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patients < 5 years of age who were admitted for lrti to the pediatric intensive care unit ( picu ) of wilhelmina children s hospital , utrecht , the netherlands , were enrolled from october through may during 20052008 .
patients were excluded if they had any of the following : asthma exacerbation , immunocompromised state , indication for antimicrobial drugs other than for lrti , and repeated picu admission for lrti during the study period .
control group participants were children < 18 years of age ( median 2.2 years ) who were admitted to the picu from october 2005 through march 2006 for reasons other than lrti .
underlying illnesses were defined as chronic pulmonary disease , congenital heart disease , immunodeficiency , malignancy , neurologic disease , or gastrointestinal disease ( 8) . to assess the severity of illness , we used the lowest ratio during the first 24 hours of the partial pressure of oxygen in arterial blood ( pao2 ) to the inspired oxygen fraction ( fio2 ) .
these ratios were acquired from the pediatric intensive care evaluation database , which contains validated clinical data for all dutch picu admissions .
nasopharyngeal aspirates were collected from all patients in the lrti group as part of the investigation of their illnesses . in the control group , nasopharyngeal aspirates were taken from intubated patients , and throat swabs were taken from extubated children as part of routine surveillance to identify transmission of respiratory syncytial virus ( rsv ) . because rsv surveillance was conducted as part of normal patient care , patient consent / ethical approval was not needed , according to the medical ethical research council of our institution .
specimens from patients in the lrti group were initially examined for rsv , influenza viruses , parainfluenza viruses , adenoviruses , rhinoviruses , coronaviruses , human metapneumovirus , and mycoplasma pneumoniae by using real - time pcr as previously described ( 1,9,10 ) .
all samples were retrospectively tested for hbov , kipyv , and wupyv also by using real - time pcr as previously described ( 11,12 ) .
after nucleic acid extraction using the magna pure lc 1.0 nucleic acid isolation system ( roche diagnostics , rotkreuz , switzerland ) , amplification was carried out in a 25-l reaction mixture on a 7500 fast real - time pcr system ( applied biosystems , foster city , ca , usa ) .
positive controls for the kipyv and wupyv pcr were provided by s. bialasiewicz and t.p .
sloots , university of queensland , queensland , australia , and the positive control for hbov was provided by t. allander , karolinska institute , stockholm , sweden .
ct values are inversely correlated with viral load ; i.e. , low ct values indicate high viral loads .
of 90 lrti patients enrolled , 78 ( 86.7% ) had sufficient material stored for hbov , kipyv , and wupyv testing .
eighty - eight control patients were enrolled , of which 83 ( 94.3% ) had sufficient material stored to be included .
the main clinical conditions of control patients were cardiac disease requiring surgery ( 33.7% ) , trauma ( 8.4% ) , sepsis ( 8.4% ) , and upper respiratory tract infection ( 8.4% ) .
a total of 57 ( 68.7% ) nasopharyngeal aspirates and 26 ( 31.3% ) throat swabs from the 83 patients who had sufficient samples were tested .
* picu , pediatric intensive care unit , wilhelmina children s hospital , utrecht , the netherlands ; lrti , lower respiratory tract infection ; iqr , interquartile range ; pao2/fio2 , ratio of the partial pressure of oxygen in arterial blood to the inspired oxygen fraction . in lrti patients ,
hbov was found in 4 ( 5.1% ) and wupyv in 2 ( 2.6% ) of the 78 patients .
table 2 shows ct values and clinical characteristics for lrti patients whose samples were positive as well as for controls whose samples were positive for these viruses .
rsv was found in 52 ( 66.7% ) , influenza viruses in 3 ( 3.8% ) , parainfluenza viruses in 2 ( 2.6% ) , adenoviruses in 4 ( 5.1% ) , rhinoviruses in 20 ( 25.6% ) , coronaviruses in 6 ( 7.7% ) , human metapneumovirus in 5 ( 6.4% ) , and mycoplasma pneumoniae in 1 ( 1.3% ) of the patients .
multiple respiratory viruses were found in 3 of the 4 lrti patients with hbov infection and in both patients with wupyv infection ( table 2 ) .
one patient had a single infection with hbov ( i.e. , no other virus was detected ) .
this patient was born at 31 weeks of gestational age and had a history of a grade iv idiopathic respiratory distress syndrome .
she was admitted to the picu at 19 months of age with a severe lrti . bacterial throat and
* picu , pediatric intensive care unit , wilhelmina children s hospital , university medical center utrecht , the netherlands ; lrti , lower respiratory tract infection ; hbov , human bocavirus ; kipyv , ki polyomavirus ; wupyv , wu polyomavirus ; ct , cycle threshold ; npa , nasopharyngeal aspirate ; adeno , adenovirus ; hmpv , human metapneumovirus ; rsv , respiratory syncytial virus ; irds , idiopathic respiratory distress syndrome ; pvl , periventricular leukomalacia ; na , not assessed ; urti , upper respiratory tract infection ; aml , acute myeloid leukemia ; bmt , bone marrow transplant ; vsd , ventricular septal defect ; alte , apparent life - threatening event . in the control group ,
hbov was found in samples from 4 ( 4.8% ) patients , kipyv was found in 4 ( 4.8% ) , and wupyv was present in 2 ( 2.4% ) samples from the 83 patients whose samples could be tested .
median ct values for hbov , kipyv , and wupyv combined were 18.1 ( interquartile range [ iqr ] 20.4 ) for the lrti group and 34.4 ( iqr 5.1 ) for the control group ( p = 0.09 ; figure ) .
the ct values indicate that , on average , the viral load in the lrti group might be higher than in the control group .
cycle threshold ( ct ) values of lower respiratory tract infection ( lrti ) and control patients with human bocavirus ( hbov ) , ki polyomavirus ( kipyv ) , and wu polyomavirus ( wupyv ) infections .
lrti patients are those admitted to the pediatric intensive care unit , wilhelmina children s hospital , university medical center utrecht , the netherlands ; control patients are patients admitted to the pediatric intensive care unit with other diagnoses .
in the present study , the prevalences of hbov , kipyv , and wupyv in picu patients with lrti ( n = 78 ) or without lrti ( n = 83 ) were similar ( 5.1% , 0% , 2.6% ; and 4.8% , 4.8% , 2.4% , respectively ) .
most hbov- and kipyw - positive lrti patients were co - infected with other viruses .
first , lrti patients were younger than controls ( lrti , 100% < 5 years ; controls , 40% > 5 years ) .
however , the positivity rates for hbov , kipyv , and wupyv in control children < 5 years were similar to rates of control children > 5 years of age ( 6/49 vs. 3/34 , respectively ) .
studies have also shown that the highest incidence of kipyv / wupyv infection occurs in children 1 year of age , slightly older than the children in our lrti group ( 1315 ) .
the young age of the lrti group may have resulted in a lower than expected positivity rate for this group .
second , all lrti patients had nasopharyngeal aspirates taken ; however , 68.7% of controls had provided nasopharyngeal aspirates .
hbov and kipyv / wupyv infections were more common in controls who had nasopharyngeal aspirate samples taken than in those who had throat swab samples taken ( 8/57 vs. 1/26 ) .
this difference in positivity for nasopharyngeal aspirates strengthens our conclusion that these viruses are not found more frequently in picu children with lrti .
sampling errors make precise quantification of viral loads difficult . nevertheless , in the lrti group , low ct values , which indicate high viral loads , were found in nasopharyngeal samples taken from 3 patients infected with hbov ( ct  15 ) and from 1 patient infected with wupyv ( ct = 19 ) .
ct values found in nasopharyngeal samples from patients in the control group were much higher , 3239 .
a possible explanation for this difference is that high viral loads in the young lrti population represent symptomatic primary infection , whereas the low viral load in the older controls might represent asymptomatic long - term shedding .
prevalences of hbov , kipyv , and wupyv infections in children in the picu is low ( <5% for lrti patients and controls ) , and these agents are unlikely to be a major cause of lrti at the picu .
however , hbov might be pathogenic in some picu patients because 1 person with a hbov single infection in a high quantity was identified .
further studies using quantitative viral detection are needed to investigate the probability that hbov , kipyv , and wupyv represent etiologic agents of lrti .
we evaluated the prevalence of human bocavirus and ki and wu polyomaviruses in pediatric intensive care patients with and without lower respiratory tract infection ( lrti ) . 
 the prevalence of these viruses was 5.1% , 0% , and 2.6% , respectively , in children with lrti and 4.8% , 4.8% , and 2.4% , respectively , in those without lrti .