Principles and Practice of Clinical Bacteriology Second Edition - Free

24 ORAL AND OTHER NON-β-HAEMOLYTIC STREPTOCOCCI
involved in many types of infections, in which the source of the infection
is almost invariably endogenous, being derived from the host’s microflora.
The streptococcal species themselves are generally thought to
be of relatively low virulence, not normally associated with acute,
rapidly spreading infections such as those caused by Streptococcus
pyogenes, although they clearly have phenotypic features that result in
the production of disease under appropriate circumstances.
Since many of these streptococci are present in the mouth, upper
respiratory tract, genitourinary tract and, to a lesser extent, gastrointestinal
tract, they are sometimes involved in pathological processes at
these sites, possibly following some local or systemic change in
host susceptibility or an alteration in local environmental conditions.
A classic example is the manifestation of dental caries that arises
following excessive consumption of dietary sugars, particularly
sucrose. Alternatively, the streptococci at a mucosal site may gain
access to the blood stream because of some local traumatic event and
set up an infection at a distant location, such as the heart valve in
endocarditis or in the brain or liver, giving rise to an abscess. The key
event for infections at distant body sites is bacteraemia.
Epidemiology
Our understanding of the importance of non-β-haemolytic streptococci
as bacteraemic pathogens has been hampered by a dearth of surveillance
activity in many countries. Robust estimates of incidence have been,
and to a degree remain, few and far between. Most quantitative studies
undertaken have been based on case series originating from localized
areas whose catchment populations are often difficult to enumerate
and, therefore, difficult to translate into estimates of incidence.
In England and Wales a comprehensive laboratory-based surveillance
network gathers reports of bacteraemia caused by all pathogens. Data
from this surveillance on non-β-haemolytic streptococci indicate an
incidence of 3.8 per 100 000 population in 2002 (HPA, 2003a).
Non-β-haemolytic streptococci comprised approximately 3% of all
bacteraemia reported through this system (HPA, 2002), broadly in line
with estimates from other European countries and the Americas, which
range from 1.5% to 5.9% (Jacobs et al., 1995; Casariego et al., 1996;
Diekema et al., 2000; Fluit et al., 2000).
Of the non-β-haemolytic streptococci, mitis group organisms
appear to be the most common cause of bacteraemia overall (Venditti
et al., 1989; Doern et al., 1996; HPA, 2003a), the rate of reports in
England and Wales being 1.4 per 100 000 population in 2002, with
S. oralis being the most common single species identified (12% of all
non-β-haemolytic streptococci). A smaller study from The Netherlands
similarly found S. oralis to be the most common non-β-haemolytic
streptococci causing bacteraemia, particularly associated with infection
in haematology patients (Jacobs et al., 1995). Studies focusing on
neutropenic patients, one of the most vulnerable patient groups,
suggest that between 13% and 18% of bacteraemias are due to nonβ-haemolytic
streptococci (Wisplinghoff et al., 1999; Marron et al.,
2001), with S. mitis featuring as one of the most prominent species
(Alcaide et al., 1996).
Interestingly, the relative contribution of each non-β-haemolytic
streptococcal group to bacteraemic episodes in England and Wales
has changed over 1990–2000. The incidence of the formerly dominant
sanguis group (S. sanguis, S. parasanguis and S. gordonii) has
declined by half, whereas those of mitis and anginosus group streptococci
have increased dramatically by three- and twofold, respectively
(Figure 2.2).
Estimates of the relative importance of healthcare exposure in the
aetiology of non-β-haemolytic streptococcal bacteraemia have
differed between studies. Two studies of S. bovis bacteraemia in Hong
Kong (Lee et al., 2003) and Israel (Siegman-Igra and Schwartz, 2003)
found none and 10% of cases, respectively, to have been hospital
acquired, whereas 6 of 31 (19%) S. milleri bacteraemias identified in
a case series from Spain were considered to be hospital acquired
Rate per 100 000 population
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002
(Casariego et al., 1996). Nosocomial infection surveillance from the
United States (Emori and Gaynes, 1993; Pfaller et al., 1997) and
England (NINSS, 2003) identified between 1% and 3% of hospitalacquired
bacteraemia to involve non-β-haemolytic streptococci.
Being opportunistic pathogens, bacteraemia involving nonβ-haemolytic
streptococci tend to be concentrated in vulnerable
individuals, namely the very young and older age groups (Figure 2.3).
Much like many other blood stream infections, non-β-haemolytic
streptococcus infection tends to have rates that are higher in males
than in females, this pattern being seen across all non-β-haemolytic
streptococcal groups (HPA, 2003b) and often also seen in studies of
infective endocarditis (Mylonakis and Calderwood, 2001; Hoen et al.,
2002; Mouly et al., 2002), with some exceptions (Hogevik et al., 1995).
Infective Endocarditis
Anginosus group
Mitis group
Salivarius group
Bovis group
Mutans group
Sanguis group
Figure 2.2 Annual rate of laboratory reports of bacteraemia caused by nonβ-haemolytic
streptococci, England and Wales. Source: Health Protection
Agency (Communicable Disease Surveillance Centre).
Infective endocarditis involving non-β-haemolytic streptococci
usually occurs in patients with preexisting valvular lesions and is
typically subacute, whereas the acute form of endocarditis which can
occur in those with previously undamaged heart valves is associated
with more virulent bacteria such as Staphylococcus aureus, S. pyogenes
or S. pneumoniae. Patients at particular risk of developing subacute
endocarditis include those with congenital heart defects affecting
valves, those with acquired cardiac lesions following rheumatic fever
Rate per 100 000 population
4
3
2
1
0
Male
Female