Legionellosis overview and Definition

It is a severe form of pneumonia which results in the inflammation of the lungs.  The bacterium is also called as the Legionella.


The bacterias are obligate and facultative intracellular parasites.  Water plays an important role in the infection of the Legionella.  Within the amoebic cells,  legionella species can avoid the endosomal lysosomal pathway and can replicate within the phagosomes. Surviving and growing in amoebic cells allows Legionella to persist in nature.

Within the amebic cells, Legionella species can avoid the endosomal-lysosomal pathway and can replicate within the phagosome. Surviving and growing in amebic cells allows Legionella to persist in nature. 

. After it is ingested, the bacterium can survive as a symbiont within what then becomes its protozoan host. The amoeba then becomes Trojan horse since, by harboring the pathogenic bacterium, the amoeba can afford it protection.

Legionella species infect human macrophages and monocytes; intracellular replication of the bacterium is observed within these cells in the alveoli. The intracellular infections of protozoa and macrophages have many similarities.

Activated T cells produce lymphokines that stimulate increased antimicrobial activity of macrophages. This cell-mediated activation is key to halting the intracellular growth of legionellae. The significant role of cellular immunity explains why legionellae are observed more frequently in immunocompromised patients. Humoral immunity is thought to play a secondary role in the host response to legionellae infection.

Clinical signs & symptoms

It is the main predominant clinical manifestation of Legionnaire disease (LD). The incubation period lasts for 2-10 days , patients develops the following non specific symptoms.

  • Fever
  • Weakness
  • Fatigue
  • Malaise
  • Myalgia
  • Chills
  • Headache
  • Lethargy
  • Confusion
  • Cerebellar ataxia
  • Agitation
  • Stupor
  • Haemoptysis
  • GI symptoms include the diarrhoea , nausea, vomiting and abdominal pain.

In neonates, Legionnaires disease can manifest as septicemia and/or pneumonia with a fulminant course, often diagnosed at autopsy.

Extrapulmonary sit includes the heart.  The other site may includes the liver, spleen, brain and lymph nodes.

  • Cellulitis
  • Sinusitis
  • Pyelonephritis
  • Pancreatitis
  • lymphadenopathy
  • Peritonitis
  • Wound infection
  • Prosthetic valve endocarditis
  • Myocarditis
  • Pericarditis
  • Postcardiotomy syndrome

Differential Diagnosis

Blood smear:

Examination of your white blood cells under a microscope.:The bacteria can often be seen inside these white blood cells.Changes in the mononuclear cells is also seen.leucocytopenia can be visualized in response to the bacterial infection.

Peripheral Blood Smear:It shows leucopenia(reduction in the number of white blood cells) and thrombocytopenia(reduction in the number of platelets).

Polymerase chain reaction (PCR): This is a newer method to detect the multiplication of the bacteria and it is used to amplify the bacterial DNA.

Antibody test. This often does not become positive until many days or a few weeks after the infection. It is usually not helpful while you are first sick.

It shows positive for acid fast stain.

Immunohistochemical staining Test: It is used in bone marrow, lymph node and spleen.  Immunostaining of bone marrow can also used to detect the presence of the disease.

Indirect Immunofluorescent antibody test: This test detects the antibodies such as IgM developed in response to the Legionellosis. This test shows negative in 1 st week of illness. Hence subsequent serum sample should be collected in further weeks for the detection of antibodies.

CT guided needle Biopsy:

CT scan might be used to guide a biopsy needle into this  area to get the tissue for lung and further investigations are made.

MRI scan:

It uses the soft tissue image of the organ. It uses the both the magnet and radiowaves and aids in the view of soft tissues of the internal organs.

Immunosorbent assay- detect the  specific IgM or IgG antibodies.


Lumbar puncture or spinal tap:

 The fluid is drawn from the spinal cord and further investigations are made.


Normal erythrocyte sedimentation problems:

A normal erythrocyte sedimentation rate-  it is defined as the rate of red blood cell which are termed as erythrocytes and their deposition or sedimentation rate.


A needle is inserted between the ribs and the sample from pleural fluid is taken.

Urinalysis commonly reveals proteinuria and hematuria.

Fine needle aspiration (FNA) biopsy:

A needle is used to aspirate small fragments of tissues. It is used for cancer in the lymph node in the lungs.

Transtracheal or transbrochial FNA:

It is done by passing the needle through the walls of the trachea or bronchi during bronchoscopy or endobronchial ultrasound.

Transthoracic needle biopsy:

It is used to find out if the tumour is present outside the tumour.


It  a procedure that uses a lighted tube inserted behind the sternum (breast bone) and in front of the windpipe to look at and take tissue samples from the lymph nodes along the windpipe and the major bronchial tube areas. If some lymph nodes can’t be reached by mediastinoscopy, a mediastinotomy may be done so the surgeon can directly remove the biopsy sample. For this procedure, a slightly larger incision (usually about 2 inches long) between the left second and third ribs next to the breast bone is needed.


Bronchoscope is a thin lighted instrument used to view the lungs. The physician passes the tube through the trachea then the bronchus and alveoli are viewed clearly.

Chest Xray:

The chest x ray shows the abnormalities present in the lung.


Avoid consuming contaminated food and water

Avoid alcohol and smoking

If already has any respiratory problems then seeks the medical help as early as possible

Follow proper sanitary protocol

Clean the surrounding

Stay hydrated and take lots of fluids and electrolytes

Heat the food before eating.