Community-Acquired Pneumonia
Streptococcus is the most common bacterial cause of community-acquired pneumonia. Other causes of community-acquired pneumonia include Haemophilus influenzae, Legionella, mycoplasma, chlamydia, and viruses. Staphylococcus may rarely cause community-acquired pneumonia, but this organism is more often a cause of hospital-acquired pneumonia. Community-acquired pneumonia occurs most commonly in very young and very old people.
Pneumococcal Pneumonia: Pneumococcal pneumonia is caused by Streptococcus pneumoniae (pneumococcus). It usually starts after an upper respiratory tract viral infection (a cold or influenza) damages the defenses of the airways enough to allow bacteria to infect the area. Shaking and chills are followed by a fever, a cough that produces sputum, shortness of breath, and chest pain on the side of the affected lung when breathing. Nausea, vomiting, fatigue, and muscle aches are also common. The sputum is often rust-colored from blood.
Pneumococcal pneumonia, although usually treatable, can be fatal, especially in the very young, the very old, and those people who have other severe illnesses. Pneumococcal pneumonia can lead to a life-threatening infection of the blood (pneumococcal septicemia). Many people develop fluid in the pleural space (the space between the two layers of the membranes covering the lungs [pleura]); this condition is called pleural effusion (see Section 4, Chapter 52). Rarely, pneumococcal infection can spread to the coverings of the brain (meninges), causing pneumococcal meningitis and leading to confusion, a stiff neck, seizures, and possibly coma.
A person who has been infected with one of the 80 known types of pneumococcus develops partial immunity to reinfection with that type but no immunity to the others. A vaccine is available that protects against 23 of the most prevalent and serious types of pneumococci that cause infection (see Section 17, Chapter 189).
Pneumococcal pneumonia may be treated with any of several types of antibiotics, including penicillins and cephalosporins, but the organism that causes pneumococcal pneumonia has developed increasing resistance to these drugs in the last decade. People who are infected with resistant strains of pneumococcus or who are allergic to penicillin are given erythromycin, a fluoroquinolone, or another antibiotic instead.
Pneumonia Caused by Haemophilus influenzae: Haemophilus influenzae is a bacterium; despite its name, it is not the influenza virus that causes the flu. Haemophilus influenzae type b strains are the most virulent and cause serious diseases, including meningitis, epiglottitis, and pneumonia, usually in children younger than 5 years. However, because of the widespread use of the Haemophilus influenzae type b vaccine, serious disease from this organism is becoming less common. This type of pneumonia is more common among Native Americans, Eskimos, blacks, people with sickle cell disease, and people with immune system disorders. Most such cases are caused by strains other than those used in the Haemophilus influenzae type b vaccine.
Signs of the infection include typical pneumonia symptoms, such as a fever, a cough that produces sputum, and shortness of breath.
Vaccination for type b strains of Haemophilus influenzae is recommended for all children. The vaccine is given in three doses--at ages 2 months, 4 months, and 6 months.
Antibiotics are used to treat Haemophilus influenzae type b pneumonia. Trimethoprim-sulfamethoxazole is commonly used and can be taken by mouth. Many other antibiotics work as well.
Legionnaires' Disease: Legionnaires' disease is caused by the bacterium Legionella pneumophila and other species of Legionella. It accounts for about 1 to 8% of all pneumonias and about 4% of fatal pneumonias acquired in hospitals. Legionella bacteria live in water, and outbreaks have occurred primarily in hotels and hospitals when the organism has spread through the air conditioning systems or water supplies, such as showers. No cases have been identified in which one person directly infected another.
Although Legionnaires' disease may occur at any age, people who are middle-aged and older have been affected most often. People who smoke tobacco, take corticosteroids, have chronic kidney failure, or have undergone organ transplantation seem to be at greater risk. Legionnaires' disease can be life threatening.
The first symptoms, appearing 2 to 10 days after the infection is transmitted, include fatigue, fever, headache, and muscle aches. A dry cough later becomes productive of sputum. People with serious infections can become extremely short of breath and may have diarrhea or mental disturbances.
Laboratory tests are performed on sputum, blood, and urine samples to confirm the diagnosis. Because people infected with Legionella pneumophila produce antibodies to fight the disease, blood tests show an increasing concentration of these antibodies. However, the results of antibody tests usually are not available until after the pneumonia has run its course.
Antibiotics, such as the fluoroquinolones, erythromycin (with or without rifampin), or azithromycin, are used for treatment. About 20% of the people who develop the disease die. The death rate is much higher among those who contract the disease in the hospital or who have an impaired immune system.
Mycoplasmal Pneumonia: Mycoplasma pneumoniae is the most common cause of pneumonia in people aged 5 to 35, but it is an uncommon cause in other people. Epidemics occur especially in confined groups such as students, military personnel, and families. The epidemics tend to spread slowly because the incubation period lasts 10 to 14 days. Most commonly, this type of pneumonia strikes in the spring.
Mycoplasmal pneumonia often starts with fatigue, a sore throat, and a dry cough and thus resembles influenza. The symptoms slowly worsen. Attacks of severe coughing may eventually produce sputum. About 10 to 20% of people develop a rash. Occasionally, anemia, joint pains, or neurologic problems (such as meningitis) develop. Symptoms often persist for 1 to 2 weeks, followed by slow improvement. Some people still feel weak and tired after several weeks. Although mycoplasmal pneumonia is usually mild and most people recover without treatment, severe cases do occur occasionally.
An x-ray shows that the person has pneumonia. Most laboratories do not offer blood tests that accurately identify mycoplasma.
When the symptoms and x-ray results lead a doctor to suspect mycoplasmal pneumonia, treatment is often started, even if mycoplasma has not been accurately identified. Erythromycin and doxycycline are the preferred antibiotics for treating mycoplasmal pneumonia. Clarithromycin, azithromycin, and the fluoroquinolones are also effective. Antibiotic treatment reduces the period of fever and lung involvement and promotes recovery. However, antibiotics do not cure mycoplasmal pneumonia immediately, because people treated with antibiotics continue to carry and spread the organism for several weeks.
Pneumonia Caused by Chlamydia pneumoniae: Chlamydia is another common cause of pneumonia in people aged 5 to 35. It also affects some older people. The disease is transmitted from person to person in tiny airborne droplets spread by coughing. The symptoms are similar to those of mycoplasmal pneumonia. Most people do not become seriously ill. X-rays show evidence of pneumonia, but the diagnosis of Chlamydia as the cause can only be established by blood tests and tests of sputum samples that are performed in research laboratories.
The antibiotics erythromycin, doxycycline, clarithromycin, azithromycin, and the fluoroquinolones are effective. If treatment is stopped too early, symptoms tend to return.
 See the sidebar Psittacosis: An Unusual Type of Pneumonia.
Viral Pneumonia: Many viruses can infect the lungs, causing viral pneumonia. Two types of influenza virus, called types A and B, cause pneumonia (see Section 17, Chapter 198). The chickenpox virus can also cause pneumonia in adults. Parainfluenza, respiratory syncytial virus, and adenovirus can cause pneumonia, sometimes more commonly in children and people who are very old. The measles virus also may cause pneumonia, especially in malnourished children. People of any age who have an impaired immune system may develop severe pneumonia from cytomegalovirus.
Viral pneumonias cause cough, which may be non-productive of sputum or productive of whitish sputum. Many people develop headache, fever, and muscle aches.
X-rays show a less dense pattern of infection than that seen with bacterial pneumonias. Stains of respiratory secretions can be used to detect some viruses, such as respiratory syncytial virus and influenza. Many viruses can be cultured, but culturing is time-consuming, expensive, and usually not helpful. It is also possible to show elevations of antibodies to specific viruses, but usually the person has recovered by the time the results are positive.
Annual influenza vaccinations are recommended for health care workers, older people, and people with chronic conditions such as emphysema, diabetes, heart disease, and kidney disease. Many viral pneumonias can be treated with drugs to kill the virus.
Viral pneumonia can cause a cough that lingers long after the infection has resolved. Additionally, many people develop secondary bacterial pneumonia after a viral infection because of the damage the viruses wreak on the lining of the airways; antibiotics may be needed when such a bacterial superinfection (an infection that develops while a person is being treated for another infection) occurs.
See the sidebar Drugs Used for Viral Pneumonias.
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