Community-Acquired Pneumonia: A Global Perspective

Richard G. Wunderink, MD

Charles Feldman, MBBCh, DSc, PhD, FRCP, FCP (SA)

Community-acquired pneumonia (CAP) is a truly global disease, and remains the leading infectious disease cause of hospitalization, morbidity, and death in both the developed and developing world. Therefore, that this journal has published several global perspectives on CAP over the years is hardly surprising. Since the last CAP-focused issue was in 2016, an update on this topic in 2020 appeared appropriate. Clinical and microbiological differences in CAP in different regions of the world clearly exist, based on differences in population ages and smoking habit, underlying comorbid conditions, rates of influenza and pneumococcal vaccination, and access to healthcare. Epidemiological patterns of pathogens in different geographical regions are also changing. The guest editors of this edition of the journal invited leading experts around the world to contribute chapters describing the current status of various different microorganisms causing CAP, both in their regions and globally.

Traditionally, Streptococcus pneumoniae (pneumococcus) has been described as the most common cause of CAP. However, in countries such as the United States, the pneumococcus is becoming a less common cause of CAP, attributed to an increasing use of pneumococcal conjugate vaccination, with both individual and herd protections, as well as a decrease in smoking in the country. These factors, together with an aging population and use of more sensitive diagnostic testing, have resulted in viruses, most commonly influenza but including other respiratory viruses, being documented more commonly in CAP than the pneumococcus. However, in Europe and sub-Saharan Africa, pneumococcus still remains one of the most common causes of CAP, in the case of sub-Saharan Africa largely due to underlying human immunodeficiency virus infection, with persistently high levels of pneumococcal infection despite successful rollout of antiretroviral therapy. With regard to viruses, coronavirus (CoV) infections are emerging infectious diseases, with worldwide importance and the potential for severe outbreaks and global pandemics. In 2003, severe acute respiratory syndrome (SARS)-CoV emerged in China and almost 10 years later, Middle East respiratory syndrome (MERS)-CoV emerged, the latter still causing episodes of severe pneumonia with high case fatality rate. The most recent outbreak of coronavirus infection, with SARS-CoV-2, emerged in China in December 2019, and has now become pandemic; however, this edition of the journal does not carry a review of the topic since the infection was still evolving when the manuscripts to be included in the edition were being finalized.

Staphylococcus aureus has been an important emergent etiology of CAP over the past two decades, with severe CAP due to methicillin-resistant S. aureus leading to critical illness and death. Similarly, gram-negative pathogens with increasing levels of antibiotic resistance, particularly Pseudomonas aeruginosa, Enterobacteriaceae, and Acinetobacter baumannii, are being increasingly recognized as important pathogens in CAP, whose presence in different geographical locations is dependent of local epidemiology. The need for early recognition of all these pathogens is essential as they are associated with more severe disease, poorer outcomes, and limited therapeutic options.

Melioidosis, caused by the facultative intracellular gram-negative pathogen Burkholderia pseudomallei, is an emerging cause of CAP across the tropics. Recent modeling and epidemiological studies have demonstrated the presence of this pathogen in previously unrecognized regions of the world, with a global mortality of 90,000 deaths per year. Of concern is that global warming may be conducive for the development of B. pseudomallei infection, particularly in combination with the pandemic of diabetes mellitus among most populations. Q-fever, caused by Coxiella burnetii, an obligate intracellular pathogen that can cause CAP, has a varying geographical distribution. It is a zoonotic infection that occurs more frequently in people in rural areas in contact with animals, but human-to-human transmission can occur via infected aerosols. Nocardia spp. are a rare cause of pneumonia globally, but can cause life-threatening infections in immunocompromised hosts, such as those with solid-organ transplant and/or taking immunosuppressive drugs. While the lungs are most commonly affected, other organs can also be involved. While a rare cause of CAP, Nocardia can cause severe CAP in patients with comorbidities and compromised immunity.

In endemic areas, dimorphic fungal infections with Histoplasma capsulatum, Blastomyces dermatitidis, and Coccidioides posadasii/immitis account for up to 30% of cases of CAP, but are often indistinguishable from the more common bacterial causes. While highly endemic in areas of North America, they have global significance not only because of established areas of endemicity on all inhabited continents, but also the increasingly interconnected world of travelers and goods, as well as global warming. Lastly, although well documented in the literature, it is not commonly recognized that Mycobacterium tuberculosis is an important and common, if not the most common, cause of CAP in some areas of the world, such as sub-Saharan Africa, Asia, and South America. Tuberculosis is clinically indistinguishable from other common causes of CAP and has been documented to occur together with other pathogenic causes of CAP. It is an important cause of severe CAP in patients requiring intensive care unit admission. Suspicion and documentation as soon as possible are critical to rapidly institute appropriate chemotherapy.

The guest editors owe a debt of gratitude to all the authors of this edition of the journal, who gave of their time to prepare truly magnificent manuscripts and to bring this edition to fruition. The truly global nature of this edition can be seen by reviewing the author list of each chapter, which in addition to having authors from different parts of the world, also notes individuals from different institutions and even different countries collaborating together in individual chapters to bring this text to finality.