Coxiella burnetii may be associated with exposure via any mammal, but most commonly cattle, goats, sheep, and pets, including cats and dogs. Infected mammals shed C. burnetii in their urine, feces, milk, and placenta. The mode of transmission is either aerosol or by tick bite; high concentrations of the organism can be found in birth products of infected animals. The incubation period is approximately 3 weeks. The acute disease is a self-limiting “flu-like” illness characterized by high fever, rigor, headache, myalgias, cough, and arthralgia. Pneumonia may be accompanied by granulomatous hepatitis. Radiologic findings include lobar or segmental alveolar opacities which may be multiple. Other manifestations may include maculopapular or purpuric rash, aseptic meningitis or encephalitis, hemolytic anemia, endocarditis, pericarditis, pancreatitis, or epididymo-orchitis. Rarely C. burnetii is associated with chronic Q fever, which is defined as infection lasting for more than 6 months and endocarditis is the predominant manifestation.

Pneumonia due to C. psittaci usually occurs after exposure to infected birds. Infection in birds is usually asymptomatic, or may cause illness associated with ruffled feathers, ocular or nasal discharge. The organism is shed in feces, urine, and respiratory secretions. Humans are usually infected by inhalation of organisms in dried feces or in bird feather dust. Cage cleaning may pose an infection risk The onset is often insidious with nonproductive cough, fever, and headache, but may be abrupt. The incubation period is usually 5 to 15 days. Clinical clues include pharyngeal erythema, splenomegaly (which tends to occur toward the end of the first week), and a rarely seen specific rash (Horder’s spots – pink blanching maculopapular eruption resembling rose spots of typhoid fever).

Francisella tularensis can cause primary tularemic pneumonia. Human infection occurs following contact with infected animals (hares or rabbits most common) or biting invertebrate vectors (most commonly ticks but also mosquitoes, horse flies, fleas, and lice). In one reported outbreak of tularemic pneumonia, a significant risk factor was mowing grass (presumably airborne transmission from close contact to rabbit habitats). Pneumonia may also occur from hematogenous spread after vector-borne (e.g., tick) infection. The incubation period following infection with F. tularensis is 3 to 5 days. The onset is usually abrupt with high fever, chills, cough (usually nonproductive, occasionally with hemoptysis), pleuritic chest pain, and diaphoresis.

Hantaviruses comprise a genus of enveloped viruses within the family Bunyaviridae and are associated with two severe, acute febrile illnesses: hemorrhagic fever with renal syndrome and hantavirus pulmonary syndrome (HPS). Hantaviruses are shed in the urine, feces, or saliva of acutely infected reservoir rodents; transmission to humans occurs via the aerosol route. Many patients describe encountering living or dead rodents or visible evidence of rodent infestation prior to illness. Typically the incubation period is 3 weeks after exposure. The clinical illness of HPS typically begins with a prodromal phase which is characterized by nonspecific manifestations such as fever, myalgias, headache, nausea, vomiting, abdominal pain, and cough. This phase typically lasts 3 to 8 days and is followed by the cardiopulmonary phase, which starts suddenly with tachypnea and shortness of breath and is followed by respiratory failure and shock. Chest radiograph shows noncardiogenic bilateral interstitial edema during this phase. Characteristically, the patient is hemoconcentrated and manifests significant thrombocytopenia.

Yersinia pestis, which causes plague pneumonia, is primarily a zoonotic infection of rodents and wild and domestic animals (most often a cat); humans are considered incidental hosts. Transmission occurs via bites of rodent fleas, scratches or bites from infected domestic cats, direct handling of infected animal tissues, or inhalation of respiratory secretions from infected animals. Infection in the United States is found mainly in the southwestern and Pacific coastal area. Incubation is usually 2 to 3 days. The disease may have an abrupt onset and usually begins with a painless cough with shortness of breath. Untreated pneumonic plague has a 40% to 90% mortality.

In the natural setting, inhalation anthrax is exceedingly uncommon and is classically referred to as woolsorter’s disease, because of the association with workers in wool mills who may inhale Bacillus anthracis spores. However, the potential use as a biologic weapon has brought increased interest to this pathogen. In the 2001 outbreak of probable bioterrorism-associated anthrax conducted through the US postal system, nine cases of inhalation anthrax were identified, resulting in four deaths. The incubation is variable; often less than 1 week but can be 6 weeks or longer. Initial symptoms are nonspecific with fever, malaise, chest pain, and nonproductive cough. This may be followed by brief improvement and then severe respiratory distress, shock, and death. Widened mediastinum (associated with hemorrhagic mediastinitis) without parenchymal infiltrates found on radiographic imaging (CT scan is most sensitive) is characteristic of inhalation anthrax. The diagnosis is often established with positive blood cultures that may initially be dismissed as contaminants.

In addition to inhalation anthrax syndrome, pneumonic plague, and pneumonic tularemia are possible agents of bioterrorism. Clustered cases occurring without the expected epidemiologic exposures to animals, insects, or environmental activities should raise the possibility of a bioterrorism event. Specific epidemiological, clinical, and microbiologic clues should lead to early suspicion and rapid activation of the health alert system, since laboratory confirmation of the agent could be delayed.

Pneumonia caused by respiratory viruses

Viruses account for an important number of pneumonias in adults, especially during the winter months and amongst the elderly. Recent studies suggest that approximately one-third of adults hospitalized for pneumonia had a viral etiology. Many of the emerging infections have been associated with newly identified viruses (many zoonotic): Middle East respiratory syndrome coronavirus (MERS-CoV), and the new avian influenza A viruses (H5N1 and H7N9). Influenza and respiratory syncytial virus (RSV) are the most commonly identified viral pathogens; others include parainfluenza virus, rhinovirus, coronavirus, and possibly human metapneumovirus (although pneumonia is uncommon). Influenza should be considered during periods of peak activity within a community and is often associated with sudden fever, myalgias, and cough. RSV is a more common cause of pneumonia in immunocompetent adults than previously appreciated. Characteristics include seasonal occurrence (winter), and association of bronchospasm. MERS-CoV is a novel coronavirus which was identified in the fall of 2012 in Saudi Arabia; it is different than the coronavirus previously associated with SARS (severe acute respiratory syndrome). Most persons who became infected developed a severe acute respiratory illness and about half died. Investigations are underway to determine the source; transmission from person to person and to healthcare providers has occurred. There have been no cases reported in the United States but clinicians are advised to consider this in patients with compatible illnesses who have traveled to the Arabian Peninsula or neighboring countries. A new strain of avian influenza (influenza A H7N9) was recently identified in China. Most of the people infected have had contact with poultry and available evidence suggests there has not been ongoing spread from person to person. Symptoms have started with high fever and cough. While mild cases have been seen, most patients have had severe respiratory illness with a mortality rate of 28% at the time of this writing.

Diagnosis

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