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

In the decades that have passed since the epidemic of acute respiratory “Legionnaire’s 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. Some newer zoonotic pathogens with primarily pulmonary manifestations include Sin Nombre virus (hantavirus pulmonary syndrome) and certain coronaviruses (Chapter 126). Zoonotic viral pathogens that can be transmitted from animal to human, and then human to human, including influenza and the coronaviruses, have the greatest potential for rapid pandemic spread; it is likely that new viral respiratory pathogens will continue to emerge. Viral pathogens are discussed in detail elsewhere; the focus of this chapter is exclusively bacterial pathogens most often associated with animals, animal products, or the environment.

Global surveillance and cooperation have enabled more rapid recognition of new infectious syndromes, and the use of molecular techniques has allowed for rapid development of diagnostic tools that are critical for understanding the animal origin of pathogens and routes of transmission. Zoonotic infections can reflect an initial event, followed by person-to-person secondary cases, typical of viral diseases like the severe acute respiratory syndrome coronaviruses (SARS-CoV-1 and SARS-CoV-2) and occasionally observed with Yersinia pestis. While avian influenza virus (H5N1) primarily spread to individuals in proximity to chickens, the novel H1N1 influenza virus in 2009–2010 demonstrated an anthropocentric cycle that led to a worldwide pandemic, despite being of zoonotic origin; aborting further influenza spead via vaccination of humans can decrease the risk of reverse zoonoses (which could help break the cycle of further hman transmission).1a New zoonoses are always emerging: an 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 been shown to cause pneumonia in humans after exposure to horses,2a dogs, and unpasteurized milk.

Companion animals have been the source of numerous zoonotic pneumonias, including Pasteurella multocida,3,4 Chlamydia psittaci,5 Rhodococcus, and Bordetella bronchiseptica hosts6–9,9a (Tables 138-1 and 122-5). In a study in Spain, ...

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