Booklet Group 22 English Version

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Immune adaptation in birds

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Immune adaptation in birds

By Ewan Hilbrands, Anna Lauxen, Bart-Jan Mazenier, DouweMul and Patrick Werndlij.

Professional assistance provided by dr. K.D. Matson



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Table of contentsThe standard avian immune system ........................................ 5

Measuring the immune system ................................................ 7

Size of immune system organs ............................................. 7

White blood cells .................................................................. 8

Presence of antibodies & cytokines ..................................... 9

Energy cost of the immune system ........................................ 10

Difference in immune system between migrating and non-

migrating birds ....................................................................... 11

Why is there a difference in immune response? ................... 13

Conclusion .............................................................................. 18

References .............................................................................. 19





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Introduction

Avian flu is set to become one of the most dangerous

diseases in the world. With the Spanish flu (H1N1) of 1918

causing approximately 20 million deaths worldwide, and

more recent reoccurrences of H1N1 strain viruses the topic

seems more important than ever. Yet, there is still much

unknown about the avian immune system.

In this flyer, we will be discussing the avian immune system,

and differences between migratory and non migratory birds;

specifically the type of adaptation they have undergone

through evolution.



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The standard avian immune system

To understand differences in the immune system between

migratory and non-migratory species we first have to get a

basic grasp of the “standard” avian immune system.

The recent years have shown remarkable progress in the field.

Most of this research was performed in poultry. From this

research we have learned that the avian immune system

looks similar to that of most mammals, but there are some

important differences.

To explore these differences, we will split the immune system

into two parts; humoral and cellular. The cellular immune

system is regulated by the thymus, which is essential for the

development of for example T-lymphocytes and macrophages.

The humoral immune system, which is responsible for

antibodies produced by B-lymphocytes is regulated by an

organ known as the Bursa of Fabricius. This organ is strictly

avian.

Another important difference can be found in the

development of T-cells in birds. While it is largely similar,

researchers found a type of T-cell that had not been seen

before in any other type of animal. The production and

regulation of T-cells is done by the thymus, Bursa of Fabricius

and spleen.



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Over the past years, research has shown that the immune

response in birds relies on lymphokines. These molecules,

produced by lymphocytes, move to the site of infection and

serve as a target for other parts of the immuneresponse. Not

much research has been done on this subject, because

scientists haven’t been able to clone the genetic code

responsible for these lymphokines yet.

Figure 1 presents an example of where in the body different

organs that make up the immune system are located. It is

notable that they are spread throughout the body rather than

being concentrated in a specific region.

Figure 1. An overview of location of organs responsible for the immune

response in birds.



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Measuring the immune system

In order to objectively talk about differences in immune

response we must establish methods to measure it. In this

section, we will discuss three different methods that have

been used in actual research.

Size of immune system organs

The first method we will discuss is looking at the size of the

different organs mentioned earlier that are relevant to the

immune system. Different species of birds will often have

different sized organs proportional to body size, though there

are some exceptions. Different situations call for differing

amounts of evolution of the organs. The general expectation

would be that larger organs indicate a strong capacity for

immune response, which we will discuss later on.

Figure 2. Size comparison of Bursa of Fabricius. Left is normal sized.



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White blood cells

The second method used by researchers is investigating the

white blood cells; the concentration of white blood cells can

be measured from a blood assay. If the concentration is lower

than average, it would indicate that the immune system is

weakened at the time of the assay. This can have multiple

reasons, discussed later. Another more specific method of

determining the immune system’s fitness is by determining

the ratio of lymphocytes and heterophils. If the heterophil

count is high compared to the lymphocyte count, the system

is considered suppressed. The advantage of using blood

assays is that only a very small amount of blood is necessary

for an assay (~150 µL), which means specific animals can be

studied multiple times over a longer duration.

Figure 3. Graphs showing relevance of heterophil to lymphocite ratio to

presence of immunoglobulin. More immunoglobulin indicates a stronger

immune system.



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Presence of antibodies & cytokines

The third method to get an idea of the fitness of the immune

system is by checking the concentration of antibodies and

cytokines. The usual way to use this method is to first check a

healthy bird’s concentrations, then inject it with LPS

(Lipopolysaccharide), which elicits a strong immune response

in animals. After the injection the concentrations can be

measured again, giving a full picture of how well the immune

system is functioning. The stronger the reaction, the healthier

the immune system. In figure 3, the right hand graph gives a

sample of measured immunoglobulin in animals. The higher

the measured amount, the stronger the animal in question is

with regards to immunity.



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Energy cost of the immune system

It goes without saying that possessing an immune system

costs energy. An organism that chooses to maintain it has to

invest energy in a plethora of immune-cells: Lymphocytes, T-

Cells, antibodies and more. Because energy is limited, it

follows that not every process can run at maximum efficiency.

Growth, reproduction and thermoregulation are all processes

that need a lot of energy.

Multiple experiments have shown that energy reallocation

can have drastic effects on life expectancy: rats raised in

germ-free conditions, eliminating the need for an immune

system, lived longer than rats raised under normal conditions.

In a different study, chicks raised in a germ-free environment

weighed more, used more energy and proteins for growth,

and the efficiency of those proteins went up. In another study,

chickens, guinea fowls and swine were administered

antibiotics, reducing the energy investment required in the

immune system. This resulted in weight gain and increased

nitrogen retention.

It is clear that maintaining an immune system is costly, but

vital. Research by T. Piersma in 1997 showed that most

species try to stay in a habitat that is unfriendly to parasites,

e.g. near the sea where the salt is unfavorable. This results in

a larger budget of energy for other things, and generally

increased fitness.



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Difference in immune system between

migrating and non-migrating birds

In the previous section we discussed the energy costs

associated with the immune system; in this section we'll be

looking at some of the practical implications of these costs. As

preparation for the migration flight, birds have been shown to

convert approximately 50% of their lean body mass (non-fat

tissue) into fat to serve as an energy store. Despite this

preparation, it has been shown that this entire store is

depleted over the course of the migratory flight (Barlein

1985; Moore and Kerlinger 1987).

This drastic change in the bird's body gives us a good idea of

the investment involved in migration. A logical conclusion to

make from this is that other energy-consuming systems are

reduced or shut down. While immunosuppresion may seem

like a vital flaw in survival strategy, it has been shown that

certain organs that play a role in the immune reponse are

more developed than in non-migratory birds; specifically the

spleen and bursa of Fabricius have been shown to be

substantially bigger.



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While their organs have adapted to perform better during

immunosuppression, a study in Swainson's thrushes

(Catharus Ustulatus) by Owen and Moore (2008) has shown

that the immune response is generally lower in birds that

have prepared for migration.

Conserving energy is one of the main benefits of

immunosuppression, but it also serves to lower the impact of

immunopathology during a period of stress. If an infection

were to occur as a result of exercise, the resulting tissue

damage would likely result in the bird's death.

Overall, while at first glance it would seem wise to invest

heavily in a strong immune system due to a larger range of

risk factors, the constraints of energy limit the adaptability of

birds.

Figure 4. Swainson’s Thrush (Catharus Ustulatus)



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Why is there a difference in immune

response?

As explained earlier, migratory birds have a more evolved set

of immune organs. What has led to this evolution though?

The difference can be explained through a multitude of

factors, including climates, food availability and exposure to

pathogens.

Knowing that having and maintaining an immune system has

energy costs associated with it, J.M Fair et al. have shown in

their experiments that when birds are exposed to antigens

their growth significantly decreases. See figure 5.

Figure 5. Growth of Japanese Quails in relation to exposure to antigens.



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When one looks at migration and the necessity of the

immune system, a logical conclusion is that it would be less

evolved due to the priority of everything else; as shown

earlier, this is not correct. In fact, the relevant organs are

more evolved. It seems likely that due to having more evolved

organs the amount of energy required can be reduced.

Another explanation could be that birds lose their Bursa of

Fabricius when they become sexually mature. Therefore

migratory birds have to adapt to a more diverse parasite

fauna before the start of their first migration. As the biggest

energy cost is already fulfilled before migration, the energy

costs during migration might be uninfluenced.

Another explanation can be found in the effect of

temperature on immune response. Migratory birds are

exposed to at least two different climates during their lifetimewhile residential birds are not. It could be that higher

environmental temperatures make it easier for the immune

system to function. This was researched by A.M. Henken et al.

The results showed no effect on the immune system itself,

but it did have an effect on food intake when exposed to

antigens. It could be that because there is more food

available to migratory birds over the course of a lifetime and

because they don’t encounter big temperature differences

there is enough energy intake to maintain their immune

system, while residential birds have less energy intake,

leaving a smaller budget.



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Because migratory birds are more likely to encounter a larger

variety of pathogens during their lifetime due to exposure to

different regions, they need a more evolved immune system

to keep themselves healthy.

Overall, migration increases food availability and allows birds

to stay closer to their preferred environmental temperature,

but comes with the extra invest of energy to maintain the

immune system.



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Figure 6. Table comparing the size of Bursa of Fabricius and spleenin similar migratory and non-migratory birds.



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Conclusion

Overall, research has shown that migratory birds have a more

evolved immune system, though during migratory periods

their immune response will still be lowered due to the

immense energy requirement of migration.

Much is still unclear about the exact workings of the avian

immune system and the adaptations they make to prepare

for migration. The threat of avian pathogens is a very real one,

making it an interesting subject to study for likely years to

come.



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References

J.M. Sharma; ‘Overview of the avian immune system’

Overview of the avian immune system, 30 ( 1991 ) 13-17

Bruce Glick; ‘The Avian Immune System’ uit: AvianDiseases, 23 (1979) 282-289

Péter L. Pap , Csongor I. Vágási, Jácint Tökölyi, GáborÁ. Czirják & Zoltán Barta, Variation in HaematologicalIndices and Immune Function During the AnnualCycle in the Great Tit Parus major, (2010), Ardea98(1):105-112

Katrina G. Salvante, Techniques for StudyingIntegrated Immune Function in Birds (2013), The Auk,

Vol. 123, No. 2 (Apr., 2006), pp. 575-586A.P.Moller, J.Erritzoe, Host immune defenseandmigration in birds,(1998), Paris, Evolutionary ecology,issue 12, p-945-953.

Bairlein F (1985) Body weights and fat deposition ofPalaearctic passerine migrants in the central Sahara.Oecologia 66:141–146

Moore FR, Kerlinger P (1987) Stopover and fatdeposition by North American wood-warblers(Parulinae) following spring migration over the Gulfof Mexico. Oecologia 74:47–54



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A.P.Moller, J.Erritzoe, Host immune defense andmigration in birds,(1998), Paris, Evolutionary ecology,

issue 12, p-945-953.

J.C. Owen, F. R. Moore, Swainson's thrushes inmigratory disposition exhibit reduced immunefunction (2008)

Trade-offs in evolutionary immunology: just what is

the cost of immunity? Robert L. Lochmiller andCharlotte Deerenberg, 2000.

J.M.Fair, E.S.Hansen,R.E.Ricklefs, Growth,Developmental stability and immune response injuvenile Japanese quails (Coturnix coturnixjaponica),(1999) St. Louis, The royal society, issue

266, p-1735-1742.

A.M.Henken, A.M.Groote Schaarsberg, M.G.Nieuwland,The effect of environmental temperature on immuneresponse and metabolism of the young chicken. 3.Effect of environmental temperature on the humoralimmune response following injection of sheep red

blood cells, (1983), Wageningen, Poultry science,issue 62, p-59-67.

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