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. nhokkanen says:

    Paragraph 4 seems to imply that MMR vaccines contain Thimerosal; they do not.

    Headlines about a 2005 NIEHS primate study by Burbacher et al. announced that mercury left the blood quickly, sidestepping the more ominous implication — because the mercury quickly lodged in the animals’ brain tissue.

    Questioning the sensibility of injecting any form or amount of neurotoxic mercury into animals or humans does not make one “anti-vaccine.” Rather, such questioning indicates one’s willingness to reexamine all scientific beliefs, no matter how dearly held or whomever profits, should new information become available that renders status quo less certain.

  2. skeptvet says:

    You are correct that MMR used to contain thimerosal. It was removed due to public concern, and it has not been put back. Some other vaccines do still contain thimerosal. The important point was that the evidence clearly shows no harm was done by thimerosal when it was in the MMR vaccine, and that since it has been removed there has been no decline in the diseases that were claimed to be caused by thimerosal.

    Questioning the sensibility of injecting any form or amount of neurotoxic mercury into animals or humans does not make one “anti-vaccine.”

    The problem here is with your use of the word “neurotoxic,” which implies that thimerosal invaccines is the cause of neurologic disease. This turns out not to be true. Thimerosal can, of course, be toxic at sufficient doses, but then so can water. It is not toxic at the dose present in vaccines. And mercury can be neurotoxic, but ethyl mercury and methyl mercury are different compounds, and a large part of the unnecessary fear of thimerosal stems from not understanding the biologically relevant difference.

    It was, at one time, perfectly reasonable to ask whether thimerosal in vaccines was safe, which is why the AAP recommended its removal from vaccines. It is now clearly demonstrated that it is safe, which is why that recommendation was later withdrawn. Continuing to claim that thimerosal in vaccines is unsafe is, then, a pretty reliable indicator of an anti-vaccine agenda since the question has already been answered from a scientific point of view.

  3. Beccy Higman says:

    It is important to understand the effect of the precise molecule rather than any given element, after all we need salt, but ingesting either sodium or chlorine in their elemental forms would be harmful; assuming you could get either past your mouth and into your digestive system. And there are plenty of examples of more complex molecules where one form is biologically essential whereas it’s mirror image is damaging or even lethal.

  4. R says:

    The thimerosal free rabies vaccine comes in 1 year and 3 year, so if I know the dog has to get it again in 1 year, then every 3 years, can he get the 1 year for the first shot?

    IMRAB 3 is recommended for the vaccination of healthy cats, dogs, ferrets, horses, cattle and sheep against disease caused by rabies virus. Three-year duration of immunity in dogs, cats and sheep
    One-year duration of immunity in horses, cattle and ferrets

    IMRAB 1
    IMRAB 1 is recommended for the vaccination of healthy dogs and cats against disease caused by rabies virus. One-year duration of immunity in dogs and cats

    Thanks in advance.

  5. R says:

    Okay, I spoke to my vet today, he told me the thimerosal free rabies vaccine was really more beneficial for cats, due to that injection site sarcoma they sometimes get.
    He said it wouldn’t make a difference for dogs.

  6. skeptvet says:

    I’m afraid your vet is giving you merely his opinion, and it is not an opinion based on scientific evidence. It is true that cats are apparently more prone to vaccine-associated sarcomas than dogs, though the incidence is still very low (1 out of every 5,000-10,000 vaccinations). However, there is no evidence at all that thimerosal has anything to do with this. There was some evidence that aluminum adjuvant might have been associated with the occurrence of these sarcomas, and most of us now use adjuvant-free vaccines such as the Merial Purevax, but even this hypothesis is tentative, and it is not yet entirely clear if the change in vaccine type will have any effect on the rate of occurrence of these tumors. So again, there is no reason to think a thimerosal-free vaccine is any safer than any other vaccine.

  7. R says:

    Yes, I think that’s what my vet meant (more or less). I didn’t mean to misinterpret what he said. I will ask if he uses Merial Purevax.

  8. R says:

    Btw: If you sold t-shirts here, I would by one! I really appreciate this site.

  9. R says:

    Buy, not by :$

  10. cia parker says:

    The MMR never had thimerosal: the mercury would have killed the live viruses in the vaccine. The MMR is dangerous for other reasons, the live viruses are dangerous, and the vaccine often causes vaccine encephalitis and many types of subsequent damage. The killed virus vaccines used to contain mercury as a preservative, and still have dangerous levels of “trace” amounts.

  11. skeptvet says:

    As for the mercury issue, there is no evidence it was ever a health risk in any vaccine, there have been not reductions is potential health problems since it was removed from routine childhood vaccines, and you need to provide actual evidence that any “trace” amounts present a health risk.

    As for encephalitis, “often causes” is a lie. Per the FDA, which is a far more reliable source than you are:

    Encephalitis has been reported approximately once for every 3 million doses of MMR vaccine. Post-marketing surveillance of more than 400 million doses distributed worldwide (1978 to 2003) indicates that encephalitis is rarely reported after MMR vaccination. In no case has it been shown conclusively that encephalitis was caused by a vaccine virus infection of the central nervous system.”

  12. cia parker says:

    Vaccines can cause encephalitic brain damage just from the reaction of the immune system to the perceived danger of the vaccine suddenly showing up in the blood, having bypassed the body’s filtering system. However, mercury in vaccines can cause vaccine encephalitis all by itself, and several generations of children have been disabled by it. I reacted to the mercury in a tetanus booster many years ago by having both arms paralyzed the same day, and was later diagnosed by MRI with multiple sclerosis, all the symptoms of which are identical to those of mercury poisoning. My newborn reacted to the mercury in the hep-B vaccine given at birth without permission, with vaccine encephalitis and was later diagnosed with autism. A mercury-containing flu vaccine paralyzed my father for the last three years of his life. Some people have a genetic inability to excrete the vaccine mercury as quickly as others do, and in us it is stored in the brain, bones, or internal organs for life, and a fever can pull it out of the bones and set off new symptoms.

    The books The Age of Autism and Evidence of Harm show how mercury in vaccines causes autism and many other severe conditions, with many scientific and epidemiological studies cited. Books by the holistic veterinarians Dr. Richard Pitcairn, Dr. Marty Goldstein, and Dr. Don Hamilton give the scientific evidence and many examples from their practices, that mercury and aluminum (etc.) in pet vaccines, as well as in human, often cause serious and even deadly autoimmune disease and cancer.

  13. skeptvet says:

    Again, you make lots of claims here without any evidence to back them up. They are claims that are contradicted by actual scientific evidence. You do your cause no good by them citing the likes of Richard Pitcairn, whose faith in the religion of homeopathy eliminates any credibility and who himself presents no scientific evidence to support claims about vaccine but only his wildly implausible opinions and biased anecdotes. The purported link between vaccines and autism has been conclusively disproven and rejected by every legitimate scientific and public health organization and mainstream autism research and advocacy groups. There simply is no truth to the claims you make, and without real evidence to support them there is no reason for anyone to accept them.

  14. Liz says:

    Thank you, Cia Parker. Anecedotal evidence is too swiftly rejected by medical and scientific communities heavily dependent on pharmaceutical revenues and attentions. Besides anecdotal evidence which puts the CDC’s one-in-a-million claim to shame, there are plenty of doctors and scientists, with legitimate credentials and research to back them up, who are raising warning flags about vaccine safety. We would do well to power past the cognitive dissonance gut-reaction and learn truth.

    It’s interesting because you ask for evidence and when I attempted to post this comment with links to all these doctors’ videos on YouTube, your site said my comment looked “spammy” and to try again. You want evidence, but don’t allow it? Odd. Anyway, I came here searching for information about pet vaccinations after seeing a recent news story about dogs, vaccinations, and autoimmune disorders (let’s see if your page will let me link it…nope, no luck, took this one out first). I was disappointed to find your site so quick to adopt the industry line of “vaccines are safe and effective, vaccines don’t cause autism, vaccines don’t contain thimerosal, everything’s fine, folks, just move along.”

    Here is a list of Doctors who are against vaccine mandates and promote caution in their administration. A few are listed twice because the ones at the top had links with them before your site disallowed that.
    1. Dr. Nancy Banks
    2. Dr. Russell Blaylock
    3. Dr. Shiv Chopra
    4. Dr. Sherri Tenpenny
    5. Dr. Suzanne Humphries
    6. Dr. Larry Palevsky
    7. Dr. Toni Bark
    8. Dr. Andrew Wakefield
    9. Dr. Meryl Nass
    10. Dr. Ghislaine Lanctot
    11. Dr. Robert Rowen
    12. Dr. David Ayoub
    13. Dr. Boyd Haley PhD
    14. Dr. Rashid Buttar
    15. Dr. Roby Mitchell
    16. Dr. Ken Stoller
    17. Dr. Mayer Eisenstein
    18. Dr. Frank Engley, PhD
    19. Dr. David Davis
    20. Dr Tetyana Obukhanych
    21. Dr. Harold E Buttram
    22. Dr. Kelly Brogan
    23. Dr. RC Tent
    24. Dr. Rebecca Carley
    25.. Dr. Andrew Moulden
    26 . Dr. Jack Wolfson
    27. Dr. Michael Elice
    28. Dr. Terry Wahls
    29. Dr. Stephanie Seneff
    30. Dr. Paul Thomas
    Doctors and Scientists with Concerns About Vaccine safety
    Shizuo Akira, MD, PhD
    David Amaral, PhD, MIND Institute, UC-Davis
    François-Jérôme Authier, Professor, PhD
    David Ayoub, MD, Radiologist
    Anne-Catherine Bachoud-Levia, PhD
    Toni Bark, MD
    David S. Baskin, PhD
    Denis Bedoret, PhD
    Russell Blaylock, MD, CCN, former clinical assistant professor of neurosurgery at the University of Mississippi Medical Center in Jackson, MS. and is currently a visiting professor of biology at Belhaven University, Jackson, MI
    T. Bobrowicz, PhD
    Kenneth Bock, MD
    Marie-Françoise Boissea, PhD
    Subbarao Bondada, PhD
    Jeff Bradstreet, MD
    Pierre Brugierese, PhD
    Julie Buckley, MD
    Thomas Burbacher, MD
    Fabrice Bureau, PhD
    Rashid Buttar, DO, FAAPM, FACAM, FAAIM
    Stephanie F. Cave, MS, MD, FAAFP
    E. Cernichiari, PhD
    Pierre Cesaroa, PhD
    Lakshman Chelvarajan
    T. Chen, PhD
    Xavier Chevalierf, PhD
    Shiv Chopra, MSc, PhD
    Stephanie Christner, DO
    T. Clarkson, PhD
    John Barthelow Classen, MD
    Cevayir Coban, PhD
    Maryline Couettea
    Andy Cutler, PhD (research chemist)
    Jeffrey Dach, MD
    Josep Dalmau, MD, PhD
    Vicky DeBold, PhD, RN
    Jamie Deckoff-Jones, MD
    Christophe J Desmet, PhD
    Mary Catherine DeSoto, PhD
    Richard Deth, PhD
    J.G. Dórea, PhD
    Peter Doshi, PhD Johns Hopkins School of Medicine
    M. Duszczyk, PhD
    Steven Edelson, MD, Director of the Autism Research Institute in San Diego
    Mayer Eisenstein, MD
    (The late) Dr. Frank Engley
    Håkan Eriksson, PhD
    Christopher Exley, PhD
    Carl Feinstein, MD
    Peter Fletcher, PhD, former Chief Scientific Officer, at the UK Department of Health
    Lisa Freund, PhD
    Paula A. Garay, PhD
    Robert F. Garry, PhD
    Thomas V. Getchell, PhD
    Romain K. Gherardi, Professor, head of the department of Histology, Henri Mondor hospital, Paris, Neuropathologic and Clinical activities at the Neuromuscular Disease Reference Center, and is coordinator of the Department of Neurosciences INSERM
    Beatrice Golomb, PhD, MD
    Jay Gordon, MD
    K.S. Grant, PhD
    John Green, MD
    Boyd Haley, PhD
    Richard Halvorsen, MD
    Diane Harper, MD, MPH, MS
    (The late) Bernadine Healy, MD
    Martha Herbert, MD, PhD, Professor of neurology at Harvard Medical
    Laura Hewitson, PhD
    Robert T. Hitlan, PhD
    Amy Holmes, MD
    Brian Hooker, PhD
    Mady Hornig, PhD
    Suzanne Humphries, MD
    Philip Incao, MD
    Ken J Ishii, PhD
    Emmanuel Ittie, PhD
    Dr. Jill James, PhD
    Bryan Jepson, MD
    Jerry Kartzinel, MD
    Matthew S. Kayser, MD
    Marcel Kinsbourne, PhD
    Kouji Kobiyama, PhD
    Sheldon B. Korones, MD
    Arthur Krigsman, MD
    Pierre Lekeux, PhD
    A. Lerner, PhD
    N. Liberato, PhD
    S.X. Lin, PhD
    Andrew D. Livingston, PhD
    Yushu Liu, PhD
    Brian J. Lopresti, PhD
    Kurt M. Lucin, PhD
    Patrick Maisona, PhD
    M. D. Majewska, PhD
    Jennifer Margulia, PhD
    Thomas Marichal, PhD
    N. Scott Mason, PhD
    A. Kimberley McAllister, PhD
    Jaquelyn McCandless, MD
    Susan McCreadie, MD
    (The late)Dr. Robert Mendelsohn, MD
    (The late) John Menkes, MD, Former head of pediatric neurology at UCLA Medical School. Menkes was also director of pediatric neurology at the Cedars-Sinai Medical Center in Los Angeles. In addition, he was a member of the Forum for Vaccine Safety with the National Institute of Medicine.
    Joseph Mercola, DO
    Claire Mesnil, PhD
    K. Meyza, PhD
    S. Midha, PhD
    P. Mierzejewski, PhD
    Elizabeth Mumper, MD, Associate professor of clinical pediatrics at the University of Virginia
    Devi S. Nambudripod, MD
    Meryl Nass, MD
    C. Nelson, PhD
    E. Newell, PhD
    Raymond Obomsawin, MSc, PhD
    Tetyana Obukhanych, PhD
    Keiichi Ohata, PhD
    M. Olczak, PhD
    Dr. Mehmet Oz
    Larry Palevsky, MD
    Elodie Passeria, PhD
    Michael S. Petrik, PhD
    Jon Poling, MD
    Diana Popa, PhD
    Massroor Pourcyrous, MD
    (The late) Bernard Rimland, MD
    Aviva Jill Romm, MD
    Catherine Sabatel, PhD
    E. M. Sajdel-Sulkowska, PhD
    Bob Sears, MD
    Martyn A. Sharpe, PhD
    Chris Shaw, Professor, PhD
    DD Shen, PhD
    K. Vijendra Singh, PhD
    Yehuda Shoenfeld, MD, FRCP
    Peter Siesjö, PhD
    Ken Stoller, MD
    Carol Stott, PhD
    Arnold J. Stromberg, PhD
    Z. L. Sulkowski, PhD
    Louise Swarbrick, PhD
    Rena C. Tabata, PhD
    Sherri Tenpenny, DO
    Jaime Tomko, PhD
    Lucija Tomljenovic, PhD
    Anju Usman, MD
    Eva Vanamee, PhD
    Chiara Villac, PhD
    John Walker-Smith, Professor
    Judy Wilyman, PhD candidate
    Tony Wyss-Coray, PhD
    Margaret C. Wong, PhD
    V.C. Yang, PhD
    Amy Yasko, MD
    Edward Yazbak, MD
    Judy Van de Water, PhD, Immunology, UC Davis
    Chiara Villac, PhD
    Walter Zahorodny, PhD, Assistant Professor of Pediatrics, University of Medicine and Dentistry of New Jersey
    A. M. Zavacki, PhD

  15. skeptvet says:

    Science is not a popularity contest, and lists of doctors who believe something isn’t evidence, merely opinion. Of course, even with this sort of illegitimate form of argument, you would lose since the overwhelming majority of doctors do support vaccination. All of the links you provide are to misinformation and propaganda that have been rebutted a million times. The problem, of course, is that anti-vaccine activists aren’t interested in facts and evidence since this is an emotion-driven movement supported by fear and mistrust, not science:

    “Fourteen studies” rebuttals:
    Mark CrislipDavid Gorski
    Steven Novella
    107 Studies that Show Vaccines do not Cause Autism

  16. v.t. says:

    Just so you know, Liz, wordpress injects “looks like spam” whenever ANYONE posts more than one link – it has to get through the wordpress filters. But I digress, you probably just think skeptvet censored you. Likewise, it’s his blog and it’s been made clear several times that link spam and propaganda site links will not be tolerated.

    You don’t have a valid argument, least of all one supported by real science and research. Take some time to read the links skeptvet offered you – you’ll find out how valid research is done.

  17. L says:

    Is the Zoetis rabies vaccine okay? Or should I ask for the Merial Purevax? I just want to avoid any trouble for a dog, if possible. Thanks

    “There was some evidence that aluminum adjuvant might have been associated with the occurrence of these sarcomas, and most of us now use adjuvant-free vaccines such as the Merial Purevax” (excerpt)

  18. L says:

    What do you think about Benadryl 1 hour prior and a shot of prednisone as a prophylactic at the same time as the rabies vaccine in an adult dog with unclear vaccination history?
    And a rabies titer 1 year later prior to the booster? If the titer was high or adequate wouldn’t that indicate the booster wasn’t indicated?

    Sorry for the questions, I’m a worrier….

  19. skeptvet says:

    The intention of premedication is to prevent acute hypersensitivity reactions. Unfortunately, they don’t actually prevent such reactions. The benadryl may suppress some of the symptoms, such as hives, but the reaction still occurs. The prednisone works through a mechanism involving protein production, so it doesn’t take affect for 4-6 hours, so premedication right before a vaccine isn’t going to be useful. And in any case, such treatment isn’t indicated in dogs that don’t have a history of such hypersensitivity reactions, and the vast majority of dogs do not have these reactions.

    As for titers, these can be useful to prove that a dog has had an appropriate protective response to vaccination. However, they don’t tell us if a dog needs to be revaccinated. A positive titer indicates protection, but a negative titer does not indicate susceptibility since there are other elements of the immune system the titer does not measure which are important in immunity. And a titer will not substitute for legal rabies vaccination requirements, so dogs who have not had the rabies vaccine as required by law will be considered unvaccinated regardless of their titers.

  20. L says:

    Thank you. Maybe I will give the Benadryl prior to the appointment and hang out in the waiting room for an hour afterwards. I might skip the prednisone, I figured it would only prevent an acute reaction anyway, I will give it some more thought.

  21. L says:

    “And a titer will not substitute for legal rabies vaccination requirements, so dogs who have not had the rabies vaccine as required by law will be considered unvaccinated regardless of their titers”.

    I know that, my concern would be that if the dog has a high or adequate rabies titer would giving him the booster overload his immune system and possibly cause adverse effects. I understand that “the law is the law”. However…..

  22. skeptvet says:

    While that’s perfectly sound logic, the problem is that the issue of “overloading: the immune system isn’t a real one. Our immune system is exposed to more antigens through normal living (eating, contact with other people, paper cuts, and so on) in about 3 weeks than in the entire childhood vaccination series (see here). The immune system competently handles far more challenges every day than it experiences through vaccination. An appropriate titer does indicate protection against rabies, and hopefully the law will eventually be changed to allow use of this measure. But either way, there is no reason to be concerned about vaccinating even if the dog has an adequate titer.

  23. L says:

    Just wanted to let you know that your support was helpful, my dog got his rabies shot today and seems fine. I did do the Benadryl and prednisone…just in case.

  24. Teresa Franklin says:

    Quoted from skeptvet: “Thimerosal can, of course, be toxic at sufficient doses, but then so can water.” I am sorry (chuckle) but that made me lol when I read it. Thimerosal is mercury, mercury is toxic at any concentration and in any of its forms (it undergoes conversion within our bodies – just like it does in fish!). You present a very one-sided story here – read all of the reports – not just the ones that support your views. Readers, be warned – and do your own research. Read the book, Mercury-free by James Hardy to begin to learn about how we are poisoning ourselves and our environment with mercury. And, to be sure, there is a connection between Autism and mercury vaccines. For example, read ‘Blood Levels of Mercury Are Related to Diagnosis of Autism: A Reanalysis of an Important Data Set’ M. Catherine DeSoto, PhD, and Robert T. Hitlan, PhD, Journal of Child Neurology, a highly cited article refuting the original study’s claim (analyses were conducted incorrectly).

  25. skeptvet says:

    mercury is toxic at any concentration and in any of its forms

    Actually not true, which illustrates your lack of familiarity with the evidence or the science here. The evidence is also overwhelming that there is no association between vaccination and Autism, and what is more thimerosal was removed from childhood vaccines many years ago and the rate of new autism cases diagnosed has not only not fallen but increased. You are making a flat-earth argument here that is clearly inconsistent with the facts.

  26. Swooper1 says:

    The cdc is currently under investigation for ordering their scientists to cover us evidence of a link between mmr and autism after senior research scientist and whistleblower William Thompson revealed their activities. Remember in the 50’s how we were told by “every reputable agency” that Tabasco doesn’t cause cancer? Yeah…
    A list of studies that show a link between vaccines and health issues-

  27. skeptvet says:

    Absolute bollocks!

    The claim that the CDC has covered up evidence regarding vaccines and autism is a bogus internet conspiracy that has been debunked numerous times (1, 2). Similarly, the evidence that vaccines are not associated with autism is overwhelming, and not a single reputable scienctific, public health, or autism advocacy group accepts this lie. You are living in a fantasy world.

  28. P Heath says:

    Swooper1 said:
    “Remember in the 50’s how we were told by “every reputable agency” that Tabasco doesn’t cause cancer?”

    That’s exactly how these things start. Rumours will spread, “facts” will be repeated. By this time next year everyone will know that Tabasco causes cancer. No amount of scientific denial will convince them otherwise. We are all going to need something else to make food less bland.

    …my dog is now officially on a Tabasco free diet. I will be able to report back soon that she eats her food faster, seems to enjoy it more and her execessive thirst has gone.

  29. Felicia Ward says:

    No evidence of harm does not mean that something is safe. It means that they really haven’t looked very close.
    John Hopkins purports to equate a minuscule amount of formaldehyde as safe for infants and children. That defies logic. A known carcinogen (classified as such by our EPA) is ok in a small dose.
    The amounts of all of the ingredients in vaccines are supposedly safe for a full grown man. Can you show where the synergistic effect of aluminum, formaldehyde and Thimerosal in the cummalative amounts on the 2018 vaccine schedule are safe? I’m still waiting on that study.
    It’s called the precautionary principle in the EU. A product has to shown safe before it can go on the market. Demonstrating safety is not the same as “no evidence of harm”.
    The studies you cite are not detailed so that we can go and read the parameters of the study like sample size, the duration of study and other methods.
    No one has to be a rocket scientist to understand that it’s not a good idea to inject heavy metals and known carcinogens into small children. One just needs some common sense and the ability to think independently.

  30. skeptvet says:

    They have looked and found no evidence of harm. They even removed thimerosal from many vaccines and saw no difference in any health issue. And there is no way to demonstrate something is safe and can never do harm. That is an impossible nothing could ever meet, and it is not how EU regulatory systems actually work. There are degrees of confidence one must have in the risk vs benefit of various products, but “proof something is safe”. isn’t the standard.

    The idea of a “toxin” is meaningless unless dose, route of exposure, and other important factors are considered. You can die from too much water or oxygen, but that doesn’t mean you should avoid them. Formaldehyde occurs naturally in fruit and in your own body. Is it. unsafe? You can create disease with some compounds that contain mercury, but that doesn’t mean thimerosal is unsafe. What you think of as “common sense” is simply uninformed fear and a lack of understanding of how the relevant aspects of biology actually work.

  31. Angie says:


    What is your take on mercury in fish based diets? My dog has been eating a fish based diet for about 3 years because she has trouble tolerating other proteins. I was told that I should be rotating her proteins, but every time I’ve tried to switch (slowly of course), she ended up with major digestive issues. But, now I concerned about the build up of mercury overtime in her little body due to being fed fish every day for a long time.

  32. skeptvet says:

    Several studies have looked at mercury levels in dog and cat foods and have not found that fish-based diets are higher than other diets or that levels are high enough to be of concern. For example;

    Determination of Total Mercury and Methylmercury Concentrations in Commercial Canine Diets.
    Top Companion Anim Med. June 2019;35(0):6-10.
    DOI: 10.1053/j.tcam.2019.02.002
    Rae A Sires 1, Andrea J Fascetti 2, Birgit Puschner 2, Jennifer A Larsen 3

    Copyright © 2019 Elsevier Inc. All rights reserved.
    Article Abstract
    Methylmercury is an organic form of mercury that is well recognized for its bioaccumulation in aquatic species. Consumption of fish contaminated with methylmercury poses a toxicological health risk to both humans and animals. Salmon is an increasingly common ingredient in commercial pet foods because of manufacturers’ interest in unconventional protein sources and inclusion of omega-3 polyunsaturated fatty acids. Previous studies have measured total mercury, but not methylmercury, in commercial pet foods. The purpose of this study was to evaluate total mercury and methylmercury concentrations in commercially available salmon-containing and nonsalmon-containing canine diets and to estimate risk of chronic exposure in dogs fed these diets long term. Total mercury was detected in 3 of 24 diets evaluated (12.5%), 2 of which did not contain any ingredients from fish. The single salmon-containing diet that contained total mercury was the lowest of the 3 but was also the only sample positive for methylmercury. None of the 3 mercury-containing diets contained fish oil. Concentrations of total mercury were similar to most data previously reported for pet foods. Using expected calorie intake for dogs of 2 body weights, the mercury concentrations determined in this study were applied to theoretical chronic exposure calculations to assess risk of toxicosis to dogs. Total mercury and methylmercury were uncommonly identified in the commercially available canine diets sampled in this study and were found in concentrations unlikely to pose risk to healthy adult dogs. Common sources of mercury in pet foods remain unknown and are not reliably of seafood origin.

  33. henry boxer says:

    I am thinking about vaccinating my Pomeranian, she is very small, female and five yrs old. I was told that they give the same amount of vaccine to a great Dane or sometimes larger animals as they would do to a miniature breed, such as my Pom.

    Any advice concerning my concerns regarding this? And also my fear of possible side-effects from the rabies vaccine, with and without thimerosal? I have read of serious but rare reactions to the rabies vaccine in dogs, and esp small breeds.

    Lastly if we vaccinate her, is there a better time to do so, and should one perhaps stay at the veterinary practice for an hour or so after the vaccination in case of any problems that might occur?

  34. skeptvet says:

    I have addressed the question of vaccine dose, and the bottom line is there is no evidence that smaller quantities of vaccine are safer or as effective as the recommended dose; that idea is based on the misconception that vaccines work like drugs, but the basic biology is very different.

    There is also no evidence that thimerosal is a risk to humans or animals. The vast majority of rabies vaccines given do contain this preservative, and no evidence has emerged that it causes problems.

    Small breeds are more prone to adverse vaccine reactions than larger dogs, though the problem is likely more about genetics than size per se. The best way to reduce the risk is to only give one vaccine at a time with at least 3-4 weeks between different vaccines.

  35. Jo Anna Kloster says:

    After reading through the discussion chain about vaccinations I have one burning question. WHEN will Zoetis, Merck, Boehringer, and whoever else I missed that makes animal vaccines conduct trials testing the efficacy of half dose rabies vaccine on smaller dogs? or even a 3/4 dose. Heck knows, they’ve had YEARS to get this done. Meanwhile, our little dogs suffer Oh, that’s right, that might mean vets wouldn’t be using as much of their precious medicine and might not make as much profit. It’s really disturbing how our pet’s health comes down to money. And how vet clinics make lots of profit with vaccinations. When a titer could confirm that non-core vaccinations are still working in our pets. I know I wandered at the end, but I’d like to know the answer to my rabies question.

  36. Lucky says:

    I have been searching for the amount of Thimerosal in micrograms in vaccines for dogs and have been unable to find it. My dog receives the Imrab 3 and is about to get his next shot. If you have information about that vaccine, I would appreciate it. If not, the level in any dog vaccine you are familiar with might help. Thanks!

  37. skeptvet says:

    You can contact the manufacturer to ask about thimerosal content. IMRAB3 TF is a thimerosal-free rabies vaccine, but I believe it is present in most others. That said, there is no evidence that this causes any health problems, cf statements from The WHO, the FDA, and the EMEA.

  38. Lucky says:

    Sorry, I thought you must have the quantity information handy because you know it is safe. Since you are certain that the ethylmercury in Thimerosal is harmless in dog vaccines, I was curious as to the amount in micrograms.

    1. There is a safety limit on how much ethylmercury a person can receive based on weight (0.1 mcg/kg of weight/day. So, a 5 kg baby – 11 pounds – can only receive up to .5 mcg per day). What is the similar safe limit established for dogs?

    2. If you can explain why the government continues to use the INGESTED Methylmercury standard for INJECTED Ethylmercury, since ethylmercury has been used in vaccines for almost 100 years, that should help to reduce the concerns. Should they be treated the same?

    3. They seem to affect cells very differently. Ethylmercury appears to be much more toxic. What did these researchers get wrong?

    “The treatment of Jurkat T cells with thimerosal caused a significant decrease in cellular viability at 1 ?M (25%, p<0.05; IC50: 10 ?M). Methyl mercury exhibited a significant decrease in cellular viability at 50 ?M (33%, p<0.01; IC50: 65 ?M).”

    From: Effect of thimerosal, methylmercury, and mercuric chloride in Jurkat T Cell Line (PMC3600517)


    “After in vivo administration, ethylmercury passes through cellular membranes and concentrates in cells in vital organs, including the brain, where it releases inorganic mercury, raising its concentrations higher than equimolar doses of its close and highly toxic relative methylmercury…”

    From: Thimerosal Induces DNA Breaks, Caspase-3 Activation, Membrane Damage, and Cell Death in Cultured Human Neurons and Fibroblasts (PMC1892749)

    I’m trying not to be concerned that my dog received Thimerosal, but that is hard to do if I don’t know the safety limit and how much is in 1 vaccine, especially when he received multiple vaccines on the same day. I appreciate your help! Thanks!

  39. skeptvet says:

    There is plenty of information about the safety of ethyl mercury as a preservative. Here are a few good sources:

    Institute of Medicine

  40. art malernee says:

    I’m trying not to be concerned that my dog received Thimerosal, but that is hard to do if I don’t know the safety limit and how much is in 1 vaccine, especially when he received multiple vaccines on the same day. I appreciate your help! Thanks!>>>>>

    switch what ever thing you want to know t about in the vaccine with “secret sauce”. then every time the usda has someone get up to give a talk to the public ask them why pet vaccines have secret sauce when fda approved human vaccines do not. If anyone has any usda written information consumer friendly or not why we cannot get online to know what ingredients are in the pet vaccines I would like to see it. The argument people will see things like thimerosol is in the vaccine and not want to vaccinate I think is a poor argument. I think people will see the usda is not consumer friendly and not trust the usda to do the right thing for consumers and not trust the vaccines.

  41. skeptvet says:

    Art, this view ignores the evidence and the assessment of the evidence by experts at the FDA, CDC, WHO, Academy of Pediatrics, and many other institutions. Whatever you feel the faults of the USDA are, the fact is that there has never been any evidence-based reason to believe thimerosal in vaccines presents health risks, and its use for decades in human and veterinary vaccines has failed to reveal any. Anxiety about thimerosal is no different from anxiety about EMF or 5G–it is fear based on either a lack of understanding of the facts or a general suspicion of “chemicals” and technology.

  42. Lucky says:

    I guess I don’t understand how this is supposed to work. I thought people ask you questions about the topic you wrote about and you TRY to answer them. You don’t have answers to any of my questions, just links?

    So, you don’t know if there is a government safety limit for Thimerosal (ethylmercury) for dogs like there is for people? You don’t know how much Thimerosal is in a dog vaccine – any dog vaccine? You don’t have an opinion on if the government should be using the ingested standard for a different mercury for injected ethylmercury? You don’t directly refute the research on cell death in the studies I asked about, you just send me a link to a different study? What are the problems with their findings that make them incorrect? Ethylmercury doesn’t kill cells?

    From the 2004 study you posted:
    “It is not yet known for sure how much (if any) vaccine-derived ethyl mercury in the blood crosses the blood–brain barrier.” That’s comforting. I feel better. If it is found, after almost 100 years of use, that ethylmercury crosses the blood-brain barrier, does that mean it has been killing brain cells?

    Sorry, one last question. Since Thimerosal has been used in vaccines since the 1930s, can you please point me to the government’s safety study that they used to approve Thimerosal for use in vaccines? From everything I have read, there is no study. Thimerosal was grandfathered. Since you know Thimerosal is safe, you must have read the study I’m trying to find. I appreciate your help!

    If not, here is an interesting 2003 Mercury in Medicine Report from a congressional study on Thimerosal in vaccines:

  43. skeptvet says:

    This blog is a platform for me to share information and evidence about science-based pet health. I often do find topics of interest from readers, but I am not providing a research service, and I am certainly not capable of preparing detailed responses to all of the hundreds of questions I get. I have provided links to reliable sources where you can find the information you are looking for, but I am not able to do that work for you.

    And to be clear, when major public health organizations, regulators, and groups of experts in pediatrics and vaccines all agree that the evidence is clear that thimerosal presents no significant health risks, it is a bit unreasonable to ask for others to work through the details to prove it to your satisfaction. Do you ask for the engineering specifications and safety testing data for every airplane you fly in, or do you believe that the experts and regulators in the aviation field have done their work properly and trust it will fly safely? If you want to recapitulate the research evidence for yourself, I’m afraid the burden is appropriately on you to do the work involved.

  44. art malernee says:

    Whatever you feel the faults of the USDA are, the fact is that there has never been any evidence-based reason to believe thimerosal in vaccines presents health risks,>>>>
    We agree on that..
    I just feel the sauce should not be a usda secret.

  45. art malernee says:

    some good news about the usda pet vaccination regulation.

    the USDA vaccine labeling is not going to have a specific revaccination interval unless it has been proven that vaccination at that interval is required as shown by experimental study. The “one year” recommendation was completely arbitrary. This supports and validates what we have already known and been doing, that DVMs have full flexibility in the frequency of administering veterinary vaccines.

    Here is the language:
    2. Need to Vaccinate vs. Minimum Duration of Immunity
    APHIS considers a “need” to revaccinate to occur when immunity has waned sufficiently that it no longer provides meaningful protection against disease. This is distinct from recommending revaccination at the interval defined in a minimum duration of immunity study. Such studies demonstrate that immunity continues to provide meaningful protection at that time; immunity may not wane until much later.
    3. Acceptable Approaches
    Address revaccination recommendations in one of the following ways:
    a. Disclose the minimum duration of immunity in the indications statement and make no other comments about revaccination.
    b. Disclose the minimum duration of immunity in the indications statement and recommend contacting the veterinarian/manufacturer for more information on revaccination frequency.
    c. For products licensed before November 2016, and which previously had labeling recommending a specific revaccination interval: In addition to the duration of immunity statement, add “Historically, annual (or other interval) revaccination has been recommended for this product. The need for this booster has not been established. Contact your veterinarian or manufacturer for more information on revaccination frequency.”
    You can find the full document on the USDA site: VETERINARY SERVICES MEMORANDUM NO. 800.54

  46. Susan Porter says:

    Please watch Megyn Kelly interview with Robert Kennedy Jr. on the Megyn Kelly show. Mercury in vaccines builds up in the brain not the serum but the serum studies are used to “prove” that ethyl mercury is safe unlike methyl mercury. There is no safe form or safe level of mercury in the brain. Please watch the interview because you have the right to know about this very important topic.

  47. skeptvet says:

    Completely untrue and based on tw of the least reliable sources possible.

  48. Bronwyn says:

    My dog has just had her triennial rabies vaccine (as mandated by law in my country.) I had concerns about the whole thimerosal debate. Thank you for putting my mind to rest with this article – I appreciate the rational, scientific, evidence based approach that you follow. Kudos too, for your patient and informative replies to the anti-vax misinformation that has found its way to your comment section.

  49. Betsy says:

    Nobody has posted about the UC Davis Study: “Thimerosol Linked to Immune System Dysfunction”

    Vaccine reactions seem only to be measured/noted in hours, days, maybe a few weeks, and anything beyond is deemed coincidence. Nobody seems to have researched why they occur. Nobody has done studies to look at this dendritic cell impact in other species, to look at long term impacts on health, such as allergies, cancer and autoimmune disease. No money in it to do so, studies too expensive to conduct, I expect. Perhaps damage to dendritic cells doesn’t show up right away, and perhaps the damage is cumulative after years of repeated vaccinations, but we won’t know because no one has looked at this as far as I can tell beyond this original study by Pessah et al.

    I spoke with the Boehringer Ingelheim rep at the vet hospital I work at about the TF version of IMRAB and he said it costs the same as the regular version. Why NOT just use it? But our vet is stubborn and cited the WHO “thimerosol” is safe declaration. Meanwhile, our clinic is overrun with atopic dermatitis and chronic diarrhea cases.

    Has anyone looked at cumulative epigenetic changes caused by thimerosol? I doubt it. How do we know that disease states in pets aren’t caused by inflammation caused by toxins such as thimerosol? We won’t know because no one has looked at it, is looking at it.

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