What's the likely diagnosis & the 1st-line treatment for this patient's condition?

40-year-old man with fever, chills & multiple skin lesions on his arms & legs for around a month. He had a h/o HIV infection & was on ART. O/E, lesions appeared as shown. His CD4 count is 80 cells/mm³, & microscopy finding of the lesion biopsy reveals lobular proliferation of small vessels & mixed cells, predominantly neutrophilic. Warthin-Starry staining finding is shown.