Recent studies in Sweden, the United Kingdom and North America have established that many healthy Cavalier King Charles Spaniels have low circulating platelet counts, especially when measured with automated cell counters. Manual counting usually reveals a higher number of platelets of relatively large size (megathrombocytes) in some but not all dogs. In the Swedish study, 102 healthy Cavaliers had mean platelet counts of 178,000/µl. However, 32 (31%) of them had platelet counts below 100,000/µl and 4 dogs had counts between 30,000-50,000/µl. The mean count for male dogs was significantly lower (147,000/µl) than that of females (202,000/µl).

In North America, many dogs of this breed are found to have thrombocytopenia during routine health profiling prior to elective procedures or at annual checkups. Typical platelet counts range from 20,000-85,000/µl with some counts as low as 5,000/µl in clinically normal dogs. Ages range from 4 months to 10 years and both sexes are affected. These dogs can be presented for routine wellness exams, dental prophylaxis, or complaints varying from vomiting and diarrhea, coughing, back or joint pain or other illness. Diagnosis can be confusing as petechial or ecchymotic hemorrhages or other signs of bleeding are usually absent and thrombocytopenia is just an incidental finding. However, some Cavaliers have clinically expressed immune-mediated thrombocytopenic purpura, making the diagnosis more complicated.

Dogs with thyroid disease, commonly seen in Cavaliers, may show a greater incidence of thrombocytopenia. Whether the chronic cardiac valvular disease prevalent at a young age in this breed could be a contributing factor is unknown, although no association has been found between the presence of cardiac murmurs and low platelet counts. Splenomegaly is not a feature of the condition. Preliminary evaluation of bone marrow megakaryocytes of severely thrombocytopenic, healthy dogs has shown no structural abnormalities, but functional studies have yet to be performed. The thrombocytopenia has persisted over several years of followup, and treatment with immuno-suppressive drug regimens has not resolved the low platelets counts. Pedigree analysis revealed that the thrombocytopenic tendency can be transmitted to subsequent generations, suggesting that this is a congenital, heritable disorder of the breed. Similar disorders occur in people.

The clinical importance of these findings warrants comment. Cavalier King Charles Spaniels that have thrombocytopenia as an incidental laboratory finding should have platelet counts rechecked by manual methods, and do not need treatment to attempt normalization of platelet numbers. A complete health profile including thyroid function is advised to identify any concurrent abnormality that may need attention.