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this retrospective study was conducted during january 2011july 2013 on a subset of a prospective study in which children were enrolled at birth and observed until young adolescence ( 8) .
we observed 144 children born during 19952004 , from whom final samples were obtained during 20042008 .   on average , 13 serum samples per child
were obtained during the study period ( technical appendix table 1 ) . at each follow - up visit , clinical symptoms or illnesses since
the ethics committee of the hospital district of southwest finland ( www.vsshp.fi/en/ ) approved the study .
igg enzyme immunoassays ( eia ) for mcpyv and tspyv were conducted as described , except that we omitted subtraction of antigen - free background in the mcpyv assay ( 5,7 ) .
the respective lower and higher eia cutoff values for igg absence and presence were 0.120 and 0.210 for mcpyv and 0.100 and 0.240 for tspyv ( 5,7 ) .
igm eias for these 2 human polyomaviruses were developed as for human bocavirus 1(hbov1 ) ( 8) .
the cutoff values were 0.207 and 0.260 for mcpyv and 0.194 and 0.240 for tspyv .
serum samples obtained during the final examination of each child were screened for mcpyv igg and tspyv igg .
the children whose final samples lacked virus igg were considered to be igg negative ; those whose final samples showed virus igg were considered to have seroconverted . each previous serum sample for each child who seroconverted
serum samples collected immediately before , at , and after the igg seroconversion were examined for viral dna ; the seroconversion sample , the subsequent sample , and the final sample were examined for igg avidity .
of the 144 children , 45 ( 31% ) showed igg seroconversion for mcpyv and 39 ( 27% ) for tspyv . before they were 1 year of age
, 4 children showed igg seroconversion for mcpyv at 0.680.94 year of age , 1 child showed seroconversion for tspyv at 0.80 year , and another child showed mcpyv igg , igm , and low avidity of igg in the first sample , which was collected at 0.63 year ( technical appendix table 2 ) .
none of these children who seroconverted early in life showed maternal igg to the corresponding virus . comparing participants at 01 year of age with those at 913 years
, the seroprevalence for mcpyv caused by acquired infections rose from 3.4% to 65% and for tspyv , from 0.7% to 53% .
seroprevalence related to polyomavirus primary infections in children in finland during follow - up , january 2011july 2013 .
seroprevalence was calculated by the formula : seroprevalence = ( no . seropositive children remaining in the study at each age category )  100 .
of the 45 children who seroconverted for mcpyv , 28 ( 62% ) showed additional markers of primary infection at the time of igg seroconversion : igm was present in 15 ( 33% ) , and low avidity of igg was detected in 23 ( 51% ) .
of the 39 tspyv  seroconverted children , 32 ( 82% ) showed corresponding markers : igm in 30 ( 67% ) and low avidity of igg in 13 ( 29% ) .
samples did not show mcpyv viremia at or flanking the seroconversion , and tspyv viremia was observed at low quantity ( < 10 copies / ml ) in the samples of 2 who seroconverted . except 4 seroconverters for mcpyv and 1 for tspyv , all showed long - term maturation of igg avidity to the corresponding virus .
maternal igg showed in 10 ( 22% ) of the 45 who seroconverted for mcpyv and in 12 ( 31% ) of the 39 for tspyv at sampling ages of 0.230.62 year ; these maternal iggs were no longer discernable at 0.491.07 year .
after the first year of life , the age at seroconversion with either virus did not appear to correlate with the presence or absence of maternal antibodies . to determine clinical correlates of mcpyv and tspyv primary infections , all infection - related symptoms and illnesses during the seroconversion interval were compared with those during the previous or subsequent interval for each patient who seroconverted ( table ) .
infection - related symptoms during the seroconversion interval were reported for 73% of children who seroconverted for mcpyv and for 82% of those who seroconverted for tspyv .
the occurrences of symptoms , however , were not notably different from those during the previous or subsequent intervals .
exanthema was reported in 7 ( 15.9% ) children at the mcpyv seroconversion interval and in 1 child in each of the adjacent periods ( 2.2% before , 2.3% after ) .
the seroprevalence of mcpyv and tspyv among children < 13 years of age increased because of acquired infections , a finding consistent with reports that primary infections with these 2 viruses are ubiquitous in childhood ( 5,7 ) .
the seroprevalence of these viruses had not reached a plateau by the end of study ; thus , some infections are expected to occur later than 13 years of age .
we expect that some children have become infected with these viruses after exiting the study .
for both viruses , prevalence of maternal antibodies during infancy was high , supporting recent findings by martel - jantin et al .
although the age of the child at seroconversion did not appear to correlate with the presence of maternal antibodies , these antibodies were absent from children who showed viral infection during infancy , raising the possibility that maternal immunity may protect infants from infection by these viruses .
however , the prevalence of mcpyv infection among infants was higher than that of tspyv , possibly caused by mcpyv shedding from parental skin ( 1012 ) .
we did not observe any symptom associated with the time of seroconversion for these 2 viruses , which implies that primary exposures to these viruses during childhood cause no symptoms .
when studying mcpyv infections in adult men who seroconverted , tolstov et al . also found no clinical associations ( 13 ) .
their observations and ours in the current study of mcpyv and tspyv support the common notion that the prototypic human pyvs , bkpyv and jcpyv , also establish persistence without initial signs ( 14,15 ) .
exanthema , a common sign of skin infection , did not appear infrequently during mcpyv seroconversion ; however , the rate did not reach statistical significance ( p = 0.0703 ) , which might be related to our limited cohort size .
we can not rule out that data for some transient skin - related signs may not have been captured during periodic patient interviews .
. larger studies of more geographically diverse populations are needed to determine whether primary infection with mcpyv or tspyv is always asymptomatic .
although igm and low - avidity igg were observed in more than half of the seroconversion samples , the frequency of viremia was extremely low , indicating serodiagnostics as the strategy of choice in diagnosing primary infections with mcpyv and tspyv .
we used comprehensive serodiagnostic methods ( igm , igg , and igg avidity ) and pcr to study merkel cell polyomavirus and trichodysplasia spinulosa - associated polyomavirus infections in children observed from infancy to adolescence . comparing seroconversion intervals with previous and subsequent intervals 
 , we found that primary infections with these 2 viruses were asymptomatic in childhood .