The evidence for non-typeable Haemophilus influenzae as a causative agent of childhood pneumonia
The evidence for non-typeable Haemophilus influenzae as a causative agent of childhood pneumonia
Mary P E Slack 0
0 School of Medicine, Gold Coast Campus, Griffith University , Southport, Queensland 4222 , Australia
Haemophilus influenzae type b (Hib) was a major cause of bacterial pneumonia in children prior to the introduction of Hib-conjugate vaccines. The widespread use of Hib-conjugate vaccines has resulted in a significant decline in the number of cases of invasive Hib disease, including bacteraemic pneumonia, in areas where the vaccine has been implemented. In many countries, non-typeable H. influenzae (NTHI) is now the most common cause of invasive haemophilus infection in all ages. NTHI are a recognized cause of bacteraemic and non-bacteraemic pneumonia in children and in adults. Less than 10% of cases of pediatric pneumonia are bacteraemic, and children generally do not expectorate lower respiratory tract secretions, so determining the microbial cause of a non-bacteraemic pneumonia is challenging. In this commentary the evidence that NTHI is a cause of pneumonia in children is briefly reviewed.
Non-typeable Haemophilus influenzae; Community-acquired pneumonia; Children
Following the introduction of Hib-conjugate vaccines,
the number of cases of invasive Hib infections, including
bacteraemic pneumonia, has declined wherever the
vaccine has been implemented. In many countries
nontypeable Haemophilus influenzae (NTHI) is now the
most common cause of invasive haemophilus infection
in all ages [
]. The clinical spectrum of invasive NTHI
infections is similar to that of Hib, in that it can cause
meningitis, bacteraemia, pneumonia, and other localized
invasive infections; however, NTHI is more likely to
cause bacteraemia or pneumonia, and Hib was
predominantly associated with meningitis in young children and
acute epiglottitis in older children (>18 months) in
highincome countries [
NTHI as a cause of pneumonia in children
While NTHI are a well recognized cause of both
bacteraemic and non-bacteraemic pneumonia in adults, the
data on the relative importance of NTHI in childhood
pneumonia is limited and, to some extent, conflicting
]. This is due, in part, to the difficulty in obtaining
samples to determine the cause of non-bacteraemic
lower respiratory tract infections that have not been
contaminated with upper respiratory tract commensals
(including NTHI). Trans-thoracic fine-needle aspiration
is the most reliable method, but it is only performed
where there is dense peripheral consolidation and is not
a routine procedure. Other less direct diagnostic
techniques, including broncho-alveolar lavage (BAL) and
sputum cultures may be used, but are still problematic.
NTHI can be present in the upper airway in the absence
of disease, and isolation of NTHI from an expectorated
or induced sample of sputum in a child with clinical and
radiographic evidence of pneumonia may be coincidental
rather than causal. In young children it can be difficult
to distinguish pneumonia from bronchiolitis [
definition of pneumonia used by investigators also varies
considerably, and is frequently not stated in publications,
making it difficult to compare results from different
studies. The Pneumococcal Vaccines Accelerated
Development and Introduction Plan (Pneumo-ADIP)
developed a case definition for childhood pneumonia, based
on cough, respiratory difficulty and tachypnea [
its use would provide standardization across different
Studies from Papua New Guinea in the 1980s [
suggested that NTHI was a major cause of pneumonia in
children in these communities. More recent studies of
community-acquired pneumonia (CAP) in children,
reviewed by Slack [
], have identified NTHI in a
proportion (range 4–26%) of lung aspirates by culture and/or
A study from The Gambia reported the results of
molecular testing of lung and pleural aspirates collected
from 55 children (aged 2–59 months) with severe
radiologically confirmed pneumonia [
]. There were 56
samples collected in total. A pathogen was isolated from
53/56 (95%) samples by one or more laboratory method,
and 12/53 (23%) were identified as H. influenzae.
Molecular characterisation of these 12 isolates showed that 3 were
NTHI and it was suggestive that an additional 3 may also
have been NTHI; however, there was insufficient DNA to
permit full multilocus sequence typing. Only 1 was Hib.
Although the data is limited, there seems to be no
doubt that NTHI is the cause of a small proportion of
non-bacteraemic cases of CAP in children and may
cause a greater proportion of such cases in developing
countries. The incidence and case fatality rate of
pneumonia in developing countries is tenfold higher
than in developed countries [
]. This probably reflects
underlying problems of poverty, malnutrition,
overcrowding, and inadequate healthcare in the developing
world. In such vulnerable hosts it is entirely possible that
low-grade pathogens such as NTHI would cause a
higher proportion of respiratory infections.
Identification of bacteraemic pneumonia relies on
isolation of the putative pathogen from a normally sterile
site—usually a blood culture. Several studies have
reported on pediatric invasive NTHI infections in the era
of Hib-conjugate vaccination.
Collins et al. [
] reported on the follow up of invasive H.
influenzae disease in England and Wales, which was
diagnosed between 2000 and 2013. Over this period there
were 1585 cases of invasive H. influenzae infection in
children aged 1 month to 10 years, in whom NTHI caused
31–51 cases per year (0.53–0.92/100,000). Detailed clinical
follow up of 214 cases of invasive NTHI infection
occurring between 2009 and 2013 revealed that 52% (n = 111)
occurred in <2 year olds and 52% (n = 110) had an
identified comorbidity. Bacteraemic pneumonia was the most
common presentation (n = 99, 46%). Bacteraemic
pneumonia was also the most common (47%) presentation in a
Canadian study [
]. In a retrospective surveillance study
in the Netherlands [
], 47% of cases in children aged
7 weeks to 5 years presented with meningitis (28 of 60
cases) and approximately 25% presented with bacteraemic
pneumonia. In a German study [
], 34% of pediatric
cases of invasive NTHI infection presented with
meningitis, 28% with septicaemia without focus, and 21%
presented with pneumonia. This is similar to the
findings from an Israeli study [
] where 25% of NTHI
invasive infections presented with pneumonia. One
possible reason for the disparity between these studies
is differing blood culture practices in children
presenting with high fever with or without localising foci such
as a pneumonia.
Although the data from these studies is not strictly
comparable, they do suggest that invasive NTHI can
present as a pneumonic infection in children aged from
6 weeks, with the majority of cases occurring in children
aged <4 years.
NTHI are a significant component of the nasopharyngeal
microbiota and the organisms can spread contiguously
from the upper respiratory tract to cause otitis media,
sinusitis and pneumonia in otherwise healthy children.
A preceding viral upper respiratory tract infection may
facilitate the development of lower airways infection,
including CAP [
]. Invasive NTHI infections, including
meningitis, bacteraemia and bacteraemic pneumonia
usually occur in children with underlying predisposing
conditions, including immunosuppression and chronic
respiratory diseases, although some cases do occur in
previously healthy children. In the study reported by
Kalies et al. [
], 62% of the cases of NTHI invasive
infection had no known predisposing condition. By contrast,
van Wessell [
] reported comorbidities in approximately
50% of the cases in children aged 1–4 years.
There is also evidence that NTHI are a major pathogen
in cases of acute non-responding or recurrent pneumonia.
In a retrospective study [
], 250 previously healthy
children who developed either recurrent or non-responding
pneumonia underwent flexible bronchoscopy with a
semiquantitative culture of BAL. NTHI was identified in 51%
of the cases of recurrent community-acquired
bronchopneumonia and 26% of non-responsive
communityacquired bronchopneumonia, often in mixed culture [
Although invasive NTHI infection in children is
uncommon and the organism is less virulent than Hib,
NTHI can cause severe disease, particularly in neonates
who are often premature, and in older children who
have significant comorbidities [
]. Neonatal invasive
NTHI infections are well described, with an incidence of
1.6–4.9/100,000 live births, accounting for
approximately 5% of all neonatal bacterial invasive infections
]. The majority of neonatal infections present as
sepsis without focus, rather than pneumonia [
]. In the
UK study [
], 16% of the cases required intensive care
and 11% died. A European surveillance study [
reported a case fatality ratio of 17.4% in <1 year olds, 9.2%
in 1–4 year olds, and 5.2% in 5–14 year olds.
It is clear that in countries where Hib has been virtually
eliminated by the use of Hib-conjugate vaccine, NTHI has
emerged as the most common cause of pediatric H.
influenzae infections, including pneumonia. A vaccine effective
against NTHI could be of value in preventing these
infections. The 10-valent pneumococcal vaccine (PHid-CV;
Synflorix, GSK Biologicals, Rixensart, Belgium) uses H.
influenzae outer membrane lipoprotein D as its carrier
protein, which is conserved among strains of H. influenzae.
Immunisation results in high IgG antibody concentrations
against protein D, but has no effect on nasopharyngeal
NTHI colonisation or H. influenzae density [
]. Its efficacy
against invasive NTHI infections has not yet been
], although a recent report suggests PHiD-CV10
may reduce the prevalence of suppurative otitis media in
children at high risk of severe middle ear disease [
Whole genome sequencing of 20 nasal or nasopharyngeal
NTHI isolates from a cohort of Aboriginal and Torres
Strait Islander children in Australia, selected to represent
common NTHI genotypes, revealed that 5 of 20 NTHI
genomes lacked the hpd genes that encode Protein D [
The challenge is to develop a vaccine that overcomes the
marked heterogeneity and phase variability of NTHI.
Because NTHI now cause as much acute middle ear disease
as pneumococci [
], and are the dominant microbe in
chronic and recurrent middle ear disease worldwide, an
effective NTHI vaccine could be cost-effective in many
pediatric populations. Further studies to identify the specific
risk factors that predispose children to invasive NTHI
infection should be undertaken [
]. Accurate identification
of NTHI and systematic reporting of cases of pediatric
NTHI infections would be of great value in establishing the
true role of NTHI in pediatric CAP. Surveillance, coupled
with improved diagnostic techniques to identify the
microbiological causes of non-bacteraemic pneumonia,
is needed to fully understand the role of NTHI in
CAP: Community-acquired pneumonia; Hib: Haemophilus influenzae type b;
NTHI: Non-typeable Haemophilus influenzae
No external funding was received for this work.
Availability of data and materials
MPES devised and wrote this commentary.
School of Medicine, Gold Coast Campus, Griffith University, Queensland 4222,
Australia. MPES is a Consultant Medical Microbiologist. She was formerly Head
of the National Haemophilus Reference Laboratory at Public Health England
and Head of the WHO Global Collaborating Centre for Haemophilus influenzae.
MPES has received funding from Pfizer, GSK and Sanofi Pasteur for participation in
advisory boards and from Pfizer, GSK and Astra Zeneca for participation in Symposia
at international scientific meetings. She has worked as a contractor for Pfizer UK.
Consent for publication
Ethics approval and consent to participate
MPES devised and prepared the manuscript.
Springer Nature remains neutral with regard to jurisdictional claims in
published maps and institutional affiliations.
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