Haemophilus influenzae is a bacterium responsible for numerous cases of the infectious diseases epiglottitis and meningitis

Haemophilus influenzae in detail - technical

Essentials

Haemophilus influenzae (Pasteurellaceae) is a Gram-negative bacillus that is an exclusively human pathogen. There are six capsular serotypes (a–f), of which type b (Hib) being a major cause of childhood infectious disease. Transmission occurs by close bodily contact, the main source being other children, and is usually followed by nasopharyngeal carriage, following which susceptible people may develop disease.

Clinical features—in infants Hib causes symptoms ranging from a mild nonspecific febrile illness (occult bacteraemia) to fully blown sepsis with meningitis, epiglottitis, pneumonia, septic arthritis, and cellulitis. Noncapsulate or nonserotypable H. influenzae cause otitis media and conjunctivitis in children, and exacerbations of chronic bronchitis, sinusitis, and pneumonia in adults. H. parainfluenzae, H. aphrophilus, H. paraphrophilus, and H. segnis are rare causes of infective endocarditis and other sepsis.

Diagnosis and treatment—Gram staining of cerebrospinal, synovial, or pleural fluid is a key investigation, but definitive diagnosis requires culture or detection of specific DNA by polymerase chain reaction (PCR) methods. Aside from supportive care, treatment requires (1) appropriate antibiotics—resistance is an increasing problem: the agent of choice for invasive Hib disease is a third-generation cephalosporin with good cerebrospinal fluid penetration (e.g. ceftriaxone or cefotaxime, but not cefuroxime); chloramphenicol with or without ampicillin remains effective in some developing countries. (2) corticosteroids—except in children in low-income countries, these reduce mortality, severe hearing loss, and neurological sequelae of Hib meningitis. Antibiotic treatment of noncapsulate H. Influenzae otitis media, sinusitis, and chronic bronchitis is widely practised but largely uinsupported by evidence.

Prevention—polyribosyl ribitol phosphate (PRP)-conjugate vaccines, often given at 2 to 6 months of age, have virtually eliminated Hib disease from North America, Europe and some other countries.

Introduction

Haemophilus influenzae is a human-adapted pathogen with no other reservoir. Typically, it inhabits the nasopharynx but may also be recovered from other mucosal surfaces including genital and intestinal tracts. Despite being a fastidious organism that is relatively difficult to grow, it was first isolated as early as 1890 by Pfeiffer who mistakenly thought it was the cause of a current influenza pandemic.

Description of the organism

The genus Haemophilus (family Pasteurellaceae) includes H. ducreyi, which causes chancroid, the sexually transmitted infection (Chapter 7.10.13), and other human-adapted species all of which are commensals: H. parainfluenzae (the most abundantly colonizing species), H. aegyptius, H. aphrophilus, H. haemolyticus, H. paraphrophilus, and H. segnis.

H. influenzae is a small (0.2–0.3 × 0.5–0.8 μm) Gram-negative nonmotile coccobacillus that grows well on rich media under 5% CO₂ and produces 2- to 3-mm-diameter grey translucent colonies after 18 to 25 h incubation. It is fastidious with the following specific growth requirements: it does not grow on nutrient agar, such as Columbia, without growth supplements X factor (haemin) and V factor (NAD). H. parainfluenzae requires only V factor. Rich media such as chocolate agar support the growth of Haemophilus spp. and is the preferred solid medium for propagation of these organisms. Precise speciation requires sequence analysis, e.g. 16S ribosomal DNA.

H. influenzae are phenotypically and genetically diverse. A minority are capsulate (serotypeable) among the majority of noncapsulate (nonserotypeable) strains. Capsulate strains produce one of six chemically and antigenically distinct polysaccharide capsules a to f. These strains are relatively nondiverse, consisting of few lineages compared to the much more diverse noncapsulate strains. This suggests that genes encoding the capsule were acquired relatively recently. The type b capsule is essential for the pathogenesis of bacteraemia and meningitis, a feature recognized in the early 1930s.

Pathogenicity

H. influenzae expresses several cell surface features essential for colonization of the nasopharynx. These are virulence factors of which the capsule is the most important. Of the six antigenically distinct structures (types a–f), type b accounts for virtually all the invasiveness of H. influenzae in children. The serotype b capsule consists of a negatively charged phosphodiester-linked linear polymer of disaccharide units of polyribosylribitol phosphate (PRP) which resists phagocytosis by interfering with binding of serum complement. The capsule also resists desiccation, perhaps promoting host-to-host transmission. Serum antibody directed against serotype b capsular polysaccharide is protective. This simple observation stimulated the development of the highly successful H. influenzae type b (Hib) vaccine, now used routinely in national childhood immunization programmes. Modern vaccines contain PRP covalently conjugated to a protein carrier such as tetanus toxoid.

Other cell surface structures involved in pathogenesis, particularly in those strains lacking a capsule (nonserotypeable H. influenzae), include lipopolysaccharide, pili, and other adhesion proteins.

Epidemiology

Haemophilus influenzae type b

Hib is a major cause of childhood infectious disease. Acquisition occurs by close bodily contact, the main source being other children, and is usually followed by carriage. The organism dwells harmlessly for months in the nasopharynx. However, in a few susceptible individuals, acquisition immediately precedes invasive disease. Carriage rates increase from birth until 4 years and are higher in developing countries especially where there is crowding, day care attendance, and contact with siblings. Hib immunization virtually eliminates carriage and produces a marked herd effect, protecting against disease. In 2000, it was estimated that as many as 60% of the world’s children were unimmunized, but Hib vaccine is now being brought to most parts of the world. However, in some parts of Asia, particularly China, Hib is far less prevalent and so the health benefits of mass vaccination may be insufficient to justify a national vaccination programme.

The main diseases caused by Hib are meningitis, primary bacteraemia, pneumonia, epiglottitis, and arthritis (Table 2); the most important is meningitis. Before the introduction of vaccination, it was the most important cause of childhood meningitis in the United States of America, accounting for 80% of cases, and in the United Kingdom it accounted for approximately 50% of cases. In contrast, it was a much less prominent cause of meningitis than Neisseria meningitidis in the ‘meningitis’ belt of Africa.

The incidence of Hib infections varies with age. Neonates are protected, after which disease peaks by 9 months of age and declines to very low levels by 4 years. Age-specific disease incidence is inversely related to serum antibodies to Hib. Male gender and ethnicity are risk factors for disease. For example, the incidence among Native Americans less than 4 years old exceeded 150 cases/100 000 per year compared to only 50 cases/100 000 per year in the population as a whole. In western countries, case fatality of Hib meningitis was about 5% and long-term morbidity (deafness and neurological and learning deficits) occurred in at least 10% of cases.

Since the implementation of Hib conjugate vaccination, the disease has virtually disappeared from North America and Europe and there has been a similar dramatic decline in The Gambia and Kenya. A striking but temporary re-emergence of Hib disease in the United Kingdom was attributed to the introduction, in 2000, of a combined Hib-acellular pertussis vaccine that induced lower Hib antibody levels.

Noncapsulate or nonserotypeable H. influenzae

Noncapsulate H. influenzae is acquired soon after birth and is carried by 20 to 80% of the population but it rarely causes bacteraemia and meningitis. However, it does cause otitis media, sinusitis, chronic bronchitis, and pneumonia. In cases of otitis media, one of the commonest childhood diseases, H. influenzae was cultured from middle ear fluid of 23%, while 23% and 26% grew Moraxella catarrhalis or Streptococcus pneumoniae, respectively. Bronchitis and sinusitis are diseases predominantly of adults. Chronic bronchitis and pneumonia associated with H. influenzae occur more frequently with advancing age.

Antibiotic resistance

Resistance of H. influenzae to antibiotics was first reported in the early 1970s. Since then, the prevalence of ampicillin-resistant β-lactamase-producing strains has risen rapidly in most parts of the world and strains resistant to tetracycline, chloramphenicol, and trimethoprim or multiresistant to these antibiotics have emerged. In recent years, resistance rates have remained reasonably constant in Europe at 10 to 20%.

Clinical features

Hib invasive disease

In infants, clinical features vary from a mild nonspecific febrile illness, reflecting so-called occult bacteraemia, to fully blown sepsis with meningitis. Infants may present with fever and irritability alone but severe cases show typical features of meningitis including altered mental status, stiff neck, and sepsis. In some cases there is disseminated intravascular coagulation with purpuric rash and septicaemic shock, reminiscent of meningococcaemia. Diagnosis is by examination of cerebrospinal fluid and blood culture. In older children, lumbar puncture should not be performed until cerebral oedema can be excluded by CT, but antibiotic treatment must not be delayed.

Epiglottitis is an acutely life-threatening medical emergency. Against a background of sepsis (fever, tachycardia, and tachypnoea), pronounced local signs and symptoms evolve rapidly, including sore throat, drooling, dysphagia, hoarseness, barking ‘brassy’ cough, and stridor. The epiglottis is inflamed and swollen, looking like a red cherry, but attempts to examine the throat may precipitate acute airway obstruction. Pneumonia is an important but relatively unrecognized feature of Hib disease. The main features are fever with signs of respiratory distress including tachypnoea, nasal flaring, and intercostal indrawing. Chest examination and radiography are diagnostic.

The child with septic arthritis shows features of sepsis, is unwilling to use the affected limb, and resists movement of the painful joint. Examination and culture of joint fluid are diagnostic. Cellulitis is rare in children and cellulitis of the neck is unusual in adults. These presentations have been increasingly associated with H. influenzae type f infection, especially since the introduction of Hib conjugate vaccine.

Noncapsulate H. influenzae

Otitis media is the main clinical presentation. This common childhood illness presents with irritability and, on otoscopy, an inflamed tympanic membrane is visible and may perforate discharging pus. Although not used routinely, tympanocentesis is the most reliable means of aetiological diagnosis. A high proportion of cases of conjunctivitis in children are caused by noncapsulate H. influenzae. Historically, H. aegyptius has also been associated with conjunctivitis. Culture of conjunctival swabs is the main test for making a diagnosis. Brazilian purpuric fever, a fulminant septicaemic illness with high case fatality, is also caused by H. aegyptius.

Noncapsulate H. influenzae sinusitis presents with local pain, a sense of pressure in the head, local facial oedema, and visible pus draining from the ostia of the sinuses. Diagnosis is by skull radiography, with special (Towne’s and Water’s) views, or CT scan, which reveals sinus opacification and a fluid level. Sinus aspiration provides an aetiological diagnosis.

In adults, exacerbations of chronic bronchitis are commonly associated with noncapsulate H. influenzae. H. influenzae and S. pneumonia are cultured from sputum of up to 50% of cases although their precise aetiological role is uncertain.

Noncapsulate H. influenzae can cause severe invasive disease such as neonatal sepsis, resembling group B streptococcal neonatal sepsis, and pneumonia in adults, particularly older people. It has also been implicated in meningitis associated with head trauma, particularly skull fracture.

Laboratory diagnosis

Culture or detection of specific DNA is essential for aetiological diagnosis. Direct examination of cerebrospinal, pleural, or synovial fluids by Gram’s stain may reveal organisms with the morphological features of H. influenzae. This is very helpful in indicating bacterial infection but is nonspecific. Blood culture using most commercial systems yields excellent growth in both anaerobic and aerobic bottles. However, growth on agar and differentiation of Haemophilus spp. requires special conditions (see above).

Antibiotic susceptibility testing using antibiotic discs requires supplemented media to support the growth of Haemophilus spp. but this may be inaccurate and should be supplemented by measurement of β-lactamase activity. Chloramphenicol disc susceptibility is frequently inaccurate and should be supplemented by an assay for chloramphenicol acetyltransferase activity.

Capsular type b antigen can be rapidly detected in cerebrospinal fluid, sterile site fluid, or urine but is seldom used since the implementation of Hib vaccination. Polymerase chain reaction (PCR) of cerebrospinal fluid has been developed for diagnosing Haemophilus spp. meningitis. Capsular typing of H. influenzae can be achieved serologically but a PCR-based method has proved more reliable.

Treatment

Antibiotics

A third-generation cephalosporin with good cerebrospinal fluid penetration is the first-line antibiotic treatment for invasive Hib disease. High-dose ceftriaxone or cefotaxime are effective for treating H. influenzae meningitis and septicaemia, but cefuroxime must not be used. In developing countries, chloramphenicol alone (depending on the prevalence of chloramphenicol resistance) or in combination with ampicillin is effective therapy.

Antibiotics are commonly prescribed for otitis media, sinusitis, and chronic bronchitis, but large meta-analyses have failed to demonstrate convincing efficacy although some subgroups are benefited. Oral amoxicillin is the first-line drug. Amoxicillin/clavulanate, trimethoprim, tetracycline (adults only), and quinolones (adults only) can also be used.

Corticosteroid treatment

Corticosteroids significantly reduce mortality, severe hearing loss, and neurological sequelae. In adults with community-acquired bacterial meningitis, corticosteroid therapy should be started with the first antibiotic dose. In children, data support the use of adjunctive corticosteroids in children only in high-income countries.

Prevention and control

PRP-conjugate vaccines are the best preventive measure for controlling Hib disease. Highly effective vaccines contain capsular antigen PRP conjugated to tetanus toxoid (PRP-T), outer membrane protein (PRP-OMP), or mutant diphtheria toxoid (PRP-CRM, HbOC). Three doses are given at intervals between the ages of 2 and 6 months. In many countries, a booster dose is given at age 1 to 2 years.

Rifampicin, 20 mg/kg orally once a day for 4 days, eradicates carriage and is believed to prevent secondary cases among close contacts. This is appropriate only where they have not received Hib vaccine.

Other nasopharyngeal Haemophilus spp.

H. parainfluenzae

H. parainfluenzae is a well-adapted commensal that colonizes virtually everyone soon after birth but is rarely associated with disease. It has been isolated in cases of infective endocarditis, neurosurgical meningitis, prosthetic device infection, and brain and liver abscesses. It is treated in the same way as H. influenzae.

Rare species

H. aphrophilus, H. paraphrophilus, and H. segnis are implicated in fewer than 2% of all cases of infective endocarditis and in brain or lung abscesses and empyema fluid. Since they are slow growing, they may be missed if blood cultures are not incubated for prolonged periods. Antibiotic treatment is the same as for H. influenzae.

Further reading