Sub-clinical enterovirus infections in Norwegian infants: A prospective cohort study on viral circulation and predictors of infection

Abstract

Background: Enteroviruses are common in infancy, but usually sub-clinical and self-limiting. Most previous data on enterovirus circulation derive from analyses of specimens from individuals with disease. Studies of enterovirus circulation in healthy populations antedate the advent of molecular technologies. Population-based studies that use molecular approaches for diagnosing are needed to obtain unbiased estimates of enterovirus circulation. Objectives: The objectives of the present studies were to estimate the prevalence of enterovirus infections in Norwegian children, and to gain insight into the molecular epidemiology of natural circulating enteroviruses. Other aims included studying associations of infection with disease, and investigating possible risk factors of infection. <br><br>

Subjects and methods: Newborns were recruited as part of a prospective cohort study in Norway aimed at identification of environmental risk factors for type 1 diabetes (T1D). The infant’s parents submitted monthly stool samples (collected from their children) for viral analysis from 3 months of age. Data on symptoms of disease, as well as putative predictors of infection, were based on parental reports at 3-months intervals. Three papers (paper I-III) were based on data from 113 children with HLA high genetic risk for T1D and enrolled from 2001 to 2003. The majority of the 1255 stool samples were collected when the children were aged 3-15 months. The last paper (paper IV) is based upon data from 639 children. 4279 samples were collected when the children were age 3-12 months from 2001 to 2006. Among these children, 394 did not have increased genetic risk for T1D. Enteroviruses were detected and quantified using a one-step real-time reverse transcription (RT)-PCR targeting the 5’ untranslated region. Enterovirus positive samples were typed directly by nucleotide sequencing of the VP1 region. <br><br>

Main results: Enterovirus infections were common in infancy. Strains of species human enterovirus A (HEV-A) and HEV-B were approximately equally represented among the children, while strains of HEV-C were rare. Poliovirus and HEV-D were not detected. Widespread circulation of a single strain of serotype enterovirus 71 (EV71) belonging to genotype C1 was discovered during a restricted period of time, but was not associated with central nervous system disease (CNS) among Norwegian infants. Complete genome 7 sequencing of this strain revealed differences in the 5’ nontranslated region and RNA dependent polymerase with respect to more pathogenic viral genotypes that may explain its reduced virulence. Logistic regression indicated a linear decrease in risk of enterovirus infection with more intensive breastfeeding. The most pronounced effect was at 3 months of age. The protective effect gradually faded until the age of about 11 months. Interestingly, carrying the HLA high risk genotype for T1D may confer protection against enterovirus infection.<br><br>

Conclusion: This is the first population based study using solely molecular methods to verify the prevalence of species HEV-A. Our findings suggest that the prevalence of enterovirus infections in general, and of HEV-A in particular, has been underestimated in epidemiological studies based on virus culture. Furthermore, our results indicate that risk for infection may be modified by diverse factors including breastfeeding and being HLA high risk for T1D. Our collective findings highlight the need for future epidemiological studies of these viruses to elucidate issues relating to viral prevalence, transmissibility and risk factors as well as their association with clinical disease. The worldwide circulation of EV71 and its potential to cause severe disease underscores the need for additional studies to identify the neurovirulent determinants of EV71 in human disease. The present work contributes importantly to the knowledge on enterovirus infections in normal infants.