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Journal of Clinical Microbiology, September 2006, p. 3371-3373, Vol. 44, No. 9
0095-1137/06/$08.00+0 doi:10.1128/JCM.01289-06
Copyright © 2006, American Society for Microbiology. All Rights Reserved.
Enteric Virus Group,1 Centre of Clinical Research Excellence in Child and Adolescent Immunisation, Murdoch Children's Research Institute,2 Department of Paediatrics, University of Melbourne, Royal Children's Hospital, Melbourne, Australia3
Received 22 June 2006/ Accepted 10 July 2006
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Human adenoviruses cause a wide spectrum of clinical disease, from uncomplicated acute respiratory and gastrointestinal infections in normal healthy individuals to chronic systemic infection in immunosuppressed hosts. Human adenoviruses are classified into 51 serotypes within six species (A to F). Species C adenoviruses are the most commonly isolated respiratory pathogens, involving AdV-1, AdV-2, and AdV-5 (13).
While an association between nonenteric adenovirus and IS has been reported (4, 7), the virus has not been characterized. It is not known whether IS-associated strains differ from prevalent strains causing symptomatic respiratory or conjunctival illness in young children. In this study, we investigated the temporal distribution and genetic relationship of adenovirus strains isolated from IS patients at the Royal Children's Hospital (RCH), Melbourne, Australia.
Twenty-one adenoviral isolates were obtained prospectively from fecal specimens collected from children less than 2 years of age, admitted to the RCH with IS from January 2003 until December 2004, as previously described (5). IS-associated isolates were detected in both years, including nine IS cases between July and November (winter and spring) 2003, with no similar clustering in 2004. In the week preceding IS, possible symptoms were observed by the parents of 8/20 IS patients. This group included seven patients with respiratory tract infection symptoms and one with fever alone.
During the study period, 198 non-IS-associated adenoviral strains were identified from respiratory, conjunctival, and gastrointestinal specimens sent for routine diagnostic identification at the RCH virology department. Adenoviruses were detected year round with no particular seasonality. Peaks were observed in January and November 2004, with 23 and 20 positive identifications, respectively. Fifty-nine isolates were selected for analysis based upon temporal relationship (within 1 to 4 weeks) with IS-associated isolates, including 32 from children less than 2 years old.
Species typing (A to F) was performed on all 21 IS- and 59 temporally linked non-IS-associated isolates. DNA was extracted from isolates adapted to A549 cells (4) and typed using species-specific primers to the fiber gene (18). Species C was the dominant type for both IS (15/21)- and non-IS (32/59)-associated adenoviruses (P = 0.12, Fisher exact test) (Table 1).
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TABLE 1. Adenovirus species distribution, and species C serotype distribution, of clinical samples (isolated from 2003 to 2004) as determined by species-specific multiplex PCR, hexon gene PCR, and sequence analysis
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DNA restriction analysis was used to evaluate the genetic diversity of AdV-2. DNA was digested individually with BamHI and SmaI (New England BioLabs, MA), and restriction patterns were assigned according to the system proposed by Adrian et al. (2, 3).
Identical BamHI profiles (represented by four bands) were seen in 17/18 isolates analyzed (Fig. 1). The 18 AdV-2 isolates exhibited five different SmaI profiles, consisting of four to seven bands (Fig. 1). Using combined BamHI and SmaI data, isolates were classified into six different genome types, termed D1 to D6 (Table 2). D2 predominated in both IS- and non-IS-associated strains. Strains isolated from patients with IS were of five different genetic types, with non-IS-associated isolates being of three genetic types.
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FIG. 1. Restriction enzyme patterns of species C AdV-2 isolates obtained from IS (lanes 2 to 7) and non-IS (lanes 8 to 19) cases. Genomic profiles were resolved by 1.2% Tris-borate-EDTA agarose gel electrophoresis. Panel A depicts DNA digested with BamHI, and panel B depicts profiles from SmaI restriction enzyme digestion. Lanes 1 and 20, DNA ladder 1V (Roche, Mannheim, Germany).
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TABLE 2. Genomic classification of AdV-2 isolates obtained from IS and non-IS cases
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No temporal clustering of IS-associated adenovirus was identified in this study, and genetic characterization did not demonstrate clustering of adenoviral species or serotypes.
Of the 79 adenovirus isolates typed, species C accounted for the majority of IS- and non-IS-associated isolates. Species C is widely represented in most populations, representing more than half of the clinical adenovirus isolations worldwide. In this study, characterization of species C isolates into serotypes identified types AdV-1, AdV-2, and AdV-5 in both IS and non-IS cases, with no significant differences identified between the groups. AdV-6 was absent as also noted in other studies (2, 3).
AdV-2, the most common IS-associated serotype in this study, is most frequently described as a respiratory pathogen but has been associated with gastroenteritis (9). In the routine clinical samples typed, AdV-2 was characterized in 6 of 13 gastroenteritis isolates and only 5 of 30 respiratory isolates. AdV-2 genomic variation was investigated further. Interestingly, five of six genetic types identified were present in IS-associated strains, while only three types were present in the non-IS-associated strains. This large genetic diversity is supported by previous analysis of species C serotypes in non-IS-associated isolates (1, 2). Despite this strain diversity, D2 was shown to be the dominant genome type in both IS- and non-IS-associated adenovirus isolates. This suggests that a common dominant type associated with IS and non-IS clinical disease manifestations is circulating in the community.
Establishment of cause by isolation of adenovirus from IS patients was beyond the scope of our study. The significantly increased isolation rate compared to that of healthy controls in a previous study (5) strongly supports an association. This reduces the likelihood that shedding of adenovirus in IS patients is coincidental. It has been speculated that adenoviral infection may predispose infants to development of acute IS. Idiopathic cases are believed to arise from hypertrophy of infected lymph nodes, including Peyer's patches acting as physical "lead points" in the pathogenesis of IS, although pathological studies do not consistently identify hypertrophied lymphatic tissue (8, 10). If adenoviral infection is in fact a precursor to IS, it can be hypothesized from our findings that susceptible children will acquire commonly circulating community strains. Given that most young children infected with the adenovirus suffer only mild illness, such as respiratory infection, we can further speculate that susceptible individuals may have an altered anatomic or immune status that, when they are infected with adenovirus, predisposes them to IS. This hypothesis is consistent with the susceptible-host hypothesis stemming from the Rotashield experience, where a small number of cases occurred despite widespread vaccine uptake (15). Host immune response may be the key determinant of the clinical course of infection after adenovirus colonization. Evaluation of host responses to adenovirus, together with exploration of adenoviral virulence determinants, remains a priority area of investigation into the most common cause of acute bowel obstruction in young children.
This work was supported by a project grant from the Murdoch Children's Research Institute, Melbourne, Australia.
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