Microbiology, 2021

916 22 • Respiratory System Infections
conditions that weaken the immune system.
When L. pneumophila bacteria enter the alveoli, they are phagocytized by resident macrophages. However, L.
pneumophila uses a secretion system to insert proteins in the endosomal membrane of the macrophage; these
proteins prevent lysosomal fusion, allowing L. pneumophila to continue to proliferate within the phagosome.
The resulting respiratory disease can range from mild to severe pneumonia, depending on the status of the
host’s immune defenses. Although this disease primarily affects the lungs, it can also cause fever, nausea,
vomiting, confusion, and other neurological effects.
Diagnosis of Legionnaires disease is somewhat complicated. L. pneumophila is a fastidious bacterium and is
difficult to culture. In addition, since the bacterial cells are not efficiently stained with the Gram stain, other
staining techniques, such as the Warthin-Starry silver-precipitate procedure, must be used to visualize this
pathogen. A rapid diagnostic test has been developed that detects the presence of Legionella antigen in a
patient’s urine; results take less than 1 hour, and the test has high selectivity and specificity (greater than
90%). Unfortunately, the test only works for one serotype of L. pneumophila (type 1, the serotype responsible
for most infections). Consequently, isolation and identification of L. pneumophila from sputum remains the
defining test for diagnosis.
Once diagnosed, Legionnaire disease can be effectively treated with fluoroquinolone and macrolide
antibiotics. However, the disease is sometimes fatal; about 10% of patients die of complications. 14 There is
currently no vaccine available.
CHECK YOUR UNDERSTANDING
• Why is Legionnaires disease associated with air-conditioning systems?
• How does Legionella pneumophila circumvent the immune system?
Q Fever
The zoonotic disease Q fever is caused by a rickettsia, Coxiella burnetii. The primary reservoirs for this
bacterium are domesticated livestock such as cattle, sheep, and goats. The bacterium may be transmitted by
ticks or through exposure to the urine, feces, milk, or amniotic fluid of an infected animal. In humans, the
primary route of infection is through inhalation of contaminated farmyard aerosols. It is, therefore, largely an
occupational disease of farmers. Humans are acutely sensitive to C. burnetii—the infective dose is estimated to
be just a few cells. 15 In addition, the organism is hardy and can survive in a dry environment for an extended
time. Symptoms associated with acute Q fever include high fever, headache, coughing, pneumonia, and
general malaise. In a small number of patients (less than 5% 16 ), the condition may become chronic, often
leading to endocarditis, which may be fatal.
Diagnosing rickettsial infection by cultivation in the laboratory is both difficult and hazardous because of the
easy aerosolization of the bacteria, so PCR and ELISA are commonly used. Doxycycline is the first-line drug to
treat acute Q fever. In chronic Q fever, doxycycline is often paired with hydroxychloroquine.
Disease Profile
Bacterial Diseases of the Respiratory Tract
Numerous pathogens can cause infections of the respiratory tract. Many of these infections produce
similar signs and symptoms, but appropriate treatment depends on accurate diagnosis through laboratory
testing. The tables in Figure 22.15 and Figure 22.16 summarize the most important bacterial respiratory
14 Centers for Disease Control and Prevention. “Legionella (Legionnaires’ Disease and Pontiac Fever: Diagnosis, Treatment, and
Complications).” http://www.cdc.gov/legionella/about/diagnosis.html. Accessed Sept 14, 2016.
15 WD Tigertt et al. “Airborne Q Fever.” Bacteriological Reviews 25 no. 3 (1961):285–293.
16 Centers for Disease Control and Prevention. “Q fever. Symptoms, Diagnosis, and Treatment.” 2013. http://www.cdc.gov/qfever/
symptoms/index.html. Accessed July 6, 2016.
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