Thimerosal–Should I worry about mercury in vaccines for my dog or cat?

Many health concerns among pet owners reflect concerns regarding human health that have been extrapolated to the health of our pets. It is natural to assume, or at least suspect, that factors which influence human health also influence the health of our animal companions. This doesn’t always turn out to be true, but it often does. Given there is much more information and scientific evidence about health risk factors available in the human health field than in veterinary medicine, sometimes it makes sense to apply this information to health issues in our pets, though

We must always bear in mind the dangers of extrapolation between species with significant biological differences.


Concerns about the safety of vaccinations for children have been partly responsible for stimulating increased concern about the safety of veterinary vaccines. I have addressed this subject several times (e.g. 1, 2), but recently I was asked by another practitioner about a very specific vaccine safety issue—the risks, if any, of thimerosal.


Thimerosal is a preservative used in vaccines since the 1930s. Such preservatives which added to vaccines following several terrible incidents of children acquiring deadly infections from contaminated vaccines. It has been widely used since, and few health concerns have been raised until quite recently.


Following some of the first allegations that vaccines, in particular the MMR vaccine, might be associated with an increased risk of autism or other developmental disorders, the hypothesis was put forward that thimerosal might be responsible because it is a mercury compound, and other types of mercury compounds are known to have toxic effects on the nervous system.


Because of public concern about this, thimerosal was removed from most vaccines for children in the U.S. in 1999 pending additional research, a recommendation endorsed by the American Academy of Pediatrics. Subsequently, additional research and epidemiologic evidence following the removal of thimerosal from vaccines concluded quite definitively that there was no evidence of any increase in risk associated with this compound. The AAP withdrew their statement on removing thimerosal from vaccines in 2002 based on new evidence.


It is now established that vaccines are not the cause of autism in children, and there is no evidence that thimerosal is responsible for any significant health effects when used as a preservative in vaccines for humans (see also 3, 4, 5).


Thimerosal is present in some veterinary vaccines. I have not found any specific published research on the issue of whether this preservative has harmful effects when used as a preservative in vaccines for dogs or cats, apart from one study suggesting it can cause local irritation that might confuse the results of one kind of allergy test. The only study regarding thimerosal in vaccines looked at whether the thimerosal in vaccines influenced the mercury content of hair in dogs. This study found that thimerosal in vaccines did not increase the amount of mercury detected in the dogs’ hair, and that this mercury was primarily associated with eating foods containing fish.


I am not aware of any scientific evidence showing any significant health risks associated with thimerosal in veterinary vaccines. According to one toxicologist I contacted, there are no clinical studies in veterinary patients, but studies in laboratory animal research suggests no risks at the levels used in vaccines. This is consistent with the robust evidence in humans that thimerosal used in vaccines is not a health risk.


This has not stopped the more extreme voices in the anti-vaccine community from insinuating that thimerosal is a health risk for our pets despite the lack of evidence for this. There is at least one veterinary vaccine marketed as “thimerosal free,” and there is no harm in using such a vaccine. But there is also no reason to think this vaccine is any safer than those which do contain thimerosal, and there is no reason to expect your veterinarian to prefer it over other brands.


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53 Responses to Thimerosal–Should I worry about mercury in vaccines for my dog or cat?

  1. skeptvet says:

    1. That study is about the effect of thimerosal in a test tube on a specific cell type from mice. It has nothing to do with the concerns about negative health effects of vaccines containing thimerosal in dogs or humans.

    2. We actually know a lot about adverse vaccine reactions and why/how they occur. Hundreds of studies in multiple species evaluating different populations and vaccines with various methods have been published. We certainly don’t know everything, and we can’t predict or prevent every adverse reaction, but the idea that we haven’t studied the issue and know little about it is wildly inaccurate. There have been many studies on the role of dendritic cells in vaccine response, and dendritic cell-based vaccines are a promising approach in human cancer treatment, so the issue is again not that scientists have ignored the subject but that you are only aware of or focused on one paper and not familiar with the deep background science that explains why immunologists and scientists in this field don’t share your concern.

    The problem is not “We don’t know anything because no one is studying this.” The problem is that people who are worried about vaccines selectively focus on a narrow topic (such as thimerosal) or a few papers and don’t have the context or sufficient understanding of the entire field of immunology and vaccine science to understand why this isn’t a concern to experts in those fields. There is no sufficient, plausible basis for your concern that would justify conducting lengthy, challenging, and expensive studies to reassure you about this specific issue.

  2. Gotcha says:

    Check out the study on thimerosal and monkey brains.

    Make sure that you get the original study and not just the summarized study. It goes into detail of exactly how thimerosal is far more dangerous than organic mercury and explains exactly what happens to the brain causing neurological problems.

    Hell, you could even just read the FOIA Simpsonwood CDC transcripts where doctors debated thimerosal and found strong correlations.

    Bottom line is it’s far more dangerous than previously believed and that’s specifically because of fallacious hair follicle/urine samples which reveal nothing about mercury accumulation in the organs..most importantly, the brain…

  3. skeptvet says:

    The CDC still disagrees with you, as does the FDA and the WHO. Funny how you somehow managed to see what all these major public health agencies and their scientists missed, huh?

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