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

Case-control study of paternal occupational exposures and childhood bone tumours and soft-tissue sarcomas in Great Britain, 1962-2010

BACKGROUND

This nationwide study investigated associations between paternal occupational exposure and childhood bone tumours and soft- tissue sarcomas.

METHODS

The UK National Registry of Childhood Tumours provided cases of childhood sarcomas born and diagnosed in Great Britain, 1962-2010. Control births, unaffected by childhood cancer, were matched on sex, birth period and birth registration sub-district. Fathers' occupations were assigned to one or more of 33 exposure groups and coded for occupational social class.

RESULTS

We analysed 5,369 childhood sarcoma cases and 5380 controls. Total bone tumours, total soft-tissue sarcomas and the subgroups osteosarcoma, rhabdomyosarcoma and Ewing Sarcoma Family of Tumours (ESFT) were considered separately. Significant positive associations were seen between rhabdomyosarcoma and paternal exposure to EMFs (odds ratio = 1.67, CI = 1.22-2.28) and also for ESFT and textile dust (1.93, 1.01-3.63). There were putative protective effects on total bone tumours of paternal dermal exposure to hydrocarbons, metal, metal working or oil mists.

CONCLUSIONS

Despite the large size and freedom from bias of this study, our results should be interpreted with caution. Many significance tests were undertaken, and chance findings are to be expected. Nevertheless, our finding of associations between ESFT and paternal exposure to textile dust may support related suggestions in the literature.

High parental occupational social contact and risk of childhood hematopoietic, brain and bone cancers

BACKGROUND

The etiology of childhood cancer is largely unknown, though some research suggests an infectious origin of hematopoietic, central nervous system (CNS) and bone cancers.

METHODS

We examined parental occupational social contact as a proxy for exposure to infectious agents and risk of childhood cancer. This population-based case-control study utilized a linkage of four Danish data-registries, and included 3581 cases (<17 years, diagnosed 1973-2012) and 358,100 age-matched controls. We examined the risks of leukemia, lymphoma, CNS and bone cancer related to high occupational social contact from (1) conception to birth and (2) birth to diagnosis.

RESULTS

Acute lymphoblastic leukemia (ALL) and bone cancer were inversely associated with high maternal social contact from conception to birth (OR: 0.86, 95% CI: 0.67-1.10) and birth to diagnosis (OR: 0.54, 95% CI: 0.34-0.86). Children of fathers with high social contact from birth to diagnosis had an increased risk of bone cancers, particularly in rural areas (OR: 1.65, 95% CI: 1.03-2.63). Parental social contact was associated with increased risk of astrocytoma, with strongest associations found in first-born children (maternal: OR: 1.54, 95% CI: 1.02-2.32; paternal: OR: 1.82, 95% CI: 1.05-3.17).

CONCLUSION

Our results support the notion of a role of infections for some cancer types.

Case-control study of paternal occupational exposures and childhood lymphoma in Great Britain, 1962-2010

BACKGROUND

This nationwide study investigates associations between paternal occupational exposure and childhood lymphoma.

METHODS

The UK National Registry of Childhood Tumours provided cases of childhood lymphoma born and diagnosed in Great Britain 1962-2010. Control births, unaffected by childhood cancer, were matched on sex, birth period and birth registration sub-district. Fathers' occupations were assigned to one or more of 33 exposure groups and also coded for occupational social class.

RESULTS

We analysed 5033 childhood lymphoma cases and 4990 controls. Total lymphoma and the subgroups Hodgkin, Burkitt and non-Hodgkin lymphoma were considered separately. No one exposure was significantly associated with increased risk within all subgroups and for total lymphoma. However, exposure to "ceramics and glass" was significantly associated with increased risk of total lymphoma, Hodgkin and non-Hodgkin lymphoma. Paternal lead exposure was associated with Burkitt lymphoma and exposure to metal fumes was associated with Hodgkin lymphoma.

CONCLUSIONS

This study provides no support for previous suggestions of an association between childhood lymphoma and paternal occupational exposure to pesticides, solvents/hydrocarbons or infections potentially transmitted by father's social contacts. An association with exposure to "ceramics and glass" was noted for the two major lymphoma subtypes together comprising 80% of total lymphoma.

Parental Occupational Exposures and Risk of Childhood Acute Leukemia

Introduction:

Acute leukemia, accounting for 20% of all cancers diagnosed in individuals younger than 19 years old, is the most prevalent childhood malignancy. Among environmental risk factors, parental occupational exposures have attracted scientific interest as potential predisposing factors for childhood leukemia. The role of parental occupational exposure to social contacts, harmful chemicals, electromagnetic fields and ionizing radiation has been investigated with conflicting and inconsistent results.

Aim:

A case-control study aiming to assess the association between parental occupational exposures to social contacts, chemicals and electromagnetic fields and the risk of offspring acute leukemia.

Material and Methods:

108 children with acute leukemia and equal number of matched controls were included. Data on parental occupations before conception, during pregnancy, during breastfeeding and after birth, and on potential risk factors was recorded. Associations between parental exposure and risk of childhood leukemia were estimated.

Results:

Parental occupational exposure during the four periods of exposure was not associated with childhood leukemia. High birth weight and family history of cancer were associated with the development of childhood acute leukemia. A weak association of maternal medication use during pregnancy and leukemia risk emerged.

Conclusions:

Since the causative factors of childhood leukemia remain unknown, further investigation is mandatory for the reduction of disease burden.

Parental occupations and risk for Crohn's disease in children

BACKGROUND AND OBJECTIVES

Lacunae exist on the identity of specific environmental risk factors that associate with Crohn's disease (CD). We hypothesized that indirect exposures acquired via the parents' occupation may confer susceptibility.

METHODS

A case-control study based on children diagnosed with CD (prior to age 20) at a tertiary care gastroenterology clinic in Montreal, Canada was carried out. Population- and hospital-based controls without IBD were selected. Information on occupations held by the parents was acquired from interview. Jobs were coded using the Canadian National Occupational Classification for Statistics. Associations were examined using logistic regression accounting for potential confounders. Odds ratios (OR) and corresponding 95% confidence intervals (CI) were estimated.

RESULTS

A total of 466 cases and 335 controls were studied. The mean (±SD) age of the cases (12.4 ± 3.2) was slightly higher than controls (10.5 ± 4.9). Gender and ethnicity were equally distributed between the groups. Logistic regression analysis suggested that children whose fathers worked as retail salespersons/sales clerks [odds ratio (OR) 3.6, 95% confidence interval (CI) 1.2-11.1], and whose mothers worked as administrative secretaries (OR 3.2, 95% CI 1.6-6.4), were more likely to be at risk for CD. Mothers who worked as either early childhood educators (OR 2.3, 95% CI 0.85-6.2) or as clerks (OR 2.8, 95% CI 0.8-9.9) also appeared to confer risks, but these associations were statistically not significant.

CONCLUSION

Parental occupations related to 'social mixing' that can potentially enhance exposure to infectious agents, appear to confer higher risk for CD in children.

A case-control study of occupational contact levels in the childhood leukaemia cluster at Seascale, Cumbria, UK

OBJECTIVES

To investigate adult occupational contact levels and risk of childhood leukaemia and non-Hodgkin's lymphoma (LNHL) in Seascale, an association found in other situations of rural population mixing (PM).

DESIGN

Matched case-control study.

SETTING

Seascale, Cumbria, UK.

PARTICIPANTS

For each case of LNHL recorded in patients under age 25 years during 1950-2006, up to 20 matched controls were chosen and parental occupational details obtained; an exception was a single working young adult, whose own occupation (and that of controls) was used.

PRIMARY OUTCOME MEASURES

Contact levels of occupations were categorised as: low/medium (reference group), high or very high contact levels, as in previous studies, with provision for certain unusual occupations. In particular, specialist policemen responsible for security and access at the nearby Sellafield nuclear complex were allocated to the highest contact category, and those Sellafield employees who worked in controlled areas to the middle (high) category. Since of possible bias, unusual contact aspects noted in the main research and development (R&D) building were reserved for a supplementary analysis. ORs were calculated for the occupational contact levels.

RESULTS

Compared to the reference group, the social class adjusted ORs for the high and very high contact categories were 8.18 (95% CI 0.95 to 70.33) and 14.90 (1.20 to 184.90), respectively, with a significant trend across the categories (p value=0.024). In the supplementary analysis with R&D workers moved to the very high contact category, the OR for the latter became 29.68 (2.12 to 415.79), and the p value for trend, 0.011.

CONCLUSIONS

The Seascale LNHL excess was most marked among those young people linked with high occupational contact levels; it is therefore not an exception to the pattern of family infection shown by other PM-related excesses. The findings have implications for the choice of controls in certain types of virus study.

Case-control study of paternal occupation and social class with risk of childhood central nervous system tumours in Great Britain, 1962-2006

BACKGROUND

Paternal occupational exposures have been proposed as a risk factor for childhood central nervous system (CNS) tumours. This study investigates possible associations between paternal occupational exposure and childhood CNS tumours in Great Britain.

METHODS

The National Registry of Childhood Tumours provided all cases of childhood CNS tumours born and diagnosed in Great Britain from 1962 to 2006. Controls without cancer were matched on sex, period of birth and birth registration sub-district. Fathers' occupations were assigned to one or more of 33 exposure groups. A measure of social class was also derived from father's occupation at the time of the child's birth.

RESULTS

Of 11 119 cases of CNS tumours, 5 722 (51%) were astrocytomas or other gliomas, 2 286 (21%) were embryonal and 985 (9%) were ependymomas. There was an increased risk for CNS tumours overall with exposure to animals, odds ratio (OR) 1.40 (95% confidence intervals (CIs) 1.01, 1.94) and, after adjustment for occupational social class (OSC), with exposure to lead, OR 1.18 (1.01, 1.39). Exposure to metal-working oil mists was associated with reduced risk of CNS tumours, both before and after adjustment for OSC, OR 0.87 (0.75, 0.99).Risk of ependymomas was raised for exposure to solvents, OR 1.73 (1.02,2.92). For astrocytomas and other gliomas, risk was raised with high social contact, although this was only statistically significant before adjustment for OSC, OR 1.15 (1.01,1.31). Exposure to paints and metals appeared to reduce the risk of astrocytomas and embryonal tumours, respectively. However, as these results were the result of a number of statistical tests, it is possible they were generated by chance.Higher social class was a risk factor for all CNS tumours, OR 0.97 (0.95, 0.99). This was driven by increased risk for higher social classes within the major subtype astrocytoma, OR 0.95 (0.91, 0.98).

CONCLUSION

Our results provide little evidence that paternal occupation is a significant risk factor for childhood CNS tumours, either overall or for specific subtypes. However, these analyses suggest that OSC of the father may be associated with risk of some childhood CNS cancers.

Case-control study of paternal occupation and childhood leukaemia in Great Britain, 1962-2006

Background:

Paternal occupational exposures have been proposed as a risk factor for childhood leukaemia. This study investigates possible associations between paternal occupational exposure and childhood leukaemia in Great Britain.

Methods:

The National Registry of Childhood Tumours provided all cases of childhood leukaemia born and diagnosed in Great Britain between 1962 and 2006. Controls were matched on sex, period of birth and birth registration subdistrict. Fathers’ occupations were assigned to 1 or more of 33 exposure groups. Social class was derived from father’s occupation at the time of the child’s birth.

Results:

A total of 16 764 cases of childhood leukaemia were ascertained. One exposure group, paternal social contact, was associated with total childhood leukaemia (odds ratio 1.14, 1.05–1.23); this association remained significant when adjusted for social class. The subtypes lymphoid leukaemia (LL) and acute myeloid leukaemia showed increased risk with paternal exposure to social contact before adjustment for social class. Risk of other leukaemias was significantly increased by exposure to electromagnetic fields, persisting after adjustment for social class. For total leukaemia, the risks for exposure to lead and exhaust fumes were significantly <1. Occupationally derived social class was associated with risk of LL, with the risk being increased in the higher social classes.

Conclusion:

Our results showed some support for a positive association between childhood leukaemia risk and paternal occupation involving social contact. Additionally, LL risk increased with higher paternal occupational social class.

Environmental factors and childhood acute leukemias and lymphomas

This review considers recent studies regarding the role of environmental factors in the etiology of childhood leukemia and lymphoma. Potential environmental risk factors identified for childhood leukemia include exposure to magnetic fields of more than 0.4 micro Tessla, exposure to pesticides, solvents, benzene and other hydrocarbons, maternal alcohol consumption (but only for certain genotypes), contaminated drinking water, infections, and high birth weight. The finding of space-time clustering and seasonal variation also supports a role for infections. There is little evidence linking childhood leukemia with lifetime exposure to ionizing radiation although fetal exposures to X-rays are associated with increased risk. Breast-feeding, consumption of fresh fruit and vegetables and having allergies all appear to be protective. Burkitt lymphoma (BL) is confined to areas of the world where malaria is endemic, with the additional involvement of the Epstein-Barr virus (EBV) as a co-factor. Environmental risk factors suggested for other types of non-Hodgkin lymphoma (NHL) include exposure to ionizing radiation (both lifetime and antenatal), pesticides, and, in utero exposure to cigarette smoke, benzene and nitrogen dioxide (via the mother). Hodgkin lymphoma (HL) is especially associated with higher levels of socioeconomic deprivation, but breast-feeding seems to confer lower risk. This is consistent with an infection or immune-response mediated etiology for HL.

Infection and pediatric acute lymphoblastic leukemia

In this review, we provide an overview of recent findings from the Northern California Childhood Leukemia Study (NCCLS) on factors related to the immune system including child's vaccination history and measures of child's exposure to infectious agents, namely daycare attendance, infection during infancy, and parental social contact in the work place. We also provide suggestions for the next stages of studies.

Host, family and community proxies for infections potentially associated with leukaemia

Three hypotheses have proposed the involvement of infections in the aetiology of childhood leukaemia, suggesting either a specific leukaemogenic infection or a series of common infections that lead to a dysregulation of the immune system. Much of the evidence for the link with infections has been based on epidemiological observations, often using proxy measures of infection. Proxy measures include population mixing, parental occupation, age distribution of incidence, spatial and space-time clustering of cases, birth order and day care during infancy. This paper discusses the proxies used and examines to what extent a commonly used proxy measure, birth order, is a fair representation of either specific infections or general infectious load. It is clear that although leukaemia, and other diseases, may be linked with infections, one needs to (1) measure specific and general infections with more accuracy and (2) understand how proxy measures relate to real infections in the population.

What do epidemiologists mean by 'population mixing'?

There is growing evidence that some chronic diseases are caused, or promoted, by infectious disease. 'Population mixing' has been used as a proxy for the range and dose of infectious agents circulating in a community. Given the speculation over the role of population mixing in many chronic diseases, we review the various methods used for measuring population mixing, and provide a classification of these. We recommend that authors fulfill two criteria in publications: measures are demonstrably associated with the putative risk factors for which population-mixing is acting as a proxy and fundamental characteristics of the chosen measures are clearly defined.

Parental social contact in the work place and the risk of childhood acute lymphoblastic leukaemia

To study the possible relation between parental social contact through occupation, a marker for a child's risk of infection, and childhood acute lymphoblastic leukaemia (ALL), the parents of 294 children with ALL aged 0-14.9 years and 376 matched controls were interviewed about their jobs after their child's birth up to the age of 3 years. Job titles were assigned to a level of social contact, and an index of occupational social contact months was created using the level and the job duration. Positive interactions between this index and rural residence associated with an increased risk of childhood ALL and common ALL (c-ALL) were observed (interaction P-value=0.02 for both, using tertiles of contact months; interaction P-value=0.05 and 0.02 for ALL and c-ALL, respectively, using continuous contact months); such findings were not observed when job durations were ignored. Our data suggest that duration of parental occupation may be important when examining the association between parental social contact in the workplace and childhood leukaemia.

Childhood Cancer and Social Contact: the Role of Paternal Occupation (United Kingdom)*

OBJECTIVE

To investigate the relationship between childhood cancer (particularly leukaemia) and paternal occupational social contact (a proxy for potential exposure to infections) using the UK Childhood Cancer Study.

METHODS

Using a national population-based case-control study, self-reported occupational data from fathers of 3596 children diagnosed with cancer between 1991 and 1996 under 15 years of age (cases) and fathers of 7011 children without cancer (controls) were analysed. Associations were assessed using odds ratios (OR) calculated by time of exposure (birth, diagnosis), diagnostic group (all cancers, leukaemia, acute lymphoblastic leukaemia (ALL), central nervous system tumours, other cancers), level of occupational social contact (high, medium, low), urban-rural status at diagnosis and occupational title.

RESULTS

From 371 occupations, 75 (20%) were classified as having high levels of occupational social contact, 31 (8%) as medium and the remaining 265 (71%) as low. No associations were apparent for fathers' occupational social contact overall, for any time period or diagnostic group. OR for leukaemia and high levels of paternal occupational social contact at birth and diagnosis being 1.02 (95% confidence interval (CI) = 0.88-1.18) and 0.91 (95% CI = 0.79-1.06), respectively. Analyses by urban-rural status at diagnosis and by occupational title revealed no notable associations.

CONCLUSIONS

The analyses of paternal occupational histories do not support the suggested association between high levels of paternal occupational social contact and an increased risk of childhood leukaemia. However, the role of participation bias should not be discounted.

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.

A case-control study of parental occupation, leukemia, and brain tumors in an industrial city in Taiwan

We conducted a case-control study in an industrial city in Taiwan to determine whether parents of newly diagnosed patients who were younger than 30 years old with leukemia or brain tumors or the patients themselves were more likely to have been employed in certain occupations or industries. Job histories were collected for parents (and for subjects if they worked) on 103 newly diagnosed cases of leukemia, 74 newly diagnosed cases of brain tumors, and 417 controls matched for age and sex. All jobs since the age of 16 that the subjects held for more than 6 months, a total of approximately 4,000 jobs, were coded for occupation and industry according the standard four-digit system used in Taiwan. Matched-pair analyses were performed comparing cases and controls among all jobs held by subjects and both parents using four-digit occupation and industry codes. Separate analyses also were performed for parental jobs held during the preconception, perinatal, and postnatal periods. Odds ratios (ORs) were adjusted for subject smoking, parental smoking, and exposure to medical radiation. Certain industry and occupation four-digit codes were significantly associated with increased odds ratios of childhood tumors. Including work during any or all periods, leukemias were more common in children of fathers who had worked (1) as wood treaters (adjusted OR 16.03, 95% confidence interval CI = 1.77-145.5), and (2) as building finishers and related trades workers (adjusted OR 4.08, 95% CI = 1.12-14.8), whereas brain tumors were more common in children of mothers who had worked (1) in electronic parts and components manufacturing (adjusted OR 13.78, 95% CI = 1.47-129.0) and 2) as textile and garment workers (adjusted OR 7.25, 95% CI = 1.42-37.0), as well as in subjects who had worked with certain electronic parts and components (adjusted OR 28.67, 95% CI = 2.88-285.6). Leukemias were more common in children of fathers who had worked in the preconception period (1) as wood treaters (adjusted OR 12.17, 95% CI = 1.36-109.2), (2) as building finishers and related trades workers (adjusted OR 4.08, 95% CI = 1.12-14.8), (3) as electronic equipment assemblers (adjusted OR 4.56, 95% CI = I 1.05-19.9), and (4) as certain other assemblers (adjusted OR 10.24, 95% CI = 1.02-102.6). In addition, leukemias were more common in children of fathers who had worked in the perinatal period (1) as wood treaters (adjusted OR 13.08, 95% CI = 1.36-125.5) and (2) as building finishers and related trades workers (adjusted OR 4.51, 95% CI = 1.04-19.6). Brain tumors were more common in children of mothers who had worked in the preconception period (1) in electronic parts and components manufacturing (adjusted OR 11.81, 95% CI = 1.20-116.3), and (2) as textile and garment workers (adjusted OR 7.25, 95% CI = 1.18-31.0).

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.

Fathers’ Occupational Contacts and Risk of Childhood Leukemia and Non-Hodgkin Lymphoma

BACKGROUND

There is evidence to suggest that childhood leukemia and non-Hodgkin lymphoma have an infective etiology. We investigated the risk of childhood leukemia and non-Hodgkin lymphoma in relation to paternal occupational contact with other individuals, a surrogate for potential exposure to infection.

METHODS

We carried out a case-control study using 792 cases from the Northern Region Young Persons' Malignant Disease Registry, United Kingdom, 1968-1997, and 2 large population-based control groups. Paternal occupations at birth were classified as having standard, high, or very high levels of contact. Conditional logistic regression was used in the analysis.

RESULTS

There was an increased risk of childhood leukemia and non-Hodgkin lymphoma in those children whose fathers' occupational contacts were high or very high compared with standard (odds ratio = 1.3; 95% confidence interval = 1.0-1.5). The excess risk in the very high group was most pronounced for 245 cases of acute lymphoblastic leukemia aged 2 to 5 years at diagnosis (1.5; 1.1-2.1). The risk with paternal occupational contacts was greater in rural areas, although it was also present in urban areas. The risks of leukemia and non-Hodgkin lymphoma were also higher among the offspring of men employed as policemen, sales representatives, or teachers.

CONCLUSIONS

Our findings are consistent with the hypothesis of an infective etiology for childhood leukemia and non-Hodgkin lymphoma, and they add to the evidence that infections could be transmitted to children by adults.

Parental occupation at periconception: findings from the United Kingdom Childhood Cancer Study

AIMS

To study the risk of childhood cancer in relation to parental occupation and related exposures.

METHODS

Self reported occupational data from mothers and fathers of 3838 children with cancer and 7629 control children were analysed. Odds ratios were calculated for 31 "occupational groups" by parent, diagnostic group (leukaemia, acute lymphoblastic leukaemia (ALL), central nervous system tumours, and other cancers) and time of exposure (periconception, birth, and diagnosis).

RESULTS

Findings did not support the hypothesis that occupational exposure of fathers to ionising radiation increases the risk of childhood cancer in their offspring. Specific examination of periconceptual chemical exposures showed small but statistically significant increased risks for leukaemia and ALL among children whose fathers were exposed to exhaust fumes, driving, and/or inhaled particulate hydrocarbons. In the remaining analyses, a fourfold increase in the risk of other cancers was observed among the children of fathers working with leather but based on small numbers. Both maternal and paternal exposure to textile dust was related to an increased risk of other cancers.

CONCLUSION

Results failed to produce any strong evidence to link parental occupational exposures with an increased risk of childhood cancer. No relation was found for paternal periconceptual exposure to ionising radiation. The consistency of the associations observed between childhood leukaemia and paternal occupational exposure to exhaust fumes, driving, and/or inhaled particulate hydrocarbons at periconception suggest a small risk for vehicle related exhaust. However, other explanations cannot be excluded and further research into the nature of the associations is required.

Population mixing, childhood leukaemia, CNS tumours and other childhood cancers in Yorkshire

We tested the hypothesis that variation in population mixing attributable to the diversity of migrants moving to an area is associated with the incidence of childhood leukaemia and other childhood cancers. An ecological analysis was performed on 954 children (<15 years) diagnosed with a malignancy between 1986 and 1996 in 532 electoral wards in Yorkshire, UK. Incidence rate ratios (IRR) were calculated for all childhood leukaemias (n=325), acute lymphoblastic leukaemia (ALL) (n=248), central nervous system (CNS) tumours (n=236) and other solid tumours (n=393) Incidence of all childhood leukaemias was significantly lower in areas of high (top decile) population mixing (IRR 0.72, 95% Confidence Interval (CI) 0.54-0.97) and higher in areas of low (bottom decile) population mixing (IRR 1.56, 95% CI 0.73-3.34), but similar patterns of incidence were not observed for central nervous system or other solid tumours. Population mixing may be a proxy for the range of infections circulating in a community and these results are consistent with the hypothesis that greater exposure to infections reduces the risk of developing childhood leukaemia by conferring efficient modulation of the immune system.

A case-control study of childhood leukaemia and paternal occupational contact level in rural Sweden

In a national case-control study in Sweden, we investigated whether in rural areas (where susceptible individuals are more prevalent than in urban areas) leukaemia risk was higher among the young children of fathers with many work contacts, as the infective hypothesis has predicted. A total of 1935 cases diagnosed in 1958-1998 together with 7736 age-matched (within 1 year) population controls (of whom 970 and 3880 respectively were aged 0-4) were linked to paternal occupational details as recorded in the census closest to the year of birth. Applying the two classifications of occupational contact level used in a study of rural Scotland, the odds ratios for children aged 0-4 years in the highest contact category (which includes teachers) in the most rural Swedish counties were 3.47 (95% CI 1.54, 7.85) and 1.59 (1.07, 2.38) respectively, relative to the medium and low (reference) category; no such excess was found in urban or intermediate counties. There was also a significant positive trend at ages 0-4 in the rural counties across the three levels of increasing occupational contact (P for trend 0.02 and 0.03, respectively), but again not in the urban or intermediate counties. No such effect or trend was found at ages 5-14 in any of the three county groupings. The findings confirm those of a recent study in rural Scotland, and also suggest that unusual population mixing (as occurred in Scotland as a result of the North Sea oil industry) is not a necessary requirement for the effect, since comparable mixing has not been a feature of rural Sweden.

Paternal occupational contact level and childhood leukaemia in rural Scotland: a case-control study

In a national Scottish study of 809 cases of leukaemia and non-Hodgkins lymphoma diagnosed in 1950-89 among children aged 0-4 years who were born in Scotland, together with 2363 matched population controls, we investigated one aspect of the infective hypothesis. This concerns whether in rural areas (where the prevalence of susceptible individuals is likely to be higher) the risk is greater among the young children of men whose work involves contacts with many different people, particularly children, as noted in certain childhood infections. A positive trend was found in rural areas across 3 levels of increasing paternal occupational contact (as recorded at birth) by each of 2 previously defined classifications; no such effect was found in urban areas. The rural trend was more marked in that part of the study period with greater population mixing, but the difference from the period with less mixing was not itself significant, leaving open whether these rural findings reflect the extreme isolation of much of rural Scotland, or the effects in such areas of a degree of population mixing. In marked contrast, among the 850 cases and 2492 controls aged 5-14, those in rural areas in the higher population mixing period showed a significantly decreasing trend with increasing paternal occupational contact level. This would be consistent with immunity produced either by earlier infection at ages 0-4 years, or directly by low doses of the infective agent that were largely immunizing at these older ages. The findings overall provide further support for infection underlying childhood leukaemia and for the role of adults.

Childhood Type 1 diabetes mellitus and parental occupations involving social mixing and infectious contacts: two population-based case-control studies

AIMS

To test the hypothesis that increased exposure to infections, through parental jobs involving high levels of social mixing, reduces the risk of childhood Type 1 diabetes mellitus.

METHODS

Two population-based case-control studies of children diagnosed with Type 1 diabetes mellitus from Yorkshire (0-15 years) and Northern Ireland (0-14 years) included 220 and 189 cases and 433 and 465 controls, respectively. Parental occupations were coded using a standard occupational classification. Each job was allocated to high, medium or low levels of social mixing according to a predefined categorization. Odds ratios (OR) for the risk of childhood Type 1 diabetes were estimated for parental social mixing by age at diagnosis.

RESULTS

Childhood Type 1 diabetes mellitus was not associated with high levels of parental occupational social mixing (Yorkshire - mothers: OR 1.07, 95% confidence interval (CI) 0.76-1.50; fathers: 1.15, 0.75-1.76; Northern Ireland - heads of household, usually the father: 0.78, 0.49-1.25). A larger proportion of mothers (39%) compared to fathers (18% Yorkshire, 17% Northern Ireland) had jobs involving high levels of social mixing. Mothers with high social mixing jobs conferred a nonsignificant reduced risk of Type 1 diabetes among children diagnosed under 5 years of age (0.58, 0.24-1.38) compared to those diagnosed at age 5 years and above (1.14, 0.77-1.69).

CONCLUSIONS

No association between parental occupational social mixing and the risk of childhood Type 1 diabetes mellitus was detected for all ages combined. Mothers were more likely to have jobs involving high levels of social mixing than fathers. The possible protective effect of maternal high occupational social mixing on children diagnosed below 5 years of age merits further investigation.