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Pneumonia is an inflammation of the lung parenchyma. Consolidation of the lung tissue may be identified by physical examination and chest x-ray. From an anatomical point of view, lobar pneumonia denotes an alveolar process involving an entire lobe of the lung while bronchopneumonia describes an alveolar process occurring in a distribution that is patchy without filling an entire lobe. Numerous factors, including environmental contaminants and autoimmune diseases, as well as infection, may cause pneumonia. The various infectious agents that cause pneumonia are categorized in many ways for purposes of laboratory testing, epidemiologic study and choice of therapy. Pneumonias occurring in usually healthy persons not confined to an institution are classified as community-acquired pneumonias. Infections arise while a patient is hospitalized or living in an institution such as a nursing home are called hospital-acquired or nosocomial pneumonias. Etiologic pathogens associated with community-acquired and hospital-acquired pneumonias are somewhat different. However, many organisms can cause both types of infections.



Bacterial pneumonias

Streptococcus pneumoniae is the most common agent of community-acquired acute bacterial pneumonia. More than 80 serotypes, as determined by capsular polysaccharides, are known, but 23 serotypes account for over 90% of all pneumococcal pneumonias in the United States. Pneumonias caused by other streptococci are uncommon. Streptococcus pyogenes pneumonia is often associated with a hemorrhagic pneumonitis and empyema. Community-acquired pneumonias caused by Staphylococcus aureus are also uncommon and usually occur after influenza or from staphylococcal bacteremia. Infections due to Haemophilus influenzae (usually nontypable) and Klebsiella pneumoniaeare more common among patients over 50 years old who have chronic obstructive lung disease or alcoholism.

The most common agents of nosocomial pneumonias are aerobic gram-negative bacilli that rarely cause pneumonia in healthy individuals. Pseudomonas aeruginosa, Escherichia coli, Enterobacter, Proteus, and Klebsiella species are often identified. Less common agents causing pneumonias include Francisella tularensis, the agent of tularemia;Yersinia pestis, the agent of plague; and Neisseria meningitidis, which usually causes meningitis but can be associated with pneumonia, especially among military recruits. Xanthomonas pseudomallei causes melioidosis, a chronic pneumonia in Southeast Asia.

Mycobacterium tuberculosis can cause pneumonia. Although the incidence of tuberculosis is low in industrialized countries, M tuberculosis infections still continue to be a significant public health problem in the United States, particularly among immigrants from developing countries, intravenous drug abusers, patients infected with human immunodeficiency virus (HIV), and the institutionalized elderly. Atypical Mycobacterium species can cause lung disease indistinguishable from tuberculosis.

Aspiration pneumonias

Aspiration pneumonia from anaerobic organisms usually occurs in patients with periodontal disease or depressed consciousness. The bacteria involved are usually part the oral flora and cultures generally show a mixed bacterial growth. Actinomyces, Bacteroides, Peptostreptococcus, Veilonella, Propionibacterium, Eubacterium, andFusobacterium spp are often isolated.

Atypical pneumonias

Atypical pneumonias are those that are not typical bacterial lobar pneumonias. Mycoplasma pneumoniae produces pneumonia most commonly in young people between 5 and 19 years of age. Outbreaks have been reported among military recruits and college students.

Legionella species, including L pneumophila, can cause a wide range of clinical manifestations. The 1976 outbreak in Philadelphia was manifested as a typical serious pneumonia in affected individuals, with a mortality of 17% (see Ch. 40). These organisms can survive in water and cause pneumonia by inhalation from aerosolized tap water, respiratory devices, air conditioners and showers. They also have been reported to cause nosocomial pneumonias.

Chlamydia spp noted to cause pneumonitis are C trachomatis, C psittaci and C pneumoniaeChlamydia trachomatiscauses pneumonia in neonates and young infants. C psittaci is a known cause for occupational pneumonitis in bird handlers such as turkey farmers. Chlamydia pneumoniae has been associated with outbreaks of pneumonia in military recruits and on college campuses.

Coxiella burnetii the rickettsia responsible for Q fever, is acquired by inhalation of aerosols from infected animal placentas and feces. Pneumonitis is one of the major manifestations of this systemic infection.

Viral pneumonias are rare in healthy civilian adults. An exception is the viral pneumonia caused by influenza viruses, which can have a high mortality in the elderly and in patients with underlying disease. A serious complication following influenza virus infection is a secondary bacterial pneumonia, particularly staphylococcal pneumonia. Respiratory syncytial virus can cause serious pneumonia among infants as well as outbreaks among institutionalized adults. Adenoviruses may also cause pneumonia, serotypes 1,2,3,7 and 7a have been associated with a severe, fatal pneumonia in infants. Although varicella-zoster virus pneumonitis is rare in children, it is not uncommon in individuals over 19 years old. Morality can be as high as 10% to 30%. Measles pneumonia may occur in adults.

Other pneumonias and immunosuppression

Cytomegalovirus is well known for causing congenital infections in neonates, as well as the mononucleosis-like illness seen in adults. However, among its manifestations in immunocompromised individuals is a severe and often fatal pneumonitis. Herpes simplex virus also causes a pneumonia in this population. Giant-cell pneumonia is a serious complication of measles and has been found in children with immunodeficiency disorders or underlying cancers who receive live attenuated measles vaccine. Actinomyces and Nocardia spp can cause pneumonitis, particularly in immunocompromised hosts.

Among the fungi, Cryptococcus neoformans and Sporothrix schenckii are found worldwide, whereas Blastomyces dermatitidis, Coccidioides immitis, Histoplasma capsulatum and Paracoccidioides brasiliensis have specific geographic distributions. All can cause pneumonias, which are usually chronic and possible clinically inapparent in normal hosts, but are manifested as more serious diseases in immunocompromised patients. Other fungi, such as Aspergillus and Candida spp, occasionally are responsible for pneumonias in severely ill or immunosuppressed patients and neonates.

Pneumocystis carinii produces a life-threatening pneumonia among patients immunosuppressed by acquired immune deficiency syndrome (AIDS), hematologic cancers, or medical therapy. It is the most common cause of pneumonia among patients with AIDS when the CD4 cell counts drop below 200/mm3.

Pathogenesis and Clinical Manifestations

Infectious agents gain access to the lower respiratory tract by the inhalation of aerosolized material, by aspiration of upper airway flora, or by hematogenous seeding. Pneumonia occurs when lung defense mechanisms are diminished or overwhelmed. The major symptoms or pneumonia are cough, chest pain, fever, shortness of breath and sputum production. Patients are tachycardic. Headache, confusion, abdominal pain, nausea, vomiting and diarrhea may be present, depending on the age of the patient and the organisms involved.

Microbiologic Diagnosis

Etiologic diagnosis of pneumonia on clinical grounds alone is almost impossible. Sputum should be examined for a predominant organism in any patient suspected to have a bacterial pneumonia; blood and pleural fluid (if present) should be cultured. A sputum specimen with fewer than 10 while cells per high-power field under a microscope is considered to be contaminated with oral secretions and is unsatisfactory for diagnosis. Acid-fast stains and cultures are used to identify Mycobacterium and Nocardia spp. Most fungal pneumonias are diagnosed on the basis of culture of sputum or lung tissue. Viral infection may be diagnosed by demonstration of antigen in secretions or cultures or by an antibody response. Serologic studies can be used to identify viruses, M pneumoniae, C. burnetii, Chlamydia species, Legionella, Francisella, and Yersinia. A rise in serum cold agglutinins may be associated with M pneumoniaeinfection, but the test is positive in only about 60% of patients with this pathogen.

Rapid diagnostic tests, as described in previous sections, are available to identify respiratory viruses: the fluorescent-antibody test is used for Legionella. A sputum quellung test can specify S pneumoniae by serotype. Enzyme-linked immunoassay, DNA probe and polymerase chain reaction methods are available for many agents causing respiratory infections.

Some organisms that may colonize the respiratory tract are considered to be pathogens only when they are shown to be invading the parenchyma. Diagnosis of pneumonia due to cytomegalovirus, herpes simplex virus, Aspergillus spp. or Candida spp require specimens obtained by transbronchial or open-lung biopsy. Pneumocystis carinii can be found by silver stain of expectorated sputum. However, if the sputum is negative, deeper specimens from the lower respiratory tract obtained by bronchoscopy or by lung biopsy are needed for confirmatory diagnosis.

Prevention and Treatment

Until the organism causing the infection is identified, decisions on therapy are based upon clinical history, including history of exposure, age, underlying disease and previous therapies, past pneumonias, geographic location, severity of illness, clinical symptoms, and sputum examination. Once a diagnosis is made, therapy is directed at the specific organism responsible.

The pneumococcal vaccine should be given to patients at high risk for developing pneumococcal infections, including asplenic patients, the elderly and any patients immunocompromised through disease or medical therapy. Yearly influenza vaccinations should also be provided for these particular groups. An enteric-coated vaccine prepared from certain serotypes of adenoviruses is available, but is only used in military recruits. In AIDS patients, trimethoprim/sulfamethoxazole, aerosolized pentamidine or other antimicrobials can be given for prophylaxis of Pneumocystis carinii infections.

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Infectious arthritis is joint pain, soreness, stiffness, and swelling caused by an infectious agent such as bacteria, viruses or fungi. These infections can enter a joint various ways: After spreading through the bloodstream from another part of the body, such as the lungs during pneumonia.

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