Inverview with Constanze MacKenzie

Authors:
Mackenzie CA, Lockridge A, Keith M

Title:
Declining Sex Ratio in a First Nation Community

Source:
Environmental Health Perspectives 113: 1295-1298, 2005.

Summary:
The birth sex ratio (number of male births divided by the total of all births) is generally thought to be stable worldwide with an approximately equal number of males and females born over a given period of time. However, a trend of declining sex ratio has been reported by many industrialized countries in recent years. Although the sex of a human embryo is ultimately determined genetically, it has been suggested that hormonal milieu at the time of conception might favor fertilization by certain sperm. Alternatively, it has been proposed that exogenous chemicals may induce sex-specific mortality due to differences in utero development between male and female embryos, thus affecting the sex ratio of the surviving offspring. Previous studies which have examined the reproductive ability of populations exposed to endocrine disrupting chemicals, due to either close proximity to an industrial plant or through other sources such as food, have provided evidence to suggest that these chemicals have the potential to cause significant changes in the sex ratio of the community.

Mackenzie et al. assessed the proportion of male births over a 20 year period for the Aamjiwnaang First Nation community in Ontario, in response to concerns of the community regarding the observation of fewer male children born in recent years. The Aamjiwnaang reserve has a population of approximately 850 individuals, and is surrounded by several large petrochemical, polymer, and chemical industrial plants. This assessment was part of a larger community-based participatory research project, and included measurements of soil, sediment, and air sampling, as well as door-to-door health surveys and interviews. Live birth data was collected from the Department of Indian and Northern Affairs database and Northern Affairs Canada, for the period 1983-2003. The proportion of male births was determined for each year, and linear regression was used to examine the trend in the proportion of male births over time. The proportion of live male births was also calculated for 5 and 10 year intervals over the duration of the study period. This data was then compared to the expected proportion of males for Canada, as well as another First Nation community. The sex ratio for the control community was found not to be statistically different from the expected Canadian male proportion, and therefore the analyses were performed using the Canadian sex ratio as the expected value.

The results of the linear regression analysis demonstrated that the proportion of male births appeared to be stable for the period 1984-1992, but the period 1993-2003 showed a declining trend in the proportion of male births observed. When the data was analyzed by 5 year intervals the proportion of male live births ranged from 0.551 (1989-1993) to 0.348 (1999-2003). The proportion of male births was significantly different from the expected sex ratio (0.512) for the most recent 5-year interval (1999-2003). A statistically significant decrease in the proportion of male births was also observed when the data were categorized into 10-year intervals for the period 1994-2003. The observed ratio for this period was 0.412.

The data presented in this study indicates a significant ongoing decline in the proportion of male live births beginning in the early 1990s through to the end of the study period in 2003. Due to the proximity of the Aamjiwnaang reserve to several industrial plants and the finding of high concentrations of several contaminants with known effects on sex ratios in the soil and sediment on the Aamjiwnaang reserve, it is plausible that the declining trend in sex ratios may be partly due to the effects of exposure to endocrine disrupting chemicals. Evidence of altered wildlife reproduction in the area including the finding of fish with intersex gonads in the Great Lakes area close to the reserve, and reduced hatching success and altered development in turtles further strengthens this hypothesis. However, it must be considered that there are numerous biological and environmental factors which have the potential to influence the sex ratio of a community, including parental age, parity, birth order, infertility, maternal nutrition, stress, and selective reproductive practices. Information regarding these other potential contributing factors was not available for the Aamjiwnaang community, and therefore not included in this analysis. The authors of this study indicated that a community health survey is under way to gather information on other covariates which may be influencing the sex ratio of the community. It should also be considered that the database used to obtain the birth data may have included the births of band members who do not reside on reserve land. In order to further investigate the potential role of endocrine disrupting compounds on the sex ratio of this community, an additional assessment incorporating the biological plausibility of the timing of exposure, the potential effects of other chemical exposures, and a consideration of other potential biological and environmental influences is required.