Bacillary angiomatosis is composed of a proliferation of capillary-sized vessels lined by epithelioid endothelial cells. Numerous neutrophils are seen in the stroma, as well as eosinophilic aggregates containing fibrin and bacilli (which may be highlighted with a Warthin-Starry stain)(Folpe).
The epithelioid endothelial cells form small vessels, and are arranged in a vague lobular pattern.
Key features include the plump pale cytoplasm of the endothelial cells (not spindled like Kaposi sarcoma), acute inflammation and eosinophilic debris.
Bacillary angiomatosis is a reactive vascular proliferation resulting from infection by Bartonella henselae or Bartonella quintana, gram-negative rods. Cats are the reservoirs for B henselae whereas humans are the reservoirs for B quintana with the vector being the human body louse (Aung).
Bacillary angiomatosis most commonly affects skin and subcutaneous tissue, and these lesions can erode into the underlying bones. Disseminated disease can involve the visceral, especially liver (peliosis hepatis), spleen and lymph nodes (Aung, Folpe).
Of note, peliosis is characterized by multiple blood-filled cavities within a visceral organ and can be associated with infections, hematological malignancies, drug use (e.g. steroids, OCPs), among others (Tsokos).
Differential diagnosis includes granulation tissue (clinical history, clear endothelial cells, neutrophils and debris will favor bacillary angiomatosis) and Kaposi sarcoma (KS will exhibit spindled cells forming slit-like spaces).
It is seen most often in HIV positive patients, but have been described in other immunocompromised patients. The lesions present as a red or purple nodule on the skin or mucosa.
Aung K, et al. Bacillary Angiomatosis:eMedicine. Last updated on Sept 6 2011. Available at: emedicine.medscape.com/article/212737-overview
Folpe AL, Inwards CY. Bone and Soft Tissue Pathology: Foundations in Diagnostic Pathology Philadelphia, PA: Elsevier; 2010: 167-8.
Tsokos M, Erbersdobler A. Pathology of peliosis. Forensic Sci Int. 2005 Apr 20;149(1):25-33.