We all realize that diseases cost both companies and growers and they both strive to avoid the consequences of disease. Diseases can be caused by microbes (viruses, bacteria, fungi or protozoa), internal or external parasites, genetic disorders or by nutrient deficiencies. Modern poultry production methods have virtually eliminated nutrient deficiencies and are addressing genetic disorders. However, both companies and growers continue to battle against microbes and parasites.
Since fewer and fewer antibiotics are being used in poultry feeds, growers and companies are depending more heavily on the immunity provided by vaccines.
Understanding Immunity
Immunity can be described as the ability of the body to recognize the presence of material normally within the body (“self”), and to eliminate foreign (“non- self”) materials. When a disease organism invades, the bird’s body usually produces antibodies and specific cells whose purpose is to engulf (or eat)and destroy foreign substances.
Substances that are identified by the bird’s body as foreign are known as antigens. In other words, antigens are substances that cause the immune system to develop a defense against an invading organism (an immune response). However, it is important to realize that antigens are chemical substances that modern science has often been able to identify and separate from or weaken in the disease causing microbes so that the bird’s body becomes immune without getting the disease. Some proteins are good antigens that are easily recognized by the immune system and will produce an effective immune response. Other materials, such as carbohydrates are less effective antigens, and the immune response may not provide good protection. Once a bird’s immune system has responded to an antigen (either from the microbe or a vaccine) antibodies circulate in body fluids. If the bird is exposed again to that microbe, it responds very quickly because it “remembers” the microbe.
The quick response of the immune system prevents the disease from happening or shortens its duration and severity.
Getting a disease
For most poultry diseases the progression is the same. This progression has three steps or phases: infection, development of immunity and recovery. When birds are not immune to a given disease, infection may easily occur, allowing the microbe to attack various parts of the body producing sickness in the bird.
Depending on the disease, some or all of the birds may die from the infection. However, the performance of even those birds that do not die is reduced by the infection. Those birds that do not die from the infection usually become immune to the disease.
However, the development of immunity in this fashion is risky because the disease may irreparably damage tissues (such as the intestine) in the bird’s body. Such immune responses are also expensive because they require nutrients that cannot be used for growth or production.
Those birds that survive the disease have an active immunity that allows their body to rapidly respond to future invasions of the same or similar microbes. While performance may return during recovery from the disease, the performance lost during exposure is often never regained, particularly if the challenge occurred early in the life of the bird.
Immunity from Hens
As the embryo develops within the egg it has no immunity of its own, but antibodies from the breeder hen are absorbed; protecting the chick from diseases. This immunity (called maternal or passive immunity) protects the young bird from diseases, but prevents the bird’s body from mounting an immune response and is short lived.
At 3 days of age about half of the passive immunity is lost. Very little passive immunity is present at 2 weeks and at 3 weeks it is completely gone.
Vaccine-induced immunity
Vaccines trigger the bird’s body to think that it’s being invaded by a specific organism, and the immune system goes to work to destroy the invader and prevent it from infecting the bird again. If the bird is exposed to a disease for which it had been vaccinated, the invading germs are met by antibodies that will destroy them.
The immunity the bird develops following vaccination is similar to the immunity acquired from natural infection. Today, modern large scale animal agriculture has vaccines against most major pathogens and are continually creating new ones.
Jon Moyle, F. Dustan Clark and Frank Jones are with the Center of Excellence for Poultry Science, University of Arkansas. For a complete list of references to this story see our website.
