The following abstract presents results of a study conducted by a contract laboratory for the National Toxicology Program. The findings may not have been peer reviewed and were not evaluated in accordance with the levels of evidence criteria established by NTP in March 2009. For more information, see the Explanation of Levels of Evidence for Developmental Toxicity. The findings and conclusions for this study should not be construed to represent the views of NTP or the U.S. Government.
The EPA Office of Solid Waste and Emergency Response (OSWER) nominated silver for developmental and reproductive toxicity testing, based upon potential exposure to metallic silver and silver compounds that may occur in the workplace, in the environment from releases of silver-containing products and waste streams, and through the chronic intentional ingestion of colloidal silver.
Based on the results of a screening study conducted in this laboratory in CD® rats, the present study was conducted using female Sprague-Dawley-derived (CD®) rats dosed by gavage with silver acetate in 1% aqueous methylcellulose (10, 30, or 100 mg/kg/day) or vehicle on gd 6 through 19. Silver acetate contains 64.6% silver by weight. Therefore the lowest dose (10 mg/kg/day as silver acetate) was equivalent to approximately 6.5 mg silver/kg/day or 1300 times the human reference dose (Rfd) of 5 mg/kg/day for argyria, a discoloration of the skin or mucosal surfaces through the deposition of silver in the tissues.
One animal was removed from the high dose group due to a misdirected dose, and another animal (confirmed pregnant) in the high-dose group was euthanized on gd 12 due to morbidity. All remaining females survived until scheduled sacrifice on gd 20. Pregnancy was confirmed in 21-25 females per group (i.e., 87.5-100.0% per group) after necropsy on gd 20.
Treatment-related clinical signs were few, noted primarily in the mid and high-dose groups and consisted of weight loss rooting after dosing, and piloerection.
Maternal body weight was comparable among groups, as was maternal body weight change. A significant (p less than 0.05) decreasing linear trend was noted for maternal body weight on gd 12, but there were no statistically significant differences between the control group and any silver acetate-treated group. Maternal body weight change corrected for gravid uterine weight, gravid uterine weight, and absolute and relative maternal liver weight were each unaffected by treatment with silver acetate.
Maternal absolute feed consumption did not exhibit any dose-related trends, but was significantly decreased at the mid dose of silver acetate, but not the low or high dose, on gd 12 to 15, 15 to 18, 6 to 20, and 0 to 20. Relative maternal feed consumption (mg/kg/day) still did not exhibit any dose-related trends, and was significantly decreased at the mid dose compared to the control group only on gd 12 to 15, and 0 to 20. Similarly, but to a lesser extent, absolute maternal water consumption was decreased at the mid dose on gd 12 to 15 and 15 to 18. Relative maternal water consumption (g/kg/day) exhibited no significant differences from the control group for any silver acetate-treated group.
There were no differences among groups for the number of ovarian corpora lutea/dam, number of implantation sites/litter, or percent preimplantation loss/litter. Postimplantation/loss (resorptions, late fetal deaths, or nonlive implants/litter), live litter size, and percent male fetuses/litter did not differ among groups. An increasing trend was observed for the percent litters with late fetal deaths. Average fetal body weight per litter (sexes combined) and average male fetal body weight per litter exhibited a significant decreasing trend, but no significant pairwise differences between the silver acetate-treated groups and the control group. No statistically significant effects were noted for average female fetal body weight.
No toxicologically relevant differences were observed in the incidences of fetal malformations or variations. The percent female fetuses with malformations per litter exhibited, a significant main effect for dose (ANOVA), but no significant trend or pairwise differences between any silver acetate-treated group and the control group. A single female fetus in the low-dose group had thirteen individual types of skeletal malformations in the vertebral column, ribs, and sternum. The observation of this extensive skeletal dismorphogenesis in a single fetus from the low dose group, but not in the mid and high dose groups, suggests that the malformations were not treatment related.
In summary, the maternal LOAEL for this study was considered to be 30 mg/kg/day silver acetate (19.4 mg silver/kg/day) based on clinical signs including weight loss. The maternal NOAEL was 10 mg/kg/day silver acetate (6.5 mg silver/kg/day), or 1300 times the current Rfd for argyria. The NOAEL for developmental toxicity for this study was 100 mg/kg/day silver acetate (64.6 mg silver/kg/day), based on the absence of any biologically or statistically significant developmental toxicity.