Atypical Infections

The expansive chapter “Atypical Infections” provides an overview of over 20 separate and distinct atypical infections impacting the musculoskeletal system. These include nontuberculous/atypical mycobacterial infection, brucellosis, cat scratch disease, fungal and higher bacterial infections (including coccidioidomycosis, North American blastomycosis, South American blastomycosis, cryptococcosis, sporotrichosis, histoplasmosis, aspergillosis, candidiasis, mucormycosis, maduromycosis, actinomycosis, and nocardiosis), hydatid disease/echinococcus, leprosy, Lyme disease, melioidosis, syphilis, tuberculosis, and COVID-19. For each infectious entity, the subchapter delves into etiology, prevalence and epidemiology, pathophysiology, clinical presentation, and disease manifestations. Imaging findings, including unique imaging characteristics of each disease are thoroughly discussed, encompassing a spectrum of modalities, including radiography, magnetic resonance imaging (MRI), multidetector computed tomography (MDCT), ultrasound, various nuclear medicine studies, and positron emission tomography/computed tomography (PET/CT). A practical differential diagnosis is provided for each disease. Finally, a synopsis of treatment options, including available medical and surgical therapies, is discussed. Key points are provided for each disease entity. The chapter commences with a historical perspective on the recognition of atypical mycobacteria as human pathogens in the 1950s. It then explores human brucellosis, underscoring its systemic zoonotic nature and emphasizing diverse transmission modes. Cat scratch disease, first described in 1931, is detailed, underscoring its typically self-limited nature but with potential for serious illness in immunocompromised individuals. Various fungal infections are discussed, including their global distribution, dimorphic nature, and increased virulence in immunocompromised hosts. Hydatid disease caused by Echinococcus tapeworms is introduced, detailing the different species involved and their transmission dynamics between intermediate and definitive hosts. The chapter proceeds to address leprosy, Lyme disease, melioidosis, syphilis, and tuberculosis, detailing the distinctive features of each infectious entity. The chapter concludes with a description of the global impact of coronavirus disease 2019 (COVID-19), emphasizing its dynamic and evolving nature as new research data emerges. Overall, this chapter serves as a comprehensive guide, offering a detailed overview of the diverse atypical infectious diseases affecting the musculoskeletal system, with a strong emphasis on their imaging findings.