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Melanie A. Fratto, Andrew K. Davis, Vanessa O. Ezenwa
Odum School of Ecology, University of Georgia
Juvenile House Finch with Mycoplasmal conjunctivitis
Melanie with a captured House Finch
Abstract
A paradox of the immune response to stress is
that under stress, the ratios of phagocytic
y
increases,, but the abilityy of blood
leukocytes
cells to kill bacteria decreases. In this study,
house finches, Carpodacus mexicanus, infected
with Mycoplasma gallisepticum and ones with
no obvious infection were captured to study this
paradox. This bacterial infection is common
among finches and can be spotted by the
development of red, swollen eyes. The effect of
stress from capturing and holding wild house
finches for two hours was assessed by counting
white blood cells and running an assay in which
plasma is exposed to E. coli to measure the
innate killing ability of blood. The ratio of
heterophils to lymphocytes is an accurate
measure of stress and is frequently used in
stress studies, and this ratio was compared in
pre-stress
pre
stress and post-stress
post stress samples from the
same bird. The results of this study show that
the two hour holding period increased the stress
levels of both infected and uninfected birds, and
that an existing infection alters the effects of
stress on immune function. This study will add
to the current understanding of how stress
affects an animal’s immune function and,
therefore risk of infection
therefore,
infection.
3 min and 2 hr plates from same bird
Background
• White blood cells are part of the first line of defense
against infection. All animals, including humans, have five
white blood cell types
types. Lymphocytes are a common type of
white blood cell that responds to attacks on the immune
system. In birds, heterophils are the main type of
phagocytic cell, meaning they engulf and destroy infectious
material. Other white blood cells present include
monocytes, basophils, and eosinophils.
• House finches are small songbirds, or passerines. They
can be found throughout the country, in urban areas and
backyards. They are also highly susceptible to a bacterial
infection called mycoplasmal conjunctivitis (Mycoplasma
gallisepticum). With this infection, the birds develop swollen,
red eyes filled with fluid, making it easy to tell whether or
not birds are infected with the disease.
The main questions addressed by this study
are:
• How does holding a bird for two hours affect
stress level as measured by the change in its
heterophil to lymphocyte ratio?
• How does stress affect immune function and
white blood cell counts in birds infected with a
pathogen versus those uninfected?
Methods
• 35 birds, 9 of which were infected, were trapped over a one month
period during the summer of 2011, using mist nets around feeders.
•An initial blood sample was collected within 3 minutes of the bird
being caught in the net and a blood smear was made.
Example white blood cells as seen under a microscope
•The birds were held for 2 hours (in paper bags with air holes), then
a second blood sample was collected and another smear made.
•The plasma from each blood sample was saved in a -80°C freezer.
• A standard white blood cell count was done for each slide.
A E.
E colili killi
killing assay was run on each
h plasma
l
sample
l and
d the
h
•An
number of colonies present on each plate after a 24 hour period
were counted.
Melanie examining plates with bacterial colonies
•Data were transformed to meet the assumptions of normality.
Statistics were calculated using analysis of variance (ANOVA) tests.
Results
Key Questions
FIG. 1: The holding period successfully increased the
stress levels of both infected (F1,16 = 7.0, p = 0.02) and
uninfected (F1,50 = 4.8, p = 0.03) birds. Heterophil and
lymphocyte levels were obtained from the blood smears.
Example plate with bacterial colonies
Conclusions
FIG. 2: Stress did not
have a strong effect
on percent bacteria
killed, but infected
birds tended to kill
more bacteria. White
blood cell counts
decreased with
stress in uninfected
birds (F1,50 = 32, p <
0.001), but there was
no effect of stress
on blood counts on
infected birds. The
same pattern was
observed for number
of heterophils, with
infected birds
showing no response
to stress,
stress and
uninfected birds
showing a significant
decline with stress
(F1,50 = 5.1, p =
0.03).
• Initial stress levels of infected birds were similar to the final
stress level of uninfected birds (Fig. 1). The infected birds’
stress level then increased to even higher levels with
holding. This shows that having an infection is a source of
stress for the bird and an added disturbance increases that
stress level further.
• In response to stress, percent bacteria killed and numbers
of WBCs and heterophils declined in uninfected birds, while
these values stayed relatively constant in infected birds
(Fig. 2).
• In combination, these results imply that the immune
systems of infected birds are already stimulated by the
infection and are not altered in the same ways by stress.
The number of WBCs may remain at a high level because
of the demands on the immune system
y
to fight
g the
infection.
Acknowledgements
•We thank Sergio and Heather for their help trapping house finches. We
also are grateful to Dr. Sonia Altizer for the use of lab space and materials.