Haemophilus influenzae Carriage and Transmission
Haemophilus influenzae is solely associated with humans and can be found in the nose and throat in young children under 5 years of age and the urogenital tract in women. It is not transmitted by or to animals. Although most people who carry this bacterium may not develop clinical symptoms, they however can unknowingly transmit it to others.
Transmission
H. influenzae is transmitted from person to person through respiratory droplets as a result of coughing, sneezing and being in close contact with an infected person. Vertical transmission from mother to neonate during birth is also possible as a result of genital tract colonisation. The precise incubation period is unknown. It has, however, been observed that children start showing symptoms of meningitis after a probable incubation period of about 2– 4 days with clinical manifestations evolving rapidly thereafter such that early treatment is essential particularly as untreated cases of meningitis can lead to death or long term disability.
Transmission rates have been found to be higher in the following places:
- nursery and daycare centres
- children living in crowded conditions such as those found in low-income urban areas with poor ventilation or with high exposure to smoke.
Risk Population
At any given time, between 1% to 5% of the population asymptomatically carry Haemophilus influenzae (Hi) in their nasopharynx. Unvaccinated populations are most likely to carry serotype B Hi but the Hi carriage rate is approximately 20% in one-year olds and increases to more than 50% in children 5 years of age although most carried strains are non-typeable (NTHi) (Mukundan et al 2007). This is probably as a result of niche displacement caused by the eradication of Hib and Streptococcus pneumoniae due to the introduction of the conjugate vaccines.
Under 5s
The burden of disease is highest in the under 5s. In the pre-vaccine era, approximately 85% of H. influenzae cases were found in the under 5 age group as children of this age have not yet had the ability to develop natural immunity towards the bacterium. Unvaccinated children below 2 years are the most vulnerable age group as they account for 65% of the total infections.
Adults
Invasive disease due to H. influenzae is not exclusive to the under 5 age group; it also affects adults over 65 years. Adults are usually susceptible to invasive infections of mucosal surfaces that include sinusitis, non-bacteremic pneumonia and acute exacerbation of chronic obstructive pulmonary diseases.
Immuno-compromised individuals
Individuals with medical conditions that include sickle cell disease, Human Immunodeficiency Virus (HIV) infection, asplenia, antibody and complement deficiency states, patients with cancer receiving chemotherapy, radiotherapy, and in post bone marrow transplant states are also susceptible to Haemophilus influenzae invasive disease.
Ethnic minority groups or indigenous populations
Haemophilus influenzae infections are higher amongst ethnic minority populations that include native Americans, Australian Aboriginals and African populations.
