Childhood leukaemia mortality and population change in England and Wales 1969-73

The Association Between Childhood Leukemia and Population Mixing: An Artifact of Focusing on Clusters?

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Background:

Studies investigating the population-mixing hypothesis in childhood leukemia principally use two analytical approaches: (1) nonrandom selection of areas according to specific characteristics, followed by comparisons of their incidence of childhood leukemia with that expected based on the national average; and (2) regression analyses of region-wide data to identify characteristics associated with the incidence of childhood leukemia. These approaches have generated contradictory results. We compare these approaches using observed and simulated data.

Methods:

We generated 10,000 simulated regions using the correlation structure and distributions from a United Kingdom dataset. We simulated cases using a Poisson distribution with the incidence rate set to the national average assuming the null hypothesis that only population size drives the number of cases. Selection of areas within each simulated region was based on characteristics considered responsible for elevated infection rates (population density and inward migration) and/or elevated leukemia rates. We calculated effect estimates for 10,000 simulations and compared results to corresponding observed data analyses.

Results:

When the selection of areas for analysis is based on apparent clusters of childhood leukemia, biased assessments occur; the estimated 5-year incidence of childhood leukemia ranged between zero and eight per 10,000 children in contrast to the simulated two cases per 10,000 children, similar to the observed data. Performing analyses on region-wide data avoids these biases.

Conclusions:

Studies using nonrandom selection to investigate the association between childhood leukemia and population mixing are likely to have generated biased findings. Future studies can avoid such bias using a region-wide analytical strategy. See video abstract at, http://links.lww.com/EDE/B431.

Epidemiological evidence for an infective basis in childhood leukaemia

An infective basis for childhood leukaemia is not a new suspicion (Kellett, 1937). The failure of microbiologists to identify any specific agent and of epidemiologists to demonstrate marked space-time clustering of the disease (Smith, 1982) have been discouraging, but neither is incompatible with an infectious origin. In several vertebrate species, the specific agents responsible for leukaemia belong to a class that is notoriously difficult to isolate. Also, many infectious illnesses do not cluster because they are uncommon responses to the relevant infection. Thus, the agent responsible for infectious mononucleosis is mainly spread not by those with the illness but by that very much larger number of infected individuals who are clinically unaffected (or only trivially so). Such infections can be considered as 'mainly immunising': they can be seen as representing the most probable broad category to which the infection underlying childhood leukaemia belongs.

An examination, with a meta-analysis, of studies of childhood leukaemia in relation to population mixing

Background:

Marked influxes of people into rural areas, termed rural population mixing (PM), have been associated with excesses of childhood leukaemia (CL), consistent with mini-epidemics of a mainly immunising, subclinical infection to which CL is a rare response. For such situations of rural PM would promote contacts between infected and susceptible individuals, the latter tending to have a higher than average prevalence in rural or isolated areas. Confusion has arisen from some workers applying the term PM to non-rural situations lacking known recent change.

Methods:

Available PM studies using the original definition of influxes were examined, a meta-analysis carried out of studies of CL in relation to exposure to high levels of rural PM, and also a detailed analysis by age group.

Results:

The meta-analysis of 17 studies shows a significant CL excess in association with rural PM: overall relative risk (RR) at ages 0–14: 1.57; 95% confidence interval 1.44–1.72; at 0–4 years 1.72 (1.54–1.91). This contrasts with the absence of an excess of CL in similarly exposed urban areas (RR 1.00; 0.93–1.07), pointing to a high level of immunity there. The mixed results of studies using other definitions of PM were summarised. The excess associated with rural PM below age 2 years (RR 1.51; 1.17, 1.92) was not appreciably different from that at later childhood ages.

Conclusion:

Much of the inconsistency among studies ostensibly about CL and PM reflects the use of definitions other than that originally proposed. The broad similarity of the CL excess below age 2 with that at older childhood ages is inconsistent with the Greaves’ delayed infection hypothesis, since any infection underlying the former is difficult to consider as delayed.

Childhood leukaemia and population movements in France, 1990-2003

In a national study, we investigated the incidence of childhood leukaemia (CL) over a 14-year period in France in relation to several measures based on the proportion of individuals who changed address between the last two national censuses. A positive association was found with the proportion of migrants who came from a distant place. The further the migrants came, the higher was the incidence of leukaemia, particularly among children aged 0-4 years in 'isolated' communes at the time of diagnosis (RR=1.4, 95% CI: 1.1,1.8 in the highest category of migration distance). Although the role of the population density was less obvious, a more marked association was found above a certain threshold. No association with the proportion of commuters was observed.

Childhood acute lymphoblastic leukaemia in relation to population mixing around the time of birth in South Hungary

In a retrospective epidemiological study of 481,984 live births in South Hungary, we investigated whether higher levels of population mixing around the time of birth is a risk factor for acute lymphoblastic leukemia (ALL) under age 5 years. Poisson regression was used to investigate the relationship between risk of ALL and the population-mixing index based on the number of incomers in each county district for each year, standardized to have a range of 0-1. Among all children, the risk of ALL increased significantly with increasing population mixing around the time of birth (trend across the range of 0-1 RR = 2.1 95% CI: 1.02-4.44). This effect was more marked for boys (RR = 3.1 95% CI: 1.13-8.51), which supports a sex-specific effect of exposures on risk of ALL.

Population-mixing at the place of residence at the time of birth and incidence of childhood leukaemia in France

The association between the risk of childhood leukaemia before age 7 years and population-mixing at the place of residence at birth was investigated by retrospectively considering all the children born in mainland French communes between 1st January 1990 and 31st December 1998. An increased risk of acute lymphoblastic leukaemia was found with higher levels of migration for children residing at birth in isolated communes with a population density > or =50 people per km2 (SIRR = 2.59, 95% CI: 1.48-4.49). No association was observed with lower population densities. For children residing in non-isolated communes at birth, the results were similar but less marked. The risk tended to increase only for population densities > or =5000 people per km2 (SIRR = 1.57, 95% CI: 0.99-2.52). The findings are consistent with epidemic models and support the hypothesis of an infectious aetiology relating to population-mixing. Population density may be seen as an indicator of the opportunity of contacts between inhabitants and should therefore be taken into account when investigating an infectious hypothesis. This is the first systematic study of population-mixing at the place of residence at the time of birth to be conducted on a national scale.

An infectious aetiology for childhood acute leukaemia: a review of the evidence

There are three current hypotheses concerning infectious mechanisms in the aetiology of childhood leukaemia: exposure in utero or around the time of birth, delayed exposure beyond the first year of life to common infections and unusual population mixing. No specific virus has been definitively linked with childhood leukaemia and there is no evidence to date of viral genomic inclusions within leukaemic cells. The case-control and cohort studies have revealed equivocal results. Maternal infection during pregnancy has been linked with increased risk whilst breast feeding and day care attendance in the first year of life appear to be protective. There is inconclusive evidence from studies on early childhood infectious exposures, vaccination and social mixing. Some supportive evidence for an infectious aetiology is provided by the findings of space-time clustering and seasonal variation. Spatial clustering suggests that higher incidence is confined to specific areas with increased levels of population mixing, particularly in previously isolated populations. Ecological studies have also shown excess incidence with higher population mixing. The marked childhood peak in resource-rich countries and an increased incidence of the childhood peak in acute lymphoblastic leukaemia (ALL) (occurring at ages 2-6 years predominantly with precursor B-cell ALL) is supportive of the concept that reduced early infection may play a role. Genetically determined individual response to infection may be critical in the proliferation of preleukaemic clones as evidenced by the human leucocyte antigen class II polymorphic variant association with precursor B-cell and T-cell ALL.

Childhood leukaemia incidence and the population mixing hypothesis in US SEER data

We evaluated the infectious aetiology hypothesis of childhood leukaemia that rapid population influx into rural areas is associated with increased risk. Using data from the US SEER program, we found that in changes in rural county population sizes from 1980 to 1989 were associated with incidence rates for childhood acute lymphocytic leukaemia (ALL). The observed associations were strongest among children 0-4 years of age, born in the same state as diagnosis, in extremely rural counties, and when counties adjacent to nonrural counties were excluded. Similar analyses for brain and central nervous system (CNS) cancer in children, a disease less linked to this infectious hypothesis, provide evidence against methodologic bias. Similar evaluations for other decades were not meaningful due to limited sample sizes and, perhaps, increased population mobility.

Population mixing and childhood diabetes

BACKGROUND

Exposure to infections, particularly in early life, may modify the risk of developing childhood diabetes. Population mixing, based on the number and diversity of incoming migrants to an area can be used as a proxy measure for exposure to infections. We tested the hypothesis that incidence of childhood Type 1 diabetes is higher in areas of low population mixing.

METHODS

Children (<15 years) diagnosed with diabetes between 1986--1994 in Yorkshire, UK (n = 994) were analysed with demographic data and denominator populations from the 1991 UK Census. Population mixing was estimated separately for 'any age' (>1 year) and children (1--15 years) for each area, using the proportion of migrants and an index of diversity based on numbers and origins of migrants. Regression models calculated the effect of 'any age' and childhood population mixing on the incidence of diabetes, controlling for population density, ethnicity and proportion of migrants.

RESULTS

Areas with low levels of population mixing of children (bottom decile), were significantly associated with higher incidence of childhood diabetes for 0-14 years (incidence rate ratio [IRR] = 1.46, 95% CI : 1.01--2.11). When stratified by age different effects were observed for childhood population mixing with raised IRR for ages 5-9 (2.23, 95% CI : 1.20--4.11) and 10-14 (1.47, 95% CI : 0.89--2.42), and decreased IRR for 0--4-year-olds (0.56, 95% CI : 0.17--1.82).

CONCLUSION

The incidence of childhood diabetes is highest in areas where limited childhood population mixing occurs and the diversity of origins of incoming children is low; those over 4 years are at greatest risk. This is consistent with an infectious hypothesis where absence of stimulation to the developing immune system increases vulnerability to late infectious exposure, which may precipitate diabetes.

Quantifying the effect of population mixing on childhood leukaemia risk: the Seascale cluster

A statistical model was developed based on Poisson regression of incidence of childhood leukaemia and non-Hodgkin's lymphoma (NHL) in relation to population mixing among all 119 539 children born 1969-1989 to mothers living in Cumbria, north-west England, (excluding Seascale). This model was used to predict the number of cases in Seascale (the village adjacent to the Sellafield nuclear installation) children, born 1950-1989 and diagnosed before 1993. After allowing for age, the incidence of acute lymphoblastic leukaemia (ALL) and NHL was significantly higher among children born in areas with the highest levels of population mixing, relative risk (RR) = 11.7 (95% confidence interval (CI) 3.2-43) and was highest among children of incomers. The model predicted up to 3.0 (95% CI 1.3-6.0) cases of ALL/NHL in children born in Seascale compared to six observed and 2.0 (95% CI 1.0-3.4) cases in children resident, but not born, in Seascale compared to two observed. Population mixing is a significant risk factor for ALL/NHL, especially in young children, accounting for over 50% of cases in Cumbria and most cases in Seascale.

Are the children of fathers whose jobs involve contact with many people at an increased risk of leukaemia?

OBJECTIVES

To investigate the hypothesis that children of men whose jobs involve contact with many people (particularly children) are at an increased risk of leukaemia.

METHODS

A population based dataset obtained from routinely collected death certificates involving 14,168 cancer deaths occurring before the age of 15 years registered in England and Wales between 1959-63 and 1970-90. Associations were assessed with the proportional cancer mortality ratio (PCMR), with all childhood cancer deaths forming the standard for comparison. The PCMRs were adjusted, by stratification, for age and year of death (in 1-year bands) and paternal social class (nine categories). Analyses were performed by estimated level of paternal occupational social contact (high, medium, and low) for all leukaemias, leukaemia subtype, age at death, year of death, and individual occupation.

RESULTS

Out of 223 occupations, 36 (16%) were identified as having potentially high levels of social contact, and 27 (12%) as having potentially medium levels of social contact. No associations were found between paternal occupational social contact and death during childhood from leukaemia (high social contact: PCMR 94, 95% confidence interval (95% CI) 87 to 102; medium social contact: 101, 95 to 106). No associations were found when the data were analysed by leukaemia subtype, age at death, year of death, or individual occupation.

CONCLUSION

The findings presented here do not support the suggestion that childhood leukaemia is related to the amount of social contact that fathers experience at work.

High-contact paternal occupations, infection and childhood leukaemia: five studies of unusual population-mixing of adults

The hypothesis has been tested that, among excesses of childhood leukaemia associated with extreme population-mixing, the incidence is higher for the children of men in occupations involving contact with many individuals (particularly children), as noted in certain childhood infections. Data on childhood leukaemia were examined from five previous studies of the author in which significant excesses had been found associated with population-mixing involving adults. Occupational titles were categorized according to the estimated level of work contacts as medium, high, very high or indeterminate. Occupations involving frequent contact with children were categorized as having a very high contact level given the high frequency of exposure to the infection postulated as underlying childhood leukaemia. There was a significant positive trend (P < 0.001) in childhood leukaemia risk at ages 0-14 years across the occupational contact categories from the reference group (comprising the medium and low plus indeterminate categories) through high to very high (i.e. high-child) contact categories in the combined data from the author's five studies of adult population-mixing; this significant trend also applied at ages 0-4 (P < 0.001) and 5-14 (P < 0.01) years. The excess in the high category was mainly because of paternal occupations connected with the construction industry and transport, suggesting a broader definition of the 'very high' contact category. No sign of these excesses was found in a limited examination of the question outside areas of population-mixing using mortality data for childhood leukaemia in the general population of England and Wales. The findings represent the first individual-based support for infection underlying childhood leukaemia that is promoted by population-mixing, as well as further support for the role of adults in transmission of the infection.

Effect of population mixing and socioeconomic status in England and Wales, 1979-85, on lymphoblastic leukaemia in children

OBJECTIVE

To examine the effects of migration, diversity of migrant origins, commuting, and socioeconomic status on the incidence of acute lymphoblastic leukaemia in childhood.

DESIGN

Poisson regression analysis of incidence rates in relation to the variables of interest.

SETTING

The 403 county districts of England and Wales during 1979-85.

SUBJECTS

Children aged under 15 years.

RESULTS

There were significant trends in the incidence of lymphoblastic leukaemia at ages 0-4 and 5-9 years with the proportion of children in a district who had recently entered the district. While there was no consistent relation between the proportion of recent incomers in the total population of a district and its incidence rate, the combination of higher migration with greater diversity of origins or distance moved was associated with higher incidence in both age groups. Incidence increased significantly at age 0-4 with the level of employment in a district and at age 5-9 with the proportion of households with access to a car. No significant trends were found with commuting.

CONCLUSIONS

The results for level of child migration and diversity of total migration provide evidence of an effect of population mixing on the incidence of childhood leukaemia which is not restricted to areas experiencing the most extreme levels of mixing.

Childhood leukemia and rural population movements: Greece, Italy, and other countries

Mortality from childhood leukemia was examined particularly in rural countries in relation to any major rural-urban migration. Significant increases have been found in other situations of rural population mixing as predicted by the infection hypothesis. The 1950s and 1960s were of most interest since it preceded the decline in mortality brought about by effective chemotherapy in many countries. The 33 countries covered were all those in the World Health Organization's mortality database. No sensitive measure of rural-urban migration is available for international comparisons. However, it seems noteworthy that Greece and Italy, the two countries with the most striking levels of rural migration in the 1950s and 1960s, also had unusually high mortality rates from childhood leukemia. Greece was most affected proportionally by these population movements and from 1958 to 1972 had the highest recorded mortality from this cause in the world. The problems of international comparisons of mortality data dictate caution in drawing conclusions. However, against a background of other work on population mixing, and in the light of certain considerations, we suggest that the marked rural population mixing in Greece and Italy may have contributed to their high mortality rates from childhood leukemia in the 1950s and 1960s.

Childhood leukaemia and non-Hodgkin's lymphoma near large rural construction sites, with a comparison with Sellafield nuclear site

OBJECTIVE

To determine whether population mixing produced by large, non-nuclear construction projects in rural areas is associated with an increase in childhood leukaemia and non-Hodgkin's lymphoma.

DESIGN

A study of the incidence of leukaemia and non-Hodgkin's lymphoma among children living near large construction projects in Britain since 1945, situated more than 20 km from a population centre, involving a workforce of more than 1000, and built over three or more calendar years. For periods before 1962 mortality was studied.

SETTING

Areas within 10 km of relevant sites, and the highland counties of Scotland with many hydroelectric schemes.

SUBJECTS

Children aged under 15.

RESULTS

A 37% excess of leukaemia and non-Hodgkin's lymphoma at 0-14 years of age was recorded during construction and the following calendar year. The excesses were greater at times when construction workers and operating staff overlapped (72%), particularly in areas of relatively high social class. For several sites the excesses were similar to or greater than that near the nuclear site of Sellafield (67%), which is distinctive in its large workforce with many construction workers. Seascale, near Sellafield, with a ninefold increase had an unusually high proportion of residents in social class I. The only study parish of comparable social class also showed a significant excess, with a confidence interval that included the Seascale excess.

CONCLUSION

The findings support the infection hypothesis and reinforce the view that the excess of childhood leukaemia and non-Hodgkin's lymphoma near Sellafield has a similar explanation.

Wartime evacuation and mortality from childhood leukaemia in England and Wales in 1945-9

OBJECTIVE

To discover whether the wartime government evacuation of children from London and other population centres to rural districts was associated with any increase in childhood leukaemia.

DESIGN

Observational study of mortality from leukaemia among the childhood population of England and Wales in relation to the unique population movements during the second world war. The 476 rural districts of England and Wales were ranked according to the ratio of government evacuees (two thirds of them children) to local children in September 1941. The districts were divided into three categories, each with similar numbers of children in 1947 but with different ratios of evacuees to local children ("low," "intermediate," "high"). Mortality from childhood leukaemia was examined in these three rural categories in 1945-9. Urban areas were also examined according to their exposure to evacuees.

SETTING

Local authority areas of England and Wales.

SUBJECTS

Children aged under 15.

RESULTS

47% excess of leukaemia at ages 0-14 years occurred in 1945-9 in the rural "high" category for evacuees relative to the "low" category, with a significant trend across the three categories. There were increases in both the 0-4 and 5-14 year age groups, but these were larger in the older age group. Rates 25% lower than average occurred in rural areas with few evacuees.

CONCLUSION

These findings suggest that wartime evacuation increased the incidence of childhood leukaemia in rural areas and that other forms of population mixing may have contributed to the increases in past decades. Overall, they add to the appreciable evidence for an infective basis in childhood leukaemia.

Childhood leukaemia and non-Hodgkins lymphoma in young people living close to nuclear reprocessing sites

An increased incidence of leukaemia and NHL has been recorded close to Britain's two nuclear reprocessing sites at Sellafield in West Cumbria and Dounreay in Northern Scotland. Two hypotheses have been advanced to explain the excesses - paternal preconceptional irradiation (PPI) and an infective basis promoted by population mixing. Subsequent studies have provided no support for the PPI hypothesis and have also shown that this was derived from the analysis of a sub-group. In contrast, support for the population mixing hypothesis has come from a series of studies of residential and occupational situations. One of these showed that the excess near Dounreay was part of a wider increase in rural areas of Scotland affected by the unusual mixing associated with away-from-home work in the North Sea oil industry, being more marked in areas of relatively high social class. It is probable that the excess in Seascale is due to related demographic factors, this parish being extreme in its high social class, geographic isolation and its proximity to Sellafield, the largest, most changing industrial site in rural Britain.