Infections of the Respiratory System

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Infections of the Respiratory System



Exposure to about 8 microbes per minute, or approximately 10,000 per day, makes the respiratory tract one of the most common sites for pathogenic infection in the body, as it is in direct contact with the physical environment and the microorganisms contained therein. Couple this with its warm, moist, barrier-layered surfaces and you have excellent conditions for trapping particles to allow for infections. From the common cold to pneumonia, the respiratory tract has been an avenue for the introduction of viruses and bacteria that have persisted with us through the ages, producing disease ranging in severity from merely irritable to lethal. The name “cold” is perhaps a misnomer that was used to describe the season in which the symptoms became more prevalent or the action of the organisms on thermogenesis, which is the process by which the body generates heat or energy by increasing the metabolic rate. The process of thermogenesis can be triggered by conditions such as wet hair, body parts exposed to drafts, or extended periods of time waiting in low temperatures.

Common cold symptoms were recorded in ancient Egypt and Hippocrates referred to the disease in the 400s bc. Early American Indian, Mayan, and Aztec cultures recorded descriptions of the cold; Aztec treatments consisted of chili pepper, honey, and tobacco mixtures.

Wet feet and wet clothes as possible causes for the common cold were stated early in the 1700s and 1800s by John Wesley and William Buchan, respectively. After the discovery of viruses in the late 1800s and their association with the common cold, acute viral nasopharyngitis became the descriptive name of this malady. While the role of bacteria in the germ theory of disease held sway early on, antibiotics failed to cure colds because of their viral cause although they did alleviate accompanying bacterial irritations and secondary infections.

Pneumonia, a respiratory tract disease, occurs as a result of inflamed alveoli filling with fluid, which prevents them from transporting gases needed to support life on a minute-by-minute basis. There are multiple causes of pneumonia, such as bacterial, viral, fungal, or parasitic infections as well as chemical or physical injury; or it may be a secondary condition to another illness such as alcohol abuse or cancer. Severe acute respiratory syndrome (SARS) is caused by the highly contagious SARS coronavirus, which first occurred in 2003 after initial outbreaks in China. According to the World Health Organization (WHO) more than 8000 people worldwide became sick in the 2003 outbreak and of these 774 died.


While the common cold negatively impacts school and work attendance, it positively contributes to visits to physicians and drugstores. This most common of afflictions suffered by humans conservatively costs just under $8 million dollars in visits to physicians and just under $3.5 billion dollars in over-the-counter and prescription drug costs per year. In addition to medically related costs, just under 200 million school days are missed by students each year and just under 130 million workdays by their parents to stay at home with them. Add this to the workdays missed by employees and cold-related work loss is in excess of $20 billion per year.

Pneumonia is common in all age groups but is the leading cause of death among the elderly and those who are chronically or terminally ill. Vaccines are effective for some pneumonia-causing agents, unlike the common cold, which has no cure, only supportive care.


The respiratory tract consists of the nasal cavity, the pharynx, larynx, bronchial tree, and lungs. Structurally and functionally the system can be divided into upper and lower respiratory systems (Figure 11.1).

The lining of the respiratory tract is a mucosal epithelium that serves as a barrier against microbial invasion; however, it is not as effective as an intact skin barrier. The mucosal lining of the respiratory tract has a moist and relatively warm environment suitable for microbes. Moreover, microbes may be trapped in the mucous layer and by way of the ciliary escalator transported to the pharynx and then swallowed. The normal flora (see Chapter 9, Infection and Disease) of the respiratory system contains a large number of bacteria that help to maintain a healthy state of the host by competing with potential pathogenic organisms. The microorganisms of the normal flora are usually harmless but they can become opportunistic pathogens when the host immune system is depressed, or when damage occurs to the mucosal membrane. Infections of the respiratory system can be caused by bacteria, viruses, and fungi. For an infection of the respiratory system to occur due to an exogenous agent the following criteria must be met:

Bacterial Infections

Most of the upper respiratory tract mucosa is colonized by normal flora. The most common bacteria found in the normal flora of the upper respiratory tract are Staphylococcus aureus and S. epidermidis. In addition, aerobic corynebacteria can be cultured from nasal surfaces. Moreover, small numbers of Streptococcus pneumoniae, Neisseria meningitidis, and Haemophilus influenzae can be found in the nasopharynx of some individuals. α-Hemolytic streptococci including S. mitis, S. mutans, S. milleri, and S. salivarius are considered important organisms in this area because it is believed that these bacteria protect against invasion by pathogenic streptococci; however, these same organisms can also cause tooth decay and periodontal disease if unchecked.

Bacterial infections of the respiratory tract can be caused by Staphylococcus, Streptococcus, Klebsiella, Haemophilus, Bordetella, Corynebacterium, Mycobacterium, Legionella, Mycoplasma, Chlamydia, and Coxiella. Infections caused by inhabitants of the normal flora can occur and appear as secondary infections after damage to the mucosal lining, usually caused by a viral infection such as the common cold. A summary of bacterial infections acquired through the respiratory system is given in Table 11.1.

TABLE 11.1

Bacterial Infections Acquired Through the Respiratory Tract

Illness/Disease Organism(s) Target of Infection Transmission
Streptococcal pharyngitis (strep throat) Group A Streptococcus Pharynx Nasal or salivary secretions; person-to-person contact
Scarlet fever Streptococcus pyogenes Pharynx, tongue Direct contact with infected person; nasal droplets; fomites such as shared drinking glasses
Drug-resistant Streptococcus pneumoniae disease (DRSP) Streptococcus pneumoniae Pharynx, lungs, alveoli Person-to-person contact
Mycoplasmal pneumonia Mycoplasma pneumoniae Lungs; mucous membranes Nasal secretions among people in crowded environments
Chlamydial pneumonia Chlamydia pneumoniae Lungs Inhalation of respiratory droplets
Pertussis (whooping cough) Bordetella pertussis Trachea—ciliated epithelial cells Inhalation of respiratory droplets
Tuberculosis Mycobacterium tuberculosis Lungs Inhalation of respiratory droplets
Staphylococcal pneumonia Staphylococcus aureus; S. pneumoniae Lungs Nosocomial; complication after influenza
Haemophilus infections Haemophilus influenzae Pharynx, bronchi, lungs Inhalation of respiratory droplets
Klebsiella pneumonia Klebsiella pneumoniae Lungs Nosocomial
Diphtheria Corynebacterium diphtheriae Respiratory membranes Inhalation of respiratory droplets
Legionellosis Legionella pneumophila Lungs Inhalation of contaminated water mist
Psittacosis (“parrot fever”) Chlamydia psittaci Lungs Inhalation of dried bird excrements or secretions
Inhalation anthrax Bacillus anthracis Lungs Inhalation of spores
Q fever Coxiella burnetii Lungs Inhalation of contaminated droplets excreted by infected animals


Streptococcal Infections

The genus Streptococcus is composed of spherical gram-positive bacteria well known for being responsible for “strep throat,” but is also capable of causing meningitis, pneumonia, endocarditis, erysipelas, necrotizing fasciitis (see Chapter 10, Infections of the Integumentary System, Soft Tissue, and Muscle), and toxic shock syndrome. The virulence of group A Streptococcus seems to be increasing worldwide.

Strep Throat (Streptococcal Pharyngitis)

Strep throat or streptococcal pharyngitis is caused by group A Streptococcus bacteria and is the most common bacterial infection of the throat. Although this infection can occur at any age, it is most common in children between the ages of 5 and 15 years. Strep throat is transmitted by person-to-person contact via nasal secretions or saliva (sneezing and/or coughing) by an infected individual or a carrier. On occasion, contaminated food, especially milk and milk products, may be the cause of the infection.

The infection occurs with higher frequency in the late fall, winter, and early spring. Most sore throats are caused by viruses, not by bacteria, and therefore the Centers for Disease Control and Prevention (CDC, Atlanta, GA) recommends that antibiotics not be used unless the test for streptococcal infection proves to be positive, because bacterial resistance to certain antibiotics has been reported. However, if results are positive for Streptococcus, patients should receive antibiotics to minimize transmission and to reduce the risk of further complications, such as rheumatic fever.

Scarlet Fever

Scarlet fever is an upper respiratory disease also caused by an infection with a group A β-hemolytic streptococcus (Streptococcus pyogenes) (Figure 11.2) and once was a serious childhood disease but now is generally treatable. The incubation period is 1 to 2 days and typically begins with a fever and sore throat, but might also exhibit chills, vomiting, abdominal pain, and malaise. The exotoxin produced by the bacteria is responsible for the “strawberry” tongue (Figure 11.3) as well as the characteristic fine rash on the chest, neck, groin, and thighs. The treatment of scarlet fever is the same antibiotic treatment as for strep throat, and complications with appropriate treatment are rare.

Streptococcus pneumoniae

Streptococcus pneumoniae is a gram-positive, encapsulated α-hemolytic diplococcus (Figure 11.4), also known as pneumococcus, and is a common cause of mild respiratory illness, but also a major source of pneumonia. Other than pneumonia the organism is also capable of causing pharyngitis, sinusitis, otitis media, meningitis, osteomyelitis, septic arthritis, endocarditis, peritonitis, pericarditis, cellulitis, and brain abscess. S. pneumoniae is a common inhabitant of the nasopharynx of healthy people, but can be the cause of disease when the organism reaches other areas such as the eustachian tubes, nasal sinuses, and lungs. Furthermore the organism can be found in larger numbers in environments where people spend a lot of time in close proximity and it can be transmitted by person-to-person contact or by inhalation. If the organism is inhaled and not removed by the ciliary escalator (such as in smokers, in whom the cilia have been damaged or degenerated), or the mucous membranes are damaged by a viral infection, the bacteria can attach or even penetrate the mucosa. Once the organism succeeds in getting to a site where it normally is not found it will stimulate the immune system of the host, resulting in the attraction of leukocytes (see Chapter 20, The Immune System). The capsule of S. pneumoniae is resistant to phagocytosis and if immunity is not present the alveolar macrophages are incapable of destroying pneumococci. In this case the bacterium spreads into the bloodstream, where it most likely causes bacteremia. The organism then can get to other areas of the body, causing the conditions mentioned earlier. The virulence of S. pneumoniae is a direct result of its capsule and the encapsulated (smooth) strains are the ones causing disease, whereas the nonencapsulated (rough) strains are avirulent.

The onset of bacterial pneumonia can vary from gradual to sudden. In severe cases patients may have fever, shaking chills, shortness of breath, cough with rust-colored or greenish mucus, and pleuritic chest pain. Fever and sputum production may be absent in elderly persons with pneumococcal pneumonia. Pneumonia can sometimes appear in a mild form, often referred to as “walking pneumonia,” in which symptoms are slight or absent. The risk for bacterial pneumonia is highest for young children, the elderly, people with chronic medical conditions (e.g., heart disease, lung disease, and diabetes), and persons who have a suppressed immune system. Because bacterial pneumonia can easily be passed from one member of the community to another, it can manifest itself in a condition referred to as “community-acquired pneumonia.” This form of the disease occurs worldwide and is a major cause of death among all age groups.

Vaccination is available for the prevention of pneumococcal pneumonia. Two vaccines are currently available: the pneumococcal conjugate vaccine, which is part of the routine infant immunization schedule, and the pneumococcal polysaccharide vaccine used for adult immunization. Vaccination with the pneumococcal polysaccharide vaccine is recommended for all adults age 65 years and older and for persons 2 to 64 years of age with certain chronic illnesses or immunocompromising conditions. Without vaccination the treatment of choice used to be penicillin; however, because of overuse of antibiotics nearly all strains of S. pneumoniae previously susceptible to penicillin show resistance to penicillin as well as to other antibiotics.

Drug-resistant Streptococcus pneumoniae Disease

Drug-resistant Streptococcus pneumoniae disease (DRSP) manifests itself as pneumonia, bacteremia, otitis media, meningitis, peritonitis, and sinusitis. These conditions can be caused by an organism that is resistant to one or more commonly used antibiotics. Overuse of antimicrobial agents is a contributing factor to the emerging resistance and spread of resistant strains. Transmission occurs by person-to-person contact and the risk groups are mostly the elderly, children 2 years of age or older, persons who attend or work at child care centers, immunocompromised patients (e.g., those with AIDS), and also persons who recently used antimicrobial agents. Outbreaks of DRSP have been reported in nursing homes, child care centers, and in institutions for HIV-infected people. A pneumococcal conjugate vaccine is available and its use is preventing many infections due to drug-resistant pneumococci. Future increased use of the vaccine, systematic surveillance, and routine testing for the drug-resistant strains might slow down the emerging drug resistance of this bacterium.

Other Common Infections

Mycoplasmal Pneumonia

Mycoplasma pneumoniae, a small bacterium that lacks a cell wall (Figure 11.5), is the cause of primary atypical pneumonia, a relatively mild pneumonia, which usually affects people younger than 40 years of age. Transmission occurs by respiratory droplets through inhalation or person-to-person contact. The incubation period lasts 10 to 14 days and epidemics can occur, especially in crowded areas such as in schools, among military personnel, in homeless shelters, and within a family. Symptoms may last 1 to 3 weeks, starting with fatigue, a sore throat, and a dry cough. It resembles influenza at the beginning, followed by worsening of the cough, which eventually produces sputum. Although it is usually a mild condition and most people recover without treatment, severe cases require antibiotic treatment. It should be noted that because of the lack of a cell wall these organisms are resistant to penicillin and other β-lactam antibiotics (see Chapter 22, Antimicrobial Drugs), which act by interrupting the formation of peptidoglycan cross-links of bacterial cell walls.

Pertussis (Whooping Cough)

Pertussis, also known as whooping cough, is a highly contagious disease caused by Bordetella pertussis, an extremely small, aerobic, gram-negative coccobacillus (Figure 11.6). It is a serious disease that can cause permanent disability and even death. Pertussis is easily spread from person to person by airborne droplets discharged from the mucous membranes of infected people. Initial symptoms occur about a week after exposure and resemble those of the common cold. The severe coughing spells start approximately 10 to 12 days later and these spells may lead to vomiting. The coughing often ends with a “whoop” noise caused when the patient is trying to take a breath. Despite the availability of, and high coverage with vaccines, pertussis is one of the leading causes of vaccine-preventable deaths worldwide. Ninety percent of all cases occur in the underdeveloped countries and most deaths involve infants who are either not vaccinated or incompletely vaccinated. Treatment with effective antibiotics, if started early, shortens the infectious period but usually does not alter the outcome of the disease.