NARCOMS Report-Seasonal Patterns in Birth Rate
Tuula Tyry, PhD––NARCOMS Program and Editorial Manager, Barrow Neurological Institute, Arizona; Breanna Bullock, BA–NARCOMS Database Coordinator, Barrow Neurological Institute,Arizona
Introduction
Epidemiological research studies patterns, causes, and control of disease in various populations. Since NARCOMS Registry participants often submit questions and comments about the epidemiological factors linked to MS prevalence and incidence, we asked Drs. Wallin and Kurtzke to write an up-to-date feature article on several epidemiological factors of current interest in MS research (Wallin & Kurtzke, 2006). They have also reviewed MS epidemiology in a previous issue of MSQR (Wallin & Kurtzke, 2003).
Sharing the same theme, this NARCOMS Report looks at seasonal patterns in the incidence of MS, a topic receiving renewed interest after a recent article published in the British Medical Journal (BMJ) by Dr. Cristen Willer and her colleagues (Willer, Dyment, Sadovnick, Rothwell, Murray et al., 2005). We will refer to some of their results while describing NARCOMS Registry population in terms of the geographic distribution of the participants’ birthplace and their month of birth. Analyses are based on all participants who lived in the US at the time of enrollment even if they were born abroad.
Geographic Distribution in Place of Birth
A total of 25,323 Registry participants have provided us with information about their place of birth. The birthplace for the remaining 5,677 (22%) is currently unknown to us. Figure 1 illustrates the geographic distribution of the participants born in the US based on the state in which they were born. The north-south gradient, suggesting that the incidence of MS within the northern hemisphere is higher in the north, has been defined relative to alatitudinal position of 42 degrees north in the Northern hemisphere and 42 degrees south in the Southern hemisphere (Islam, Gauderman, Cozen, Hamilton, Burnett et al., 2006). A total of 10,247 (40.4%) NARCOMS participants were born north of 42 degrees Northern latitude (e.g., north of a line drawn roughly from the southern border of Oregon to the southern border of Massachusetts). The remaining 15,088 (59.6%) NARCOMS participants were born south of that line. Among the 1,123 who were born outside US, 485 (43%) were born north of 42 degrees Northern latitude (CANADA, Northern Europe) and 31 (3%) born south of 42 degrees Southern latitude (Australia, New Zealand, Southern part of South America). These 516 participants are included in the NARCOMS–USA North group. Subsequent analyses on birth month were done separately for those born further away from the equator (NARCOMS–USA North group) and those born within 42 degrees of northern and southern latitudes (NARCOMS–USA South group).
Birth Month Among Those Born Further Away From the Equator
The Willer et al., 2005 study analyzed data obtained from three separate MS populations: Patients with MS in Canada (n = 17,874), patients with MS in Great Britain (n = 11,502), and a combined pool of patients with MS in the Northern Hemisphere (Canada, Great Britain, Denmark, and Sweden, n = 42,045). In each of these groups the birthrates in May were higher than in November. Furthermore, in all groups November birthrates were significantly lower than in the general population born in the corresponding geographical area during the same era. In theBritish MS population also the May birth rate was significantly higher than in the general population. The largest seasonal differences were found in Scotland, where the incidence of MS is the highest as well. The May-November differences described above are illustrated together with NARCOMS and US general population data.
According to Willer et al., the pronounced seasonal difference observed in the MS populations studied may reflect an interaction between genetics and environmental factors relating to climate. Explicitly mentioned factors for an increased incidence of MS among May births as compared to November births were differences in maternal folate level, as it relates to the development of the central nervous system and/or immune system, and maternal Vitamin D deficiency due to lack of exposure to ultraviolet sunlight during colder months.
When the NARCOMS population was analyzed as a whole, the monthly birthrates were remarkably similar to the US general population as well as the birth rates among a sub group of white Caucasians (90% of NARCOMS participants are white Caucasian). Unlike the MS and general populations in Canada, the US general population and the NARCOMS population did not demonstrate an increase in spring (May) birthrates as compared to fall (November). According to Dr. William James, the overall pattern of monthly births in the US has been different from the traditional pattern in Europe (high peak in the spring and a smaller peak in the fall) since the mid 1900s and instead continues to demonstrate one peak season corresponding to summer months (James, 1990). Interestingly, when the cohort of NARCOMS participants born in northern latitudes was analyzed separately, May birth rate (8.7%) was indeed higher than November birth rate (7.9%). The difference, however, was still much smaller than in any of the four comparisons reported in the Willer et al., 2005 article and only slightly wider than seen in the subgroup of US general population born in the northern latitudes (8.6% vs. 8.1%). None of the differences were statistically significant. In the subgroup of NARCOMS participants born closer to the equator, the birth rates in November were actually higher than in May and once again the seasonal differences were quite similar to those found in a subgroup of US general population born in a corresponding region.
Conclusions
The distribution of NARCOMS participants based on place of birth seems to reflect the north-south gradient with 43.6% of the participants having been born further away from the equator. Based on 1955 vital statistics data of the general US population, only 29.4% were born in the Northern latitudes. Data from 1955 was used for comparison in order to reflect the birthrate trends during a period of time when most NARCOMS participants were born (average age approximately 50 years). The effect of north-south gradient may be somewhat obscured since this analysis did not take into account where the participants had spent their early childhood––another proposed epidemiological risk factor. On the other hand, NARCOMS Registry participants saturate the Northeastern US, at least partially due to uneven initial recruitment. This may have inflated the number of participants reporting a birthplace in that region.
This brief analysis provided only limited support to the proposed increase in incidence of MS in May births as compared to November births and the statistically nonsignificant difference was seen only among those in the northernmost part of the US. The seasonal difference in monthly birth rates appears to be a prominent factor only further north where the seasonal changes in climate have a more dramatic effect on all aspects of life, including behavioral patterns and family planning. More comprehensive analyses of the NARCOMS data are already under way and will address the various racial and cultural factors that may have a bigger role in the more heterogeneous US population than in Canada and Northern Europe.
References
Islam, T., Gauderman, W. J., Cozen, W., Hamilton, A.S., Burnett, M. E., & Mack, T. M. (2006). Differential twin concordance for multiple sclerosis by latitude of birthplace. Annals of Neurology, 60(1), 56-64.
James, W. H. (1990). Seasonal variation in human births. Journal of Biosocial Sciences, 22(1), 113-9.
Wallin M. T. (2006). An update on MS risk factors. MSQR, 25(3): 6-15.
Wallin, M. T & Kurtzke, J. F. (2003). Trends in multiple sclerosis (MS) prevalence and Incidence: Geographic, racial, and ethnicity risk factors. MSQR, 22(3), 1, 3-7.
Willer, C. J., Dyment, D. A., Sadovnick, A. D., Rothwell, P. M., Murray, T. J., & Ebers, G. C. (2005). Timing of birth and risk of multiple sclerosis: Population-based study. British Medical Journal, 330(7483), 120.