What is thrombocytopenia?
Thrombocytopenia refers to a low number of thrombocytes, also known as platelets. Platelets are small cells that help the blood to clot by clumping together and forming plugs in damaged blood vessels.
Platelets are produced by the bone marrow and released into circulation. Thrombocytopenia can occur if the platelets are destroyed or used up (such as with hemorrhage or bleeding) faster than the bone marrow can produce them.
Thrombocytopenia can also occur due to primary bone marrow disorders, such as leukemia, or drugs that cause bone marrow suppression.
What is immune-mediated thrombocytopenia (ITP or IMTP)?
“Immune-mediated” or “autoimmune” describes a situation in which the body reacts against its own tissues. ITP is a disorder in which your pet’s immune system targets and destroys platelets. The immune system makes antibodies that coat the surface of the platelets, which causes the spleen to recognize them as foreign and destroy them. ITP is an uncommon blood disorder in dogs, and rare in cats.
In many cases, there is no obvious inciting cause for ITP, and it is called primary, or idiopathic. Evan’s syndrome is considered secondary if an underlying cause of immune system dysregulation can be identified.
"In dogs, females are twice as likely as males to develop ITP,
and it is most common in middle-aged dogs."
Primary ITP is more common in dogs than in cats. In dogs, females are twice as likely as males to develop ITP, and it is most common in middle-aged dogs. Certain breeds are over-represented, including the cocker spaniel, miniature and toy poodles, and Old English sheepdogs.
In cats, there are no known breed, sex, or age predispositions. Secondary ITP is more common than primary ITP in cats, though both are considered rare.
Causes of secondary ITP include:
- Infectious diseases, such as Rocky Mountain Spotted Fever, ehrlichiosis, babesiosis, and hemotropic mycoplasmosis, in which blood parasites directly target the blood cells (these parasites are often spread by ticks or other insects)
- Infectious diseases such as leptospirosis, heartworm disease, canine distemper virus, and canine infectious hepatitis in dogs; and feline leukemia virus, feline immunodeficiency virus, and feline infectious peritonitis in cats
- Systemic inflammatory conditions such as severe bacterial infections, pancreatitis, bee stings, or snake bite envenomation
- Other autoimmune diseases such as autoimmune hemolytic anemia (AIHA or IMHA, which is destruction of red blood cells) or lupus
- Cancer
- Recent vaccination has been implicated as a potential cause, but no studies have been done that support the association
- Drug reaction
If a pet has both immune-mediated thrombocytopenia and anemia, it is called Evan’s syndrome.
What are the symptoms of ITP?
Dog with petechiae (small clumps of bruises).In mild cases, no obvious symptoms may be present. As the platelet count drops lower, spontaneous bruising can occur. Small clumps of bruises called petechiae or larger bruises called ecchymosis can be observed, particularly on hairless areas of skin like the belly and ear, as well as the gums and whites of the eyes.
Large internal bleeds are typically not found, though blood can be seen in the urine, stool, vomit, or nose bleeds. If bleeding is within the eye, blindness can occur; if bleeding is within the central nervous system, seizures or other neurologic dysfunction can occur; and if bleeding is within the lungs, breathing difficulty can be observed.
If blood loss is significant, the gums can start to look pale. Other non-specific signs can also occur, such as fever, weakness, lethargy, decreased appetite, enlarged lymph nodes, and enlarged liver and/or spleen.
How is ITP diagnosed?
Your veterinarian will perform a physical exam. If they identify bruising or abnormal bleeding, then tests will be done to evaluate your pet’s blood clotting abilities. Thrombocytopenia is diagnosed with a test called a complete blood count (CBC). This test measures the red and white blood cells, as well as the platelets. The cells will also be evaluated under the microscope to determine their size and shape. If thrombocytopenia is detected, then additional testing can be done to determine if the condition is primary or secondary.
Recommended additional laboratory testing may include a biochemical profile to evaluate organ function and electrolyte levels, a urinalysis to assess the urinary tract, and screening for infectious diseases. Your veterinarian can test for tick-borne diseases, including ehrlichiosis, babesiosis, heartworm disease, and leptospirosis, depending on disease prevalence in your geographical location.
Imaging such as X-rays of the chest and abdomen, as well as an abdominal ultrasound, can look for the presence of cancer.
How is ITP treated?
If no underlying cause is found and thrombocytopenia is suspected to be primary ITP, then treatment is based on suppressing the abnormal immune response. Immunosuppressive medications will be the mainstay of treatment.
Typically, prednisone (or prednisolone in cats) is the first-line treatment. If no response is seen after a week or if clinical signs worsen, then additional immunosuppressive medications can be considered, such as azathioprine, cyclosporine, and mycophenolate.
Melatonin can be added as an adjunctive therapy, as there is some evidence it can help modulate the immune system. If there is any suspicion of bleeding from the gastrointestinal tract, then antacids and anti-ulcer medications should be given.
"Immunosuppressive medications will be the mainstay of treatment."
Blood products such as whole blood or platelet-rich plasma can be considered, but the platelets do not survive long after transfusion. If blood loss is significant, or there is concurrent autoimmune hemolytic anemia, then a transfusion may be of more benefit to replace the depleted red blood cells.
Other options may be available at referral centers. In patients that do not respond well to medication, splenectomy can be considered. It has not been well studied in pets though, so it is not considered first line.
In pets with secondary ITP, treatment will also be directed at the underlying cause. Antibiotics may be started while awaiting further diagnostics.
Is there anything else I need to know?
Patients with very low platelet counts are at risk of bleeding. They may need strict cage rest to avoid any sort of trauma. If treatment is successful and the platelet numbers return to normal, then a very slow taper of the medications will be required. This tapering will likely take 4 to 6 months, with rechecks and dose adjustments every couple of weeks. Faster tapers are associated with higher rates of relapse.
Vaccinating pets who have experienced Evan’s syndrome or other autoimmune diseases may not be recommended due to a potential risk of causing a recurrence, but that decision will be based on a risk/benefit discussion with your veterinarian.
What is the prognosis for ITP?
The prognosis is good if the condition is treated quickly, and most patients recover. Relapses are possible and can occur in up to 30% of cases. Some patients require life-long immunosuppressive medications. Secondary infections from immunosuppression and other side effects of these medications are possible (such as diabetes with prednisone). If there is significant blood loss requiring a transfusion, or if there is bleeding into the intestinal tract, these patients may be more likely to suffer a relapse.
In those patients with secondary ITP, prognosis is more variable. If the inciting cause can be successfully treated, then prognosis is similarly favorable.
