Causes
Bacteria are the most common cause of pneumonia. However, pneumonia can also be caused by viruses, fungi, and other agents. It is often impossible to identify the specific culprit.
Many bacteria are grouped into one of two large categories by the laboratory procedure used to look at them under a microscope. The procedure is known as Gram staining. Bacteria are stained with special dyes, then washed in a special solution. The color of the bacteria after washing determines if they are gram-negative or gram-positive bacteria. Knowing which group the bacteria belong to helps your doctor determine how severe the disease is, and how to treat it. 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:
- Streptococcus pneumoniae (also called S. pneumoniae or the pneumococcus) is the most common cause of pneumonia . This gram-positive bacterium causes about 20 - 60% of 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, causes about 2% of CAPs and between 10 - 15% of hospital-acquired 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 commonly cause infections 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 (hospital-acquired pneumonia). It is a common cause of 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, Proteus (found in damaged lung tissue), and Enterobacter.
Atypical Pneumonia
Atypical pneumonias produce mild symptoms and a dry cough. Organisms that cause atypical pneumonias include:
- Mycoplasma pneumoniae (M. pneumoniae) is the most common atypical pneumonia organism. Mycoplasma is a very small bacterium that lacks a cell wall. Pneumonia caused by M. pneumoniae spreads when someone carrying the infection is in close contact with others for a long period of time. It is most often found in school-aged children and young adults. The condition, commonly called "walking pneumonia," is usually mild.
- 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, and it is usually mild. It is less common, but usually more severe, in the elderly.
- Legionella pneumophila causes Legionnaires' disease. You can catch it by breathing in drops 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 in people who have been near whirlpools and saunas. Legionella pneumophila is not passed on from person to person. Some experts believe the organism causes 29% to 47% of all pneumonia cases.
Viral Pneumonia
A number of viruses can cause pneumonia either directly or indirectly. They include:
- Influenza (Flu). Pneumonia is a major serious complication of the flu and can be very serious. It can develop about five days after flu symptoms start. The flu weakens the body's defense systems, making it easier for bacteria to grow in the lungs.
- 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 as well as people with damaged immune systems. Studies indicate that RSV pneumonia may be more common in adults than previously thought, especially the elderly.
- Severe acute respiratory syndrome (SARS). SARS is a respiratory infection caused by a newly-described coronavirus, which appears to have jumped from animals to humans. The disease was first reported in China in 2003.
- Human parainfluenza virus. This virus is a leading cause of pneumonia and bronchitis in children, the elderly and in patients with damaged immune systems.
- Adenoviruses. Adenoviruses are common and usually are not problematic, although they have been linked to about 10% of childhood pneumonia.
- Herpesviruses. In adults, herpes simplex virus and varicella-zoster (the cause of chicken pox) can cause pneumonia in people with impaired immune systems.
Aspiration Pneumonia and Anaerobic Bacteria
The mouth contains a mixture of bacteria that is normally harmless. However, if this mixture reaches the lungs, it can cause a serious condition called aspiration pneumonia. This may happen after head injury or general anesthesia, or when a patient takes drugs or alcohol. In such cases, the gag reflex doesn't work as well as it should, so bacteria can enter the airways to the lung. Unlike other organisms that are inhaled, bacteria that cause aspiration pneumonia do not need oxygen to live. These bacteria are called anaerobic bacteria.
Opportunistic Pneumonia
Impaired immunity leaves patients vulnerable to serious, life-threatening pneumonias known as opportunistic pneumonias. They are caused by organisms that are harmless to people with healthy immune systems. Infecting organisms include:
- Pneumocystis carinii, renamed Pneumocystis jiroveci in 2002, is an atypical organism. Originally thought to be protozoa, it is now classified as a fungus. P. jiroveci is very common and generally harmless in people with healthy immune systems. It is the most common cause of pneumonia in AIDS patients.
Click the icon to see an image of pneumocystis carinii.
- Fungi, such as Mycobacterium avium
- Viruses, such as cytomegalovirus (CMV)
Click the icon to see an image of CMV.
In addition to AIDS, other conditions also put patients at risk for opportunistic pneumonia. They include cancers such as lymphoma and leukemia. Long-term use of corticosteroids and drugs known as immunosuppressants also increase a person's risk for these pneumonias.
Occupational and Regional Pneumonias
Exposure to chemicals can also cause inflammation and pneumonia. Where you work and live can put you at higher risk for exposure to pneumonia-causing organisms.
- Workers exposed to cattle, pigs, sheep, and horses are at risk for pneumonia caused by anthrax, Brucella, and Coxiella burnetii, which causes Q fever/
Click the icon to see an image of inhalation anthrax.
- Agricultural and construction workers in the Southwest are at risk for coccidoidomycosis (Valley fever). The disease is caused by the spores of the fungus Coccidioides immitis.
- Those working in Ohio and the Mississippi Valley are at risk for histoplasmosis, a lung disease caused by the fungus Histoplasma capsulatum.
Click the icon to see an image of coccidoidomycosis.
- Workers exposed to pigeons, parrots, parakeets, and turkeys are at risk for psittacosis, a lung disease caused by the bacteria Chlamydia psittaci.
- Hantavirus causes a dangerous form of lung disease. This rare virus is carried by rodents. It does not spread from person to person. Cases have occurred in New Mexico, Arizona, California, Washington, and Mexico.
Click the icon to see an image of the hantavirus.
Severe Acute Respiratory Syndrome (SARS)Severe Acute Respiratory Syndrome (SARS) is a contagious respiratory infection that was recognized as a worldwide threat in 2003. It was first identified as a new disease by World Health Organization (WHO) physician Dr. Carlo Urbani. Urbani diagnosed it SARS in a 48-year-old American businessman, who had traveled from the Guangdong province of China through Hong Kong to Hanoi, Vietnam. The businessman died from the illness. Dr. Urbani died from SARS just a month later, on March 29, 2003 at the age of 46. SARS spread fast. Within 6 weeks of Urbani's discovery, the disease had infected thousands of people around the world on every continent except Antarctica. Schools had closed throughout Hong Kong and Singapore, and national economies were affected. The WHO officially identified SARS as a global health threat, and issued an unprecedented travel advisory. It wasn't clear at the time whether SARS would become a global pandemic, or would settle into a less aggressive pattern. The latter seems to have happened. As of a May 2005, there was no known SARS transmission anywhere in the world, according to the U.S. Centers for Disease Control and Prevention (CDC). The SARS outbreak is a dramatic example of how quickly world travel can spread a disease. According to reports from the CDC and WHO, more than 8,000 people became sick with SARS during the outbreak. Of that group, 774 died. The outbreak is also an example of how quickly a networked health monitoring system can respond to an emerging threatCauses And Risk Factors SARS is a serious form of atypical pneumonia that causes acute respiratory distress and sometimes death. It is caused by a new member of the coronavirus family (this family also includes the virus that causes the common cold). The discovery of the SARS-related virus represents one of the fastest identifications of a new organism in history. SARS is clearly spread by droplet contact. When someone with SARS coughs or sneezes, infected droplets are sprayed into the air. Like other coronaviruses, the SARS virus may live on hands, tissues, and other surfaces for up to 6 hours in these droplets and up to 3 hours after the droplets have dried. While droplet transmission through close contact has been responsible for most cases of SARS, there is evidence that SARS might also spread by infected droplets carried on hands and other objects the droplets had touched. Airborne transmission was a real possibility in some cases. Live virus had even been found in the stool of people with SARS, where it has been shown to survive for up to four days. And the virus may be able to live for months or years when the temperature is below freezing. With other coronaviruses, re-infection (contracting the same disease after recovery or during initial illness) is common. Preliminary reports suggest that this may also be the case with SARS. Estimates are that the incubation period is usually between two and ten days, although there have been documented cases where the onset of illness was considerably faster or slower. People with active symptoms of illness are clearly contagious. It is not known, however, how early contagion begins before symptoms appear, or how long contagion might linger after the symptoms have disappeared. Prevention The best way to prevent SARS is to avoid direct contact with people who have SARS, until 10 days after their fever and other symptoms are gone. Reduce travel to locations where there is an uncontrolled SARS outbreak. Always wash your hands. The CDC has identified hand hygiene as the cornerstone of SARS prevention. A soap and water wash works well, or you can use an alcohol-based instant hand sanitizer. Cover your mouth and nose when sneezing or coughing. Respiratory secretions should be considered to be infectious, Clean commonly touched surfaces with an EPA-approved disinfectant. In some situations, masks and goggles may be useful for preventing the spread of airborne or droplet infection. Gloves might be used in handling potentially infectious secretions.Vaccine In December 2004, the U.S. National Institute of Health began a small clinical trial to test a preventive SARS vaccine.Interim results from the trial showed the vaccine is safe and well tolerated. Chinese researchers began testing a SARS vaccine in May 2004. Symptoms The hallmark symptoms of SARS are fever greater than 100.4°F (38.0°C) and a dry cough, with difficulty breathing or other respiratory symptoms. The following symptoms, listed in order of how often they appeared, were found in more than half of the first SARS patients:
Less common symptoms (also in order) include:
Signs And Tests: Listening to the chest with a stethoscope (auscultation) may reveal abnormal lung sounds. In most people with SARS, progressive chest X-ray changes or chest CT changes demonstrate the presence of pneumonia. Much attention was given early in the outbreak to the development of a quick, sensitive test for SARS. Specific tests for the SARS virus include the PCR for SARS virus, antibody tests to SARS (such as ELISA or IFA), and direct SARS virus isolation. All current tests have some limitations. General tests used in the diagnosis of SARS might include:
Treatment: People suspected of having SARS should be evaluated immediately by a physician. Antibiotics are sometimes given in an attempt to treat bacterial causes of atypical pneumonia. Antiviral medications have also been used. High doses of steroids have been employed to reduce lung inflammation. In some serious cases, serum from people who have already gotten well from SARS (convalescent serum) has been given. Evidence of general benefit of these treatments has been inconclusive. Other supportive care such as supplemental oxygen, chest physiotherapy, or mechanical ventilation is sometimes needed. Prognosis: The overall worldwide death rate due to SARS at the end of the outbreaks was about 14 -15%, although it was much higher, up to 50%, in those over age 65. Many more were sick enough to require breathing assistance from a machine (mechanical ventilation). And many others required ICU care. Today, intensive public health policies are proving to be effective in controlling outbreaks. Many nations have stopped the epidemic within their own countries. All nations must be vigilant, however, to keep this disease under control. Complications:
Call Health Care Provider: Call your health care provider if you suspect you or someone you have had close contact with has SARS. |
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