Low Blood Platelets (Thrombocytopenia) in Cats

What is thrombocytopenia?

Medically known as thrombocytopenia, low platelets (PLT) is a decreased number of platelets in the blood. Normal platelet levels should be around 200,000 µL (microlitre).

What are platelets?

Platelets (thrombocytes) are disc-shaped, nonnucleated cell fragments that circulate in the bloodstream. Their function is to stop blood loss (known as hemostasis). Three mechanisms work together, stopping the flow of blood.

1. Platelet adhesion – When damage to a blood vessel occurs, circulating platelets form a clump over the damaged vessel to block it off.
2. Coagulation – Fibrinogen is activated by protein factors in the blood to form fibrin strands. These strands help to mesh the platelet plug, strengthening it.
3. Vasoconstriction – When a blood vessel becomes damaged, vasoconstriction, makes the blood vessel smaller, which restricts blood loss from the damaged site.

The bone marrow forms all of the cellular components of the blood. Platelets are produced by cells known as megakaryocytes, these giant cells undergo a process known as fragmentation, releasing platelets into the bloodstream. Platelets circulate in the blood for seven days before macrophages destroy them.

Thrombocytopenia can be divided into two types, primary or secondary.

• Primary or idiopathic thrombocytopenic purpura: No known reason for low platelets.
• Secondary thrombocytopenia: Associated with underlying diseases such as cancer, infection, autoimmune disorders, certain drugs and toxins.

Causes of low platelets in cats

There are several causes of low platelets which can be divided into four main classes:

1. Decreased production of platelets
2. Premature destruction of platelets
3. Sequestration in the spleen
4. Platelets are used up faster than they can be produced

Decreased production of platelets

The most common cause of low platelet count in cats is caused by diseases that affect the production of platelets in the bone marrow.

Leukemia: A cancer of the blood and bone marrow, as the cancer cells take over the bone marrow, there will be less platelet-producing megakaryocytes.

Myelodysplasia syndrome (bone marrow failure disorders): Stem cells in the bone marrow which is responsible for the production of red and white cells, as well as platelets, begin producing abnormal cells

Infectious disease: Leptospirosis, feline immunodeficiency virus, feline infectious peritonitis, feline panleukopenia and feline leukemia virus. The feline panleukopenia vaccine may result in a temporary decrease in platelet production.

Idiopathic: Unknown cause.

Premature destruction of the platelets

Autoimmune disorders (immune-mediated thrombocytopenia): The cat’s immune system destroys platelets.

Neoplasia:

• Lymphoma – Cancer of the lymphatic system.
• Hemangiosarcoma – A sarcoma arising from the lining of the blood vessels.

Certain drugs: Methimazole, griseofulvin, albendazole, carboplatin, chloramphenicol, propylthiouracil, cytotoxic drugs (carboplatin, azathioprine, doxorubicin).

Sequestration in the spleen

This organ filters unwanted material from the blood and fights infection and stores up to 30-40% of platelets. If the spleen becomes enlarged (splenomegaly), it will begin to function abnormally, sequestering a greater number of platelets, up to 90%, and therefore reducing the number of platelets circulating in the blood.

Some forms of cancer can lead to an enlarged spleen. The liver may also sequestrate several platelets; however, it’s not at the same level as the spleen.

Platelets are used up quicker than they can be produced

Disseminated intravascular coagulation (DIC): A condition in which systemic activation of clotting occurs, leading to blood clots forming throughout the body, which can cause blockages in the vascular system as well as using up large numbers of platelets to form the clots, major blood loss can also cause a decrease in platelets.

Symptoms of low platelets

Many cats with mild thrombocytopenia are often asymptomatic, and the condition only comes to as an incidental finding.

As the platelets are there to stop bleeding, one of the obvious symptoms of low platelets is increased bleeding, either from an external wound (cut or a scratch), during surgery, or nosebleeds, bleeding gums, anal bleeding although this may not always be present.

• Lethargy
• Blood in urine
• Blood in the stool
• Red spots in the white of the eyes due to retinal hemorrhage
• Red spots on the gums and skin
• Purple areas on the skin (bruising)
• Fever

Other symptoms may vary depending on the underlying cause.

Diagnosis

Your veterinarian will perform a complete physical examination of your cat and obtain a medical history from you. Your veterinarian will want to know the following:

• Any medications your cat is on
• Exposure to toxins
• Vaccination history
• Other symptoms

Diagnostic workup:

• Complete blood count: A blood test that measures the cellular components of the blood (red blood cells, white blood cells and platelets, cats with thrombocytopenia will have low blood platelets.
• Biochemical profile: The biochemical profile is a series of tests on a sample of blood to evaluate the functional capacity of the liver and kidneys.
• Blood smear: A thin layer of blood is smeared on a slide and the cellular components are evaluated under a microscope for abnormalities.
• Prothrombin time (PT or INR): A sample of blood is obtained and blood cells are separated from the plasma (the liquid part of the blood) by centrifugation. The plasma is added to a tissue factor (also known as a platelet tissue factor or factor III) to activate the clotting factor cascade, and the time it takes the sample to clot is measured.
• Blood serum test: To look for antibodies to Rocky Mountain spotted fever.
• Bone marrow aspirate or core biopsy – A needle extracts a sample of bone marrow from the humerus (upper bone in the front leg), femur (thigh bone) or pelvis. An increase in the number of megakaryocytes suggest increased platelet use, increased platelet destruction or sequestration in the spleen. Decreased megakaryocytes indicate decreased platelet production, which may be due to cancer or viral infection.
• X-ray or ultrasound: To check the size and shape of the organs as well as look for tumours.
• Biopsy: Of spleen or liver mass.
• FIV and FeLV blood tests.

Treatment

Primary thrombocytopenia:

Mild cases may require no treatment at all. Corticosteroids may be prescribed to slow down immune-mediated platelet destruction or transfusion of platelets or whole blood, where indicated.

Secondary thrombocytopenia:

The goal is to find and address the underlying cause. Restrict activity to reduce the risk of injury and/or bleeding for cats with extremely low platelets. Transfusion of platelets or whole blood where indicated.

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