Immune-Mediated Thrombocytopenia in Dogs

Overview

Immune-mediated thrombocytopenia (ITP) is an autoimmune disease that occurs when the dog’s immune system does not recognize its platelets, also known as thrombocytes, and attacks it. This disorder leads to the deficiency of blood platelets, which are cell fragments responsible for forming blood clots to prevent excessive blood loss.

Understanding Immune-Mediated Thrombocytopenia in Dogs

The dog’s immune system learns to differentiate body cells and foreign invaders. Thus, in a normal system, only foreign cells (also known as antigens) triggers an immune response in which case the body produces antibodies to neutralize the antigen. When an animal has immune-mediated thrombocytopenia, this process is disrupted since the immune system mischaracterizes the platelet cells.

Platelets are blood cells manufactured in the bone marrow and released into the bloodstream. These cells adhere to the damaged blood vessel wall and binding with each other, effectively creating a clot to stop bleeding. When there’s diminished platelet count in the blood, the dog suffers an increased risk for bleeding and extensive blood loss.

Types of Immune-Mediated Thrombocytopenia

Immune-mediated thrombocytopenia in dogs is categorized into two types depending on their causes.

Primary Immune-Mediated Thrombocytopenia

Also referred to as idiopathic thrombocytopenic purpura, this type has no known underlying cause. Young to middle-aged dogs as well as small breeds are more predisposed to this type of ITP, although in rare cases, large breeds could be affected as well.

Secondary Immune-Mediated Thrombocytopenia

In contrast, this type of ITP occurs due to underlying factors like:

Autoimmune diseases

If the dog suffers from other autoimmune diseases like rheumatoid arthritis, lupus erythematosus, and immune-mediated hemolytic anemia, these disorders could also trigger immune-mediated thrombocytopenia.

Neoplasia

Neoplasia refers to abnormal growth of cells and tissues in the dog’s body that can either be benign or malignant. The antibody released against the tumor’s antigen may also trigger the destruction of the platelet. This usually occurs when the antigen is related to the platelet membrane. Common neoplasms that could cause immune-mediated thrombocytopenia in dogs include lymphoma, leukemia, mammary adenocarcinoma, multiple myeloma, and hemangiosarcoma.

Infectious agents

Some viral, bacterial, parasitic, and protozoal infection could trigger the dog’s immune system to produce antibodies that could also bind to the platelet membrane, causing its destruction. This could happen with infections like babesiosis, dirofilariasis leishmaniasis, tick-transmitted infection such as ehrlichiosis, and other bacterial or fungal infections.

Drug therapy

Any medication could potentially cause immune-mediated thrombocytopenia in dogs but some drugs are known to be closely associated with this condition. These drugs serve as haptens—molecules that could elicit the release of antibodies when they bind with larger carriers like platelets. Drugs that have been recorded to cause this reaction include cefazedone, auranofin, and trimethoprim-sulfonamide drug combination. Symptoms of thrombocytopenia usually occur weeks or months after the start of drug therapy.

Vaccines

Vaccine-related thrombocytopenia has been suspected, although there has been no solid evidence of vaccines being the underlying cause of this condition.

Non-Immune-Mediated Causes of Thrombocytopenia

When evaluating a dog suspected to suffer from thrombocytopenia, the veterinarian will make sure to rule out non-immune-mediated causes. Other conditions that could lead to thrombocytopenia include bone marrow disorders, bone marrow suppression due to radiation or chemotherapy, splenic sequestration, and myelophthisis. In some cases, tumors may cause thrombocytopenia through non-immune-mediated mechanisms as well. Additionally, infections like parvovirus and canine distemper can cause a decrease of platelets in the blood.

The Cavalier King Charles Spaniel is also predisposed to non-immune-mediated thrombocytopenia. However, in some cases, the veterinarian will sometimes perform a manual platelet count to rule out macrothrombocytes or large platelets as the cause for the false low platelet count.

Signs and Symptoms

Immune-mediated thrombocytopenia in dogs cause signs that are mostly associated with bleeding. Signs include:

Rapid breathing
Bruising
Nasal bleeding
Oral bleeding
Dark stool indicative of gastrointestinal bleeding
Pale gums

At the onset of symptoms, dogs need to be tested and diagnosed right away. Left untreated, signs could progress, severe internal bleeding could ensue, and it may end very fatally for the animal.

Diagnosis

Here’s the usual process that the vet may employ to get to a conclusive diagnosis:

History

Diagnosing the condition usually starts with a thorough medical history of the patient as well as evaluation of the symptoms and the time of its onset. The vet will most likely ask questions like:

Is the dog is currently taking medications that could affect platelet count like aspirin?
Is the dog suffering from other illnesses? Did it undergo blood transfusion in the past?
Has the pet traveled to other places recently? Some places have endemic diseases like dirofilariasis and ehrlichiosis.
Has the dog suffered from tick or flea infestation?

Physical Exam

Aside from the initial interview of the pet owner to determine the dog’s medical history, the veterinarian will likely proceed with a physical exam to check for evidence of bleeding. In some instances, the veterinarian may even do a rectal examination to obtain a feces sample and check for the presence of blood in it. The abdomen may also be palpated to check for evidence of spleen enlargement.

Diagnostic Tests

After the physical exam, the veterinarian will most likely request some diagnostic tests like:

CBC and Blood Smear Evaluation

A complete blood count is done to check if there is a marked decrease of platelets in the blood. Normal platelet count for dogs ranges from 175,000 to 500,000 per microliter of blood. If the platelet count is less than 30,000 microliter, this indicates spontaneous bleeding solely associated with thrombocytopenia.

A blood smear evaluation is also done in conjunction with CBC if the platelet count is low. This is done to verify the result of the CBC since the platelet could be clumping together or the dog could have large platelets so the analyzer used in CBC may mistake them for red blood cells.

Checking the blood sample under the microscope will also help assess platelet abnormalities. For instance, the presence of platelet fragments is suggestive of immune-mediated thrombocytopenia. Also, hereditary conditions or infection that could cause thrombocytopenia could be determined with a microscopic evaluation of the blood sample.

Blood Test

Aside from a complete blood count, the blood may also be tested to check for fungal or rickettsial disorders that may be causing thrombocytopenia. Also, a coagulation test may be needed to check if the dog’s system can stop blood loss.

Imaging

A radiograph or abdominal ultrasound may also be employed especially if a neoplasm is the suspected cause of the condition.

Bone Marrow Aspiration

After thorough diagnostic testing and the cause of thrombocytopenia is still not apparent, the veterinarian may request this procedure. This test will check if there’s a problem with the dog’s bone marrow and if it is producing platelets inadequately.

Treatments

There are several treatment approaches and in most cases, the vet may opt for an aggressive initial therapy to increase the chances of survival for the dog.

Supportive Therapy

Initial therapy is focused on stopping platelet destruction and bleeding, as well as treating any underlying condition. Dogs admitted to the hospital are usually kept in padded cages to minimize risk to injury that could cause bleeding. Furthermore, they are given gastrointestinal (GI) protectants if they exhibit signs of GI bleeding. Activities that would increase the risk of trauma are minimized or eliminated.

Immunosuppressive Therapy

Immunosuppressive drugs are often used to block the immune response that leads to the destruction of the platelets. Glucocorticoids like prednisone and prednisolone are often used for this purpose. However, these drugs may have side effects, which include increased urination, appetite, and thirst. Additionally, the dog may become restless and more anxious after the administration of these drugs. This is why animals medicated with these steroids are under close monitoring and drug use is gradually tapered off depending on subsequent test results.

Blood Transfusion

If the dog is suffering from anemia due to hemorrhage, the veterinarian may order a blood transfusion to stabilize the condition of the animal before moving on to further treatment or testing. Platelet concentrates, platelet-rich plasma or whole blood can be transfused to the dog, although pets transfused with whole blood may run the risk of volume overload.

Other Treatments

Dogs that suffer from secondary immune-mediated thrombocytopenia require treatments that address the underlying cause. For instance, splenectomy (removal of the spleen) might be necessary for animals that have enlarged spleen.

Cost to Treat: $500 to $1,000 per treatment

Prognosis

The prognosis for dogs suffering from this condition ranges from good to guarded. Approximately 80% of dogs recover from this illness. Unfortunately, the remaining 20% either die due to acute hemorrhage or are euthanized due to severe side effects of long-term use of medications. Pets that have blood in their stool or those that need transfusion may have a harder time recovering from the condition.

Even if the dog responds to treatment well, there’s always the possibility of a relapse. Also, those that receive immunosuppressive medication are under intense and constant monitoring as they may be prone to infection or may develop other medical condition since the dog is immunocompromised. Fortunately, this type of medication is given in decreasing doses over time and the dog may be off it eventually.

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Sources:

http://vhc.missouri.edu/small-animal-hospital/small-animal-internal-medicine/diseases-and-treatments/immune-mediated-thrombocytopenia/
https://www.vin.com/apputil/content/defaultadv1.aspx?pId=8768&catId=18823&id=3850070
https://www.ncbi.nlm.nih.gov/pubmed/30102418

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