Brit. J. prev. soc. Med. (1975), 29, 22-26

Seasonal variation in anencephalus in Canada J. MARK ELWOOD Department of Epidemiology and Community Medicine, University of Ottawa, Royal Ottawa Hospital, 1145 Carling Avenue, Ottawa KIZ 7K4, Ontario

Elwood, J. M. (1975). British Journal of Preventive and Social Medicine, 29, 22-26. Seasonal varation In anencephalus in Canada. A study of the monthly numbers of stillbirths and of deaths due to anencephalus in Canada from 1954 to 1962 showed a weak tendency to a winter excess of affected births. The seasonal trend was more marked in the Prairie provinces and in Quebec than elsewhere; the maximum rate of anencephalus occurrence was in October to December in the Prairies and in British Columbia, and in January to March in other regions.

Anencephalus is the most common fatal congenital abnormality in Canada. Large variations in incidence with time and between provinces have been described (Elwood, J. M., 1974). The Canadian experience is matched by the great variability in incidence reported in other studies, and these findings, taken with the evidence for hereditary transmission, have led most investigators to propose mixed genetic-environmental aetiological hypotheses. Early studies (e.g., McKeown and Record, 1951) in the British Isles showed a winter excess of anencephalic births, suggesting the maximum action of a causative agent in spring and early summer. Later studies showed that this variation was not seen in all regions (Laurence, Carter, and David, 1968) and could show secular changes in one area (Leck, 1972). In contrast to the British studies, no seasonal variation was seen in studies of anencephalus incidence in Rhode Island (MacMahon, Pugh, and Ingalls, 1953), Boston (Naggan, 1969), or Missouri (Silberg, Watson, and Martin, 1968). A small study of 33 cases in Charleston (Alter, 1963) showed a winter excess. Studies in Quebec (Horowitz and McDonald. 1969), Muiitoba (Choi, Klaponski, Ateah, and Nelson, 1972), and Halifax (Alter, 1963) showed no significant trends. The present work presents data for Canada, 1954-62, showing a weak tendency to a winter excess of affected births.

1962 in Canada, excluding the Northwest Territories and the Yukon. The numbers of deaths per month in each province recorded under the stillbirth code Y.38.0 'anencephalus' and the equivalent death code 750 'monstrosity' were obtained by special tabulations from Statistics Canada data, and rates are expressed as total deaths per thousand total births. It has been shown (Elwood, J. M., 1974) that Canadian mortality statistics give an estimate of the incidence of anencephalus with up to a 10-15 % error; there seems no reason to suppose that this error would change with the month of birth.

RESULTS During the period 1954-62 there were 5,100 deaths (stillbirths plus infant deaths) attributed to anencephalus in Canada, a rate of 1*21 per thousand total births. The monthly mortality rate, based on the monthly number of deaths and the number of births in the same month (Table I), shows a variation from 1*37 per thousand in January to 1*01 in May. The departure from homogeneity is highly significant (X2 = 35*47, df = 11, P 1 003 NS 9 93 Dec.-Feb. 305 393

mratio of incidence rates 2ratio of number of cases 3insufficient data to calculate ratio

Seasonal variation in anencephalus in Canada

in Belfast and four Scottish cities (Elwood, J. H. and MacKenzie, 1971) and a positive association with hours of sunshine and daylight in Scotland (Record, 1961). In all regions of Canada maximum daily temperatures and sunshine hours are in July, and minima in January-February. Maximum rainfall is in October to December in the Maritime areas and in British Columbia, and in June to August elsewhere (Canada, Department of Transport, 1967). Thus there is no apparent association of the anencephalus risk period with these meteorological variables. Most studies of an association with infectious diseases have been negative, although there are some reports of an increased risk after influenza during pregnancy (see review by Leck (1963)). In Canada, influenza is most common in JanuaryFebruary, as are notifiable diseases such as streptococcal sore throat and infectious hepatitis (Canada, Dominion Bureau of Statistics, 1954 et seq.). Renwick (1972, 1973) has suggested that such a seasonal variation as this could be due to the maximum consumption of blight-damaged potatoes at the end of the winter, when stored potatoes are consumed, and that the lack of seasonal variation in previous North American studies may reflect less storage of potatoes than in Great Britain. Sufficiently detailed information on potato storage habits is not available to check this hypothesis, although it might be expected that such an effect would be more marked in the Malitimes than in the Prairies, as the former area has greater potato production, consumption, and blight prevalence (Elwood, J. M., 1973). Thus this study shows that during the period 1956-62 a winter excess of anencephalic births occurred in Quebec and the Prairies. There was a smaller non-significant winter excess in other areas of Canada. This variation is similar to that reported in some areas of the British Isles. I wish to thank Mr. H. Page, Mr. N. Tompkins, and Dr. G. B. Hill (Statistics Canada) for supplying the mortality data. Dr. S. D. Walter and Dr. J. M. Last gave valuable criticism, and Miss M-C. Brunger (nursing student) assisted in the study. I am supported by a Physicians' Services Incorporated Fellowship, and this work was supported in part by Health and Welfare Canada grant No. 606-1038-29.

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CANADA, DOMINION BUREAU OF STATISTICS (1954-64). Annual Reports 84-201, Notfiable Diseases. Queen's Printer, Ottawa. - (1957). Report 82-516, Influenza in Canada. Queen's Printer, Ottawa. CARTER, C. 0. and EvANs, K. (1973). Spina bifida and anencephalus in Greater London. J. med. Genet., 10, 209. CHoI, N. W., KLAPONSKI, F. A., ATEAH, E., and NELSON, N. A. (1972). Some epidemiological aspects of central nervous system malformations in Manitoba. In: Drugs and Fetal Development, edited by M. A. Klingberg, pp. 511-525. Plenum Press, New York. EDWARDS, J. H. (1961). The recognition and estimation of cyclic trends. Ann. hum. Genet., 25, 83. ELWOOD, J. H. (1970). Anencephalus in Belfast: incidence and secular and seasonal variations, 1950-66. Brit. J. prev. soc. Med., 24, 78. - and MACKENZIE, G. (1971). Comparisons of secular and seasonal variations in the incidence of anencephalus in Belfast and four Scottish cities, 1956-66. Brit. J. prev. soc. Med., 25, 17. ELWOOD, J. M. (1973). Anencephaly and potato blight in Eastem Canada. Lancet, 1, 769. - (1974). Anencephalus in Canada, 1943-70. Amer. J. Epidem. (In press). HOROWITZ, I. and McDONALD, A. D. (1969). Anencephaly and spina bifida in the province of Quebec. Canad. med. Ass. J., 100, 748. LAURENCE, K. M., CARTER, C. O., and DAVID, P. A. (1968). Major central nervous system malformations in South Wales H: Pregnancy factors, seasonal variation, and social class effects. Brit. J. prev. Soc. Med., 22, 212. LECK, I. (1963). Incidence of malformations following influenza epidemics. Brit. J. prev. soc. Med., 17, 70. - (1972). The etiology of human malformations: Insights from epidemiology. Teratology, 5, 303. - and REcoRD, R. G. (1966). Seasonal incidence of anencephalus. Brit. J. prev. soc. Med., 20, 67. McKEOwN, T. and RECORD, R. G. (1951). Seasonal incidence of congenital malformations of the central nervous system. Lancet, 1, 192. MACMAHON, B., PUGH, T. F., and INGALLS, T. H. (1953). Anencephalus, spina bifida, and hydrocephalus. Incidence related to sex, race, and season of birth, and incidence in siblings. Brit. J. prev. soc. Med., 7, 211. NAGGAN, L. (1969). The recent decline in prevalence of anencephaly and spina bifida. Amer. J. Epidem., 89,154. RECORD, R. G. (1961). Anencephalus in Scotland. Brit. J. prev. soc. Med., 15, 93. RENWICK, J. H. (1972). Hypothesis-Anencephaly and spina bifida are usually preventable by avoidance of a specific but unidentified substance present in certain potato tubers. Brit. J. prev. sac. Med., 26, 67.


J. Mark Elwood

(1973). Prevention of anencephaly and spina bifida in man. Teratology, 8, 321. Rooms, S. C. and MoRmis, M. (1971). Infant mortality from spina bifida, congenital hydrocephalus, monstrosity, and congenital diseases of the cardiovascular system in England and Wales. Ann. hum. Genet., 34, 295. SILBERG, S. L., WATSON, F. R., and MARIN, J. C. (1968). Seasonal variation of congenital malformations. Canad. J. publ. Hlth, 59, 239.

SMITHLS, R. W., CHNN, E. R., and FRANKN, D. (1964). Anencephaly in Liverpool. Devel. Med. Child.

Neurol., 6,231. WILUAMsON, E. M. (1965). Incidence and family aggegation of major congenital malformations of central nervous system. J. med. Genet., 2, 161. Requests for reprints to: Dr. J. Mark Elwood, Department of Epidemiology, Harvard School of Public Health, 677 Huntington Avenue, Boston, Mass. 02115, U.S.A.

Seasonal variation in anencephalus in Canada.

A study of the monthly numbers of stillbirths and of deaths due to anencephalus in Canada from 1954 to 1962 showed a weak tendency to a winter excess ...
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