Pneumonia

Pneumonia

An acute nonspecific infection of the small air sacs (alveoli) and issue of the lung, with fever, productive cough (meaning that sputum is coughed up), acute chest pain, chills, and shortness of breath; clinically diagnosed by abnormal chest sounds and chest radiographs.

An acute nonspecific infection of the small air sacs (alveoli) and issue of the lung, with fever, productive cough (meaning that sputum is coughed up), acute chest pain, chills, and shortness of breath; clinically diagnosed by abnormal chest sounds and chest radiographs. Pneumonia is often a secondary infection that follows a primary viral respiratory infection. About 2 million people have pneumonia and dehydration from severe diarrhea are the leading causes of death. Certain specific types of pneumonia are nationally notifiable diseases in the U.S. For example, 1,085 U.S. cases of legionellosis and25 U.S. cases of psittacosis were reported to the CDC during 2001.

Etiologic Agent

Pneumonia may be caused by Gram-positive or Gram-negative bacteria, mycoplasmas, chlamydias, viruses, fungi, or protozoa. Community-acquired bacterial pneumonia is most frequently caused by Streptococcus pneumoniae (pneumococcal neumonia). S. pneumoniae is the most common cause of pneumonia in the world. Other bacterial pathogens include Haemophilus influenzae, Staphylococcus aureus, Klebsiella pneumoniae, and occasionally other Gram-negative bacilli and anaerobic members of the oral flora. Atypical pathogens include Legionella (legionellosis), Mycoplasma pneumoniae (mycoplasmal pneumonia; primary atypical pneumonia). Psittacosis (ornithosis; parrot fever), a type of pneumonia caused by Chlamydia psittaci, is normally acquired by inhalation of respiratory secretions and desccated droppings of infected birds (e.g., parrots, parakeets). Fungi such as Histoplasma capsulatum (histoplasmosis), Coccidioides immitis (coccidioidomycosis), Candida albicans (candidiasis), Cryptococcus neoformans (cryptococcosis), Blastomyces (blastomycosis), Aspergillus (aspergillosis), and Pneumocystis jiroveci (previously considered to be protozoan) may be etiologic agents of pneumonia, especially in immunosuppressed patients; a condition known as mucormycosis (zygomycosis). Viral pneumonia may be caused byadenoviruses, respiratory syncytial virus (RSV), parainfluenza viruses, cytomegalovirus, measles virus, chickenpox virus, and other viruses. Hospital-acquired bacterial pneumonia is most often caused by Gram-negative bacilli, especially Klebsiella, Enterobacter, Serratia, and Acinetobacter species. Pseudomonas aeruginosa and S. aureus are also frequent causes nosocomial pneumonias. Pneumonia is the most common fatal infection acquired in hospitals.

Reservoirs and Mode of Transmission

In most cases, infected humans; other reservoirs include infectd psittacine birds (parrots and parakeets) in psittacosis, soil and bird droppings in histoplasmosis and cyptococcosis. Depending on the pathogen involved, transmission is by droplet inhalation, direct oral contact, contact with contaminated hands and fomites, or inhalation of yeasts and fungal spores.

Diagnosis

A good quality sputum specimen (coughed up from the patient’s lungs) must be sent to the microbiology laboratory for C&S. It must be sputum – not saliva. A laboratory work-up of saliva will not provide clinically relevant information. Laboratory personnel can differentiate between saliva and sputum by preparing and examining a Gram-stained smear of the specimen. Sputum will contain numerous white blood cells (WBCs) and few epithelial cells, whereas saliva will contain few (if any) WBCs and numerous epithelial cells.

 

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