Exposure to thimerosal, a mercury-containing preservative that is used in vaccines and immunoglobulin preparations, has been hypothesized to be associated with increased risk of autism spectrum disorder (ASD). This study was designed to examine relationships between prenatal and infant ethylmercury exposure from thimerosal-containing vaccines and/or immunoglobulin preparations and ASD and 2 ASD subcategories: autistic disorder (AD) and ASD with regression.
A case-control study was conducted in 3 managed care organizations (MCOs) of 256 children with ASD and 752 controls matched by birth year, gender, and MCO. ASD diagnoses were validated through standardized in-person evaluations. Exposure to thimerosal in vaccines and immunoglobulin preparations was determined from electronic immunization registries, medical charts, and parent interviews. Information on potential confounding factors was obtained from the interviews and medical charts. We used conditional logistic regression to assess associations between ASD, AD, and ASD with regression and exposure to ethylmercury during prenatal, birth-to-1 month, birth-to-7-month, and birth-to-20-month periods.
There were no findings of increased risk for any of the 3 ASD outcomes. The adjusted odds ratios (95% confidence intervals) for ASD associated with a 2-SD increase in ethylmercury exposure were 1.12 (0.83–1.51) for prenatal exposure, 0.88 (0.62–1.26) for exposure from birth to 1 month, 0.60 (0.36–0.99) for exposure from birth to 7 months, and 0.60 (0.32–0.97) for exposure from birth to 20 months.
In our study of MCO members, prenatal and early-life exposure to ethylmercury from thimerosal-containing vaccines and immunoglobulin preparations was not related to increased risk of ASDs.
Comments
Mercury poisoning: a diagnosis easier to bear
Dear Editor,
We read with great interest the article by Price et al.(1) on exposure to thimerosal, a ethyl-mercury containing preservative used in vaccines, and risk of autism spectrum disorder (ASD). Organic mercury products, such as methyl- mercury and ethyl-mercury, can have severe neurotoxic effects, as shown both in laboratory studies and in cases of accidental exposure in humans (Minamata disease). As a consequence, it has been raised the hypothesis that neurological disorders, including ASD, could depend on exposure to environmental or pharmacological mercury. The study from Price et al. showed that ethylmercury exposure from thimerosal-containing injection administered prenatally or during infancy was not related to increased risk of ASD. In fact, pathogenesis of ASD still remains poorly understood and etiologic hypothesis are often regarded with interest by parents, in the hope to improve the cure of their children. But the work of Price should remember the importance of not diverting the attention from consolidate care of ASD children while following uncertain interpretation of the disease.
One, among unfortunately too much examples, can be the case of a young girl, referred to our hospital at the age of two years because of developmental regression. The neurological examination showed opposite behaviour, non communicative speech limited to stereotyped babbling, limited interaction with the environment, stereotyped hand movements and mild signs of self injury. The chromosomal map and X-fragile test were negative and a MRI of the head failed to show any alterations. At the age of four, stereotypies were dominant, particularly of the hands; comprehension of verbal orders was absent; apraxic gait started to appear, as well as purposeless smiling. EEG showed paroxysmal discharges. Given the clinical features, the girl was discharged with the diagnosis of Rett syndrome. However, the girl's parents decided to seek other opinions, contacting different physicians also in the field of alternative medicine. One of these practitioners provided an alternative interpretation of the symptoms, diagnosing a neurotoxic effect due to heavy metals and vaccinations. However, mercury poisoning, described as Minamata disease, is characterised by quite different clinical features, including finger, eyelid and tongue tremors, gastroenteritis, stomatitis, lethargy and irritability. Nevertheless, hair and blood mercury levels in hairs and blood were measured. The mercury dosage in hair samples revealed high exposure to methylmercury in the girl and in her family (girl 9.8 mcg/g, father 7.0 mcg/g, mother 4.7 mcg/g, sister 4.1 mcg/g). These concentrations are typical of the “frequent fish consumers” according to WHO references(2). This was explained by the large amount of fish which the family consumed, since the girl’s father was employed in the local harbour. This theory was supported by the fact that the mercury concentration in the local sea was higher than average, inducing some physicians to suggest a possible etiologic link between Rett syndrome and Methylmercury poisoning(3). However, it is important to note that no confirmed cases of mercury poisoning were ever registered in this area. Anyway, on the basis of this new diagnosis, the girl started selenium therapy to decrease the mercury levels and was given melatonin with multivitamin products with the hope of reacquiring the lost skills. During the following 3 years contacts with the patient and her family were impossible. The family returned to the Pediatric Department of our hospital requesting a lumbar puncture and the measurement of mercury levels in liquor. Considering the clinical history and the previous clinical diagnosis of Rett syndrome and as well the doubtful scientific basis of the mercury poisoning hypothesis, the lumbar puncture was not performed. On the other hand, a molecular test for the diagnosis of Rett syndrome (not available at the first diagnosis) was performed, which showed the deletion of 41 bases pairs in the exon 3 of the gene MECP2. The diagnosis of Rett syndrome, clinically formulated 5 years before, was finally confirmed by molecular evidence. The genetic diagnosis allowed us to deny a major role of mercury poisoning in the disease.
Environmental factors such as vaccines or some foods, which are produced by human activity, are often perceived as non- natural, and thus are looked at as a potential cause of illness. Large epidemiological studies are very important to demonstrate the safety of these human interventions on public health because sometimes the weight of environmental factors could be overestimated. This is important especially in that cases where is possible to find out a definite diagnosis, allowing in this way parents and physicians to concentrate their efforts on the care of the child.
References:
1. Cristofer S. Price, William W. Thompson, Barbara Goodson, Eric S. Weintraub, Lisa A. Croen, Virginia L. Hinrichsen, Michael Marcy, Anne Robertson, Eileen Eriksen, Edwin Lewis, Pilar Bernal, David Shay, Robert L. Davis, and Frank DeStefano. Prenatal and Infant Exposure to Thimerosal From Vaccines and Immunoglobulins and Risk of Autism Pediatrics 2010; 126: 656-664
2. WHO-IPCS, Environmental Health criteria 101, Methylmercury, WHO, Geneva, 1990
3. Kobal AB, Miklavcic V, Byrne AR, Velickovic Perat M. Methylmercury exposure and Rett’s syndrome? In 8th International Child Neurology Congress, Ljubljana, Slovenia, 13-18 September 1998, Monduzzi; Bologna, Italy
Conflict of Interest:
None declared
Conclusions should be based on appropriate methodologies
While the Price study is a well-conducted careful investigation, this case-control design would not detect any true differences between the groups based on what most prominent researchers suspect to be true about ASD – that it is a disease that involves genetic susceptibility to environmental triggers. Epidemiological case-control studies that fail to include study variables about individual susceptibility are not sufficient to understand interactions with ubiquitous exposures.
Their study sample of children born between 1994 and 1999 were among a cohort of children who were exposed to thimerosal from routine childhood vaccines - a ubiquitous population exposure. It is doubtful that exposure to thimerosal would differ between any group of individuals during that time no matter what disease was being investigated.
The finding of no differences in exposure between cases and controls in the Price et al., study is not surprising and certainly does not exonerate thimerosal as a potential trigger in susceptible individuals. I would suspect that many ASD children differ on a genetic susceptibility in their innate ability to detoxify thimerosal and not their differential exposure to it. The design used in the Price et al study cannot possibly determine this distinction because susceptibility was not a study variable. Given a gene/environment interaction (G x E) model of disease, a design that looks only at environmental exposure is not appropriate and null results should not mislead scientists or policy makers into asserting the safety of thimerosal for all. A proper case-control design would test a G x E hypothesis – e.g. among genetically susceptible individuals, thimerosal exposure may influence the development of ASD.
I am disappointed that the conclusions put forth by the authors did not mention the important caveat that the case-control design they used, would not be sufficient to investigate the prevailing consensus of G x E in the development of ASD. My major concern is that the null results of this and other studies will translate into a potentially erroneous public health messages that thimersiol is safe for all, when in fact approximately 10% of the population have hypersentive reactions to thimerisol – which is one reason it was removed from many over-the-counter products in the first place. To date there has never been a proper biological study to determine the safety of thimerisol in the general population.
Conflict of Interest:
None declared
Thimerosal, Autism and Hormesis
Dear Sirs:
The recently released case control study by Price et al. (1) examined a possible association between autism and ethylmercury derived from the vaccine preservative thimerosal and surprisingly found fewer than expected autism case among subjects who received higher thimerosal doses. This apparent protective effect was a robust finding, reaching significance in all six cohorts receiving >100 µg of thimerosal, representing a reduction in autism risk of 40 to 78% in their covariate-adjusted model. Clearly the dose of thimerosal has a significant impact on the likelihood of developing autism, albeit a puzzling beneficial one, leading the authors to state that they “…are not aware of a biological mechanism that would lead to this result”. However, the findings of this study strongly suggest a hormetic response to thimerosal, in which low doses produce a beneficial response to a toxic substance, reflecting mobilization of adaptive mechanisms which protect against successive exposures. Indeed, the episodic nature of thimerosal administration is particularly well- suited to engendering a hormetic response, with sufficient elapsed time between doses for metabolic and epigenetic mechanisms to augment resistance.
A number of studies have reported hormetic responses following exposure to inorganic or organic forms of mercury (Helmcke et al., 2010; Calabrese, 2008; Calabrese, 2005; Toimela and Tahti, 2004; Prati et al., 2002; Brousseau et al., 2000; Fournier et al., 2000; Contrino et al., 2002; Zdolsek et al., 1994; Contrino et al., 1988; Nordlind and Henze, 1984; Dieter et al., 1983), and an earlier preliminary report similarly described a protective influence of thimerosal vs. autism (Jones, 2003).
Hormesis is a dose-response phenomenon characterized by low-dose stimulation and high-dose inhibition, commonly exhibited by toxic exposures. Many xenobiotics, including thimerosal, promote oxidative stress and cells possess a large number of adaptive mechanisms which augment antioxidant resources in response to an initial exposure, resulting in enhanced tolerance to the deleterious effects of subsequent exposures. Adaptation can involve short-term (e.g. metabolic shifts) or long-term mechanisms (e.g. epigenetic-mediated changes in gene expression). Hormetic responses underlie the oft-described benefit of pre- conditioning or post-conditioning stress (Calabrese et al., 2007), and they offer obvious survival advantages, especially in response to a challenging environment.
However, the capacity to mount an effective hormetic response is not necessarily equal across the population, and it importantly depends upon the relevant metabolic pathways, including their genetic underpinnings. Significant impairment of antioxidant and methylation metabolite levels is well-documented in autism (Adams et al., 2009; Al-Gadani et al., 2009; Pastural et al., 2009; Paºca et al., 2009; Vojdani et al., 2008; Geier and Geier 2006; James et al. 2006; James et al., 2004), along with a higher prevalence of risk-inducing single nucleotide polymorphisms (SNPs) in genes which support these pathways (Paºca et al., 2009; James et al. 2006). Thus exposure to a toxin such as ethylmercury might lead to a protective hormetic response in most individuals, but this response would be less robust or absent in other individuals, increasing their risk.
Notably, thimerosal is utilized as a preservative precisely because of its toxicity at high concentrations, and there is an obvious need to fully characterize its activity, including both positive and negative effects on neurodevelopment, across a complete range of doses. Such studies should also take genetic variations in into account, for example by utilizing animal strains known to harbor SNPs affecting the capacity for a hormetic response.
In any case, the positive findings of Price et al. may be more significant than the negative findings, and may offer an important clue as to the origin of autism.
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Conflict of Interest:
None declared
Can this study design produce a useful result?
The logic of this study is puzzling and after many years of the mercury/autism controversy will do nothing to quieten it. It is as if Sir Richard Doll had tried to lay the smoking controversy to rest by concluding that as not all smokers get lung cancer smoking does not cause lung cancer. This rather trite point will be evident to many people and it is surprising that a study itself many years in the pipeline should be open to such basic criticisms. To learn anything much we need an unexposed population. Most people will also assume that the alleged protective effect of mercury against autism reported in the study is an artefact of the design.
Conflict of Interest:
Autistic son