Affects: Cats, Dogs
Immune-mediated thrombocytopaenia (IMT) is a disease common in dogs and rare in cats. The disease is characterised by a low platelet count caused by destruction of the platelets from the immune system. IMT is the most common cause of thrombocytopaenia in dogs.
Signs And Symptoms: Common symptoms of IMT include lethargy, anorexia, pyrexia, haemorrhage, and bruising. Destruction of platelets occurs when immunoglobins attach to the surface of the platelet, which causes macrophages to initiate phagocytosis. IMT is differentiated from other forms of thrombocytopaenia by the immune-mediated component of the condition.
Risk Factors: Bitches are twice as likely as male dogs to be affected. Breeds with documented predispositions are: cocker spaniels, miniature and toy Poodles, Old English Sheepdogs, Golden Retrievers, and German Shepherd Dogs. Neutered dogs have a significantly increased risk of developing IMT.
Diagnosis: Dogs usually present with lethargy, asthaenia, anorexia, or mild pyrexia. Signs of platelet dysfunction such as petechiae or ecchymosis, epistaxis or rhinorrhagia and in severe cases with haematuria; gingival or ocular haemorrhages are used to identify and diagnose thrombocytopaenia. Up to half of dogs may present with splenomegaly.
Patients can be screened to check for drugs and toxins that may cause haemostasis or secondary IMT. PCR serology tests may be performed to screen for diseases. Serum chemistry, coagulation panels, and Coombs tests can be performed to rule out or diagnose comorbidities including: von Willebrand's disease, haemophilia A, disseminated intravascular coagulation, immune-mediated haemolytic anaemia, or renal failure.
Radiographic and ultrasonographic procedures can rule out splenic hemangiosarcoma, mast cell tumours, pancreatitis, infection, endocarditis, and other diseases with similar symptoms. A biopsy of bone marrow can identify platelet production problems. Biopsies of diseased tissue can identify neoplasia through cytological evaluation.
Treatment: Treatment varies based on whether or not it is primary or secondary IMT; however, all cases require immunosuppression.
Therapeutic Plasma Exchange: Therapeutic plasma exchange (TPE) is a novel treatment for immune-mediated diseases and little research has been done for the effectiveness of TPE and patients with IMT. TPE requires removing the patient's plasma which contains antibodies and replacing it with fluids such as sodium chloride, frozen plasma, packed red blood cells, and hetastarch. Kopecny et al. found this treatment to be effective in three out of four dogs with IMT who were unresponsive to other treatments. Francey et al. found that dogs treated with TPE for IMT had similar prognosis to those treated differently; however, Francey et al. discussed its safety as a treatment and recommended further research.
Prognosis: Studies have identified short-term survival rates of 74–97% with a recurrence rate of 24–58%. Melaena and high blood urea nitrogen were associated with worse outcomes. Cummings and Rizzo identified a 60% survival rate in cases comorbid with melaena and high blood urea nitrogen. A study following up on 45 dogs identified with IMT after 1 year found a mortality rate of 11.9%. 89.6% of dogs survived to discharge and 31% of those discharged relapsed, with an average of 78 days for dogs with multiple relapses and 149 days for dogs with only one relapse. The longest time between discharge and relapse was over 3 years. Seven dogs experienced relapse more than once and one dog relapsed five times before being euthanised. In dogs with a platelet count below < 30,000 cells/μL there is an increased risk of spontaneous haemorrhage; however, haemorrhage cannot be predicted in dogs with IMT.