Risk Factors And Clinical Signs Of Legionnaires Disease Biology Essay

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Legionnaires disease has become recognized as the most common cause of atypical pneumonia in hospitalized patients. It is the second most common cause of community-acquired bacterial pneumonia and travel-acquired pneumonia

This bacteria cause respiratory disease in humans when a susceptible host inhales aerosolized water containing the bacteria or aspirates water containing the bacteria.

Legionelloses can occur both sporadically and in epidemics. ( page 311, med microbiology,H ayser et al)

Infection results from inhalation of droplets containing the pathogens. The incubation period is two to 10 days. The clinicalpicture is characterized by a multifocal, sometimes necrotizing pneumonia. Occurrence is more likely in patients with cardiopulmonary primary diseases or other immunocompromising conditions.

Risk factors for Legionnaires disease include the following:Smoking, Diabetes, Cancer, particularly hematological or pulmonary malignancy,AIDS,End-stage renal disease,Chronic cardiopulmonary disease, Advanced age, Alcohol abuse, Surgery

Clinical signs

The vital signs may reveal high fever and tachypnea. Chest auscultation findings may be normal or may reveal rales, rhonchi, or signs of consolidation. Pericarditis and endocarditis may be present. Hepatomegaly may be seen in rare cases. The neurologic examination findings or the patient's mental status may be abnormal. The patient may have blood-streaked sputum. Mild, generalized abdominal pain and bloating may be present


Investigations of outbreaks have documented aerosol transmission from contaminated water sources, including the following (Note: No person-to-person spread of Legionella is documented.):

Cooling systems


Decorative fountains


Respiratory therapy equipment

Whirlpool spas

Outbreaks of legionellosis have occurred after persons have breathed mist that

come from a water source ( e.g., air conditioning cooling towers, whirlpool spas,

showers) contamined with Legionella bacteria. Persons may be exposed to these

mists in homes, workplaces, hospitals, or public places. Legionellosis is not

passed from person to person, and there is no evidence of persons becoming

infected from auto air conditioners or household window air-conditioning units.

Legionella pneumophila Page 10 on 11

General Information Fo 7.5-72-15/00


iii. Pathogenesis

The classical presentation begins with an incubation period of 2-10 days.

Patients often experience a prodrome of 1-2 days of mild headache and myalgias, followed by high fever, chills, and multiple rigors.

Cough is present in 90% of cases; cough usually is nonproductive at first but may become productive as the disease progresses.

Other pulmonary manifestations include dyspnea, pleuritic chest pain, and hemoptysis, which may be present in as many as one third of cases.

Gastrointestinal symptoms include nausea, vomiting, diarrhea, and anorexia.

Neurologic symptoms include headache, lethargy, altered mental status, and rarely, focal symptoms.

Musculoskeletal symptoms include arthralgias and myalgias

Nonpulmonary symptoms are prominent early in the disease.

Infection begins with the inhalation of the Legionella pneumophila bacterium.

Once the Legionella reach the alveoli they come in contact with an alveolar

macrophage. This only takes place, however, if the bacteria are virulent enough

to overpower the host immune response. After the bacteria reach the alveolar

macrophage, coiling phagocytosis begins, and the macrophage takes the bacteria

into a food vacuole inside the cell. Here, Legionella stops the fusion of the

monocyte and the lysosome. As a result, the bacteria are able to multiply inside

the macrophage, where they eventually lyse the cell and infect other cells.

However, some strains of the bacteria have been known to manifest a disorder

called Pontiac Fever. This is a very mild infection which causes influenza-like

symptoms and will go away without treatment. Incidence of Legionellosis or

Legionnaire's Disease have increased over the past decade or so because of the

use of central air conditioning, especially in office buildings, hotels, and


iv. Laboratory diagnosis

Diagnosis of legionellosis can be difficult because signs and symptoms are nonspecific

and not distinguish L. pneumophila infections from other common causes

of pneumonia. L. pneumophila infections are considered to be fairly common but

it is underdiagnosis and under-reporting. The underdiagnosis of legionellosis

can in part be attributed to the need for rapid, specific and sensitive diagnostic

testing methods. Laboratory diagnosis of L. pneumophila is typically based on

either cultivation, serological, direct fluorescent antibody (DFA) staining

Legionella pneumophila techniques, urinary antigen detection tests or molecular based method such as real time PCR assays.


Legionellae are fastidious organisms requiring cysteine and other essential

growth promoting factors for their successful isolation from clinical material.

Suitable specimens for culture include bronchial washings and bronchoalvealor

lavage which are the specimens of choice. When is possible, specimens should

be collected before antibiotic therapy is commenced. Pleural aspirates, lung or

other tissue may also be used if available. Expectorated sputum and racheal

aspirates are less satisfactory as they contain relatively few legionellae and are

likely to be heavely contamined with oral flora.

Legionellae usually require 48 hours incubation before growth is visible for up to

five days or more. The isolation of any legionella species from clinical specimen

is considered to be significant. A negative culture however does not exclude the

diagnosis of legionellosis.

B. Serological test

Ideally, paired sera collected as soon as possible after the onset of illness and 3

to 6 weeks later should be tested. There may be up to nine weeks delay before

seroconversion can be detected. Seroconversion is defined as a four-fold

increase in the titre of antibody: against heat killed L. pneumophila serogroup 1

titre more or equal to 1:128.

Rem: A single high titre more or equal to 1:512 is a sensitive indicator of

infection with legionella but may represent past infection. High titres may be

present in up to 2% of the healthy population.

C. Direct Fluorescent antibody (DFA)

Microscopic examination of specimens using direct fluorescent antibody (DFA)

staining was the first method used to detect legionellae in lung tissue (from

autopsy or biopsy specimens) and respiratory secretions. Legionellae can be

detected in respiratory secretions by DFA for several days after the start of

antimicrobial therapy. DFA staining has also been used for serologic

identification of Legionella isolates.

D. Urine Antigen Detection

Urinary antigen testing has led to the recognition of outbreaks of Legionnaires'

disease and allowed a rapid public health response. In addition, urine antigen

testing permits early diagnosis and initiation of appropriate antibiotic therapy.

immunochromatographic (ICT) membrane assay. The test is simple to perform

and does not require special laboratory equipment, and results can be obtained

within 15 min.

pneumophila serogroup 1.

E. Nucleic acid based tests

Various PCR tests that have been developed for legionellae target either random DNA sequences for L. pneumophila, the 5S rRNA gene, the 16S rRNA gene, or the mip gene. The most widely used test was a commercially produced kit designed to detect

legionellae in the environment. Recently, several researchers have reported on

the use of real-time PCR combined with the use of a hybridization probe to

confirm the product identity for rapid detection of legionellae in clinical

specimens. But a very limited number of laboratories test for legionellae by PCR

at this time.