Ebook Comparison of Blood and Brain Mercury Levels in Infant Monkeys Exposed to Methylmercury or Vaccines Containing Thimerosal
Public perception of the safety and efficacy of childhood vaccines has a direct impact on immunization rates (Biroscak et al. 2003, Thomas et al. 2004). The current debate linking the use of thimerosal in vaccines to autism and other developmental disorders (IOM 2001, 2004) has led many families to question whether the potential risks associated with early childhood immunizations may outweigh the benefits (Blaxill et al. 2004). Thimerosal is an effective preservative that has been used in the manufacturing of vaccines since the 1930s. Thimerosal is comprised of 49.6 % mercury by weight and breaks down in the body to ethylmercury and thiosalicylate (Tab and Parkin 2000).
Recent reports have indicated that some infants can receive ethylmercury (in the form of thimerosal) at or above the Environmental Protection Agency (EPA) guidelines for methylmercury (MeHg) exposure, depending on the exact vaccinations, schedule, and size of the infant (Ball et al. 2001). Clements et al. (2000) calculated that children receive 187.5 micrograms of ethylmercury from thimerosal containing vaccines given over the first 14 weeks of life. According to the authors, this amount approaches or, in some cases, exceeds the EPA guidelines for MeHg exposure during pregnancy (0.1 µg/kg/day). Other estimates (Halsey 1999) have indicated that the schedule could provide repeated doses of ethylmercury from approximately 5 to 20 µg/kg over the first 6 months of life. Studies in preterm infants indicate that blood levels of mercury following just one vaccination (hepatitis B) increase by over 10-fold to levels above the EPA guidelines (Stajich et al. 2000).
EPA guidelines for MeHg are based on several decades of studies of humans and animal models of developmental toxicity (Burbacher et al. 1990; National Research Council 2000). Since little data exist for ethylmercury, the use of the MeHg guidelines would seem appropriate if the two compounds have similar toxicokinetic profiles and neurodevelopmental effects. The results from the few studies that have provided a direct comparison of these two compounds have been summarized recently in a review article by Magos (2003), who concluded that:
- Mercury clears from the body faster after the administration of ethylmercury than after the administration of MeHg
- The brain-to-blood mercury concentration ratio established for MeHg will overestimate mercury in the brain after exposure to ethylmercury
- Because ethylmercury is decomposed faster than MeHg, the risk of brain damage is less for ethylmercury than for MeHg.
These conclusions are based on only a few studies, none of which included measurements of both blood and brain mercury levels in infant subjects.
The current study was initiated to provide a direct comparison of the blood and brain levels of mercury in infant nonhuman primates exposed orally to MeHg or via i.m. injections of vaccines containing thimerosal. Nonhuman primates have been used extensively in previous studies of MeHg toxicokinetics and developmental neurotoxicity (Stinson et al. 1989; Vahter et al. 1994, 1995; Burbacher et al. 1986, 1990; Gunderson et al. 1986, 1988; Rice and Gilbert 1982, 1990, 1995). The routes of administration (oral for MeHg and i.m. injection for thimerosal ontaining vaccines) were chosen to mimic the two routes of mercury exposure for humans. The dosages and schedule of administration of mercury were chosen to be comparable to the current immunization schedule for human newborns, taking into consideration the faster growth (approximately 4 to 1) of the macaque infant (Gunderson and Sackett 1984). The results of this study provide important new information regarding the comparative toxicokinetics of these two compounds in newborns and infants.
CONTENT
Abstract
Introduction
Methods
- Subjects
Mercury Dosing Schedule
Blood Draw Schedule
Sacrifice Schedule
Blood and Brain Hg Measurement
Kinetic Analysis
Results
- Infant Growth and Health Status
Oral Methylmercury Kinetics
Intramuscular Thimerosal Kinetics
Discussion
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