Cardiac thrombi are less common in dogs than in cats. In dogs, they are often seen in the right atrium and only two cases of left atrial thrombus have been reported in dogs. 

A case report of a 14-year-old mixed-breed female dog describes the first reported case of a free-floating thrombus in the left atrium. The dog presented with dyspnea that persisted for several days and signs of polyuria and polydipsia for almost a year. She also had generalized alopecia for about three years. The clinical findings showed strong inspiratory and expiratory respiratory sounds. The chest x-ray and transthoracic echocardiography revealed a moderate dilated left atrium, low-grade pulmonary insufficiency, and a round mass floating in the left atrium. 

In addition to those findings, an ACTH stimulation test was performed and confirmed hyperadrenocorticism in this dog. Unfortunately, the pet owners declined further diagnostics and treatments and the dog died 19 days later. The most probable cause of intracardiac thrombus in this dog was due to hyperadrenocorticism and preexisting heart disease. However,  the therapeutic approaches for intracardiac thrombi are not available and each case should be treated one by one.

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The rare occurrence of a left atrial thrombus in a dog