As you and your dog go about your daily chores and routines, the immune system is constantly on patrol, scouring the body for signs of impending invasion and weeding out abnormal or cancerous cells. Just like a healthy heart is needed for a long life, a healthy immune system is also critical to maintain a life free from crippling or life-shortening disease. But what can stop it doing its important role in our canine companions?

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The Immune System

The immune system is a complex of organs and body systems working together to try and prevent disease. It can be thought of as an army defending a castle. The skin acts like an outer wall, and the lining of the airways and digestive tract act like an inner wall. These have ‘doorways’ where invading organisms are either allowed or prevented from getting into the blood and lymphatic system. If an invasion breaks through these barriers, then there are a series of guards and soldiers waiting to defend and counterattack.

The lymphatic system comprises a network of lymph nodes connected by lymphatic pathways, spread across the body. The nodes are filtration and storage centres for lymphatic fluid which is rich in infection-fighting cells. Some nodes are associated with the internal organs, while others lie close to the surface. Anyone that has had tonsilitis will be familiar with the tonsils at the back of the throat. Superficial nodes can be felt across your dog’s body but as with us, they are usually not obvious unless they are swollen in response to some stimulation.

Other body systems also play a part in the immune system

Bone marrow produces white blood cells which patrol our body like soldiers. These white blood cells are primed and ready to kick into action if an invading bacteria, virus or abnormal cell are discovered. Some white blood cells produce antibodies, a protein which blocks the invasion of a specific organism, while others hunt down infected cells. These ‘soldier’ cells are trained in the thymus, a gland near the heart.

Additional components of the defence system are the spleen and liver. The dog’s spleen stores blood, keeping an army of red and white blood cells ready to be released when needed. It also has a filtration system, acting like a guard to filter out abnormal cells from the blood. Meanwhile, the liver acts like an armoury, producing many of the proteins required in an immune response. It also works like a security camera, with its own invasion detection system. With this many parts of the body involved in a functional immune system, there are many potential sites for failure to occur.

Innate vs Acquired Immune Response

In a healthy immune system, there are two types of immune response: innate and acquired. The innate immune system is present from birth. It encompasses the normal function of those structures that make up the immune system, as well as the natural response that is triggered in the presence of any unwanted material within the body. It is the fastest part of the immune response and is responsible for the inflammation and fever that occur in the early stages of an infection. But it is not specific to the type of invasion and can be overwhelmed.

Acquired immunity (sometimes called adaptive immunity) is slower and has to be learned

This begins when the body deals with a disease for the first time or after a vaccine is given. It involves the production of disease-specific proteins, notably antibodies, that are targeted towards that specific invasion. When the same disease is met again, the previously trained white blood cells will respond more efficiently and quicker.

A lack of acquired immunity, or an underdeveloped innate immunity could be described as a weak immune system. This compromises an individual’s ability to respond appropriately to disease. Dogs with a weak immune system will typically develop chronic or recurrent external or internal infections which could shorten their life expectancy. They may also be prone to allergies or multiple immune-derived diseases. Several immune compromising disorders are known to occur in dogs.

Congenital Disorders

There are several hereditary genetic conditions that prevent the immune system from working correctly. They are usually passed through affected blood lines and by definition affect the individual from birth. These congenital conditions affect the immune system directly and are referred to as primary disorders.

Some breeds, therefore, have an increased risk of immune compromise

A rare mutation in some Irish setters causes their white blood cells to be unable to fight off infection (canine leukocyte adhesion deficiency). Beagles, shar-peis and German shepherds have been linked to an inability to produce antibodies (immunoglobulin deficiency). Basset hounds, Jack Russell terriers and rottweilers have been linked to an absence of ‘trained’ white blood cells with no antibody production (severe combined immunodeficiency). Rottweilers also have an increased susceptibility to parvovirus through an unknown form of selective immune deficiency. In this case, they can respond adequately to other infections but not parvovirus.

Acquired Disorders

More commonly, immune compromise occurs as the result of an acquired condition. Any disease or injury that affects the function of the individual components of the immune system have the potential to reduce the immune response. External burns or wounds can compromise the skin barrier, while damage to the lining of the airway or digestive system can reduce the physical defences found there.

In elderly animals, the function of the immune system declines

Both the innate and acquired immune responses can be less efficient. This may occur due to the thymus, the cell training centre, shrinking with age; but also as  white blood cells reduce in number or become slower to react. This makes elderly dogs more prone to disease and potentially less capable of fighting off infections.

Reduced white blood cells

The bone marrow’s ability to make white blood cells can be disrupted by radiation, among other factors, but infection is probably the most common cause of “sudden onset” immune malfunction.

One of the most common infections to trigger immune deficiency in dogs is parvovirus. This virus attacks multiple systems in the body, one of which is the bone marrow. Here it prevents new white blood cells from growing, leading to a reduced number of patrolling white blood cells in the blood. Parvovirus also causes severe damage to the lining of the digestive system, essentially breaking down the internal wall of the ‘castle’ allowing invasion.

Another virus of dogs, canine distemper virus, can also attack white blood cells, leading to a decline in antibodies. As with parvovirus, this disease can allow other infections, known as secondary infections, to attack the body in the wake of the initial virus infection. As a result, infected animals can die from overwhelming infection or multi-infections. 

Damage from within

Other diseases that can compromise the immune system include some types of blood or bone marrow cancer, and some forms of autoimmune disease. Autoimmune disease results when your dog’s immune system attacks its own cells. Immune-mediated neutropenia is one form of this where the white blood cells (specifically the neutrophil) are destroyed. These diseases lead to a reduction in the number of circulating white blood cells. Diabetes mellitus, a common hormonal problem, also impacts the white blood cells, making them less functional.

Some commonly used medications impact the immune system

Many anti-cancer treatments can rapidly reduce the numbers of white blood cells, weakening the body’s defence system. Steroids at high doses can impact the blood cells in circulation, as well as have an impact on liver health. These drugs are used at high doses to suppress the immune system in cases of autoimmune disease so they can be the treatment for, or the cause of, different types of immune compromise. Your vet will discuss the risks of using these drugs if they need to be prescribed to your dog.

Conclusion

The immune system is both complex and hard-working, always on the lookout to keep the body healthy. Multiple conditions exist that can affect your dog’s ability to fight off disease. If your dog has one of these conditions, your vet can discuss with you how this might impact their life and what treatment options may be available. They can also advise how best to protect your dog from future infections.

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