10 UNIVERSITY OF
Florida Cooperative Extension Service
Disease Prevention in Commercial Poultry1
Gary D. Butcher and Richard D. Miles2
Prevention of disease in commercial poultry
requires the producer to actively enforce an
effective/comprehensive biosecurity program and to
maintain an intact and functional immune system in
Biosecurity is a commonly used poultry industry
term that can be defined simply as "informed common
sense." That is, one develops a basic understanding
of the principals of disease transmission and combines
this knowledge with good old "common sense." The
objective would be to have a program design such
that diseases are not brought onto the poultry farm
and poultry are not brought to diseases. An effective
biosecurity program allows one to keep diseases off
poultry farms; or if disease organisms are present,
such a program would eliminate them or at least
reduce them to a level of little or no significance.
Poultry veterinarians have been attempting to
control diseases by improving biosecurity practices.
This emphasis on controlling diseases by biosecurity
practices rather than relying on vaccines and/or
antibiotics has resulted due to changes in the industry
itself. As poultry farms became larger and more
intensive, disease outbreaks became more costly; as
the lifespan of broilers decreased due to improved
genetics and feeding, birds did not have sufficient
time to recover from diseases and make it to
Veterinarians often find it difficult to convince
many farm managers of the importance of biosecurity
programs. The lack of support for these disease
prevention programs, which many farm managers may
see as costly, time consuming, and just more
unnecessary work, is probably due to the failure of
previous programs. However, the failure of previous
efforts was likely due to poor design and improper
implementation of the programs. A comprehensive
biosecurity program cannot eliminate the possibility of
disease, but it can reduce the probability. In addition,
often it is not possible to demonstrate direct benefits
from a biosecurity program from just one flock.
Improved production usually occurs gradually over
Understanding how diseases are transmitted is an
important factor in developing a biosecurity program.
Studies have consistently demonstrated that
approximately 90 percent of the time poultry diseases
spread from one farm to another by contaminated
people, poultry equipment, and farm vehicles.
Exceptions to this include direct ovarian transmission
(example: Mycoplasma gallisepticum), eggshell
penetration (example: Salmonella), and hatcher
contamination (example: Aspergillus sp.). Airborne
transmission of poultry diseases is not considered to
be an important means of disease transmission. For
example, Mycoplasma gallisepticum is horizontally
transmitted by direct contact between carriers and
susceptible chickens, and by airborne dust or droplets
very short distances(such as between cage rows or
pens within a house, but not between houses or
1. This document is Circular 1079, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida.
Publication date: June 1993.
2. Gary D. Butcher, DVM, PhD, Associate Professor, and Poultry Veterinarian, College of Veterinary Medicine; and Richard D. Miles, PhD,
Professor, and Poultry Nutritionalist, Poultry Science Department; Institute of Food and Agricultural Sciences, University of Florida, Gainesville.
The Institute of Food and Agricultural Sciences is an equal opportunity/affirmative action employer authorized to provide research,
educational information and other services only to individuals and institutions that function without regard to race, color, sex, age, handicap,
or national origin. For information on obtaining other extension publications, contact your county Cooperative Extension Service office.
Florida Cooperative Extension Service / Institute of Food and Agricultural Sciences / University of Florida / John T. Woeste, Dean
JTVw PCF FIMRIVA ur'"AmRc
Disease Prevention in Commercial Poultry
M. gallisepticum is spread from one house to
another, or from one farm to another by
contaminated equipment, vehicles, and people.
Studies with other poultry diseases have further
demonstrated that airborne transmission is not a
significant means of disease transmission between
Another important factor in developing a
biosecurity program is determining the stability of
poultry disease organisms in the environment. Table
1 includes the common poultry diseases and the time
period for which they remain viable in the poultry
house environment following the removal of chickens.
Table 1. Diseases of chicken
away from chicken.
away from birds
Hours to days
Days to weeks
Days to weeks
Hours to days
and lifespan of disease
Down time, which includes the period of time
between successive flocks when no chickens are
present on the premises, has been used as a means to
reduce the level of disease organisms in poultry farms.
Increasing down time, however, may be of only
limited value in reducing/eliminating specific diseases.
As Table 1 illustrates, fragile organisms such as M.
gallisepticum orHemophilusparagallinarum (infectious
coryza) remain viable for approximately 3 days outside
the chicken. In these cases, a one- week down period
would be very effective in eliminating these diseases
prior to bringing new chickens onto the premises.
However, for disease organisms such as infectious
bursal disease virus or the coccidia, which are very
resistant to environmental factors, increasing down
time would be of limited value. In these cases
thoroughly cleaning and disinfecting the premises
would be necessary to reduce the level of these
L-TM The objective of any poultry management
program should be disease prevention through
effective biosecurity practices. If there is a
breakdown in biosecurity, and a disease outbreak
occurs, be sure the chickens are immunologically
competent. This will limit the resulting losses.
Poultry producers need to understand the function of
the immune system to assure that its integrity is
maintained and that full advantage of its protective
capability is utilized.
The avian immune system is divided into non-
specific and specific immune mechanisms.
Non-specific immune mechanisms include the
innate or inherent ways in which the chicken resists
disease. This protective system is often not
considered when designing a poultry health program.
Many programs tend to rely primarily on vaccinations
and/or antibiotics to maintain flock health. The
importance of non-specific immune mechanisms
should be realized. Examples have been included.
Genetic factors -- birds may not have
complementary receptors to allow many disease
organisms to infect them. For example, some
strains of chickens are genetically resistant to the
lymphoid leukosis virus.
Anatomic features -- many disease organisms
cannot penetrate intact body coverings (skin and
mucous membranes) or are trapped in the mucus
secretions. Some nutritional deficiencies (biotin
deficiency) or infectious diseases compromise the
integrity of the body coverings, allowing
penetration of disease organisms.
Normal microflora -- the skin and gut normally
maintain a dense stable microbial population.
This stable microflora prevents invading disease
organisms from gaining a foothold. Improper use
of antibiotics or poor sanitation can disrupt the
balance of the microflora.
Respiratory tract cilia -- parts of the respiratory
system are lined with cilia, which remove disease
organisms and debris. If the air in the poultry
house is of poor quality due to high levels of dust
or ammonia, the ciliary system may be
overwhelmed and become ineffective.
Other factors involved in innate resistance include
nutrition, environment (avoid heat/cold stress), age
(young/old animals are more susceptible to disease),
Disease Prevention in Commercial Poultry
inflammatory processes, metabolic factors,
complement, and interferon.
The reason that good management practices are
important in maintaining poultry health is better
understood when the non-specific immune
mechanisms are defined. For example, poor
sanitation or the overuse of antibiotics may lead to a
disruption of the normal microflora; poor nutrition
may lead to deficiencies that allow disease organisms
to penetrate the protective body coverings; selection
of disease resistant strains of chickens may preclude
or lessen the effects of certain diseases.
In contrast, specific immune mechanisms
(acquired system) are characterized by specificity,
heterogeneity, and memory. This system is divided
into non-cellular humorall) and cellular components.
The non-celluar components include
immunoglobulins (antibodies) and the cells that
produce them. Antibodies are specific (specificity) for
the foreign material (antigen) to which they attach.
For example, the antibody against Newcastle disease
virus will attach only to the Newcastle virus, noi to
the infectious bronchitis virus (heterogeneity). There
are three classes of antibodies that are produced in
the chicken after exposure to a disease organism: Ig
M, Ig G, and Ig A.
Ig M appears 4 to 5 days following exposure to a
disease organism and will disappear in 10 to 12 days.
Ig G is detectable 5 days following exposure, peaks at
3 to 3 1/2 weeks, and then slowly decreases. Ig G is
the important protective antibody in the chicken, and
is measured by most serologic test systems. Thus, if
you are interested in determining antibody titer levels
following vaccination, you should collect sera after 3
to 3 1/2 weeks. If sera is evaluated prior to this time,
the antibody titer levels are still increasing, which
makes interpretation of the vaccination program
difficult. Ig A appears 5 days following exposure,
peaks at 3 to 3 1/2 weeks, and then slowly decreases.
This antibody is found primarily in the mucus
secretions of the eyes, gut, and respiratory tract, and
provides "local" protection to these tissues.
The cells that produce antibodies are called B-
lymphocytes. These cells are produced in the
embryonic liver, yolk sac, and bone marrow. The
cells move to the bursa of Fabricius (BF) at 15 days
incubation and remain there through 10 weeks of age.
The BF programs these cells, which then move to the
gland, and thymus. Destruction of the BF at a young
age by Gumboro disease virus or Marek's disease
virus prevents the programming of B-cells. Thus, the
chicken will not be able to respond as effectively to
diseases or vaccinations by producing antibodies.
When a disease organism enters the body, it is
engulfed by a phagocytic-type cell, the macrophage.
The macrophage transports the disease organism and
exposes it to the B-lymphocytes. The B-cells respond
by producing antibodies 5 days following exposure.
The lag period occurs because the B-cells must be
programmed and undergo clonal expansion to
increase their numbers. If the chicken is exposed a
second time to the same disease, the response is
quicker, and a much higher level of antibody
production occurs (memory). This is the basis for
Antibodies do not have the capability to kill
viruses or bacteria directly. Antibodies perform their
function by attaching to disease organisms and
blocking their receptors. The disease organisms are
then prevented from attaching to their target cell
receptors in the chicken. For example, an infectious
bronchitis virus that has its receptors covered with
antibodies will not be able to attach to and penetrate
its target cells, the cells lining the trachea. The
attached antibodies also immobilize the disease
organism that facilitates their destruction by
The cellular component includes all the cells that
react with specificity to antigens, except those
associated with antibody production. The cells
associated with this system, the T-lymphocytes, begin
as the same stem cells as the B-cells. However, the
T-lymphocytes are programmed in the thymus rather
than the BF.
The T-lymphocytes include a more heterogeneous
population than the B-cells. Some T-cells act by
producing lymphokines (over 90 different ones have
been identified); others directly destroy disease
organisms. Some T-cells act to enhance the response
of B-cells, macrophages, or other T-cells (helpers);
others inhibit the activity of these cells (suppressors).
The cellular system was identified when it was shown
that chickens with damaged BF could still respond to
and eliminate many disease organisms.
A chicken may become immune to a disease
organism by producing antibodies itself or by
blood, spleen, cecal tonsils, bone marrow, Harderian
obtaining antibodies from another animal. When the
Disease Prevention in Commercial Poultry
chicken produces its own antibodies following
exposure to a foreign material, the process is called
active immunity. This occurs after the bird is exposed
to a vaccine or a disease. Active immunity is harmed
by anything that damages the cellular or humoral
When the chick receives pre-made antibodies
from the hen through the egg, this is termed passive
immunity. These antibodies are not produced by the
chick. Maternal antibodies are present in the yolk,
albumin, and fluids of the egg. If the hen has a high
antibody titer level to a disease, the chick should also
be immune for several weeks. However, since the
immune system of the chick is not stimulated, there
will be no antibodies produced by the chick and no
The flock manager must be aware of the maternal
antibody levels in the chicks to schedule vaccinations.
If chickens are vaccinated when maternal antibody
titer levels are elevated, the vaccine may be buffered
excessively, resulting in a reduced response.
Conversely, if vaccinations are delayed and maternal
titer levels are low, a severe vaccine reaction may
result. Chickens may also be susceptible to diseases
as maternal titer levels will be low, and approximately
12 days is required following vaccination before
minimal protective antibody levels are produced.
In summary, the immune system of the chicken is
very helpful in preventing disease and helping to
ensure that maximum productive potential is realized.
We must learn how to take advantage of all parts of
the system when designing health programs.
Routinely, serum samples are submitted to a
poultry diagnostic laboratory to determine antibody
titer levels as an aid in the diagnosis of disease or as
part of a routine monitoring program. However, it is
important to keep in mind that the ELISA serologic
test system commonly used measures only Ig G levels
in the blood. No determination is made of Ig A
(local protection), Ig M (early protection), cell-
mediated immunity, or the non-specific immune
system. Although serology can be very useful in a
poultry health program, it is important to understand
its limitations. Table 2 lists common poultry diseases
and the part of the immune system considered to be
the primary protective mechanism in controlling the
ELISA serology, commonly used in the poultry
industry, has limitations. Some of these include the
Measures Ig G response only, not Ig A, Ig M,
CMI, or the non-specific immune mechanisms.
Must have paired sera to make determinations
Must have an organized bleeding schedule
Antigenic specificity may lead to inaccurate
Serum samples must be properly selected
(randomly, sufficient number).
Selection of birds is critical (representative of the
disease problem diagnostic; or of the flock -
monitoring). Lack of consistency of results
The development of an infectious disease depends
on three variables: 1) resistance of the chicken, 2)
virulence of the disease organism, and 3) dosage of
the organism to which birds are exposed. Through
effective biosecurity practices, the dosage of the
disease organism is reduced or even eliminated; and
through proper vaccination practices, the resistance of
the bird can be increased. The only factor over which
there is little control is the virulence of the disease
organism in the field.
Primary Protective Mechanism/Disease
Ig A Ig CMI
Inf. Bursal Dis. x
Newcastle Dis. x x
Inf. Bronchitis x
Marek's Dis. x
Fowl Pox x
Table 2. Common poultry diseases and that part of
the immune system mechanism that primarily controls