Overview
Lung infections are a group of infections that can be caused by bacteria, viruses, and perhaps by mycoplasmas and fungi and may affect different locations of the respiratory tract.
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Pneumonia
Pneumonia is an infection of the lung parenchyma and is one of the most common types of lung infections. It occurs when normal respiratory defenses are impaired such as the cough reflex is impaired, because when the cough reflex is impaired, the airways can not remove microorganisms and foreign particles when coughing, increasing the risk of pneumonia.
Damage to the mucociliary escalator is also a risk factor for pneumonia because the airways that carry air into the lungs are lined by ciliated airway epithelium. Cilia are important for removing mucus that is present in the conducting portion of the lung with trapped microorganisms. therefore, damage to the mucociliary elevator increases the risk of pneumonia. Usually, the escalator is damaged due to viral infection, so the damage to the mucociliary escalator increases the risk of developing superimposed bacterial pneumonia on top of viral infection.
Mucus plugging also increases the risk of pneumonia by blocking the airway.
symptoms of pneumonia
- cough with yellow-green or rusty sputum - Yellow-green represents pus and rusty sputum represents the blood
- Fever - Because the microorganisms get leak into the blood and cause fever
- Upper respiratory symptoms - such as runny nose, sore throat, hoarseness, and headaches
- Tachypnea (rapid respiratory rate) with pleuritic chest pain - inhalation leads to the pleura stretches, when the pleura is stretched, causing a pain known as pleuritic chest pain. Pleura is inverted, so due to the inflammatory response, the particular nerves in this region sensitize to the presence of bradykinin and prostaglandin E2, which are the major pain mediators.
- Decreased breath sounds
- Increased WBC level
- Confusion or falls
Diagnosis of pneumonia
The diagnosis is often made by chest X-rays. Gram stain of sputum can help to identify the bacteria along with the culture, and blood culture is also useful since the organisms are often found in the blood. Three patterns can be seen on the chest X-ray. The first pattern is called lobar pneumonia (pneumonia causes consolidation and occupies an entire lobe) and the second pattern is known as bronchopneumonia or lobular pneumonia (consolidation runs through the small airways and is distributed in patches along with the small airways). Finally, the third pattern is interstitial or atypical pneumonia (inflammation within the interstitium but without consolidation)
1. Lobar pneumonia
Acute bacterial infection of a large portion of a lobe or an entire lobe with fibrinous inflammation. Usually due to a bacterial infection. It is primarily a disease in young to middle-aged adults. It is rare in infants, less common in the elderly, and much more common in men than women.
Causes of lobar pneumonia
- Streptococcus pneumoniae
This bacteria causes more than 90% of the cases. The most common cause of community-acquired pneumonia and secondary pneumonia (bacterial pneumonia superimposed on viral pneumonia)
- Klebsiella pneumoniae
It is an enteric flora and aspirated, and has a thick mucoid capsule so patients present with currant jelly sputum because they cough up this capsule. Affects malnourished individuals.
- Staphylococci
- H.influenza (less common)
Stages of lobar pneumonia
There are four stages during the progression of untreated bacterial pneumonia, but these classic pathologic findings are rare today due to the application of effective antibiotics.
- Congestion
The infection causes congestion of blood vessels by dilating the blood vessel and increasing the blood flow, leading to edema. Grossly, the lung is heavy and constricted. Stage one patients usually have fever, rigor, and cough.
- Red hepatization
This stage lasts for 2-4 days. Because of the increased permeability of microvasculature, a large number of red blood cells and neutrophils move into the alveoli of the lung with massive fibrin exudation, which makes normal spongy lungs turn red and solidify. These lesions often spread to the pleura and cause serofibrinous pleurisy. Grossly, the lung looks-like liver (red, firm, and airless)
- Grey hepatization
At this stage, the congestion has atrophied, and the red blood cells are lysed, resulting in a grey-colored lung. It is hard to detect bacteria in this stage because of the specific antibody generation. Under microscopy, further accumulation of fibrins and neutrophils can be observed.
- Resolution
In this final stage, the alveolar exudate is eliminated, the fibrin is broken down by the enzymatic action and the lungs gradually return to normal. The macrophages are the predominant cells in the alveolar spaces, while neutrophils decrease in number, and the exudate is resorbed, and ingested by macrophages. Type 2 pneumocytes are the stem cells of the lung that helps to regenerate the lining of the alveolar air sacs.
Complications of lobar pneumonia
Complications are usually rare
- Suppurative pleuritis and empyema (pus in the plueral space)
- Pulmonary carnification
- The organization of the exudate converts a portion of the lung into solid tissue
- Bacteremic dissemination (septicemia, pyemia, or infectious shock)
- Lung abscess
2. Bronchopneumonia or Lobular pneumonia
Bronchopneumonia refers to scattered patchy consolidation (0.5-2cm) centered on bronchioles, caused by bacteria. Generally bilateral and multi-located based on the lobule. Bronchopneumonia is classically "hospital-acquired" pneumonia and it can be seen in persons who already suffer from another disease. Children (less than 2 years) and old people (more than 65 years) have a high risk of developing bronchopneumonia.
When the architecture of the lung is destroyed, leads to abscess formation but generally, large intervening areas are normal.
Symptoms of Bronchopneumonia
- Insidious onset
- Fever
- Cough
- Purulent sputum
- Respiratory difficulty
- Irregular course
Causes of Bronchopneumonia
- Staphylococcus aureus
It is the second most common cause of secondary pneumonia (bacterial pneumonia superimposed on a preexisting viral infection of the lung because the virus knocks out the mucociliary escalator that increases the risk to develop bacterial pneumonia, which is called secondary pneumonia). Usually, complicated by empyema or abscess
- Pseudomonas aeruginosa
- Legionella pneumophila
Usually, transmitted by water sources and causes community-acquired pneumonia. causes superimposed pneumonia on COPD
- Haemophilus influenzae
A common cause of secondary pneumonia and associated with developing pneumonia superimposed on chronic obstructive pulmonary disease (COPD)
- Moraxella catarrhalis
It causes community-acquired pneumonia and usually, this pneumonia is superimposed on COPD
Complications of Bronchopneumonia
Complications are more frequent
- Respiratory failure (more common)
- Heart failure (more common)
- Pyemia
- Abscess
Comparison
3. Interstitial (atypical) pneumonia
Interstitial pneumonia refers to diffuse interstitial infiltrates within the lungs and is caused by viruses. The interstitium is a connective tissue of the alveolar sac, therefore, inflammation occurs within the wall of the alveolar sacs without significant consolidation. Interstitial pneumonia is often called atypical pneumonia because the signs and symptoms are relatively atypical. A classical patient with pneumonia is expected to have a high fever, pleuritic chest pain, elevated WBC count, etc however, in interstitial pneumonia, the symptoms are much more upper respiratory symptoms, hence patients have minimal sputum production, cough, and low fever.
Causes of interstitial pneumonia
- Mycoplasma pneumoniae
The most common cause of interstitial pneumonia. Can not be seen on gram stain due to a lack of cell wall. Usually, infect young adults and may cause autoimmune hemolytic anemia.
- Influenza virus
Increase the risk of superimposed Staphylococcus aureus or Haemophilus influenzae bacterial pneumonia. often affects the elderly and immunocompromised. The vast majority of people get influenza virus pneumonia but it does not lead to death, however, the influenza virus weakens the normal defenses and allows for secondary bacterial pneumonia, which leads to death.
- Chlamydia pneumoniae
The second most common cause of interstitial pneumonia in young adults
- Cytomegalovirus (CMV)
Causes interstitial pneumonia in patients who is under post-transplant immunosuppression or chemotherapy
- Respiratory syncytial virus (RSV)
Major victims are infants
- Coxiella burnetii
An important cause of atypical pneumonia in farmers and veterinarians and classically associated with high fever (Q fever). Usually, interstitial pneumonia fever is low but Coxiella-caused pneumonia leads to high fever. Spores of Coxiella are deposited on cattle by ticks or present in cattle placentas
Tuberculosis (TB)
A chronic granulomatous communicable disease results from inhalation of aerosolized Mycobacterium tuberculosis. Not all M. tuberculosis infections cause TB disease, and many infections are asymptomatic. Initial exposure to M. tuberculosis leads to primary TB which creates focal caseating necrosis in the lung usually, the lower lobe, and the hilar lymph node. These foci undergo fibrosis and calcification, leading to the formation of the ghon complex.
Route of infection
- Via respiratory tract
Direct inhalation of airborne droplets of organisms generated by expectoration or by exposure to contaminated secretions of infected persons. This is the commonest and most important pathway.
- Through Gastrointestinal tract
- Through local skin - very rare
Over time, the ghon complex can become reactivated, leading to secondary TB. Usually, reactivation occurs due to AIDS and aging. Secondary TB often occurs in the apex of the lung due to the high oxygen tension in the apex of the lungs. When the ghon complex gets reactivated, leads to the development of foci of caseous necrosis and may result in miliary pulmonary TB, which creates small regions of TB scattered across the entire lung or it can develop pneumonia due to tuberculosis.
Symptoms of TB
- Fevers
- Night sweats
- cough with hemoptysis
- Weight loss
- Caseating granuloma on biopsy
Tuberculosis can also spread to multiple distant sites in the body and it can spread to any tissue.
- Spread to meninges - creates meningitis (symptoms are headache, malaise, mental confusion, and vomiting)
- Spread to the intestine - creates intestinal TB
- Spread to kidney - creates sterile pyuria
- Spread to cervical lymph nodes - leads to TB of lymph nodes
- Spread to bone (usually, lumbar vertebrae)- creates potts disease
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