This is the first population-based study of respiratory bacterial carriage prevalence and risk factors among Australian Aboriginal adults and older children. Pneumococcal carriage prevalence was 67.4% in children age 2-15 years and 26.0% in adults. The prevalence of H. influenzae carriage was 57.4% in 2-15 year old children and 22.9% in adults. Of the three pathogens, M. catarrhalis was the most prevalent in children (73.7%), and the least prevalent in adults (17.1%). Pneumococcal carriage among adults was associated with increasing age, male sex, recent chest infection, recent rhinorrhoea, frequency of sitting at an outside fire, and concurrent carriage of H. influenzae or M. catarrhalis. Among older children, pneumococcal carriage was strongly associated with younger age and concurrent carriage of H. influenzae or M.catarrhalis and less strongly associated with recent rhinorrhoea. A large number of pneumococcal serotypes circulate in this population. Higher valency PCVs would cover a greater proportion of serotypes in children than adults.
Compared to other population-based studies of pneumococcal carriage, the 42% overall prevalence in this population (mean age 25 years) was midway between that observed among Gambian villagers (65%, median age 15 years) [17
] and in a Brazilian slum (36%, median age 16 years) [18
] or Kilifi, Kenya (20%, mean age 9 years) [19
]. Lower carriage (> 2 years) has been reported in the UK (16%) [20
] and Taiwan (10%) [21
Pneumococcal carriage among children (67%) in this population (mean age 8 years) was comparable to 82% in 5 to 14 year old Gambians [17
] and greater than among 5 to 19 year olds in Kilifi (25%), 5 to 17 year old Brazilian slum residents (45%), and Brazilian adolescents (10%) [22
]. Pneumococcal carriage among adults in this population (26%) was lower than in Gambia (50%), similar to a Brazilian slum (20%), and greater than in Kilifi (3%), the UK (7%), or Taiwan (0%). An earlier study from Central Australia reported pneumococcal carriage in 34% of a selected group of Aboriginal adults [23
]. The differences in prevalence noted above are difficult to explain due to a lack of data on methodology and comparable risk factors.
carriage in children (63%) was greater than that reported in north Indian primary school children (42%, 5-10 years) [24
] or Kilifi (22%, 3-9 years) [19
]. Carriage of M. catarrhalis
(74% children, 17% adults) was greater than that reported in previous studies of healthy children (51%) and adults (5%) in Belgium [25
] and hospitalized children (7%, 4-15 years) and adults (1%) in Denmark [26
The decreasing pneumococcal carriage with increasing age of children has been described in other studies, albeit with varied patterns in different populations [17
]. Of note, the studies cited here have been conducted in populations before introduction of 7PCV. Carriage prevalence of H. influenzae
and M. catarrhalis
among children also fell with increasing age.
Interestingly, the increased pneumococcal carriage with increasing adult age (38% ≥55 years versus 20% among 16 to 34 year olds) that we observed was not seen in The Gambia or Kilifi. The reason for increased carriage in older adults is not clear. It is unlikely that the trend of increasing pneumococcal carriage is due to chance (test for trend 2002, p = 0.05; 2004, p = 0.003; overall, p = 0.0005). Patterns of social interaction, particularly exposure to children, are usually less intense in older adults but may vary in different settings. Biological (e.g. immunosenescence), environmental (e.g. smoke exposure, stress), and chronic disease factors may also be important.
Odds of pneumococcal carriage were substantially increased if H. influenzae
or M. catarrhalis
were also detected. The Kilifi study reported a similar magnitude of association between pneumococcal carriage and concurrent carriage of H. influenzae
]. The association with detection of other pathogens may reflect either facilitation of pneumococcal carriage by H. influenzae
or M. catarrhalis
, or simply non-species specific risk of nasopharyngeal carriage.
Our data, as well as contemporary [19
] and historical [28
] studies, describe increased pneumococcal carriage associated with upper respiratory infections. Our data also indicate an association of recent chest infection with increased pneumococcal carriage in adults. Episodes of pneumonia are associated with increased pneumococcal carriage [29
We found 2.2 times the odds of pneumococcal carriage among adult males compared to females. An association of carriage with gender has not been reported in most studies [17
], an exception being one study of adolescents [22
]. Some of the factors potentially associated with both gender and carriage, such as contact with children, chronic illness, smoking or sitting at an outside fire, were included in multivariate analyses.
Pneumococcal carriage and respiratory infection have been associated with exposure to air pollution of different types [22
]. We found the frequency of sitting at an outside fire increased the risk of pneumococcal carriage. Sitting and cooking at outside fires is common in rural Aboriginal communities. Thus, our data strengthen the growing body of evidence linking smoke exposure to increased risk of respiratory infection.
The large number of pneumococcal carriage serotypes which were detected suggests that pneumococcal transmission is substantial in this population. Our data suggest that the indirect effects of higher valency PCV may be greater than 7PCV (particularly for young infants who acquire S. pneumoniae from their siblings), with a more limited incremental effect among adults.
Our study has a number of limitations. Our sample collection methods underestimate carriage prevalence. Non-probability sampling may have introduced selection bias. Sampling aimed to identify equal numbers of typical adult men and women, with children numbering two thirds that of adults. However, sampling was biased towards adult women, and given lower carriage in women, adult carriage may be underestimated. Analyses of risk factors need to be interpreted in the light of possible sampling bias. Despite non-probability sampling, valid prevalence estimates are likely to fall within the given confidence intervals. Finally, this study is limited to a three year period following the introduction of 7PCV. Although carriage of serotypes 19F and 6B in 2004 remained substantial, it may have fallen in subsequent years. Another carriage study across the NT documented little change in childhood carriage of 19F and 23F between 2003 and 2005 but a reduction in carriage of 6B was observed [32
The implications for public health relate to the introduction of PCV and prevention of pneumococcal transmission and disease. High overall carriage prevalence indicates a large proportion of the population contributing to pneumococcal transmission, which is consistent with the high rates of IPD observed among Aboriginal adults [7
]. Such a reservoir of potentially transmitting individuals may maintain circulation of pneumococcal serotypes after introduction of PCV and limit the magnitude, and increase the time to development, of indirect effects. High rates of bacterial carriage also suggest potential for replacement disease following reduction of VT carriage. Ongoing pneumococcal transmission among Aboriginal adults may explain data showing no reduction in rates of IPD in Aboriginal adults following introduction of 7PCV despite a 30% to 45% reduction in IPD among all Australian adults following 7PCV introduction [33