Table 2.
Common pathogens that cause pneumonia in otherwise healthy children aged 2–59 months
| Estimated percentage* | Comments | |
|---|---|---|
| Bacterial (20–50%) | ||
| Streptococcus pneumoniae | 17–37% | Estimates based on proportion of radiographically confirmed pneumonia prevented by vaccination with 7-valent and 9-valent vaccine (vaccine probe studies),21, 26, 27 and supported by lung aspiration studies25 |
| Haemophilus influenzae | 0–31% | Increasing use of highly efficacious vaccine against disease by H influenzae type b may decrease its role as a pathogen |
| Non-type b may play a greater role in non-severe pneumonia than type b28 | ||
| Found to be a significant cause of pneumonia in all vaccine probe studies,29, 30, 31 except one,32 and in lung aspiration studies25 | ||
| Staphylococcus aureus | 1–33% | Presents clinically as a severe, necrotising pneumonia with rapid progression25 |
| Non-typhoidal salmonellae | 0–28% | Bacteraemia may present with features consistent with a clinical diagnosis of pneumonia33, 34, 35 |
| Estimates are based on studies from tropical Africa33, 36 | ||
| Associated with non-severe pneumonia in some malaria-endemic regions of Africa34 | ||
| Mycoplasma pneumoniae | 5% | Limited diagnostic capacities in low-income countries37, 38, 39, 40 |
| Proportion of pneumonia associated with infection increases with age, the greatest burden is in children aged >3 years41 | ||
| Assumption that infections do not cause significant morbidity or mortality lacks evidence to be either validated or invalidated42, 43 | ||
| Chlamydophila pneumoniae | 3–10% | Limited diagnostic capacities in low-income countries37, 38, 39 |
| Proportion of pneumonia associated with infection increases with age, the greatest burden is in children aged >3 years41 | ||
| Poor quality serological data for very young children44 | ||
| Moraxella catarrhalis | 0–9% | Often not the focus of pneumonia microbiological studies45 |
| Klebsiella pneumoniae | 0–4% | One study noted a higher proportion of 14% in children with previous antimicrobial use45 |
| Rare exception in malnourished children46 | ||
| Viral (9–64%) | ||
| Respiratory syncytial virus | 1–39% | Particularly important in young infants47 |
| Influenza viruses | 0–22% | Important cause throughout age range47 |
| Increasingly documented in the tropics48 | ||
| Adenoviruses | 0–54% | Limited diagnostic testing and use of poor or insensitive tests47 |
| Parainfluenza viruses | 0–46% | Occurrence in alternating years means that single-year studies have limited value49 |
| Human metapneumovirus | 2–8% | Recent but well-documented cause of pneumonia50, 51, 52, 53 |
| Others (including bocavirus, coronaviruses, and rhinoviruses) | 4–30% | Recent PCR-based studies more consistently identify new viruses, but their significance remains to be defined54, 55, 56 |
These estimates of aetiological burden have wide ranges. Variation may be real, due to increased proportions of aetiologies due to high HIV prevalence, as well as seasonal (eg, influenza) and geographical (eg, Salmonella) variability. However, the primary source of variability may be due to measurement, either enrolment criteria (hospitalised versus outpatient enrolment), inadequate diagnostic testing of blood cultures with low yield (eg, blood culture), or misclassification (eg, urine antigen testing). Previous antimicrobial administration also may result in underestimation of some agents, and poor laboratory quality can also play an important part.57 Zero percentages (except in the case of Klebsiella) are often due to lack of diagnostic testing and the use of poor or insensitive tests, which are the important reasons for failure to consistently identify these pathogens, although in some cases true seasonal or geographical variations may contribute.