Meningococcus (Neisseria meningitidis) is a bacterium that exclusively colonizes and/or infects humans; there is no animal reservoir.
IMMUNIZATION AGAINST BACTERIA ASSOCIATED WITH MENINGITIS
Meningococcus (Neisseria meningitidis)
is a bacterium that exclusively colonizes and/or infects humans; there is no
animal reservoir. It is present as part of the normal microbiota of the pharynx
in approximately 10% of individuals but can rarely spread through the
bloodstream and to the brain through poorly understood mechanisms, causing
meningitis and septicaemia. These are life-threatening, systemic infections;
overall mortality from meningococcal disease is approximately 10%, assuming
symptoms are recognized and treatment is commenced without delay, but rises
considerably if there are delays. Diagnosis is therefore considered a medical
emergency. At least 12 subtypes or serogroups of meningococcus have been
identified, but groups B and C account for the majority of cases in Europe and
the Americas. In the UK, group B accounts for approximately two-thirds of
reported cases with group C accounting for the remaining third. Neisseria meningitidis group A does not normally
cause disease in the UK but is an endemic cause of meningitis in other parts of
the world, particularly sub-Saharan Africa in an area between Senegal (West Africa)
to Ethiopia (East) that has been termed the ‘meningitis belt’. It is believed
that reasons for the hyperendemic incidence of group A meningococcal meningitis
in this part of the world include high incidence of upper respiratory tract
infections during the dry (dusty) season, combined with overcrowding, migration
and pilgrimages.
Whilst group C meningitis is most common in the under 1-year-old group
in the UK, mortality is highest in adolescents. Although meningitis accounts
for the majority of invasive meningococcal disease, in 15-20% of cases septicaemia
predominates and is associated with significantly higher mortality. There is
currently no vaccine available for group B meningococcus but vaccines are
available for groups A and C. As with the Hib vaccine (see below), the
preparations are intended to invoke protective immunity towards the
polysaccharide component of the bacterium. Early vaccines, composed of purified
polysaccharide, worked in adults but had poor efficacy in infants —the most
at-risk group. The new MenC conjugate vaccine comprises capsular polysaccharide
components conjugated to a carrier protein (usually diphtheria or tetanus
toxoid). This vaccine is effective in the very young and is therefore suitable
for protecting infants. The vaccine is normally administered along with DTaP
and Hib at 3, 4 and 12 months and a single dose is sufficient to immunize
individuals over 12 months of age; it has also been used to provide prophylaxis
for teenagers, adolescents and young adults. Group A vaccine is available for
those travelling to areas of the world where the infection is epidemic.
Haemophilus influenzae can cause infections ranging from bronchitis and otitis
media to life-threatening, invasive disease (meningitis and bacteraemia).
Invasive infections, which are most common in young children, are normally
caused by encapsulated strains of the bacterium that can be serologically differentiated
into six typeable capsular serotypes (a-f). Before the introduction of
vaccination, H. influenzae type b (Hib) was
the most prevalent of these in invasive disease. Non-invasive haemophilus
disease is most often caused by non-encapsulated strains that are not amenable
to typing based on capsular serology. Although the most common form of invasive
Hib disease is meningitis, accounting for 60% of cases, Hib can also cause
other infections, including pneumonia and pericarditis.
The fatality rate for treated Hib meningitis infections is approximately
5% and complications include deafness and intellectual impairment (in c. 10% of
cases). Hib often forms part of the normal microbiota of the nasopharynx in
healthy individuals and the frequency of carriage before the Hib vaccine was
introduced was approximately 4 in every 100 for preschool children. Carriage of
this bacterium is now rare because of the effectiveness of the Hib vaccination.
Hib meningitis is rare in children under 3 months and peaks in its incidence at
around 10-11 months of age; infection is uncommon after 4 years of age. Before
the introduction of Hib vaccination the incidence of the disease in the UK was
estimated at 34 per 100 000.
The vaccine utilizes purified
preparations of the polysaccharide capsule of the major serotypes of the
bacterium associated with disease. Polysaccharides are poorly immunogenic and
must be conjugated onto a protein carrier (diphtheria or tetanus toxoids) to
enhance their efficacy. The Hib vaccine is given as part of a combined product
of H. influenzae type b, diphtheria, tetanus, acellular
pertussis and inactivated polio vaccine (DTaP/IPV/ Hib), or as the Hib/MenC
conjugate vaccine.
Streptococcus pneumoniae (pneumococcus) is an encapsulated Gram-positive coccus. As with
meningococcus and Hib, the capsule is an important virulence factor for this
bacterium (non-capsulated strains are normally avirulent). Many different
capsular types have been characterized but approximately 66% of the serious
infections in adults and 80% of invasive pneumococcal infections in children
are caused by about 10 capsular types. Pneumococci are often part of the normal
microbiota of the nasal cavity and associated tissues but commensal strains are
often avirulent. Conversely, invasive infections (i.e. meningitis and septicaemia)
are often caused by strains not considered to be normal commensals. S. pneumoniae is a versatile pathogen which can
cause sinusitis or otitis media (infections of the sinuses or middle ear). The
bacterium may also cause deep lung infections (pneumonia), which accounts for
its species name, and is also capable of causing systemic infections including
bacteraemic pneumonia, bacteraemia and meningitis. The incidence of infection
by pneumococci is highest in the winter, and transmission by aerosols or direct
contact with respiratory secretions is believed to require either frequent or
prolonged close contact. Two distinct vaccines have been developed to provide
protection against this bacterium: the pneumococcal polysaccharide vaccine
(PPV) which contains purified capsular polysaccharide from 23 capsular types of
bacterium and the pneumococcal conjugate vaccine (PCV) which comprises capsular
polysaccharides from 7 common capsular types. Importantly, PCV is conjugated to
protein in a manner similar to the Haemophilus influenzae type
b (Hib) and MenC vaccines. While PPV is an effective vaccine in adults, the
effectiveness of pneumococcal prophylaxis is considerably improved in children
by protein conjugation; PCV is immunogenic in children and the childhood
vaccination schedule recommends that doses be given at 2 and 4 months of age
with a booster at 13 months. The PPV vaccine is recommended for adults over 65
years and at-risk groups aged 2 years or over. It will provide additional
prophylaxis to individuals who have already received the PCV vaccine because it
protects from additional serotypes.
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