Atypical disseminated histiocytic sarcoma in a 7‐month‐old dog.

A 7‐month‐old intact female bearded collie dog was admitted after a 2‐week history of progressive cough, inappetence, and lethargy, with no response to previous treatment with doxycycline and steroids. Mild attenuation of lung sounds in the right middle hemithorax was the only abnormality detected on physical examination. Abdominal ultrasound and thoracic radiographs were performed and revealed multifocally distributed nodules and masses, well‐circumscribed and of variable size in the kidneys and pulmonary parenchyma. Ultrasound‐guided fine needle aspirates of the renal and pulmonary masses were taken. A cytologic evaluation of these lesions pointed towards a malignant mesenchymal neoplasia. Euthanasia was elected due to the poor prognosis and rapid progression. The post‐mortem histopathology, a positive result to IBA1 immunoperoxidase staining, and a lack of detection of infectious agents, and negative E‐cadherin immunostaining enabled the final diagnosis of a disseminated histiocytic sarcoma. We report an atypical form, both in breed and age, of canine disseminated histiocytic sarcoma. While all breeds can be affected, there is a clear predisposition in some, and no cases have been previously described in bearded collies. Moreover, to the authors' knowledge, this is the youngest dog with this histiocytic disorder described to date. Disseminated histiocytic sarcoma should be considered as a differential diagnosis of multinodular tumors in dogs, regardless of the anatomic location and age of the dogs, even in puppies. [ABSTRACT FROM AUTHOR]