Bacillary Angiomatosis

Bacillary angiomatosis is a vasoproliferative infection caused by Bartonella henselae or, less commonly, Bartonella quintana. It occurs almost exclusively in significantly immunocompromised patients, most typically those with advanced HIV infection and a low CD4 count.

It produces characteristic vascular skin and, in disseminated disease, visceral lesions that can closely mimic Kaposi sarcoma both clinically and, to a lesser extent, histologically.

The critical clinical distinction is that bacillary angiomatosis, unlike Kaposi sarcoma, is a treatable bacterial infection responding well to appropriate antibiotic therapy. Accurate diagnosis is therefore essential given the fundamentally different management implications.

Pathophysiology

• Bartonella henselae causes this condition. It is the same organism responsible for cat scratch disease in immunocompetent hosts, transmitted via cat scratch or bite, with the cat flea maintaining transmission between cats.
• Bartonella quintana also causes it. It is transmitted by the human body louse, historically associated with trench fever and more prevalent in homeless and lice infested populations.
• The organism induces a proliferative vascular response in the host through direct stimulation of endothelial cell proliferation and angiogenesis. This is distinct from the malignant, human herpesvirus 8 driven vascular proliferation underlying Kaposi sarcoma despite the sometimes similar clinical appearance.
• The profound immunosuppression characteristic of the typically affected population (advanced HIV infection with a low CD4 count, or other significant immunocompromise) is believed to permit the unusual, exuberant vascular proliferative host response to infection that characterises this condition.
• This contrasts with the typically self limiting, granulomatous lymphadenitis pattern of infection with the same organisms seen in immunocompetent individuals with cat scratch disease.

• Cutaneous lesions present as characteristic friable, vascular, red to purple papules or nodules, which bleed readily on minor trauma, ranging from a single lesion to numerous widespread lesions across the skin surface.
• The clinical appearance can closely resemble the violaceous lesions of Kaposi sarcoma, pyogenic granuloma, or, in some cases, other vascular or infective skin conditions.
• Visceral involvement, particularly hepatic and splenic (historically termed bacillary peliosis hepatis when affecting the liver), presents with fever, abdominal pain, hepatosplenomegaly and deranged liver function tests, reflecting the same vasoproliferative process affecting these internal organs.
• Bone involvement, presenting with localised pain and characteristic lytic lesions on imaging, together with, less commonly, central nervous system and other organ involvement, complete the spectrum of recognised disseminated disease manifestations.
• Constitutional symptoms including fever, malaise and weight loss frequently accompany more extensive or disseminated disease.
• A relevant exposure history (cat contact, or, particularly relevant for Bartonella quintana, homelessness or body louse exposure) should be sought, together with assessment of the degree of underlying immunosuppression given the strong association with advanced HIV infection specifically.

• Skin biopsy with histopathological examination is the key diagnostic investigation, showing characteristic lobular vascular proliferation with plump, protuberant endothelial cells, together with a mixed inflammatory infiltrate.
• Warthin Starry silver staining demonstrates the characteristic clusters of bacilli, an essential distinguishing histological feature from Kaposi sarcoma. Kaposi sarcoma lacks these organisms on staining despite the sometimes overlapping clinical appearance of the two conditions.
• Bartonella polymerase chain reaction testing on biopsy tissue offers specific molecular confirmation of the causative organism, and serology, while supportive, has more limited utility in this specific immunocompromised population given a frequently blunted or delayed antibody response reflecting the underlying immune impairment.
• HIV testing (where status is not already established) and CD4 count assessment are essential given the strong association with advanced HIV infection.
• Further imaging (abdominal ultrasound or computed tomography, and, where bone involvement is suspected clinically, plain radiography or further cross sectional imaging) is directed according to the specific pattern of symptoms suggesting visceral or disseminated involvement.
• Given the clinical overlap with Kaposi sarcoma, a low threshold for biopsy of any new vascular skin lesion in a significantly immunocompromised patient is essential, with the histological and specific staining findings providing definitive distinction between these two conditions with fundamentally different management approaches.

• Erythromycin 500 mg orally four times daily, or doxycycline 100 mg orally twice daily, is given for a minimum of three months for cutaneous disease (extended to a minimum of four months, and generally longer, for more extensive or visceral disease).
• This is standard first line antibiotic treatment, reflecting good evidence of efficacy against this organism and the need for a prolonged course given the risk of relapse with shorter treatment durations.
• Rifampicin may be added in combination for severe or extensive disease, particularly with central nervous system or significant visceral involvement, guided by specialist infectious disease input given the complexity of optimal combination and duration decisions in more severe presentations.
• Initiation or optimisation of effective antiretroviral therapy is an essential component of full management in HIV associated disease, addressing the underlying immunosuppression that permits this unusual disease manifestation and supporting durable resolution and prevention of recurrence alongside specific antibiotic treatment.
• Close monitoring of clinical response underpins management, with resolution of cutaneous and, where relevant, visceral lesions expected over the course of appropriate antibiotic treatment. A low threshold for extending treatment duration or reassessing the diagnosis where response is inadequate is also needed in this generally very treatable, though potentially serious if unrecognised or inadequately treated, condition.
• Given the recognised risk of relapse, particularly if antibiotic treatment is curtailed prematurely or the underlying immunosuppression remains inadequately addressed, sustained antiretroviral therapy with ongoing immune reconstitution represents an important long term preventive measure alongside completion of an adequate initial antibiotic treatment course.