Your dog’s body is an amazing and complex machine comprised of many systems that work together to help it function. One of the most important systems is the immune system. It leads the fight against infection and disease, keeping our dogs healthy. While this system is efficient, it doesn’t always respond the way it should — and that’s what leads to immune-mediated diseases. 

What is an Immune-Mediated Disease?

There are instances when the immune system mounts an attack on a dog’s own cells. When these inappropriate responses occur, the conditions are referred to as immune-mediated diseases. Inflammation can be a naturally occurring condition created by the body, but instead of the body directing the inflammation at a foreign invader as it’s meant to, it’s directed towards body tissue.

Sometimes the cells of the body are targeted because the immune system is set off by an infection, toxin, or cancer. To determine if there’s an underlying trigger for these immune-mediated conditions, a veterinarian will perform diagnostic evaluations that may include blood tests, urinalysis, radiographs, ultrasounds, and possibly other specialized tests. 

There are circumstances where no trigger is discovered. In these instances, the disease is referred to as idiopathic. Since there isn’t a specific trigger that can be identified and treated directly, such as infection or cancer, treatment is just designed to suppress the inappropriate immune response. This suppression helps the immune system operate more optimally.

Immune-Mediated Thrombocytopenia in Dogs 

Bone marrow produces the platelets (also called thrombocytes) required for blood clots to form and help stop bleeding. When platelet numbers are decreased below a certain level, the result can be severe bruising and bleeding.

When the immune system isn’t functioning properly and destroys these platelets, the auto-immune disease is called immune-mediated thrombocytopenia (ITP). This particular condition is estimated to affect about 5% of dogs^[1].

The causes of ITP generally fall into two types, either primary or secondary. If the underlying cause cannot be identified, it’s referred to as primary ITP. In this instance, there is likely a defect in the immune system. 

The predisposing factors for primary ITP include:

• Age. Can occur at any age but appears more commonly in young adults to middle-aged dogs (typically less than 6 years old). 
• Breed. Can affect any breed, but smaller breeds seem to be affected more often. There seems to be a breed disposition for ITP in Cocker Spaniels, Poodles, and Old English Sheepdogs^[1].

When there’s a known underlying factor that causes the immune system to destroy the platelets, it’s referred to as secondary ITP.

Predisposing factors include:

• Infections such as tick-borne diseases
• Medications such as chemotherapeutic drugs 
• Cancers 
• Hereditary factors
• Other disorders such as bone marrow disorders

Without treatment, clinical signs will progress. The common symptoms include:

• Weakness
• Lethargy
• Bruising
• Bleeding in the mouth
• Bleeding from the nose
• Dark tarry-looking poop
• Pale gums
• Elevated respiratory rate

With treatment, generally, the outcome for this disease is good. Studies indicate that 63% to 80% survive long term and have a good quality of life^[1].

ITP can relapse. Studies report ranges from 9% to 39%^[2]. Regular monitoring is critical because the timing of a potential relapse varies widely between dogs.

Immune-Mediated Hemolytic Anemia in Dogs  

When the body’s immune system attacks and destroys its own red blood cells, it’s referred to as immune-mediated hemolytic anemia (IMHA). When the circulating number of red blood cells declines, anemia results, and oxygen delivery is drastically reduced.

While the causes are not well understood, they typically result from an improperly functioning immune system (called primary IMHA) or some other underlying condition (known as secondary IMHA). 

About 80% of the cases are primary^[3] and appear to be associated with certain breeds due to a genetic component. Some predisposed breeds include American Cocker Spaniel, Irish Setter, and Miniature Schnauzer^[3][4].

Several clinical symptoms are caused by this condition. They will continue to progress without treatment.

• Weakness
• Lethargy
• Jaundice (yellowing of the skin and/or whites of eyes)
• Dark urine
• Pale gums and other mucous membranes
• Lack of appetite
• Vomiting
• Increased respiratory rate

In many cases, the outcome is favorable with appropriate treatment. Survival rates range from 30% to 70%^[3], with a relapse rate of 11% to 15%^[4]. Generally, dogs that respond well to the initial treatment phase will do very well long term. Some pets can be taken off of all medications, while some require lifelong treatment. All in all, most have an excellent quality of life.

Immune-Mediated Polyarthritis in Dogs

Immune-mediated polyarthritis (IMPA) is a common condition often difficult to diagnose, where the immune system causes an inflammatory response within the joints. During this inflammatory response, white blood cells are sent to the joint, where they release chemicals and enzymes into the synovial fluid. This disrupts the joint-protective functions of the fluid. Typically, two or more joints are affected. The spine can also be affected. Any breed, sex, age, or size of dog can be impacted.

Diagnosis can be challenging since IMPA has a variety of clinical presentations. Early in the disease, some dogs only have a mild fever, a poor appetite, and only very subtle swelling of the joints. The most common signs are:

• Reluctance to walk and move
• Vocalizing with movement
• Shifting leg lameness (limping on more one leg either at the same time or separately)
• Abnormal gait
• Painful and swollen joints
• Lethargy

In the long term, most dogs survive with a good quality of life. Early diagnosis is important to protect pets from irreversible joint damage. Some dogs require lifelong medications to prevent relapses, while others can be tapered off medications. Regular follow-ups and monitoring are critical for long-term care.