Blood and Immune System of Birds
The avian circulatory system is a life-sustaining collection of vessels and blood, powered by a rapidly beating, four-chambered heart. It functions to pump blood around the body and supply the bird’s cells and tissues with oxygen and nutrients while removing waste products produced through metabolism. However, it also plays another crucial role in transporting antibodies and immune cells that fight off infections and destroy abnormal cells.
From the frozen poles to the thin air of high altitudes, birds exist in a wide variety of climates and habitats, some of which require specialized blood properties for effective respiration in such extreme environments. They also face a host of parasites and pathogens like viruses and bacteria, which they overcome through innate and adaptive immune responses.
In this guide, we’ll take a closer look at bird blood and learn about the efficient avian immune system.
Avian Blood – Composition and Functions
Red and White Blood Cells
Bird blood contains erythrocytes (red blood cells) and leukocytes (white blood cells). The erythrocytes have an oval shape, complete with a nucleus. Their size varies greatly depending on species, but they are typically larger than the round mammalian red blood cells. The percentage of erythrocytes in the blood is known as hematocrit, and it varies depending on species and individual health.
White blood cells are vital for the avian immune system. There are various types of leukocytes, and these cells perform multiple roles, including starting an immune response, capturing and digesting foreign bodies and pathogens, and inflammatory responses.
Types of white blood cells
- Lymphocytes
- Heterophils
- Eosinophils
- Basophils
- Monocytes
Hemoglobin and Oxygen Transport
Birds have very high metabolic rates, and high-intensity activities like flight massively increase their oxygen demand. An efficient system of air sacs and rigid lungs creates the interface between oxygen and blood, but special structures within the blood are required for transporting this vital gas.
Hemoglobin is a special molecule in a bird’s red blood cells that collects oxygen and transports it through the circulatory system. All birds have these vital iron-containing molecules, but some high-altitude species have special hemoglobin with an even higher oxygen affinity.
Platelets and Clotting
Bird blood also contains thrombocytes or platelets, which are like the body’s natural band-aid. When tissue is damaged and small blood vessels rupture, thrombocytes gather at the scene and react with collagen, which changes their shape and helps them block the leakage. They also become ‘sticky’ and adhere to the walls of the blood vessels and other cells to help blood clot.
Birds have very high metabolic rates, and high-intensity activities like flight massively increase their oxygen demand. European Roller
The Avian Immune System
Innate Immunity
From the moment a bird hatches out of its egg, it has systems in place that protect it from infection. The skin creates a physical barrier to air and waterborne pathogens, but birds also rely on mucus membranes and chemicals like enzymes and acids to neutralize threats taken in through the mouth.
When pathogens get past the bird’s external lines of defense and infection does occur, the immune system launches an inflammatory response. Inflammation happens when white blood cells and other protective elements are released into tissues around the wound or infection site to attack pathogens and prevent them from spreading into surrounding areas.
Adaptive Immunity
If an inflammatory response fails, birds have another option to target harmful pathogens. The adaptive immune system targets known enemies like specific pathogens using specialized immune cells called lymphocytes and proteins called antibodies.
Immune System Organs
Various organs are involved in the avian immune system. A bird’s spleen is proportionally smaller than in mammals, but it remains a crucial immune system component. This organ filters blood to destroy pathogens and diseased cells and produces lymphocytes.
Birds produce two types of lymphocytes. T-lymphocytes (T cells) target abnormal or infected cells and are programmed in the thymus, which is located in the neck. B-lymphocytes (B cells) are produced in the bursa of Fabricius, located near the cloaca. These special cells produce antibodies.
Immune Responses to Pathogens
Disease Recognition
Before it can launch a response, the avian immune system must identify the pathogen, cell, or toxin as a threat. T-cell receptors, antibodies, and receptors in platelet cells detect potential threats (antigens) and sound the alarm.
Vaccination and Immunity
Various vaccines are available for domesticated birds, including poultry and exotic pet species. Parrots, for example, may be vaccinated against polyomavirus, while Chickens may be vaccinated against Newcastle disease, Marek’s disease, and fowl pox.
Vaccinating birds not only protects the individual but also protects other members of the flock and helps prevent outbreaks among other birds in the area.
African Grey Parrot. Various vaccines are available for domesticated birds, including poultry and exotic pet species
Nutritional Support for Blood and Immunity
Diet and Immune Health
All birds need access to an appropriate, species-specific diet to grow and remain healthy. A lack of energy and nutrients results in stress, which can leave birds vulnerable to infection. Fighting off an infection also requires nutrients, so malnourished birds are at a clear disadvantage when infection occurs.
Supplements and Fortified Feeds
Birds rely on various vitamins and minerals to maintain a healthy immune system. Vitamins A, C, D, and E are considered particularly important for immune functioning in Chickens.
Free-ranging commercial poultry is not always practical, so these birds are often fed prepared foods made primarily from farmed grains. These foods may be fortified with trace minerals and vitamins to support immune functioning and other physiological processes.
Various natural feed additives, including roots and essential oils, are often given to poultry to improve their health, production, and immune response.
Many natural products have demonstrated benefits, although at least one study has found that some natural additives can have negative effects on chicken production.
Common Avian Blood and Immune Disorders
Diseases and Disorders
Various ailments can negatively affect bird immune systems and blood, including the following examples:
Stress
Various forms of stress, including social and environmental stress, can have an immune-suppressing effect on birds.
Marek’s Disease Virus
This highly virulent virus has many serious symptoms. It affects the lymphocytes and reduces or completely disables the immune response of chickens, leading to an increased risk of serious infections.
Blood Parasites
A surprisingly diverse array of blood parasites have been identified in birds, and they infect a large proportion of individuals. Examples include Plasmodium (avian malaria) and other protozoans spread by mosquitos and other biting insects. The effects of blood parasites range from very mild to fatal, depending on factors like the species involved and the bird’s condition.
Signs of Immune Health
Birds with circulatory ailments may be affected by limited oxygen delivery and show symptoms like shortness of breath and inability to perform intense exercise.
A compromised immune system may be more difficult to identify because affected birds would be vulnerable to various pathogens and diseases. These may manifest in a wide variety of signs and symptoms depending on the nature of the disease.
Birds rely on various vitamins and minerals to maintain a healthy immune system. Vitamins A, C, D, and E are considered particularly important for immune functioning in Chickens
Research and Advances in Avian Immunology
Since Louis Pasteur developed the first avian vaccine for bird cholera in 1879, scientists have developed many more life-saving vaccines and learned much more about the functioning of the avian immune system.
Most research has been conducted on the domestic chicken due to its importance as a food resource and the fact that poultry carry pathogens that can infect humans.
However, scientists are also learning more and more about the immunology of wild birds and studying the impacts of novel threats like the evolving avian influenza virus.
Summary
Inside each bird, there is an incredibly complex collection of systems that work together to ensure survival and the chance to reproduce. Without healthy blood and a functioning immune system, birds could not survive, and our world would be a much quieter place!
There is still much to learn about bird health and immune responses, both for the conservation of wild populations and for protecting poultry farming for food security.
As the demand for poultry increases and viruses continue to evolve, new technologies must be developed to safeguard birds now and into the future.