Primary Immune-Mediated Thrombocytopenia Tentatively Diagnosed in Four Cats

Feline primary immune-mediated thrombocytopenia (pIMT) is uncommon, and to the best of the authors' knowledge, scarcely reported. pIMT is mostly diagnosed based on excluding primary causes inducing secondary IMT, and observing positive response to immunosuppressive therapy. Definitive diagnosis of IMT requires immunoassays, mostly unavailable, demonstrating anti-platelet or anti-megakaryocyte antibodies. Here we describe four severely thrombocytopenic cats, aged 1.5 to 10 years, presented to the Hebrew University Veterinary Teaching Hospital, tentatively diagnosed with pIMT. Clinical signs at presentation included pale mucous membranes, extreme weakness to collapse, and bleeding. Platelet counts raged from 5 to 11x10³/µL (reference interval, 156.4-626.4x10³/µL). All cats were treated with prednisolone. In two cats, both recovered, prednisolone was the single immunosuppressive agent used. Another cat, partially and insufficiently responded to prednisolone, thus, additionally, received mycophenolate-mofetil (MMF), leading to remission. The remaining cat, with insufficient response to prednisolone, additionally received, over time, MMF, followed by leflunomide, resulting in a partial response. The thrombocytopenia had partially improved, however long-term prednisolone therapy was still required, eventually resulting in diabetes mellitus. This is the first report of MMF treatment in feline pIMT. With only four cases over the 2-year study period, feline pIMT was an uncommon cause of thrombocytopenia in our hospital. In some cats, profound thrombocytopenia at presentation led to severe hemorrhagic anemia, mostly due to gastrointestinal bleeding, posing a serious life-threatening risk. Prednisolone treatment alone induces remission in some cats, but when response to prednisolone is insufficient, or with recurrence, additional immunosuppressive drugs are needed. The overall prognosis in this small cohort was good.