The international
journal Science of the Total Environment has just published
a compelling study from
the Republic of Korea, where autism
prevalence is high. The study identifies a strong relationship
between prenatal and early childhood exposure to mercury and
autistic behaviors in five-year-olds.
Lead author Jia Ryu and coauthors
acknowledge mercury’s potential for neurotoxicity straight away but
choose to characterize previous findings on the mercury-autism
relationship as “inconsistent.” They attribute the seeming lack of
consistency, in part, to methodological issues, especially flagging
problematic cross-sectional study designs that measure autistic
behaviors and mercury levels (in either blood or hair) at a single
point in time. To rectify these methodological weaknesses, Ryu and
coauthors report on data from a multi-region longitudinal study in
the Republic of Korea called the Mothers and Children’s
Environmental Health (MOCEH) study.
The ongoing MOCEH
study examines environmental exposures during pregnancy and
childhood and their effects on children’s growth and development. A
unique feature is that it includes five different blood samples:
maternal blood from early and late pregnancy; cord blood; and
samples from children at two and three years of age. In addition,
the study asks mothers to complete three follow-up surveys and—when
their child reaches age five—the 65-item Social Responsiveness Scale
(SRS), which assesses autistic behaviors.
Key Results
Ryu and colleagues present available
data for 458 (26%) of the 1,751 mother-child pairs originally
recruited into the MOCEH study. What are their key findings?
–First, the researchers report a
significant linear relationship between mercury exposure and
autistic behaviors (as indicated by a scaled score called an SRS
T-score). Strikingly, they find that with a doubling of blood
mercury levels at four time points (late pregnancy, cord blood,
and at two and three years of age), SRS T-scores are
significantly higher.
–Ryu et al. also look
specifically at SRS T-scores greater than or equal to 60. Sixty
and above is the accepted threshold for detecting “mild to
moderate” deficits of social behavior related to autism; scores
of 76 or more are in the “severe” range. In these analyses, the
same linear relationship holds for late pregnancy and birth
(i.e., cord blood). With a doubling of blood mercury levels at
these two time points, there is a 31% and 28% increase,
respectively, in the risk of an SRS T-score of 60 or more.
–Finally, the researchers
identify a stronger association between late-pregnancy
mercury exposure and autistic behaviors in five-year-old
boys versus five-year-old girls, perhaps due to mercury’s
endocrine-disrupting properties.
Limitations
Although the Korean study has
many methodological strengths, it displays some curious
omissions and contradictions. For example, the authors note,
almost in passing, that they measured total mercury “instead of
methylmercury,” but they discount any potential relevance of
ethylmercury from thimerosal-containing vaccines. In support of
this dismissive stance, they cite a 2014 meta-analysis that
failed to find any “material associations” between thimerosal
exposure and ASD or attention-deficit hyperactivity disorder
(ADHD). They do not mention that the authors of the
meta-analysis actually urged readers to interpret their results
with caution “since the number of epidemiological studies on
this issue [is] limited and still at an early stage.”
Ryu and coauthors also repeat
the canard that methylmercury is the only form of organic
mercury capable of inducing neurotoxic effects because of
ethylmercury’s shorter blood
half-life compared to methylmercury. This statement is
disingenuous in light of the well-documented fact that
methylmercury and ethylmercury share numerous mechanisms
of toxicity. Moreover, strong evidence from
infant primates shows that injected thimerosal has a much slower
half-life in the brain than in the blood, and infants exposed to
injected thimerosal accumulate harmful inorganic mercury (an end
product of thimerosal) in the brain to a far greater extent than
infants exposed to orally ingested methylmercury. Due to these
differences, blood mercury levels are not a suitable indicator
of injected thimerosal’s potentially adverse effects on the
brain. Finally, it is biologically plausible that simultaneous
exposure to both forms of mercury (ethyl and methyl) may result
in additive
and synergistic neurotoxic effects.
The Korean Context
Interestingly, fully a third of
Ryu’s study population (32.5%) had an SRS T-score of at least 60
(versus about 18% of U.S. children). In 2011, a South Korean study of
over 23,000 seven-to-twelve-year-old children made headlines by
reporting a national ASD prevalence of 2.64%—or 1 in 38
children. The Republic of Korea’s immunization
rates have been steadily climbing (along with autism rates)
since the 1990s, with nearly universal (98%-99%) coverage, for
example, for measles and DTP3 in one-year-olds. Is it simply a
coincidence that “neuro-psychiatric
conditions” represent the single largest cause of “years of
healthy life lost due to disability” (YLD) in the Republic of
Korea? As per the nation’s crowded immunization
schedule, Korean infants receive three doses of the
thimerosal-containing Hepavax-Gene® hepatitis B vaccine at
birth, one month, and six months. Analyses of Korean children’s
mercury levels have paid insufficient
attention to developmentally vulnerable infants’ exposure to
ethylmercury in the form of thimerosal. Surely, identifying and
lowering allsources of mercury exposure is a logical
next step to improve children’s health and quality of life.
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