Understanding Immunity and Vaccines

By Jon Moyle, F. Dustan Clark and Frank Jones, Center of Excellence for Poultry Science, University of Arkansas. 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.
calendar icon 29 November 2007
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More on the Authors
Dr. Dustan Clark
Extension Poultry Health Veterinarian
Dr. Frank Jones
Extension Section Leader
Jon Moyle


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. While important, this article will not address parasite issues, but will provide some understanding of microbial disease, immunity and 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 (“nonself”) 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 (Varela, 2007). 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 (Cutler, 2002). The quick response of the immune system prevents the disease from happening or shortens its duration and severity.

Disease Processes

When a bird is exposed to a disease microbe, there is one of three outcomes, either:
  • The bird gets the disease,
  • The bird is protected by immunity from hens or
  • The bird is protected by immunity from vaccines.

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 (Cutler, 2002).

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 (Klasing, 1998).

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 (Cutler, 2002).

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.

Understanding Vaccines

Today, modern large scale animal agriculture has vaccines against most major pathogens and are continually creating new ones. However, vaccines come in a bewildering array of forms including: live or killed vaccines, recombinantvector vaccines and DNA vaccines.

Live or Killed Vaccines

Several vaccines (i.e. Gumboro Disease, Newcastle Disease, Infectious Bronchitis and others) come in live or killed (inactivated) forms. While both live and killed products produce results, it is important to realize the advantages and disadvantages of both types.

It should be obvious that if birds are given the disease causing microbe (the pathogen), they will develop the disease we are trying to prevent. However, if birds are given a weakened (or attenuated) and diluted form of the pathogen they will develop immunity, but not develop the disease. This is the concept behind live attenuated (weakened) vaccines (Okonek and Peters, 1997). Attenuated or modified live vaccines are created by weakening the disease microbe, usually by culturing the pathogen in the laboratory until it loses or reduces its ability to produce disease and then providing a small dose of the organism during vaccination (Varela, 2007). However, to be effective the live attenuated organism must stimulate an immune response by growing within the bird; usually causing brief, mild symptoms (a vaccine reaction).

Live vaccines are the most effective type of vaccine for a rapid, strong, long lasting immune response. Live vaccines also tend to be less expensive and are less likely to cause allergic reactions than other types of vaccines. (Whiting, 2005) They can be administered by injection, spray/ fog, in the water or by eye drops (intraorbitally). However, live vaccines come with their own problems. Because they contain living organisms, they must be handled with care. Excessive heat, sunlight, freezing, chlorinated water and other conditions can kill off live organisms, rendering them useless. Live vaccines can also cause severe reactions in animals that have weakened immune systems or are infected with other disease organisms. In addition, if live vaccines are not handled with proper biosecurity, the organism may spread to numerous other avian species, causing (sometimes severe) reactions. Finally, while rare, the organism could revert back to the “wild” form, causing the disease.

Killed (or inactivated) vaccines are an alternative to live vaccines. Killed vaccines contain no living organisms, eliminating the potential of reversion to a “wild,” diseasecausing form. Killed vaccines are also safer than live vaccines for weak or immune compromised animals. In addition, killed vaccines are more stable in storage than live vaccines. However, killed vaccines produce a much weaker, more unstable immunity than live vaccines and multiple doses may be required to maintain protection. Killed vaccines are also more likely than live vaccines to cause allergic reactions in birds. Finally, giving killed vaccines is much more labor intensive since they must be administered by injection.

Recombinant-vector vaccines

Recombinant-vector vaccines are made by removing the genes from the pathogen that direct cells to produce antigens and then put these genes (recombine them) into the DNA of a non-pathogenic microbe (called a vector). The newly engineered vector is then used to infect the host, where the vector will replicate and express the antigens of the virulent pathogen resulting in an immune response (Prescott et al., 2005). The biggest advantage to this vaccine type is that the newly created vector is live, so that it can be used in a similar manner to other live vaccines, but usually producing milder symptoms following vaccination. The fowl pox virus is one microbe that is used as a vector. One commercial recombinant-vector vaccine combines fowl pox and Marek’s Disease. The vaccine protects birds from fowl pox as a live virus, but also contains genes (DNA) from Marek’s Disease Virus so that birds are protected from both diseases.

DNA vaccines

DNA vaccines produce what is sometimes called genetic or DNA immunization. DNA vaccines are made by isolating the genes (the DNA) that direct the pathogen cell to make antigens. This DNA is then injected directly into muscle tissue. The DNA is then incorporated into the cells within the animal’s body, allowing the animal cells themselves to produce antigens and in turn immunity against the disease (Babiuk, 2007). At present there are no commercial available DNA vaccines for poultry. However, testing suggests the following advantages DNA vaccines: 1. They provide long- lived immunity with a single injection; 2. DNA from several pathogens could be combined so that animals could be protected from multiple diseases with a single injection and 3. DNA vaccines are extremely stable, eliminating the need for refrigeration or special handling (Henahan, 1997). However, many unknowns remain about the practicality of these vaccines in field situations, so it remains to be seen if DNA vaccines against poultry diseases will appear.


In summary, immunity is the ability of the bird’s body to recognize its own tissues (self) and to eliminate foreign (non-self) materials in an immune response. Substances that cause immune responses are called antigens. Since disease outbreaks are expensive, it is important to prevent them and vaccination provides such protection. Live vaccines use altered or diluted microbes to produce long-lasting immunity with a single exposure, but produce symptoms in the bird (vaccine reactions). Killed vaccines do not produce vaccine reactions, but offer much less protection and may require multiple injections. Recombinant-vector vaccines are made by isolating the DNA that encode for antigen production in the pathogen and then placing that DNA in a non-pathogenic, which allows that organism to produce the antigen as it grows in the animal’s body. At present, the use of DNA vaccines seems to hold the potential to help fight most diseases, but questions remain about how these vaccines will perform under field conditions.


Babiuk, L. A. 2007. Modern vaccines. Veterinary Infectious Disease Organization, Saskatoon, SK, Canada http://www.agriculture.de/acms1/conf6/ws5bvacc.htm
Cutler, G. J. 2002. Immunity. In: Bell, D. D. and W. D. Weaver, Jr., eds. Commercial Chicken Meat and Egg Production Kluwer Academic Publishers, Norwell, MA, USA, Pp 443-449.
Henahan, S. 1997. DNA vaccine outlook.
Klasing, K. C. 1998. Nutritional modulation of resistance to infectious diseases. Poultry Science 77:1119-1125.
Okonek, B. A. M. and P. M. Peters. 1997 Vaccines—How and Why.
Prescott, L. M., J. P. Harley and D. A. Klein, eds. 2005. Microbiology. pp. 740-744.
Varela, R. 2007. Vaccines: Understanding immunity and the principles behind vaccination.
Whiting, T. 2005. Understanding immunity and vaccination. Manitoba Agriculture, Food and Rural Initiatives http://www.gov.mb.ca/ agriculture/livestock/dairy/cda20s01.html

November 2007

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