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Many different microorganisms can infect the lung parenchyma; this chapter focuses on zoonoses, human commensals, and other unusual bacterial pathogens. Routes of spread to the lungs are few, clinical presentations overlap, radiologic appearance often is nonspecific, and pathophysiologic mechanisms are limited. Making a clinical diagnosis of pneumonia is relatively easy; defining the causative agent can be difficult. The search for the specific etiologic agent is driven by the desire to administer a specific antimicrobial drug as soon as possible hopefully to avoid progressive functional impairment and to contain spread to other individuals or to a community. Identifying a pathogen may, in turn, expand epidemiologic considerations and public health awareness.

In the almost four decades that have passed since the epidemic of acute respiratory disease due to Legionella erupted among delegates to the American Legion Convention in Philadelphia in 1976, physicians, microbiologists, and epidemiologists have become increasingly prepared for the diagnostic challenges of patients with unusual pneumonias. More recent examples of this multidisciplinary approach include the rapid diagnosis of Q fever (due to Coxiella burnetii), early identification of cases of Chlamydia pneumoniae and Chlamydia psittaci, and the etiology of acute respiratory infections due to obscure viral agents like Hantavirus pulmonary syndrome and the newer Coronavirus respiratory ­illnesses including severe acute respiratory syndrome (SARS). SARS was an excellent example of a zoonotic infection with an initiating event and subsequent person-to-person spread. Similarly, Middle East respiratory syndrome coronavirus (MERS) has been linked to dromedary camel exposure, with subsequent inter-human spread. The complexity of the definition of an etiologic agent in the diagnosis of pneumonia of obscure origin is highlighted in many of the examples mentioned here. Most are discussed in detail elsewhere in this text; the focus will be exclusively on bacterial pathogens most often associated with animals or animal products.

Zoonotic infections can reflect an initial event, followed by ­person-to-person secondary cases, typical of viral diseases like SARS coronavirus, and occasionally observed with Yersinia pestis. While the avian influenza virus (H5N1) has primarily spread to individuals in proximity to chickens, the novel H1N1 influenza in 2009 to 2010 demonstrated an anthropocentric cycle that led to a worldwide pandemic, despite being of zoonotic origin. New zoonoses are continually emerging: A recent outbreak in a monkey colony of deadly titi monkey adenovirus, not known to infect humans, was shown to infect a researcher and a family member, without further spread.1 Similarly, Streptococcus equi subsp. zooepidemicus (Streptococcus zooepidemicus)2 has recently been shown to cause pneumonia in humans after exposure to horses, dogs, and unpasteurized milk.

Companion animals have been the etiology of numerous zoonotic pneumonias, including Pasteurella multocida,3,4 Chlamydia psittaci,5 Rhodococcus, and Bordetella bronchiseptica (especially in immunocompromised hosts69). In a study in Spain, contact with pets was associated with an increased risk for community-acquired pneumonia, which tended to be higher as the number of pets increased; ...

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