Causes of pneumonia in children

Causes of community-acquired (home) pneumonia in children

The etiology of community-acquired pneumonia in 50% of cases is represented by mixed microflora, and in most cases (30% of cases) community-acquired pneumonia is caused by a viral-bacterial association. This cause is more often observed in children of early and preschool age. In a small percentage of cases (5-7%), the etiology is represented by a viral-viral mixed microflora and in 13-15% - a bacterial-bacterial association, for example, an association of Streptococcus pneumoniae with acapsular Haemophilus influenzae. In the remaining 50% of cases, the etiology of community-acquired pneumonia is only bacterial. The type of causative bacteria depends on the age of the child.

In the first 6 months of life, the etiologic role of pneumococcus and Haemophilus influenzae is insignificant, since antibodies to these pathogens are transmitted from the mother in utero. The leading role at this age is played by E. coli, K. pneumoniae and S. aureus and epidermidis. The etiologic significance of each of them is small and does not exceed 15-20%, but they cause the most severe forms of the disease in children, complicated by the development of infectious toxic shock and lung destruction. Moraxella catarrhalis occurs in 3% of cases. Another group of pneumonias at this age is pneumonia caused by atypical pathogens, mainly Chlamydia trachomatis, which children become infected with from the mother, either intranatally (less often antenatally), or in the first days of life. In addition, infection with Pneumocystis carinii is possible (especially in premature babies).

Starting from the age of 6 months and up to 6-7 years inclusive, pneumonia is mainly caused by Streptococcus pneumoniae, which accounts for up to 60% of all pneumonia cases. Often, as already mentioned, acapsular Haemophilus influenzae is also cultured. Haemophilus influenzae type b is detected less frequently (in 7-10% of cases). This pathogen usually causes severe pneumonia complicated by lung destruction and pleurisy. Diseases caused by S. aureus, S. epidermidis and S. pyogenes usually develop as a complication of severe viral infections such as influenza, chickenpox, measles, herpes infection, and do not exceed 2-3% in frequency. Pneumonia caused by atypical pathogens in children of this age is mainly caused by M. pneumoniae and C. pneumoniae. It should be noted that the role of M. pneumoniae as a cause of pneumonia in children has clearly increased in recent years. Mycoplasma infection is often diagnosed in the second or third year of life. C. pneumoniae is usually detected in children over 5 years of age.

The etiology of pneumonia in children over 7 years of age is practically no different from that in adults. Most often, pneumonia is caused by S. pneumoniae (up to 35-40% of all cases), M. pneumoniae (23-44%), C. pneumoniae (15-30%). Such pathogens as H. influenzae type b, Enterobacteriaceae (K. pneumoniae, E. coli, etc.), S. aureus and S. epidermidis are practically not detected.

Viruses can also cause community-acquired pneumonia. They can be both an independent cause of the disease and (much more often) create viral-bacterial associations. The most important is the PC virus, which occurs in approximately 50% of cases of viral and viral-bacterial diseases; in 25% of cases, the cause of the disease is parainfluenza viruses of types 3 and 1. Influenza viruses A and B and adenoviruses play a minor role. Rhinoviruses, enteroviruses, coronaviruses are detected less frequently. It should be noted that pneumonias caused by measles, rubella, and chickenpox viruses have been described.

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Hospital-acquired (nosocomial) pneumonia in children

Hospital pneumonias differ significantly from community-acquired pneumonias in the spectrum of pathogens and their resistance to antibiotics. The spectrum of bacterial and fungal pathogens of hospital non-ventilator-associated pneumonia depends to a certain extent on the profile of the hospital where the patient is located. Thus, in patients of the therapeutic department, hospital pneumonia can be caused by pneumococcus, but more often by S. aureus, or S. epidermidis, or K. pneumonia. In premature infants in the second-stage hospital - S. aureus, or S. epidermidis, or K. pneumoniae, or (in rarer cases) Pneumocystis carinii.

Bacterial etiology of ventilator-associated hospital pneumonia depending on the department of patient stay

Nature of the department	Pneumonia pathogens
Resuscitation, intensive care	Ps. aeruginosa S. aureus et epidermidis E. coli K. pneumoniae Acinetobacter spp. Candida spp.
Surgery, burns department	Ps. aeruginosa K. pneumoniae E. coli Acinetobacter spp. S. aureus et epidermidis Anaerobes
Oncohematology	Ps. Aeruginosa K. Pneumoniae E. coli and other enterobacteria S. aureus et epidermidis Aspergillus spp
Therapeutic departments	S. aureus et epidermidis K. pneumonia S. pneumoniae
Second stage departments for nursing premature babies	S. aureus et epidermidis K. pneumonia Pneumocystis carinii

In the etiology of hospital pneumonia (as well as in the etiology of community-acquired pneumonia) in children, respiratory viruses occupy a significant place (up to 20% of cases). These pathogens cause the disease independently or more often in the form of a viral-bacterial association, in 7% of cases - in the form of an association of Candida fungi with viruses or viruses and bacteria. Among the viruses that cause hospital pneumonia, influenza A viruses dominate, less often - influenza B. Parainfluenza viruses, adenoviruses and Coxsackie B viruses are observed even less often, and PC viruses and Coxsackie A viruses are detected in isolated observations.

Among ventilator-associated hospital pneumonias, early and late pneumonias are distinguished. Their etiology is different. Pneumonias that develop in the first 72 hours after intubation usually have the same etiology as community-acquired pneumonias in patients of the same age. This is primarily due to the fact that their pathogenesis is primarily determined by microaspiration of the contents of the oropharynx and, accordingly, the microflora that contaminated and colonized the mucous membranes of the upper respiratory tract. Thus, in children aged 2 weeks to 6-7 months of life, early VAPs are usually caused by E. coli, K. pneumoniae, S. aureus et epidermidis. In children aged 6-7 months to 6-7 years - S. pneumoniae, although there may be pneumonias caused by H. influenzae. In children and adolescents over 7 years of age, pneumonias are usually caused by M. pneumonia and somewhat less often by S pneumoniae.

In late VAP (when pneumonia develops after 72 hours of ventilation), the etiology of hospital pneumonia is dominated by pathogens such as Ps. aeruginosa, S. marcescens, Acinetobacter spp, as well as S. aureus, K. pneumoniae, E. coli, Candida, etc. The reason for this is that late VAP is caused by hospital microflora colonizing the respiratory equipment, and therefore non-fermenting gram-negative bacteria, primarily Pseudomonas aeruginosa, are of primary importance here. The etiology of ventilator-associated pneumonia is presented in Table 76-2.

Causes of ventilator-associated hospital pneumonia in children

Ventilator-associated pneumonia	Pneumonia pathogens
Early	The etiology corresponds to the age etiological structure of community-acquired pneumonia
Late	Ps. aeruginosa Acinetooacter spp S. Marsensens S. Aureus K. Pneumoniae E. Coli Candida spp

It is especially worth mentioning the etiology of pneumonia in patients with immunodeficiency. In children with primary cellular immunodeficiencies, HIV-infected and AIDS patients, pneumonia is most often caused by Pneumocystis carinii and Candida fungi, as well as M. avium-intracellulare and the herpes virus, cytomegalovirus. In humoral immunodeficiencies, pneumonia is most often caused by S. pneumoniae, as well as staphylococci and enterobacteria, and in neutropenia - by gram-negative enterobacteria and fungi.

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Causes of pneumonia in patients with immunodeficiency

Patient groups	Pneumonia pathogens
Patients with primary cellular immunodeficiency	Pneumocystis Fungi of the genus Candida
Patients with primary humoral immunodeficiency	Pneumococcus Staphylococci Enterobacteria
Patients with acquired immunodeficiency (HIV-infected, AIDS patients)	Pneumocystis Cytomegaloviruses Herpes viruses Mycobacterium tuberculosis Candida fungi
Patients with neutropenia	Gram-negative enterobacteria Fungi of the genus Candida, Aspergillus, Fusarium

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Pathogenesis of pneumonia in children

In the pathogenesis of pneumonia, a certain role is played by the low level of anti-infective protection in children (compared to adults). This is especially characteristic of young children, so their tendency to develop pneumonia is higher. In addition, the relative insufficiency of mucociliary clearance is important, especially with the development of a respiratory viral infection, with which, as a rule, pneumonia begins in a child, especially at an early age. It should also be noted that the mucous membrane of the respiratory tract tends to edema and the formation of viscous sputum during the development of inflammation, which also disrupts mucociliary clearance in a child.

There are 4 known main pathogenetic mechanisms for the development of pneumonia: microaspiration of oropharyngeal secretions, inhalation of aerosol containing microorganisms, hematogenous spread of microorganisms from an extrapulmonary source of infection, and direct spread of infection from adjacent affected organs.

Of the above mechanisms, microaspiration of oropharyngeal secretions is of the greatest importance in children. It plays a major role in the pathogenesis of both community-acquired and hospital pneumonia. Airway obstruction is also of great importance in microaspiration (especially in cases of broncho-obstructive syndrome, which is so common in young and preschool children). A combination of these mechanisms is often observed. Aspiration of large amounts of contents from the upper respiratory tract and/or stomach is typical for newborns and children in the first months of life and occurs during feeding and/or vomiting, as well as regurgitation.

When microaspiration (or aspiration, or inhalation of an aerosol containing microorganisms) coincides with a violation of the mechanisms of non-specific resistance of the child's body, for example, with ARVI, favorable conditions are created for the development of pneumonia. Hematogenous spread of microorganisms from an extrapulmonary focus of infection and direct spread of infection from adjacent affected organs are also of great importance for pathogenesis. However, more often these mechanisms play an important role in the development of secondary pneumonia.

Factors predisposing to microaspiration and, consequently, to the development of pneumonia:

• age up to 6 months, especially premature babies;
• encephalopathy of various origins (post-hypoxic, with brain malformations and hereditary diseases, convulsive syndrome);
• dysphagia (vomiting and regurgitation syndrome, esophageal-tracheal fistulas, achalasia, gastroesophageal reflux);
• broncho-obstructive syndrome in respiratory, including viral, infections;
• mechanical violations of protective barriers (nasogastric tube, endotracheal intubation, tracheostomy, gastroduodenoscopy);
• repeated vomiting with intestinal paresis, severe infectious and somatic diseases;
• conducting artificial ventilation; development of a critical condition due to the underlying disease;
• the presence of developmental defects (especially heart and lung defects);
• neuromuscular blockade.