The challenges of Hib

Haemophilus influenzae type b (Hib) is a bacterium responsible for severe pneumonia, meningitis and other invasive diseases almost exclusively in children aged less than 5 years. It is transmitted through the respiratory tract from infected to susceptible individuals. Vaccines are the only public health tool capable of preventing the majority of serious Hib disease.¹


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Signs and symptoms

Invasive Hib disease presents most frequently as meningitis (approximately 50%–65% of invasive Hib infections), but may also present as bacteraemic pneumonia, bacteraemia, cellulitis, epiglottitis, septic arthritis, osteomyelitis and pericarditis. Diseases caused by direct spread from the nasopharynx are usually considered non-invasive and include non-bacteraemic pneumonia, otitis media, sinusitis and conjunctivitis.¹

The incubation period ranges between 2 to 10 days. Most common symptoms of Hib meningitis are acute onset of fever, headache, seizures and one or more of the following signs: neck stiffness, altered consciousness or other meningeal signs (e.g. photophobia).¹

In infants <6 months of age, the signs and symptoms may be non-specific but often include a bulging fontanelle. Even with adequate medical treatment, 5% of children with Hib meningitis die and 20%–40% of survivors suffer severe sequelae including blindness, deafness, and learning disabilities. Symptoms of Hib pneumonia include cough, fever, rapid breathing (>40 breaths per minute) and lower chest wall in-drawing.¹

Symptoms of acute epiglottitis include high fever, sore throat, difficulty in swallowing and stridor. It may lead to acute respiratory obstruction. This form of Hib disease was common in older children in North America and Europe in the pre-vaccination era; it is rarely reported from developing countries where Hib disease occurs in younger children.¹

Diagnosis

Hib meningitis and pneumonia cannot be differentiated from meningitis and pneumonia caused by other bacterial pathogens on clinical examination alone. In clinically suspected Hib meningitis the etiological diagnosis can be achieved by: isolating Hib from cerebrospinal fluid (CSF); a positive latex agglutination or polymerase chain reaction (PCR) test for Hib from CSF; finding purulent CSF with a gram stain showing gram negative coccobacilli; or by growth of Hib from blood cultures. Infection in other sites may be proven through demonstration of Hib or Hib-specific components in the affected body fluid or tissue. It is difficult to identify the bacterial etiology of pneumonia. Blood cultures and cultures of lung aspirates are accurate way of identifying a bacterial agent as a cause of pneumonia.¹

Prevention

Hib infections can be prevented through vaccination. The following types of Hib vaccines are available in Australia:

• Monovalent Hib vaccines
• Hexavalent combination vaccines which contain Hib in combination with diphtheria, tetanus, pertussis, poliovirus and hepatitis B antigens

Bacteriology

Haemophilus influenzae is a Gram-negative coccobacillus that is a normal part of upper respiratory tract flora. It can be isolated in two forms: capsular and non-capsular. Strains isolated from respiratory tract specimens (sputum, middle ear or sinus fluid) are non-capsular and are known as non-typeable Haemophilus influenzae (NTHi). Six capsular types (a to f) have been described and, before the introduction of vaccination against Haemophilus influenzae type b (Hib), almost all H. influenzae isolates from sterile sites (blood, cerebrospinal fluid, joint or pleural fluid) were of the b capsular type.²

Epidemiology

Hib was previously the commonest cause of bacterial meningitis in Australian children. Aboriginal and Torres Strait Islander children, especially in remote and rural areas, had a much higher incidence of Hib infection and presented at a younger age than non-Indigenous children. Hib epiglottitis was particularly rare among Indigenous children.³ 

In Australia, Hib conjugate vaccines were first introduced into the routine vaccination schedule in 1993, leading to a more than 95% reduction in the reported incidence of Hib disease. Introduction of Hib conjugate vaccines into routine vaccination programs has led to a dramatic decline in the incidence of Hib disease in many regions of the world.³

The sharp decline in Hib disease incidence since then is seen both among the Indigenous and non-Indigenous populations. In the pre-vaccination era, there were at least 500 cases of Hib disease and 10–15 deaths annually among Australian children aged <6 years. At present, the number of cases reported in Australia for all ages is around 20 per year, a reduction of over 95% from the pre-vaccination period.³

Treatment

Hib disease is treated with appropriate antibiotics (including ampicillin or 3rd generation cephalosporins like ceftriaxone and cefotaxime for meningitis, oral amoxicillin for non-immunocompromised children aged 2–59 months with pneumonia, parenteral ampicillin and gentamicin for immunocompromised children and those presenting with severe pneumonia), symptomatic treatment, and supportive treatment of sequelae (including oxygen for meningitis patients presenting with  hypoxaemia and airway support for epiglottitis). The choice of therapy depends on the presentation of Hib disease and local patterns of antibiotic resistance.¹