Epidemiology of severe pediatric adenovirus lower respiratory tract infections in Manitoba, Canada, 1991-2005

The Aboriginal and Non-Aboriginal children live in different settings in Manitoba. In 2001, 34.7% of Manitoba's Aboriginal people lived within the municipal boundaries of the City of Winnipeg, while the remaining lived on-reserve scattered throughout the province or in other urban settings. Non-Aboriginal children live mainly in urban areas [28].

Despite their wide geographical distribution, both populations ultimately seek the same Children's Hospital.

This retrospective study involved children 0 to 4 years of age who presented to the pediatric outpatient and emergency departments at Winnipeg Children's Hospital (Manitoba, Canada) between February 1991 and October 2005 with LRTI and positive respiratory culture for adenovirus.

The study sample was sourced from adenovirus culture positive specimens. Subsequently, a chart review (emergency department, outpatient and inpatient records) was done on those adenovirus positive culture cases and those with a diagnosis of LRTI were selected for analysis.

Recognition of pediatric LRTI was based on the physician diagnosis of pneumonia, bronchitis and bronchiolitis. (All medical codes in the Health Sciences Center Historical Abstracting System, WinRecs that contain the words "pneumonia", "bronchitis" or "bronchiolitis" were included.) Only the first adenovirus respiratory isolate, during a single clinical event, was included in the analysis. The studied cases did not include readmissions. Participants' demographic, clinical and radiologic findings were collected. Clinical outcome data were assessed according to days of hospitalization, use of mechanical ventilation, and mortality. Aboriginal ethnicity was identified by the demographic data. Approximate annual incidence (detection rate) of adenovirus LRTI among Aboriginal and Non-Aboriginal children was calculated from 1999 to 2005 as the ratio of the number of infected children 0-4 years old in the target year to the number of 0-4 year-old children in the corresponding ethnic group living in Manitoba in 2001 (i.e., 20,725 Aboriginals and 50,305 Non-Aboriginals) [28]. The numbers of 0 to 4 year-old children in 2001 were used as the base values in this calculation because 2001 is close to the median year of the study period (1999 - 2005) and the lack of data on the relevant population sizes in the other study years.

The study was approved by Bannatyne Campus Research Ethics Board at the University of Manitoba, Winnipeg, Canada (H2006:166).

Although testing for respiratory viruses was not a standard policy for all LRTI admissions during the study period, this was a usual practice. All nasopharyngeal aspirate specimens received at Cadham Provincial Laboratory (CPL), during the study period had received culture for adenovirus. Moreover, cultures for influenza A and B, parainfluenza 1, 2, 3, and 4, respiratory syncytial virus (RSV), respiratory enteroviruses and (on request) rhinoviruses were also done for all specimens. Rapid antigen detection for influenza A and RSV was conducted during their respective seasons. Influenza B rapid antigen testing was offered during any heavy B seasons.

Nasopharyngeal aspirate specimens were collected in virus transport media. During daytime, the samples were immediately transported at 4°C with a cold pack to CPL, which was adjacent to the Children's Hospital. At nights and weekends, the samples were stored at the microbiology laboratory of the Children's Hospital. Nevertheless, the procedures of sample collection and transportation were consistent throughout the study.

Specimens were processed as described [30]. Cultures were observed for 3 to 4 weeks for cytopathic effects or hemadsorption, and were further tested by indirect immunofluorescence staining. The various kits used throughout the study were conducted exactly according to manufacturer directions. Therefore, the impact of changes in laboratory techniques during this time is expected to be negligible.

Serotyping was performed by experienced staff. The common respiratory serotypes in children (types 1-7) were identified by neutralization assays with type-specific reference antisera [14, 31]. Other serotypes were not tested.

One hundred ninety three children were identified with LRTI and positive respiratory culture for adenovirus (Table 1). The patients were predominantly infants (mean age ± SD = 11.1 ± 8.1 months). One hundred and twelve (58%) affected children were Canadian Aboriginals.

Figure 1 shows the yearly variation in the incidence of adenovirus infection in Aboriginal and Non-Aboriginal children. It displays the average incidence after removing the season's effect (here, season = month). Higher incidences were noted in 2001 and 2004.

The series of monthly records of the incidence of serotypes appeared to be stationary, i.e., the incidence was relatively stable over the studied years (test of linear trend had p = 0.751 for Aboriginals and p = 0.382 for Non-Aboriginals).

Of note, from 1999 to 2005, there were 177 adenovirus isolates, representing 3.0% of total LRTI and 15.1% of confirmed viral LRTI.

During the study period, seven adenovirus serotypes were isolated from the nasopharyngeal aspirates (Table 1). Serotypes 1-3 were the most common, accounting for about two-thirds of the isolates. Ten isolates were un-typed. Although adenovirus infections occurred year-round, infections peaked in the fall and winter (Figure 2), especially serotypes 1-3, 5 and 7 (data not shown). Two outbreaks (notable rises in the incidence of infections) occurred (Figure 3A-B), one in 2001 (predominantly serotype 3) and one in 2004 (predominantly serotypes 2 and 1).

Two-thirds of the children were hospitalized (median = 5 days, range = 1.0 to 189 days). Most of them had pneumonia (46%) or bronchiolitis (38%). Wheezing and hypoxia were documented in about half of the patients. Extra-pulmonary manifestations included vomiting (20%), diarrhea (20%) and conjunctivitis (7%).

Many patients received antibiotics (72%), bronchodilators (66%), oxygen (46%) and steroids (31%). Mechanical ventilation was used in 11%.

Two-thirds (141 of 193) of the patients had lobar consolidation on the chest x-ray. All lobes were equally affected and the involvement was multi-lobar in 56 (28%) and bilateral in 33 (17%) patients. Residual changes on follow-up evaluations were present in 16 patients, including bronchiectasis in 8 children.

Co-infections with other respiratory viruses were found in 19 (10%) patients (11 RSV, 6 parainfluenza, 1 influenza A and 1 CMV). The co-infection did not adversely affect the length of hospitalization (p = 0.096) or the incidence of bronchiectasis (p = 0.797).

All patients were discharged home and there were no deaths. There were no significant differences between Aboriginal and Non-Aboriginal children in the length of hospitalization (p = 0.185) or the use of mechanical ventilation (of 21 patients who had ventilation, 11 were Aboriginal and 10 were Non-Aboriginal, p = 0.624). Moreover, the length of hospitalization and the use of ventilation did not differ across serotypes (p = 0.612 and p = 0.072, respectively).

Only 27 (14%) of the 193 children had documented long-term outpatient follow-up and 11 (6%) had bronchoscopy. All children had chest x-ray and 10 had chest computerized tomography (CT) scan.

Bronchiectasis (single or multiple lobes) was diagnosed on the chest CT scan in eight patients (4%); three had serotype 1, two had serotype 3, one had each of the serotypes 2, 7 and 5 (Table 2). There was no statistically significant association between bronchiectasis and serotypes (p = 0.822). Bronchiectasis distributed equally among Canadian Aboriginal and Non-Aboriginal children (p = 0.638). The prevalence of bronchiectasis among both ethnic groups was estimated, with 95% confidence, between 0.02 and 0.08.