Pneumonia infection

Pneumonia is an infection of the pulmonary alveoli or walls of the alveolar sacs. The diagnosis of pneumonia is quite obvious; however, as many organisms can cause pneumonia, determining the cause in a particular case can be very difficult.

Numerous microorganisms can cause pneumonia, but they most commonly cause its bacteria. Common causes depend on the patient’s immune status, the location where the patient developed pneumonia, the patient’s age, and the type of pneumonia the patient exhibits (e.g., typical of interstitial pneumonia). Clinical and epidemiological factors are used to determine the most likely cause of each particular pneumonia case.

Chest X-ray of a patient with Q fever. The arrow indicates the consolidation of the lower right part of the lung and the thickened pericardium.

Pneumonia can be classified depending on the rate of pneumonia development. Pneumonia with an acute onset develops within 24-48 hands is common in typical pneumonia patients. The patient’s only complaint may be an infection of the upper respiratory tract. Still, the manifestations of typical pneumonia develop rapidly – high fever, shivering, dyspnea, tachycardia, productive cough with purulent sputum, and lung consolidation as seen on X-ray.

Interstitial pneumonia (atypical pneumonia) has a subacute onset; it may take several days to a week before the patient develops signs and symptoms of pneumonia – lower-grade fever, shivering, paroxysmal cough with or without mucoid sputum production, and pulmonary infiltrates seen on chest X-ray.

Chronic pneumonia takes weeks to months to develop full symptoms. Patients usually present a history of night sweats, low-grade fever, significant weight loss, productive cough with purulent sputum, dyspnea;  Ghon hotspots can be seen on an X-ray of the lungs.

The symptoms of aspiration pneumonia are similar to other acute-onset pneumonia, except that patients experience recurrent rigor rather than cold-induced rigor. The consolidation of lung segments is seen on X-ray. About half of patients with aspiration pneumonia will have unpleasant-smelling sputum.

Some causes of pneumonia that give unique signs and symptoms:

• Legionnaires’ disease caused by Legionella sp . may result in pneumonia with relative bradycardia, abdominal pain, vomiting, diarrhea, hematuria, mental confusion, abnormal liver, kidney test results, and increased serum creatinine phosphokinase levels.
• Psittacosis cause by Chlamydophilae psittaci  (formerly known as Chlamydia psittaci ) may result in pneumonia with relative bradycardia, epistaxis,  Horder points, splenomegaly, or normal or low white blood cell counts. The disease is associated with people who care for parrots.
• Q fever caused by Coxiellae burnetti can cause pneumonia with relative bradycardia, hepatomegaly, endocarditis, and abnormal liver function tests. Q fever is associated with farmers who have recently given birth to animals.
• Erythema nodosum and hilar adenopathy can be seen in patients with pneumonia due to fungi like  Histoplasma capsulatum and  Coccidioides immitis.
• Fungal pneumonia most commonly causes Blastomyces dermatitidis. It can also produce rough verrucous skin lesions.

• In the United States alone, two to three million pneumonia cases are reported annually.
• In the U.S. alone, pneumonia patients are responsible for over 10 million doctor visits, half a million hospitalizations, and 45,000 deaths a year. Expensive influenza and pneumonia are the seventh leading cause of death in the United States.
• A patient with pneumonia usually had a previous viral infection of the upper respiratory tract.
• Inhalation and aspiration are the two most common ways of acquiring infectious pneumonia.
• Pneumonia is more common in the winter months and in people over 65.
• Elderly patients are more likely to be hospitalized and have a higher mortality rate after the onset of pneumonia.
• Aspiration pneumonia is an endogenous infection.
• The following conditions predispose a person to aspiration pneumonia: altered level of consciousness, alcoholism, seizures, anesthesia, central nervous system disorders, trauma, dysphagia, esophageal disorders, and nasogastric probes.

• When a microorganism enters the alveoli, IgG can be opsonized in the fluid lining the alveoli and then digested. Macrophage via the Fc receptor.
• If no specific antibody to the organism is present, the macrophage can still phagocytose the microorganism using C-reactive protein or complement receptors or via receptors for pathogen-associated molecular patterns (PAMPs). Mannan, lipopolysaccharides, lipoteichoic acid, N-formylated methionine-containing peptides, muramyl peptides, and peptidoglycans are examples PAMPs that alveolar macrophages can use to phagocytose bacteria.
• When a microorganism phagocytoses, macrophages destroy the microorganism, if possible, and present microbial antigens on the surface for B and T lymphocytes.
• When activated, B and T lymphocytes can produce more antibodies and activate macrophages. Macrophages simultaneously release factors that help carry polymorphonuclear leukocytes out of the bloodstream and initiate an inflammatory response. Polymorphonuclear cells, antibodies, and complement components are useful in destroying microorganisms.

Bacterial survival in areoles

Many bacteria that cause pneumonia can initially survive in the alveoli due to the following self-defense mechanisms:

• Capsules (e.g., Streptococcus pneumoniae, Haemophilus influenzae ) prevents phagocytosis by alveolar macrophages.
• Viruses and  Chlamydia invade the host cell before alveolar macrophages can phagocytose them.
• Mycobacterium tuberculosis can survive in alveolar macrophages even after macrophages phagocytose them.

If organisms survive in the alveoli, microbial growth can cause tissue injury, stimulating the host to amplify the inflammatory response. Tissue injury can also occur due to exotoxins produced by bacteria, cell lysis caused by viruses, or the death of alveolar macrophages and the release of their lysosomal contents into the alveoli due to the growth of microorganisms in phagocytes.

Vascular permeability increases and polymorphonuclear cells arrive in the area with many serum components, trying to retain and eliminate microorganisms. While microorganisms damage the alveoli, other alveolar macrophages are recruited into the area of inflammation. Lung-associated lymphoid tissue (mediastinal lymph nodes) becomes enlarged after B and T lymphocytes’ activation. Chest X-ray may show enlargement of mediastinal lymph nodes in patients with pneumonia.

Typical or lobar pneumonia

Accumulation of microorganisms, immune system cells, and serum components can cause alveolar filling. This inflammatory response is called opacity or consolidation on chest X-ray and is commonly seen in patients with pneumonia caused by S.pneumoniae. This type of pneumonia is called typical or lobar pneumonia. The inflammatory response to infection produces factors that allow microorganisms to leave the lungs and cause systemic effects such as fever. Examples of microbial factors that may have systemic effects include gram-negative bacterial endotoxin that causes fever and septic shock and gram-positive bacterial cell wall components that can lead to fever and septic shock.

Organisms like Mycoplasma pneumoniae and influenza virus do not initially cause the accumulation of large amounts of fluid in the alveoli. However, after infection with these organisms, inflammation in the interstitial spaces (alveolar walls) occurs, resulting in interstitial or atypical pneumonia. Thoracic X-ray in patients with this type of pneumonia shows fine granular diffuse infiltrates.

Other organisms like Staphylococcus aureus, gram-negative rod-shaped bacteria, and anaerobic bacteria from abscesses or microabscesses. In these infections, the immune system can isolate itself from organisms and create localized abscesses or microabscesses that usually show a well-localized circular lesion with necrotic translucent centers on chest X-ray.

Chest X-rays show new consolidations or infiltrations and help diagnose pneumonia. When the alveolar sacs are filled with inflammatory cells and fluid, chest X-rays show consolidated well-defined density areas – unilateral (inhalation or aspiration pneumonia), bilateral (hematogenous spread to the lungs), localized, or uniform. When a chest X-ray shows inflammation and thickening of the alveolar septa surrounding the alveoli, rather than filling the alveolar sacs with inflammatory material, a diagnosis of interstitial pneumonia is more likely.

Some organisms form abscesses in the lungs (e.g., S.aureus, Enterobacteriaceae, Pseudomonas aeruginosa, and anaerobic organisms); in such cases, a chest X-ray is useful for detecting abscesses. If present, certain radiological samples may have diagnostic value, e.g.:

A sign of a convex fissure as seen in Klebsiella pneumoniae infection (but also in other infections such as S. pneumoniae, P.aeruginosa, S.aureus, Legionella, and sometimes bronchoalveolar carcinoma.

• Infection Klebsiella pneumoniae causes consolidation of the upper lobes, resulting in a “bulging fissure sign,” i.e., by dilating the affected part of the lung.
• S.aureus  lung infections can cause multiple bilateral nodular infiltrates with central cavitation. In children, chest X-rays may show thin cavities (pneumatocele), bronchopleural fistulas, and empyema.
• P.aeruginosa  infections can result in microabscesses that can merge into large abscesses.
• Infections with gram-negative rods (e.g., Klebsiella, Proteus, E.coli ) often cause lung necrosis.
• Consolidation of individual lung segments may indicate aspiration pneumonia.

Pathogen identification

To identify the specific pathogen that causes pneumonia, clinical and epidemiological data must be considered to limit the number of possible causes of pneumonia.

Gram staining of sputum in a patient with pneumonia due to Streptococcus pneumoniae. Gram-positive diplococci are seen in the sputum, and many polymorphonuclear leukocytes are present.

Gram staining of sputum of a patient with suspected pneumonia can help identify the cause of pneumonia. Some pathogens are poorly Gram-stained or not stained at all; other colors can be ordered: Dieterle silver color ( Legionella sp .), acid-fast Mycobacteria ), Gomori methenamine silver staining (fungi and Pneumocystis ).

Laboratory tests at diagnosis

Additional laboratory tests that can help establish a definitive diagnosis:

• Sputum culture,
• culture of blood samples for bacteria, fungi, or viruses,
• serology for the detection of antibodies against pathogens (e.g., cold agglutination for M.pneumoniae; detection of antibodies to the capsule S.pneumoniae ),
• antigenic tests to detect certain antigens produced by pathogens (e.g., polysaccharides for S.pneumoniae  and H.influenzae ),
• skin tests to detect delayed hypersensitivity to certain pathogens (e.g., Mantoux test for M.tuberculosis, B.dermatitis, H.capsulatum, C.immitis).
• PCR (polymerase chain reaction) for sputum samples and rapid determination of pathogens,
• Urinalysis for  Legionella  antigens.

Two vaccines can be given to adults to help prevent pneumonia. The S.pneumoniae vaccine contains 23 types of capsular antigens and is used in people over 65 years of age. The influenza vaccine can be given annually to all people over the age of 50 to help prevent viral pneumonia or secondary bacterial pneumonia that can occur after an influenza virus infection. Chemoprophylaxis to prevent influenza infections is useful in preventing secondary bacterial pneumonia.

Conjugated heptavalent S.pneumoniae  The vaccine is important in preventing these infections in younger children. The conjugated H.influenzae type b (Hib) vaccine prevents H.influenzae infections in childhood. Premature neutropenic infants can prevent respiratory syncytial virus infections or infants with different predispositions to infection by periodic injections of immune globulin for respiratory syncytial virus or humanized murine monoclonal antibody (palivizumab). Annual immunization of children with influenza vaccine prevents infections in vaccinated children and appears to prevent the virus’s spread by close contact.

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