Updates in Immune-Mediated Thrombocytopenia (ITP) in Dogs and Cats
By Tara Ghormley, DVM, DACVIM
Immune-mediated thrombocytopenia (ITP) is the most common disorder of primary hemostasis in dogs and a frequent cause of spontaneous bleeding. While well documented in canine patients, it is rare in cats. Understanding the pathophysiology, diagnostic approach, and treatment options is essential for improving patient outcomes.
What Is ITP and Who Gets It?
ITP can occur at any age, though female dogs are affected twice as often as males. Certain breeds are predisposed, including Cocker Spaniels, Poodles of all sizes, German Shepherds, and Old English Sheepdogs.
Clinical signs often appear suddenly. Dogs may present with petechiae, ecchymoses, epistaxis, melena, hematemesis, or hyphema. Neurological signs can develop if intracranial hemorrhage occurs. Non-specific symptoms such as lethargy, weakness, or anorexia are also common.
Some dogs are diagnosed incidentally when a routine CBC reveals low platelets. Mild cases may have no clinical bleeding, but severe thrombocytopenia can lead to life-threatening hemorrhage .
It is important to remember that not all low platelet counts indicate ITP. For example, Greyhounds naturally have lower platelets, and breeds such as Cavalier King Charles Spaniels, Norfolk Terriers, Cairn Terriers, and Akitas may have inherited macrothrombocytopenia without any bleeding risk.
Pathophysiology of ITP
The immune system attacks the patient’s own platelets through several suspected mechanisms:
Autoantibodies bind to platelet surface proteins, marking them for destruction.
Splenic macrophages remove antibody-coated platelets from circulation.
Cytotoxic T cells directly kill platelets, sometimes without detectable antibodies.
Megakaryocytes (platelet precursors) may also be targeted, reducing platelet production.
Primary vs. Secondary ITP
Primary ITP is an autoimmune disorder in which antibodies are directed at platelet antigens such as glycoproteins IIb/IIIa. It likely reflects an underlying dysregulation of the immune system. It is rare in cats.
Secondary ITP develops due to another condition. Triggers include neoplasia (lymphoma, hemangiosarcoma, multiple myeloma), infectious diseases (Ehrlichia, Leishmania, Anaplasma, Babesia), or drugs (notably sulfonamides). It may also accompany systemic lupus erythematosus or immune-mediated hemolytic anemia (Evan’s syndrome).
Interestingly, the severity of thrombocytopenia doesn’t always match the risk of bleeding. Dogs with platelets <30,000/µL are at risk of spontaneous hemorrhage, but larger, more active platelets may compensate. Severe bleeding is more likely when platelet function or vascular integrity is also impaired.
Diagnosing ITP
No single test confirms ITP. Diagnosis requires a careful, stepwise approach:
Confirm the platelet count on a blood smear to rule out clumping. Roughly 1 platelet per 100× field equals about 20,000/µL. Primary ITP is usually diagnosed when the platelet count is <20,000/µL without another identifiable cause.
If low platelets are confirmed, exclude other known causes such as hemorrhage, systemic inflammation, or disseminated intravascular coagulation (DIC). This is done with basic CBC, chemistry panel, and coagulation testing. A pathologist review of a blood smear is always recommended. Urinalysis is optional as hemorrhage may occur with cystocentesis. However, occult UTI has been implicated in secondary ITP development.
Optional tests which may help determine primary vs. secondary disease include: thoracic radiographs, abdominal ultrasound, and vector-borne disease testing (Ehrlichia, Babesia, Anaplasma; Leishmania in endemic regions). These may be limited by owner finances.
Cats require additional testing for FIV, FeLV, and vector-borne pathogens.
Platelet-bound antibody assays have high sensitivity and low specificity. Negative results make ITP unlikely, but immunosuppressive treatment before testing can yield false negatives. These tests are only available at a limited number of academic facilities and therefore rarely performed.
Bone marrow sampling is not routinely needed unless other cytopenias or abnormal cells are present.
Treatment Approaches
One of the challenges in ITP is the lack of strong, evidence-based protocols. Most recommendations come from clinical experience and small studies .
Secondary ITP
The priority is addressing the underlying cause. If the patient is severely affected, a short course of immunosuppressive medication may be added. Mild to moderate secondary ITP does not always require immunosuppression once the underlying disease is treated.
Glucocorticoids
Prednisone remains the first-line therapy for primary (and possibly secondary) ITP. High doses (about 2 mg/kg/day, max 60 mg/day; dose by body surface area for dogs >25 kg) suppress macrophage activity and antibody production. Effects of glucocorticoids are usually seen within 1-3 days. Higher doses may increase the risk of GI upset or ulceration.
Second-Line Immunosuppressants
These are added when steroids fail, cause severe side effects, or when relapse occurs:
Cyclosporine: a calcineurin inhibitor that suppresses IL-2 and T-cell activation. Dose: 5–7 mg/kg PO q12h. Side effects include gingival hyperplasia, hepatotoxicity, GI upset, and opportunistic infections. Safe for cats.
Mycophenolate mofetil: an azathioprine alternative that inhibits purine synthesis. Dose: 8–10 mg/kg PO q12h. Causes dose-dependent GI upset, rare hepatotoxicity, but often fewer side effects than cyclosporine; safe for cats.
Azathioprine: another purine synthesis inhibitor (2 mg/kg q24h). Cannot be used in cats. Side effects include anemia, hepatotoxicity (15% incidence), myelosuppression, and pancreatitis.
Leflunomide: reduces T-cell proliferation and antibody production. Dose: 2–4 mg/kg/day. Main risk is hepatotoxicity.
Using three immunosuppressants together does not improve outcomes and significantly increases the risk of severe opportunistic infections.
Adjunctive Therapies
Additional therapies which maybe required or improve outcomes are listed below. These are not suitable for every patient and should be added to a treatment regimen on an individual basis.
Vincristine: Promotes platelet release from bone marrow and reduces destruction. Often added in severe bleeding (<15,000 platelets/µL or active hemorrhage). Dose: 0.02 mg/kg IV once. Monitor for neutropenia and avoid in MDR1-affected dogs.
IV immunoglobulin (IVIg): Blocks Fc receptors and induces T-regulatory cells. Shown to accelerate platelet recovery when combined with steroids. Used mainly in severe or refractory cases; costly and may induce hypercoagulability.
Blood products: Whole blood or platelet concentrates are reserved for life-threatening hemorrhage. They may not significantly raise platelet counts but can be lifesaving in patients with severe bleeding. Routine transfusions based on platelet counts alone are not recommended or helpful.
Antifibrinolytics (aminocaproic acid, tranexamic acid): May help in life-threatening hemorrhage, though evidence is limited.
Gastroprotectants: Omeprazole, pantoprazole, or sucralfate can reduce steroid-associated GI ulceration, though data is mixed. Current consensus supports their use in patients with melena.
Thrombopoietin analogs (e.g., romiplostim): Expensive and rarely studied in veterinary medicine but may help refractory cases. There are limited reports of use.
Splenectomy: Considered in refractory or recurrent disease once infectious causes are excluded. Has been reported to induce remission but carries risks especially in actively bleeding patients. Can lead to poor outcomes if an infectious cause is present and not appropriately treated.
Doxycycline: Added when vector-borne disease is suspected/confirmed, typically for 2–4 weeks. Empiric treatment should be based on patient’s exposure risk, including travel history, geographic location, and flea/tick preventative use.
Monitoring and Relapse
Inpatients with hemorrhage require close monitoring, including CBC every 1–3 days until bleeding resolves. Outpatients are monitored every 1–2 weeks initially. If platelet count is stable, immunosuppressant taper should be instituted, with CBC prior to every dose reduction. Chemistry panels are repeated to check for drug side effects, particularly when second agents are added. Jugular venipuncture should be avoided due to bleeding risk .
Relapse occurs in 9–47% of cases, often early in therapy. Causes include rapid tapering of immunosuppressants, new disease triggers, or persistent/undiagnosed underlying illness. Mild relapses may be managed by returning to the last effective immunosuppressant dose and tapering more slowly. Severe relapses often require re-induction with high-dose steroids plus a second agent (or change in second agent), and sometimes vincristine, IVIg, or romiplostim.
Vaccination and ITP Relapse
Data are limited on the risk of vaccines and relapse of disease. Decisions should weigh disease risk against relapse potential. If vaccination is needed, administer only one vaccine per visit, after immunosuppressives have been discontinued, and recheck platelets 2–5 weeks later. Indoor-only cats may not require routine vaccines if they have a history of ITP.
Prognosis
Short-term survival is encouraging, with 74–93% of dogs surviving an initial ITP episode. Mortality ranges from 10–30%, with higher risk in patients with megakaryocyte hypoplasia. Severe hemorrhage (melena, CNS bleeds, pulmonary hemorrhage, severe anemia, elevated BUN) is a poor prognostic indicator.
The DOGiBAT score (Daily Canine Bleeding Assessment Tool) helps quantify hemorrhage severity across nine sites, each scored from 0–2, for a maximum of 18. Higher scores correlate with need for transfusion and longer hospitalization. Importantly, the platelet count itself does not reliably predict outcomes.
Feline ITP
True primary ITP is very rare in cats. When it does occur, presentation and treatment are similar to dogs, though spontaneous bleeding is less common. Platelet clumping can cause pseudothrombocytopenia on automated machines, making blood smear confirmation essential. Cats are treated with prednisolone as first-line therapy, with cyclosporine or chlorambucil as second agents. Azathioprine should never be used in cats due to toxicity.
Key Takeaways
ITP is the leading cause of primary hemostatic disorder in dogs, but rare in cats.
Diagnosis requires exclusion of other causes—there is no single definitive test for primary ITP.
Corticosteroids are the first-line treatment; additional immunosuppressants are added if needed.
Severe cases may require vincristine, IVIg, blood products, or newer options like romiplostim.
Relapse is common, making careful monitoring and cautious tapering essential.
Prognosis is generally good in the short term, though severe bleeding and certain pathologies worsen outcomes.