogenesis. Both lymphocyte sub-populations undergo somatic mutation during embryonic development. T-cells rearrange the T-cell receptor genes once and express the receptor proteins over days 12-14, whereas B-stem cells undergo rearrangements of the variable region of the immunoglobulin light and heavy chain genes and express surface IgM as early as day 12 and IgG by day 14. The secondary immune organs such as the spleen, caecal tonsils and Meckel's diverticulum are incomplete at hatch. Although B-cells are detected in caecal tonsils at hatch, only IgM is expressed. There are T-cells in the lamina propria and epithelium of the gut and other secondary immune organs, but they only develop cytotoxic ability some time after hatch.

What delays access to feed in newly hatched chicks?

In commercial poultry operations, hatch is delayed over a 2-day period and the chicks are transferred from the incubator only when the majority of them clear the shell. Following removal from the incubator, other practices such as sexing, vaccination and packaging are carried out before they are boxed for transportation. In practice, some chicks of a hatch may often spend up to 36-48 hours without any access to feed or water. This causes poor viability and retarded growth. Thus, the time from hatching to the onset of receiving nutrition is a critical period in the development of hatchling poultry.

Source of nutrients for newly hatched chicks

Many factors influence the early development of immune system in newly hatched chicks. One of the important factors is initial feed intake. In the newly hatched chick, yolk provides immediate post-hatch energy and protein for maintenance and growth. The absorption of essential nutrients and maternal antibodies from the yolk sac are critical for survival during the early stages. The residual yolk is usually used up within 4 days after hatching but a recent study indicated that residual yolk is used up more quickly in chickens that have access to feed immediately after hatch than those fasted for 48 hours. The reason is that the anti-peristaltic movement, which moves the yolk from the yolk stalk to the duodenum, appears to be stimulated by the presence of feed in the gut.

Immune system benefits from early feeding

The time from hatching to the onset of feeding is a critical period in the development of hatchling chicks. Around 2-5% of hatchlings do not survive this critical post-hatch period because of limited body reserves, and many survivors exhibit stunted growth, inefficient feed utilisation, reduced disease resistance or poor meat yield.

These limitations can be alleviated by the administration of nutrients in the hatchery immediately post-hatch, a technique termed early nutrition'. Appropriate nutrition and access to feed after hatch can accelerate yolk utilisation, enhance gastrointestinal tract development and trigger the secretion of pancreatic enzymes. These factors help nutrient assimilation, contribute to muscle growth and improve performance right through to market weight.

Feed provides nutrients for the growth and development of both primary and secondary lymphoid organs. The immune system of the hatchling particularly the mucosal immune system also requires feed intake for rapid development. It has been reported that delayed access to feed impairs not only intestinal development but also development of gut-associated lymphoid tissue (GALT) like the bursa of Fabricius, caecal tonsils and Meckel's diverticulum. Prolonged feed restriction is harmful to immune system development.

We studied the effect of early nutrition on the immune organ development of chickens during the first three weeks of hatch. It was observed that withholding feed for 24 hours or more resulted in poor growth of bursa. Chicks fed immediately after hatch had significantly heavier bursas, indicating that early access to feed boosts immune organ development.

Post-hatch deprivation of feed for up to 24 hours resulted in poor growth and development of spleen during the first week of hatch although by 21 days of age, spleen weight was found comparable with chicks fed immediately after hatch. Feed deprivation up to 48 hours resulted in poor spleen weight at all the ages tested (Figure 4). Prolonged feed restriction is harmful to the development of the immune system.

Improved immunity with early feeding

Fasting interferes with immune development in other ways too. It stimulates the secretion of corticosteroids, resulting in inhibition of immune cell proliferation, including immune cells that are required for the hatchling to respond to a vaccine. One of our recent studies indicates that the antibody titre at 19 days of age (in response to sheep red blood cells inoculation (SRBC) at 14 days) was significantly higher in the chicks fed immediately after hatch than those fasted for either 24 or 48 hours (Figure 5). Similarly, the ELISA antibody titre (at 21 days) against RD vaccination (on day 5) was significantly higher in the chicks allowed access to feed immediately than those fasted for 24 or 48 hours.

Early nutrition gets chicks off to a good start in life

The time from hatching to the onset of feeding is a critical period in the development of hatchling poultry. The residual yolk may be sufficient to keep the chick alive for 3 to 4 days after hatch but it does not provide sufficient nutrients for optimum growth and development of immune organs or for optimum immunity. Appropriate nutrition and access to feed immediately after hatch can accelerate yolk utilisation and stimulate the growth and development of immune systems. So, why delay? Providing nutrients immediately post-hatch contributes to poultry enterprise profitability.