Cases in Medical Microbiology and Infectious Diseases. Melissa B. Miller
virus (RSV), can also be spread by direct contact with mucous membranes, but this mode of transmission is much less common than inhalation. Organisms that are part of the endogenous microbiota of the oropharynx may, under certain conditions (such as aspiration of oropharyngeal secretions), be able to cause clinical disease. Animal exposure may result in some of the less common but more severe bacterial causes of respiratory infection, including inhalation anthrax, pneumonic plague, tularemia pneumonia, and hantavirus pulmonary syndrome. These zoonotic agents are also potential agents of bioterrorism. For the purposes of our discussions, we will divide these types of infections into two groups, upper tract and lower tract infection.
The most common form of upper respiratory tract infection is pharyngitis. Pharyngitis is seen most frequently in children from 2 years of age through adolescence. The most common etiologic agents of pharyngitis are viruses, particularly adenoviruses, coronaviruses, enteroviruses, and rhinoviruses, and group A streptococci. Pharyngitis due to group A streptococci predisposes individuals to the development of the poststreptococcal sequelae rheumatic fever and glomerulonephritis. Because rheumatic fever can be prevented by penicillin treatment of group A streptococcal pharyngitis, aggressive diagnosis and treatment of pharyngitis due to this organism is needed.
Otitis media is a common infectious problem in infants and young children. The most frequently encountered agents of this infection are the bacteria Streptococcus pneumoniae, Haemophilus influenzae, and Moraxella catarrhalis. These organisms, along with selected viruses and anaerobic bacteria from the oral cavity, are the most important pathogens in sinusitis.
S. pneumoniae, H. influenzae, M. catarrhalis, and adenoviruses are the common etiologic agents of conjunctivitis. Less commonly, Chlamydia trachomatis can cause conjunctivitis in neonates. External otitis, a common problem in swimmers, is more common in warm-weather months. Staphylococcus aureus and Pseudomonas aeruginosa are the most common agents of this relatively benign condition. Malignant external otitis is a serious medical condition seen primarily in diabetics, the elderly, and the immunocompromised. The infection can spread from the ear to the temporal bone. The most common etiology of malignant otitis externa is P. aeruginosa. Two other life-threatening infections of the upper respiratory tract are rhinocerebral mucormycosis (zygomycosis) and bacterial epiglottitis. Rhinocerebral mucormycosis is most common in diabetics, especially those with ketoacidosis. In this infection of the sinuses, fungi within the zygomycetes, such as Mucor and Rhizopus spp., invade blood vessels, resulting in necrosis of bone and thrombosis of the cavernous sinus and internal carotid artery. Treatment of this infection requires aggressive surgical debridement of the infected tissue in addition to antifungal therapy. Epiglottitis is most commonly caused by H. influenzae type b but can also be associated with S. pneumoniae, other streptococci, and staphylococci. In this disease, the airway may become compromised because of swelling of the epiglottis, with death due to respiratory arrest. With the widespread use of H. influenzae type b vaccine, the incidence of this disease has greatly decreased, but it is still occasionally seen in both children and adults.
Three childhood infections with respiratory manifestations or complications that were common in the early part of the 20th century—diphtheria, whooping cough, and measles—are now rare diseases in the developed world. This is due to the development and use of vaccines in children that are effective against the etiologic agents of these diseases, Corynebacterium diphtheriae, Bordetella pertussis, and measles virus, respectively.
Viruses play an important role in upper respiratory tract infections. The common syndrome of cough and “runny” nose is usually due to rhinoviruses, but enteroviruses and coronaviruses are frequent causes. More severe upper respiratory infections such as the “croup” are due to RSV, influenza viruses, parainfluenza viruses, and metapneumovirus. These viruses can also cause lower tract infection and are important causes of morbidity and mortality in the very young and very old.
When discussing lower respiratory tract infections, it is important to look at four different groups of patients: patients with community-acquired infections; patients with health care-associated infections; patients with underlying lung disease; and immunocompromised individuals, especially those with AIDS.
Common agents of community-acquired lower respiratory tract infections include S. pneumoniae; Klebsiella pneumoniae, especially in alcoholics; Mycoplasma pneumoniae, especially in school-age students through young adulthood; Mycobacterium tuberculosis, especially in individuals born in countries with a high prevalence of tuberculosis; RSV in infants and young children; and influenza A virus. The dimorphic fungi Histoplasma capsulatum and Coccidioides posadasii/immitis usually cause mild, self-limited diseases in patients residing in specific geographic locales. S. pneumoniae, H. influenzae, S. aureus, and M. catarrhalis may cause bronchitis and/or pneumonia in adults following viral pneumonia. Aspiration due to seizure disorders, semiconscious states from excessive consumption of alcohol or other drugs, or impairment of the gag reflex, as may occur following a stroke, may result in aspiration pneumonia or lung abscess caused by the organisms residing in the oral cavity. The anatomic location of the lung process depends on the patient’s position at the time of aspiration.
Health care-associated infections due to the organisms listed above certainly occur. Particular emphasis is placed on preventing the spread of M. tuberculosis in all patient populations and on preventing health care-associated spread of RSV in pediatric patients. Health care-associated pneumonia due to methicillin-resistant S. aureus and multidrug-resistant Gram-negative bacilli, such as P. aeruginosa and Acinetobacter baumannii, is a concern for intubated patients. Because of their ability to survive within hospital water and air conditioning systems, the potential for outbreaks of pneumonia due to Legionella spp. is a constant threat.
Patients with chronic obstructive pulmonary disease brought on by frequent smoking develop bronchitis. S. pneumoniae, M. catarrhalis, H. influenzae, and P. aeruginosa are common causes of this type of infection. Patients with cystic fibrosis have chronic airway infections that are primarily responsible for their premature death. S. aureus and mucoid strains of P. aeruginosa are the most important agents of such chronic airway disease. Both of these patient populations have an increased risk of developing allergic bronchopulmonary aspergillosis. Patients with cavitary lung disease, frequently due to prior M. tuberculosis infection, are at increased risk for another type of infection, an aspergilloma or fungus ball caused by Aspergillus spp. This fungus grows in the form of a ball in the preformed cavity. A distinction between actual tissue invasion with this fungus and noninvasive disease is clinically difficult but is important.
The diagnosis of the etiology of lung infection in immunocompromised patients is one of the most daunting in clinical microbiology and infectious disease. It has been greatly facilitated by the use of the flexible bronchoscope, which provides a relatively noninvasive means to sample the airways and alveoli. Immunocompromised patients are typically at risk for essentially all recognized respiratory tract pathogens. However, a distinction must be made between different types of immunosuppression—defects in cell-mediated immunity, humoral immunity, and neutrophil number or function—because different types of immunosuppression predispose patients to infection with different pathogens. The most common comorbidity for lower respiratory tract infections is cigarette smoking, which causes impaired removal of pathogens due to defective mucociliary clearance. Although smoking results in a significantly increased rate of both bronchitis and pneumonia, smokers are not normally described as immunosuppressed.
In AIDS patients, Pneumocystis jirovecii, Cryptococcus neoformans, S. pneumoniae, and multidrug-resistant M. tuberculosis are all seen more frequently than in other patient populations. Solid-organ transplant recipients have a greatly increased risk for pneumonia with cytomegalovirus, herpes simplex virus, Legionella spp., P. jirovecii, and Nocardia spp. Prophylactic antibiotics are frequently taken by these patients to prevent pulmonary infections with P. jirovecii. Prophylactic therapies are not as widely used for other agents for a variety of reasons, including expense, questionable efficacy of the prophylactic measures, or the rarity with which the organism is encountered. Profoundly neutropenic patients,