Pneumonia

Description

Alternative Names

Causes

Bacteria are the most common causes of pneumonia, but these infections can also be caused by other microbial organisms. It is often impossible to identify the specific culprit.

Many bacteria are categorized by the laboratory procedure used to visualize bacteria under a microscope. Bacteria are stained to determine if they are gram-negative or gram-positive bacteria. This gives the physician an idea of the severity of the pneumonia and how to treat it since different bacteria are treated with different drugs.

Gram-Positive Bacteria. These bacteria appear blue on the stain and are the most common organisms found in pneumonia. They include the following:

• The most common cause of pneumonia is the gram-positive bacterium Streptococcus pneumoniae (also called S. pneumoniae or the pneumococcus). It accounts for about 20% to 60% all community-acquired bacterial pneumonias (CAPs) in adults. Studies also suggest it causes between 13% to 38% of CAP in children.
• Staphylococcus (S.) aureus, the other major gram-positive bacterium responsible for pneumonia, accounts for about 2% of community-acquired pneumonias and between 10% and 15% of hospital-originated pneumonias. It is the organism most often associated with viral influenza, and can develop about five days after the onset of flu symptoms. Pneumonia from S. aureus most often occurs in people with weakened immune systems, very young children, hospitalized patients, and drug abusers who use needles. It is uncommon in healthy adults.
• Streptococcus pyogenes or Group A Streptococcus.

Gram-Negative Bacteria. These bacteria stain pink. Gram-negative bacteria are common infectious agents in hospitalized or nursing home patients, children with cystic fibrosis, and people with chronic lung conditions.

• Haemophilus (H.) influenzae is the second most common organism causing community acquired pneumonia and accounts for 3% to 10% of all cases (generally occurring in patients with chronic lung disease, older patients, and alcoholics).
• Klebsiella pneumoniae may be responsible for pneumonia in alcoholics and in other people who are physically debilitated. It is also associated with recent use of potent antibiotics.
• Pseudomonas aeruginosa is a major cause of pneumonia that occurs in the hospital (nosocomial pneumonia). It is a common pneumonia in patients with chronic or severe lung disease.
• Moraxella catarrhalis is found in everyone's nose and mouth. Experts have identified this bacterium as an uncommon cause of certain pneumonias, particularly in people with lung problems, such as asthma or emphysema.
• Neisseria meningitidis is one of the most common causes of meningitis (central nervous system infection), but the organism has been reported in pneumonia, particularly in epidemics of military recruits.
• Other gram-negative bacteria that cause pneumonia include E. coli (a cause in newborns and also associated with recent antibiotic use), Proteus (found in damaged lung tissue), and Enterobacter.

Atypical Pneumonia

Atypical pneumonias are generally caused by tiny nonbacterial organisms and produce mild symptoms with a dry cough. Hospitalization is uncommon with pneumonia from these organisms, but there are exceptions. They include the following:

• Mycoplasma pneumoniae (M. pneumoniae) is the most common nonbacterial pneumonia. Mycoplasma is a very small organism that lacks a cell wall. It spreads from prolonged, close contact and is most often found in school-aged children and young adults. The condition is usually mild and is commonly known as walking pneumonia. Estimates of its prevalence in community acquired pneumonias in adults range from 1.9% to 30%. In one study, it accounted for over a third of pneumonia cases in children.
• Chlamydia pneumoniae (C. pneumoniae) is now thought to cause 10% of all community-acquired cases of pneumonia. This atypical pneumonia is most common in young adults and children, where it is usually mild. In one study, it was the cause of 14% of cases in a group of children with pneumonia. While less common in the elderly, it can be very severe in this population.
• Legionnaire's disease, first diagnosed in 1976, is caused by the organism Legionella pneumophila, and is acquired by breathing droplets of contaminated water. Outbreaks have most often been reported in hotels, cruise ships, and office buildings where people are exposed to contaminated droplets from cooling towers and evaporative condensers. They have also been reported after exposure to whirlpools and saunas. Legionella is not passed on from person to person, but it may be much more common than once thought. Some experts even believe it causes 29% to 47% of all pneumonia cases. (Legionella is sometimes categorized as an atypical pneumonia.)

Viral Pneumonia

A number of viruses can cause pneumonia either directly or indirectly, and include the following:

• Influenza. Pneumonia is the major serious complication of viral influenza (the "flu") and can be very serious. It can develop about five days after flu symptoms start. Influenza is associated with pneumonia directly or indirectly by altering the mucous blanket and making a person susceptible to bacterial pneumonia.
• Respiratory syncytial virus (RSV). Most infants are infected with RSV at some point, but it is most often mild. Still, RSV is a major cause of pneumonia in infants and people with damaged immune systems. Some evidence suggests that it may increase the risk for the development of asthma. Studies indicate that RSV pneumonia may also be more common than previously thought in adults, especially the elderly.
• Severe acute respiratory syndrome (SARS). SARS is a respiratory infection caused by a new coronavirus, which appears to have jumped to humans from animals. It is a serious infection first identified in China that is transmitted by close contact with an infected individual.
• Human parainfluenza virus (HPV). HPV is second to RSV in causing pneumonia and bronchitis in children and is also an important cause of pneumonia in the elderly and in patients with damaged immune systems.
• Adenoviruses. Adenoviruses are common and ordinarily are not problematic, although they have been implicated in about 10% of childhood pneumonia.
• Herpesviruses. In adults, herpes simplex virus and varicella-zoster (the cause of chicken pox) are generally causes of pneumonia only in people with impaired immune systems.

Aspiration Pneumonia and Anaerobic Bacteria

The mouth harbors a mixture of bacteria that is harmless in its normal location but can cause a serious condition called aspiration pneumonia if it reaches the lung. This can happen during periods of altered consciousness, often when a patient is affected by drugs or alcohol, or after head injury or anesthesia. In such cases, the gag reflex is diminished, allowing these bacteria to enter the airways to the lung. These organisms are generally different from the usual microbes that enter the lung by inhalation. Many are often anaerobic (meaning they can live in the absence of oxygen).

Opportunistic Pneumonia

Impaired immunity leaves patients vulnerable to serious, even life-threatening, pneumonias known as opportunistic pneumonias. They are caused by microbes that are harmless to people with healthy immune systems. Infecting organisms include the following:

• Pneumocystis carinii, an atypical organism that is very common and generally harmless in people with healthy immune systems. It is the most common cause of pneumonia in AIDS patients.
• Fungi, such as Mycobacterium avium.
• Viruses, such as cytomegalovirus (CMV).

In addition to AIDS patients, other conditions also put patients at risk for opportunistic pneumonia. They include lymphomas, leukemias, and other cancers. Long-term use of corticosteroids and drugs known as immunosuppressants increase the susceptibility to these pneumonias.

Occupational and Regional Pneumonias

Exposure to chemicals can also cause inflammation and pneumonia. A number of people are exposed to pneumonia-causing organisms specific to particular occupations or regions.

• Anthrax. Workers exposed to cattle, pigs, sheep, and horses are at risk for pneumonia caused by anthrax, brucellosis, and Q fever. (Anthrax, of course, has become a major focus of concern because of its use in terrorist attacks in the US.)
• Coccidoidomycosis. Agricultural and construction workers in the Southwest are at risk for coccidoidomycosis, and those working in Ohio and the Mississippi Valley are at risk for histoplasmosis.
• Psittacosis. Workers exposed to pigeons, parrots, parakeets, and turkeys are at risk for psittacosis.
• Hantavirus. Hantavirus causes a dangerous form of lung disease and is carried by rodents, but is still rare. It does not appear to be contagious; cases have occurred in New Mexico, Arizona, California, Washington, and Mexico.
• Coccidioides immitis. People in the southwest are also exposed to the fungus Coccidioides immitis, the cause of Valley fever, which is a lung infection that can cause pneumonia in susceptible individuals.