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Perm State
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Pointers
IMMUNE RESPONSES
H. S. Siegel
Professor
Department of
Poultry Science
i
Response to stress by poultry
depends on the integration of the
nervous and hormonal systems.
Corticosteroids, which are pro
duced J>y the adrenal glands, and
stress reduce the size of lymphatic
glands and the numbers of circulat
ing lymphocytes, the cells respon
sible for producing antibodies and
other immunological functions. At
the same time, the number of
heterophils, a second cell species,
is increased.
The avian immune system has
multiple functions. The “B
lymphocytes” (bursa-derived)
produce antibodies when activated
to plasma cells. The “T
-lymphocytes” (thymus-derived)
function in cell-mediated immuni
ty and act as effectors to modulate
proliferation and function of other
immune cells.
Some T-cells directly- attack
foreign molecules. In addition,
macrophages, monocytes, and gra
nulocytes in blood and tissues are
antigen-presenting cells or
scavenger-type cells that absorb
foreign material or bacteria.
Mechanism And
Effects Of
Corticosteroids
On Immune Cells
Corticosteroids form a complex
with receptors in lymphoid cells
and then pass into the nucleus of
the cell to influence genetic orga
nization. Stress increases the
amount of corticosteroid bound in
lymphoid tissue cells.
Cellular and hormonal
responses begin shortly after anti
gens, such as Salmonella or E. coli,
enter the body. The time and sequ
ence of this response depend on the
antigen type, the method by which
the antigen enters the body, and the
amount of antigen.
For example, within 13 minutes
after intravenous injection of Sal
monella pullorum antigen, plasma
corticosteroid begins to rise and
reaches a peak within 3 hours. This
response is due to ACTH stimula
tion by the brain neurohormone,
corticotropin-releasing hormone.
Certain T-lymphocytes move
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from the blood to the spleen ot
birds three to six hours after anti
gen is given, and are genetically
modified through the major histo
compatibility complex.
Stress And
Immunity
Stressors such as heat, cold,
crowding, toxins, or behavioral
activities depress circulating anti
bodies and cell-mediated
immunities.
A. Thermal Stress Exposure
to temperature above 95 degrees
Fahrenheit or injection with
ACTH has been shown to depress
circulating antibody against a vari
ety of antigens. However, the
response is modified by prestress
administration of suppressors of
corticosteroid synthesis, which
demonstrates that immunosup
pression is controlled by corticos
teroids. Suppression of cell
mediated types of immunity has
been demonstrated in fowl
exposed to heal or cold. However,
effects of acute thermal stress are
not always consistent. Such varia
tions have been ascribed to various
factors.
1. Genetic differences in the
ability of some birds tc produce
antibody; or genetic differences in
ability to respond to stress hor
mones. Lines selected for high
antibody response appear to be
more intensely affected by high
temperature.
2. Nutrition interacts with
environmental temperature to
increase or decrease immune sta
tus. Restricting feed intake to
growing birds if temperature falls
below 45 degrees F., or exposing
birds to sudden temperature
change, results in depressed anti
body levels. Deficiencies of a
number of vitamins, amino acids,
or minerals have been found to be
detrimental to immune function.
3. Antigen concentration may
also influence whether stress alters
immunity. Stress depresses
immune response when low doses
of vaccines are given, but fails to
have an effect when high doses are
given. For this reason, producers
should be careful to give recom
mended doses and concentrations
of vaccines and not try to dilute the
dose or allow the vaccines to
degrade due to poor storage condi
tions. Also, the effects of vitamin
deficiency on immunity are greater
at low antigen doses than at higher
doses.
inary
4. The immune system inter
acts with hormonal and nervous
systems. In birds, a basic response
to cold is increased metabolism.
Primary regulators of metabolism
are the thyroid hormones, which
also increase antibody production.
However, blood levels of thyroid
hormones decline immediately
following antigen injection.
Therefore, while antibody levels
may increase if birds are exposed
to moderately low temperatures,
when temperatures, become low
enough to exceed the ability of the
bird to produce sufficient heat,
stress assumes greater importance.
Such a condition occurs if energy
reserves are insufficient, if the nor
mal insulative abilities of the bird
are circumvented, or if the temper
ature control of the bird is
immature.
B. Social Environments
Stressful social environments,
such as crowding, hysteria, or
aggressive behavior, reduce anti
bodies against a variety of bacter
ial antigens including Salmonella
and E. coli, and against such viral
antigens as Newcastle and
Marek’s disease.
Birds selected for high adrenal
response to social stress bind high
er levels of corticosteroid in their
lymphatic tissues and therefore are
more susceptible to stress-induced
immune depression. Antibody to
Salmonella pullorum degrades
more rapidly in less aggressive
families in socially unstable
flocks.
Effects of stress on disease
resistance seem contradictory.
Resistance to mycoplasma infec
tion, Newcastle disease, hemor
rhagic enteritis, or Marek’s disease
is depressed in birds exposed to
Stress And
Disease Resistance
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social conflict, but social conflict
actually increases resistance to E.
coli or S. Aureus in fowl.
Similarly, resistance to parasitic
infection is increased by corticos
teroid feeding or social stress. On
one hand, cold exposure increases
susceptibility of chickens to Sal
monella infections, while similar
conditions increase resistance to
Pasteurella multocida and New
castle disease.
The reasons for these contradic
tions are complex. The effects of
corticosteroids on a particular
pathology and the associated
immune responses are important
in addition to depressing antibody
and cell-mediated immunities,
corticosteroids also reduce inflam
mation. Therefore, in diseases
where the pathology involves local
Government Must Support U.S.
Agriculture During Transition Period
WASHINGTON. D.C. The
U.S. Government must maintain
appropriate policies and pro
grams, including the domestic and
export funding that will allow
agriculture to compete with the
major exporting countries that will
seek to maximize their market
positions during the GATT transi
tion period, Wayne Boutwell, pre
sident of the National Council of
Farmer Cooperatives, told the
House Ways and Means Trade
Subcommittee recently.
The agriculture leader told law
makers that “agriculture, perhaps
more than any other sector of the
U.S. economy, understands that
its growth potential lies in the glo
bal marketplace. As that market
place becomes less restrictive, the
comparative advantage of the U.S.
food and agriculture system will
reach its full potential,” Boutwell
said. “However, it is crucial that
U.S. trade policies stay in step
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or general inflammation, stress
often appears beneficial, even
when immunity is lower.
However, where defense
depends on inflammation to local
ize infection, stress-caused
increases in corticosteroids leads
to invasion of the tissue, especially
when immunity is also comprom
ised. Moreover, stress-induced
reductions in blood lymphocytes
and increases in heterophils (see
the first paragraph) will increase
resistance to bacteria but decrease
resistance to viruses.
Therefore, although immune
responses are fundamental to dis
ease resistance and do reflect the
bird’s response to its environment,
changes in disease resistance are
not always indicative of stress.
with the evolving marketplace.”
Boutwell identified several is
sues of concern to members of the
National Council of Farmer Co
operatives, including the disci
plining process for export subsi
dies, the transition for commodi
ties that have tariffied Section 22
protections, special treatment un
der the tariffication process, scien
tific standards for sanitary and
phytosanitary regulation, and'the
appropriate role of government in
agriculture and food policy.
‘The recurring theme through
most of these concerns is the wil
lingness of the U.S. government to
support the interests of U.S. agri
culture,” Boutwell said. “U.S.
agriculture must have government
support within the prescribed lim
its if it is to compete effectively
during and after the transition per
iod. With these issues addressed,
NCFC is prepared to support the
implementing legislation.”
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