Abstract
The pneumococcus
Streptococcus pneumoniae (the pneumococcus) is a Gram-positive bacterium and is classified into more than 90 pneumococcal serotypes in 46 serogroups (Henrichsen, 1995). However, the majority of invasive and non-invasive disease is associated with a much smaller number of serotypes. The pneumococcus is one of a number of bacterial pathogens that are highly promiscuous; it is able to exchange DNA with members of its own and other related species by transformation. This means that genes encoding virulence factors, including the polysaccharide capsule, can be exchanged, leading to the existence of capsule switch (Jefferies et al., 2004; Spratt & Greenwood, 2000). This is important as, at present, available pneumococcal vaccines and some of those that are under development are based on pneumococcal capsular polysaccharide.
Pneumococcal disease
Pneumococcal infection remains a major cause of otitis media, pneumonia, septicaemia and meningitis in the UK (Kyaw et al., 2003; Miller et al., 2000). It causes substantial morbidity and mortality, especially in the young and old. Otitis media is particularly common and is the leading indication for the administration of antibiotics in children in developed countries (McCaig et al., 2002). Both invasive pneumococcal disease (IPD) and non-invasive pneumococcal disease have a significant impact on the quality of life of children, adults and the elderly (Brouwer et al., 2005; Zimmerman, 2005). They result in absence from school or work, require attendance at the general practitioner or hospital and necessitate medication. They therefore also have an impact on the general economy due to the costs involved (De Graeve & Beutels, 2004).
Importantly, pneumococci have a carrier state; pneumococci are harboured in the nasopharynx from a young age and the carriage rate can be as high as 40 % (Long, 2005). A number of studies have found a link between colonization of the nasopharynx with pneumococci and an increased risk of acute and recurrent otitis media (Dhooge et al., 1999; Marchisio et al., 2003; Syrjanen et al., 2001). It is from the nasopharynx that the pneumococcus can also enter the bloodstream to cause septicaemia or meningitis. Certain risk factors make individuals prone to pneumococcal infection, including diabetes, asplenia, chronic lung disease, alcohol abuse, cancer and HIV/AIDS (Kyaw et al., 2003, 2005).
Surveillance of IPD has improved substantially throughout the UK in recent years due to interest in the potential for new pneumococcal vaccines (George & Melegaro, 2001; Kyaw et al., 2003; McChlery et al., 2005). There remains a considerable burden of IPD in the UK, particularly during the winter months, despite the availability of antibiotics and pneumococcal polysaccharide vaccines (PPVs) (George & Melegaro, 2001; Kyaw et al., 2003). In England and Wales, the overall incidence of IPD is 8.6 per 100 000 population (George & Melegaro, 2001), the highest burden being amongst the very young and elderly, with an incidence in excess of 30 per 100 000 (George & Melegaro, 2001; Miller et al., 2000; Sleeman et al., 2001). In Scotland, the overall incidence of IPD is 11 cases per 100 000 population, although the incidence rises to 51 cases per 100 000 in those 1 year of age and 45 cases per 100 000 in those aged over 65 years (Kyaw et al., 2003). The ten most common pneumococcal serogroups associated with IPD in Great Britain are shown in Table 1.