Thrombocytopenia is a somewhat rare condition in horses, which is defined as having a low platelet count. It is associated with horses having s platelet count of less than 100 x 10³ per microliter. It was found to occur in approximately 1.5% of hospitalized horses in one study conducted. It is generally caused by one of two reasons:

• Disorders which result in bone marrow not being able to produce enough platelets
• Conditions where platelets are used up or destroyed at a faster rate then they normally would.

Conditions which are associated with horses having low platelet counts include:

Idiopathic thrombocytopenia (ITP)

Certain drugs and antibiotics, colchicine and indomethacin, quinidine, hydralazine, H2-blocking agents, tolbutamide, isoniazid, and thiazide diuretics, are a few that have a history of association with drug-induced decrease in platelet counts.

Leukemia, lymphoma, or other cancers that may have spread to the horse's bone marrow.

Platelet consumption is seen in horses suffering from disseminated intravascular coagulation (DIC), thrombocytopenic purpura (TTP) and hemolytic uremic syndrome (HUS).

Chemotherapy or radiation therapy, which may affect the bone marrow's ability to produce platelets

Autoimmune disorders

Viral infections

Aplastic anemia

Exposure to toxic chemicals, such as pesticides, arsenic, or benzene

Sepsis, especially if associated with an infection with a Gram-negative bacteria