WORLD OF MICROBIOLOGY AND IMMUNOLOGY Legionnaires’ disease345•
source of infection, particularly in hospital-acquired cases of
Legionnaires’ disease. There is no evidence of person-to-per-
son transmission of Legionnaires’ disease.
Once the bacteria are in the lungs, cellular representa-
tives of the body’s immune system (alveolar macrophages)
congregate to destroy the invaders. The typical macrophage
defense is to phagocytose the invader and demolish it in a
process analogous to swallowing and digesting it. However,
the Legionellabacteria survive being phagocytosed. Instead of
being destroyed within the macrophage, they grow and repli-
cate, eventually killing the macrophage. When the
macrophage dies, many new Legionellabacteria are released
into the lungs and worsen the infection.
Legionnaires’ disease develops 2–10 days after expo-
sure to the bacteria. Early symptoms include lethargy,
headaches, fever, chills, muscle aches, and a lack of appetite.
Respiratory symptoms such as coughing or congestion are
usually absent. As the disease progresses, a dry, hacking cough
develops and may become productive after a few days. In
about a third of Legionnaires’ disease cases, blood is present
in the sputum. Half of the people who develop Legionnaires’
disease suffer shortness of breath and a third complain of
breathing-related chest pain. The fever can become quite high,
reaching 104°F (40°C) in many cases, and may be accompa-
nied by a decreased heart rate.
Although the pneumonia affects the lungs,
Legionnaires’ disease is accompanied by symptoms that affect
other areas of the body. About half the victims experience
diarrhea and a quarter have nausea and vomiting and abdomi-
nal pain. In about 10% of cases, acute renal failure and scanty
urine production accompany the disease. Changes in mental
status, such as disorientation, confusion, and hallucinations,
also occur in about a quarter of cases.
In addition to Legionnaires’ disease, L. pneumophila
legionellosis also includes a milder disease, Pontiac fever.
Unlike Legionnaires’ disease, Pontiac fever does not involve
the lower respiratory tract. The symptoms usually appear
within 36 hours of exposure and include fever, headache, mus-
cle aches, and lethargy. Symptoms last only a few days and
medical intervention is usually not necessary.
The symptoms of Legionnaires’ disease are common to
many types of pneumonia and diagnosis of sporadic cases can
be difficult. The symptoms and chest x rays that confirm a
case of pneumonia are not useful in differentiating between
Legionnaires’ disease and other pneumonias. If a pneumonia
case involves multisystem symptoms, such as diarrhea and
vomiting, and an initially dry cough, laboratory tests are done
to definitively identify L. pneumophilaas the cause of the
infection.
If Legionnaires’ disease is suspected, several tests are
available to reveal or indicate the presence of L. pneumophila
bacteria in the body. Since the immune system creates anti-
bodies against infectious agents, examining the blood for these
indicators is a key test. The level of immunoglobulins, or anti-
bodymolecules, in the blood reveals the presence of infection.
In microscopic examination of the patient’s sputum, a fluores-
cent stain linked to antibodies against L. pneumophilacan
uncover the presence of the bacteria. Other means of revealingthe bacteria’s presence from patient sputum samples include
isolation of the organism on culturemedia or detection of the
bacteria by DNAprobe. Another test detects L. pneumophila
antigens in the urine.
The type of antibiotic prescribed by the doctor depends
on several factors including the severity of infection, potential
allergies, and interaction with previously prescribed drugs.
For example, erythromycin interacts with warfarin, a blood
thinner. Several drugs, such as penicillins and cephalosporins,
are normally ineffective against the infection. Although they
may be deadly to the bacteria in laboratory tests, their chemi-
cal structure prevents them from being absorbed into the areas
of the lung where the bacteria are present. In severe cases with
complications, antibiotic therapy may be joined by respiratory
support. If renal failure occurs, dialysis is required until renal
function is recovered.
Appropriate medical treatment has a major impact on
recovery from Legionnaires’ disease. Outcome is also linked
to the victim’s general health and absence of complications. If
the patient survives the infection, recovery from Legionnaires’
disease is usually complete. Similar to other types of pneumo-The Bellevue-Stratford Hotel in Philadelphia, where an outbreak at a
Legionnaires’ convention gave the disease its name.womi_L 5/6/03 3:32 PM Page 345