Active Immunity Protects a Chick from Diseases

Unlike humans, chickens do not have a system of organized lymph nodes to protect them from bacteria, viruses, and other disease-causing microbes. Instead they have several lymphoid organs that work together to gradually develop active immunity that protects the chick from diseases. Pathogens that overwhelm a chick before its active immunity becomes fully functional disrupt the process of immunity building.

Lymphoid Tissue

Lymph — a watery fluid derived from the blood that accumulates in the spaces between the chicken’s body cells — lubricates all body tissues. Lymph contains specialized white blood cells that are technically called lymphocytes, but are popularly known as killer cells.

These killer cells neutralize or destroy invading microbes, which they recognize as antigens. An antigen is any protein that differs from the proteins naturally occurring within a chicken’s body.

When lymphocytes detect an antigen, they respond by producing substances to fight off the invader. These substances, or antibodies, break down the antigen’s defenses to make the killer cells’ job easier.

The word “lymphoid” refers to any tissue that enhances a chick’s immunity by producing lymphocytes and antibodies. Lymphoid tissue occurs primarily in a chick’s thymus, cloacal bursa, and spleen.

Thymus

A chicken’s thymus consists of two strings of several paired, flattened lobes. These stringed lobes run along both sides of the neck nearly its entire length. The number of pairs in a given chicken may be anywhere from five to eight.

As a chicken ages, the thymus has accomplished its mission and naturally shrinks. It may atrophy prematurely if overcome by disease, notably infectious anemia. This viral infection, however, more typically afflicts commercial broilers than backyard flocks.

The thymus is considered a primary lymphoid organ, because it produces functional immune cells that are stockpiled by other (secondary) lymphoid organs. The other primary lymphoid organ is the cloacal bursa.

Cloacal Bursa

The cloacal bursa is a grape-shape organ immediately above the cloaca. It is largely responsible for controlling immunity and activating antibody production in chicks. Sometimes you’ll see it called the bursa of Fabricius, after Italian anatomy professor Hieronymus Fabricius, who first described the organ in 1621.

Inside this organ are about two dozen parallel folds. Together they bear some 10,000 follicles that provide an ideal environment for the development of antibodies.

A chick’s cloacal bursa reflexively takes in fluids, a phenomenon known as cloacal drinking. It’s one way the chick is inoculated against pathogens in its environment.

The cloacal bursa reaches its maximum size in chicks between 6 and 12 weeks old. It then begins to shrink until it nearly disappears. It is no longer functional by the time the chicken is sexually mature at 4 to 5 months.

Certain diseases and nutritional conditions can cause early atrophy of the cloacal bursa, permanently compromising the bird’s immune system. As described in The Chicken Health Handbook, viral diseases that can cause premature atrophy include avian influenza, infectious bronchitis, infectious bursal disease, Marek’s disease, and highly virulent (velogenic) Newcastle disease. A disease caused by the intestinal bacterium Escherichia coli (such as air sac disease or omphalitis) can also result in early atrophy. Other causes include the protozoal disease cryptosporidiosis and toxicity resulting from moldy feed. Nutritional causes include vitamin A deficiency and dehydration.

Spleen

The spleen is a dark red, round organ normally about 3⁄4-inch in diameter. Viewed through a microscope it appears as a combination of red pulp and white pulp. Both the red pulp and white pulp produce antibodies.

Additionally, the red pulp destroys worn-out red blood cells. The white pulp forms white blood cells called leukocytes. These colorless cells circulate in blood and body fluids and help counteract pathogens and other foreign invaders.

Many diseases affect the spleen, causing it to swell (paratyphoid), turn mushy (histoplasmosis), or atrophy (infectious anemia). However, the spleen is not a vital organ. If it ceases to function, other organs take over its job.

GALT, BALT, AND HALT

In addition to occurring primarily in the thymus, cloacal bursa, and spleen, lymphoid tissue is dispersed elsewhere in the body. The most important locations are in the bone marrow, intestines (GALT), respiratory system (BALT), and head (HALT).

GALT consists of clusters of organized lymphoid nodules occurring along a chicken’s digestive tract, particularly in parts of the small intestine. The function of this gut-associated lymphoid tissue, or GALT, is to protect the chick against intestinal diseases. As a chick matures, the accumulations of GALT increase, reaching a maximum by the time the bird reaches 16 weeks of age. Along with the thymus and cloacal bursa, GALT then begins to regress. By the time the bird reaches 1 year of age, most of the tissue clusters are gone.

BALT is similar lymphoid tissue occurring in the chick’s respiratory tract, particularly in the trachea and bronchi. This bronchial-associated lymphoid tissue, or BALT, helps protect the chicken against inhaled pathogens. Like GALT, it gradually develops as a chick grows, becoming well developed by the time the bird reaches 8 weeks of age.

HALT, or head-associated lymphoid tissue, includes the nasal glands, tear glands, mucous membranes of the eyes. It also includes a relatively large Harderian gland behind each eyeball (named after physician Johann Jacob Harder, who discovered the glands in 1694). HALT protects the eyes, nasal cavity, and upper airways.

Natural Vaccination

The chick’s complex system of lymphoid tissue helps the bird develop active immunities through the process of trickle infection. Think of it as natural vaccination, resulting from gradual exposure to pathogens in the chick’s environment.

A chick that’s raised in the unnatural environment of a brooder, apart from other chickens, doesn’t have the benefit of gradual exposure to pathogens. When it eventually integrates into a mature flock, microbes its body doesn’t recognize can overhwhelm its immune system, resulting in disease.

A simple solution is to expose brooded chicks early to the microbes they are likely to encounter later in life. This method of natural vaccination is easily done by giving them a little soil to scratch in while they are still in the brooder. The sooner chicks have an opportunity to peck and scratch in natural soil, the quicker they will develop immunities to the pathogens they will encounter in their future environment.

When you know how a growing chick develops active immunity, you can understand why chickens raised in two different environments are likely to have differing sets of immunities. So, no matter how exciting it may seem at the time, introducing into an existing flock mature chickens from elsewhere often results in heartbreak. A safer way to acquire new chickens is to raise baby chicks.