Season-of-birth phenomenon in health and longevity: epidemiologic evidence and mechanistic considerations

In many human populations, especially those living in regions with pronounced climatic differences between seasons, the most sensitive (prenatal and neonatal) developmental stages occur in contrasting conditions depending on the season of conception. The difference in prenatal and postnatal environments may be a factor significantly affecting human development and risk for later life chronic diseases. Factors potentially contributing to this kind of developmental programming include nutrition, outdoor temperature, infectious exposures, duration of sunlight, vitamin D synthesis, etc. Month of birth is commonly used as a proxy for exposures which vary seasonally around the perinatal period. Season-of-birth patterns have been identified for many chronic health outcomes. In this review, the research evidence for the seasonality of birth in adult-life disorders is provided and potential mechanisms underlying the phenomenon of early life seasonal programming of chronic disease and longevity are discussed.

Impacts of nutrition on reproduction in female red deer: phenotypic flexibility within a photoperiod-mediated seasonal cycle

Red deer (Cervus elaphus) are widely distributed throughout cold northern temperate latitudes, where they have evolved to cope within highly seasonal continental environments. Naturalisation of red deer to the more moderate seasonal (but variable climatic) environment of New Zealand has been spectacularly successful, and they are widely farmed in the country’s pastoral environment for venison and antlers. The species is genetically programmed to exhibit photoperiodic control of voluntary feed intake, growth and reproduction, ensuring that energy demands are aligned with seasonally available resources and offspring are born in summer when climate is favourable for survival. However, despite genetic control of their endogenous seasonal cycles, there appears to be a strong ability for environmental factors such as nutrition to generate large phenotypic variation of seasonal traits. This may have contributed to their successful naturalisation to a wider range of seasonal environments than would be expected within their ancestral range. While precise timing of conception and duration of gestation length are the two fundamental mechanisms by which the strict seasonality of birth is maintained in seasonally breeding mammals, red deer exhibit considerable variation in both these traits. The present paper examines the outcomes of recent studies on farmed red deer on the impacts of lactation on conception date, the influence of nutrition during pregnancy on gestation length, and early life growth effects on the onset of female puberty. These studies have collectively demonstrated that while red deer are assumed to be under fairly rigorous genetic control of seasonality traits, they have a repertoire of phenotypic variation at various points of the reproductive cycle that may potentially allow a degree of adaptation to climatic variation that influences annual feed supply. This may explain the success of red deer in colonising a range of new environments that differ seasonally from their ancestral environment.

Seasonality of birth defects in West Africa: could congenital Zika syndrome be to blame?

The link between Zika virus infection during pregnancy and microcephaly and other neurodevelopmental defects in infants, referred to as congenital Zika syndrome (CZS), was recently discovered. One key question that remains is whether such neurodevelopmental abnormalities are limited to the recently evolved Asiatic ZIKV strains or if they can also be induced by endemic African strains. Thus, we examined birth registries from one particular hospital from a country in West Africa, where ZIKV is endemic. Results showed a seasonal pattern of birth defects that is consistent with potential CZS, which corresponds to a range of presumed maternal infection that encompasses both the peak of the warm, rainy season as well as the months immediately following it, when mosquito activity is likely high. While we refrain from definitively linking ZIKV infection and birth defects in West Africa at this time, in part due to scant data available from the region, we hope that this report will initiate broader surveillance efforts that may help shed light onto mechanisms underlying CZS.

Seasonality of birth defects in West Africa: could congenital Zika syndrome be to blame?

The link between Zika virus infection during pregnancy and microcephaly and other neurodevelopmental defects in infants, referred to as congenital Zika syndrome (CZS), was recently discovered. One key question that remains is whether such neurodevelopmental abnormalities are limited to the recently evolved Asiatic ZIKV strains or if they can also be induced by endemic African strains. Thus, we examined birth registries from one particular hospital from a country in West Africa, where ZIKV is endemic. Results showed a seasonal pattern of birth defects that is consistent with potential CZS, which correspond to a range of presumed maternal infection that encompasses both the peak of the warm, rainy season as well as the months immediately following it, when mosquito activity is likely high. While we refrain from definitively linking ZIKV infection and birth defects in West Africa at this time, in part due to scant data available from the region, we hope that this report will initiate broader surveillance efforts that may help shed light onto mechanisms underlying CZS.