Respiratory infection rates differ between geographically distant paediatric cystic fibrosis cohorts

Discussion

In this study we found significant and consistent differences between the rates of lower respiratory tract infections reported at annual review for two geographically distant cohorts of young children with CF. This was despite both groups being treated similarly with regard to disease surveillance, infection control, protocols for procedures, treatment of exacerbations and use of antimicrobial prophylaxis. Although the pathogens most commonly identified at both centres were those traditionally associated with infections occurring during infancy and early childhood [17], the relative difference in frequency was surprising and was corroborated by findings of increased numbers of individuals with detectable lower respiratory free NE and increased presence and extent of structural lung disease.

Geographical variability in the prevalence of lower airway infection in patients with CF has been described previously. A comparison between adults attending British and American CF clinics revealed significant differences in the composition, abundance and diversity of the microorganisms present in the lungs of these two patient populations [18]. In a single-centre study we showed that children with CF residing in regional areas had an increased risk for first P. aeruginosa acquisition compared with their metropolitan-based counterparts [15]. In contrast, multicentre cohort studies conducted throughout Europe [19] and the USA [4] were unable to detect differences in the centre-specific prevalence of Burkholderia cepacia complex or common respiratory pathogens, respectively, due to the high diversity of species or strains among these pathogen populations. Notably, these studies focussed their investigations on either entire bacterial communities or a single pathogen, and generally used upper airway specimens to study strain distribution. Our study is therefore one of the first reports to investigate region-specific prevalence of pathogens in the lower airways of similarly matched cohorts of young children with CF. The present study is unique in that it evaluated very young children at two centres united by a common disease surveillance programme and treatment protocols. Interestingly, these findings may contribute to explaining consistently better lung function for Perth subjects versus Melbourne subjects recorded in the national data registry [20], suggesting that these differences in disease outcomes may have their origins in part during the pre-school years.

In comparison with registry data, which record all exacerbation events, our surveillance cultures were performed once annually at a time when patients were relatively healthy. Although routine surveillance provides an opportunity to detect infection prior to the onset of symptoms, a limitation of this approach is the reduced likelihood of detecting species, such as Stenotrophomonas maltophilia and Alcaligenes xylosoxidans, that tend to only transiently infect the CF airways [21] and may explain why these organisms were rarely identified in surveillance cultures. Nevertheless, surveillance testing also allows the tracking of pathogen trends irrespective of exacerbation rates.

We did not take into account seasonality, which is known to influence acquisition of organisms such as P. aeruginosa [22], but this is unlikely to have explained centre differences in infections as surveillance BALs were performed throughout the year and not during exacerbations, which are more likely during the winter months. A study of the US and Australian CF registries found that higher ambient temperature was associated with earlier P. aeruginosa infection and lower lung function [23]. In addition, data from a large US study suggest that macro-environmental factors, such as temperature, humidity and air pollution, may contribute to the risk of P. aeruginosa acquisition [24]. It is possible that differences in macro-environmental factors between Perth and Melbourne centres contributed to the difference in infection rates between the two centres.

The prevalence of S. aureus and H. influenzae infections has increased in recent years [25]. While this may reflect a change in strain prevalence or population exposure, such increases could also result from changes to diagnostic and/or reporting procedures. A detailed examination of S. aureus infections among this cohort revealed high diversity among the bacterial strain population and failed to identify any dominant clones that were specifically associated with respiratory infection in CF [26]. In addition, strain prevalence rates among the AREST CF cohort did not surpass those reported for the Australian CF population as a whole. As the aggressive use of antibiotics is a strong driver of changes to microbial community diversity, the emergence or expansion of specific species can often be attributed to alterations in the treatment being provided [27], but we were unable to detect such differences between centres, with the vast majority being prescribed amoxycillin/clavulanic acid as prophylaxis and a similar-sized minority being admitted for intravenous antibiotics to each centre or undergoing therapy to eradicate infection with P. aeruginosa.

For both centres involved in the AREST CF surveillance programme, early detection and eradication of P. aeruginosa infection has become routine practice, and it is clinical policy to use anti-staphylococcal prophylaxis for the first 2 years of life. That said, the prescription of the relevant antimicrobials is still dependent on preference of the treating physician and the willingness of parents to administer long-term treatment to well-looking children. Hence, subtle variations both within and between centres likely existed [10], but it is unclear whether these are substantial enough to explain such significant differences in the detection of lower respiratory infections treated by these antibiotics or the increased pulmonary inflammation and structural changes identified in the lungs of these very young subjects.

The prevalence of meconium ileus was higher in infants with CF from Melbourne (19%) compared with Perth (6%). It is still unclear whether individuals with meconium ileus at presentation have a worse prognosis than those without meconium ileus [28–30]. However, a previous study from the AREST CF group found that meconium ileus at presentation was a significant risk factor for the presence of bronchiectasis on chest CT at 3 years old [31]; therefore, centre differences in the prevalence of meconium ileus may have contributed to the observed differences in clinical status. We could not identify any procedural differences in sample collection or laboratory protocol that could explain our findings. Both the accredited laboratories meet national standards and are experienced laboratories for detection of CF infections. Our assessment of the administration of and adherence to treatment was, however, limited. An additional limitation of our study was that our analysis was restricted to the culture-based detection of a select number of respiratory pathogens and therefore did not take into account the entire lung microbiota that is currently under intense investigation utilising culture-independent techniques. Whether or not the nature of these complex polymicrobial communities influences disease progression is not yet known and so the presence and prevalence of organisms not studied here will be an important consideration for future investigations.

In addition to the detection of common respiratory pathogens, a large number of BAL specimens also grew mixed oral flora despite considerable care having been taken to avoid contamination from the upper airways. The significance of mixed oral flora to pulmonary disease in CF remains unclear. In a 3-year study investigating the ability of oropharyngeal cultures to predict infection within the lower airways most BAL cultures contained low numbers of organisms associated with the upper airways [32]. This was in contrast to earlier findings that BAL specimens did not contain mixed oral flora from a similar investigation comparing upper and lower CF airway microbiology [33]. Although not traditionally considered as pathogens, we have recently shown that the presence of mixed oral flora in the lower CF airways is associated with an increased inflammatory response [12]. While both centres aimed to record upper respiratory tract flora regardless of whether or not another pathogen was also reported, it is difficult to explain such disparity between the two groups without assuming some differences between diagnostic interpretation and reporting exist. However, having considered that variation in sample collection, processing and reporting may have occurred through employment of different bronchoscopists or diagnostic laboratories we reviewed all methodologies and were unable to identify any variation in procedures. Further research incorporating culture-independent techniques would assist in identifying what role, if any, mixed oral flora plays in early CF airway disease.

Although the extent of bronchiectasis was more severe among Melbourne patients, the use of two different scanners means that there may be subtle differences in image quality. However, the same investigator reviewed all CT scans, thus reducing the likelihood of differences in interpretation between centres. Volumetric chest CT scans provide a more comprehensive and detailed assessment of structural change within the airways than limited-slice imaging. Limited-slice protocols, which were used more in the Melbourne centre, are more likely to underestimate structural disease prevalence and extent, so it is unlikely that differences in scanning techniques accounted for these results.

In summary, we demonstrated that two CF treatment centres performing identical disease surveillance and utilising similar standardised treatment protocols had different prevalence of early respiratory infection. The centre with the higher infection rates also had a higher prevalence of free NE and worse chest CT findings. This study provides evidence for potential geographical variability in the acquisition of early lower respiratory infection in CF. Differences in rates of infection during the first few years of life may be associated with poorer disease outcomes, and warrant specific surveillance and intervention. Future, larger studies may identify that geographical differences in acquisition of infection contribute to the variability in outcomes that occurs between CF centres.