Vaccines

Animal Studies

Pharmacokinetics and Tissue Distribution of Thimerosal, Ethyl Mercury, and Methyl Mercury in Animals

The National Institute of Allergy and Infectious Diseases and the National Institute of Environmental Health Sciences cosponsored studies in infant monkeys to compare the tissue distribution, metabolism, and excretion of mercury after exposure to thimerosal and methyl mercury. In this study, one group of animals received a dose of thimerosal in combination with a dose of thimerosal-free infant vaccine by weekly injection beginning at birth and continuing for 3 weeks. The vaccines were included in the study to mimic infant exposure as closely as possible. A second group of animals received methyl mercury orally on a weekly basis beginning at birth and continuing for 3 weeks.

Blood levels of mercury were determined for each animal after each of the four exposures. After the fourth dose, animal tissues were analyzed to determine levels of mercury in target tissues, such as brain and kidney. Assays to determine mercury levels measure inorganic vs. organic mercury but do not distinguish whether the organic mercury is in the form of ethyl mercury from thimerosal or methyl mercury. This study was not designed to measure any type of damage due to mercury exposure.

Results from these studies indicate

• Mercury, in the form of methyl mercury (oral ingestion) and thimerosal (intramuscular injection with vaccines) were both readily absorbed and distributed into blood and brain.
• Total (organic plus inorganic) mercury was cleared from both blood and brain faster after thimerosal exposure than after methyl mercury exposure.
• Levels of total mercury measured in blood and in brain were lower after thimerosal exposure than after methyl mercury exposure.
• The proportion of brain mercury that was inorganic was higher in animals exposed to thimerosal compared with methyl mercury.
• The absolute amount of inorganic mercury in brain was higher in thimerosal exposed animals compared with methyl mercury; the significance of this is not known since inorganic mercury is brain is thought by many scientists to not cause damage.
• During weekly doses of methyl mercury, total mercury in blood continued to accumulate, while during weekly doses of thimerosal, there was little accumulation of total mercury in blood.

This study shows that methyl mercury is not a suitable reference for risk assessment from exposure to thimerosal.

References

These results were published in. Environmental Health Perspectives, Volume 113, Number 8, August 2005