Veterinarian lauded for his skepticism

Dr. Brennen McKenzie fights misinformation with evidence-backed science

Published: April 03, 2023
Photo courtesy of Dr. Brennen McKenzie
A small animal practitioner in Northern California for 23 years and author of the SkeptVet blog, Dr. Brennen McKenzie was awarded the VIN Veritas Award this year for his efforts in debunking pseudoscience in veterinary medicine. He's pictured here with Brodie, one of his two dogs.

Should I spay or neuter my dog? Are vaccines safe? What should I feed my pet? Veterinarians hear the same questions over and over. Dr. Brennen McKenzie, a small animal practitioner in Northern California, streamlined the process of responding early in his career by writing handouts with answers based on the current science.

When he encountered questions about therapies for which he found high-quality clinical research was sparse or nonexistent — such as herbs and supplements, homeopathy and acupuncture — what began as a practical strategy became a vocation.

His write-ups morphed into SkeptVet, a blog he launched in 2009 to promote science-based medical interventions for companion animals, and discourage unproven approaches. Today, SkeptVet is a go-to source for debunking misinformation and pseudoscience in veterinary medicine. 

His efforts earned McKenzie the 2022 VIN Veritas Award, which the Veterinary Information Network bestows annually on colleagues "who demonstrate extraordinary courage and integrity in the service of science and truth." VIN is an online community for the profession and parent of the VIN News Service. 

McKenzie talked with VIN News recently about evolving attitudes toward science, the risks for veterinarians of being too confident about what they think they know, and his new focus on the science of canine aging — where once again, he finds himself squaring off against some anti-science ideas.

A condensed, edited version of the conversation follows. The full interview is available as a podcast.

Let's start with a big question: What is evidence-based veterinary medicine? Shouldn't all medicine be evidence-based? What's the distinction?

You're right. All medicine should be evidence-based, but the reality is, it's not, in the sense that we mean. Evidence-based medicine technically means that we integrate scientific research evidence into our clinical decision-making in an explicit and intentional way. The three pillars of evidence-based medicine are scientific-research evidence, the expertise and skills of a clinician, and the goals and values of the client. Those are the three things that all have to come together to take care of a patient.

[Veterinarians] make use of our own expertise, experience and knowledge all the time. And we, most of the time, take into account what the client wants and their perspective. We tend to use scientific evidence in a haphazard, background way, not in an explicit and intentional way. What I mean by that is, we have a whole base of knowledge — things we know about clinical diseases and about treatments — but we don't know where that knowledge came from. It's all floating around, left over from school or from books we've read or CE [continuing education] conferences. We don't really know what the provenance of the information is.

There are different levels of evidence, and some give us more accurate, more reliable information than others. So, evidence-based medicine is encouraging clinicians to ask themselves, "What information do I need to help this particular patient?" and then to go see what scientific information is out there that might be relevant to that question and ask themselves how reliable that information is.

What are the challenges to sorting through the available information?

The biggest problem that we have is excessive reliance on our own personal experience or on the anecdotes and experiences of others. A classic example is the use of metronidazole for acute idiopathic diarrhea in dogs. We've done that for decades on the basis of some very loose theoretical reasoning for why it might be helpful, and we've all done it a million times, and most of those dogs get better. So, we all think that it works and it's safe.

Now we have three or four good clinical research studies that show that, for the most part, it's probably no better than a placebo. The vast majority of acute idiopathic diarrhea cases get better all by themselves, and we just get the credit because we happen to give them something a few days before they got better. It's very difficult for us to let go of the perception that this treatment works.

We trust our own day-to-day experience as representative of causal relationships, and that goes for all kinds of medicine. I talk a lot about alternative medicine, and part of the reason for that is that there are some special features to the culture and the philosophical underpinnings of alternative medicine that encourage this problem. In conventional medicine and science-based medicine, we're taught that we should respect the scientific evidence and give it primacy over our personal experience. We still don't do it very well, but we understand the importance of that. There is a privileging of the individual personal experience of life and belief about the world in the philosophy and the culture of alternative medicine that makes it even harder to let go of practices that have been around for a long time that we've all done and feel like work for us, even when the scientific evidence is pretty clear that they don't.

How did you become interested in alternative and complementary medicine? Is it because that's where you see this tendency toward over-trusting personal experience?

My journey in terms of science communication and advocacy started primarily with just being an ordinary clinician and wanting to help my clients. I started blogging largely because once I put the time and energy into researching these things and writing articles and things for my clients, I thought, "Well, why don't I just make this available to other people to help them with their practice?" I think it became more about the alternative medicine movement because, partly, those were things that I hadn't already had answers to because they weren't discussed in depth in my training. I spent more time and energy researching them and trying to understand them on my own. There was maybe more of a need [to say], "Hey, you know, science has some insight to give us on these practices and it's often different from what you're hearing from people who are providing or marketing particular therapies or strategies."

You've used the term "Age of Endarkenment." What do you mean by that?

Obviously, it's a reference to the Age of Enlightenment. The Enlightenment was a philosophical movement in the 17th through the early 19th century, and it involved many, many areas — not just science, but politics and art and religion; lots of other things. But it was a necessary precursor to the ability of science to give us all of the improvements in human well-being that have occurred over the last couple of hundred years. Before science could work and give us these tools, we had to have some shift in our attitudes. We had to believe that human reason and empirical observation were effective ways of getting to the truth about nature. And, in fact, that those were better tools than relying on old-fashioned texts, or tradition, or history, or revelation or the things that we had primarily used.

We also had to have a shift in the idea that progress was possible — that we could make the world a better place. There was a pervasive idea, at least in Western culture, that the world was sort of degraded from a past golden age, and that we didn't have any hope of making it better. We tended to focus on the next world. The Enlightenment said, "No, we can actually make life better for people, and that's a worthwhile goal. And the way to do it is [through] reason and observation." And here we are, a couple of hundred years later, living longer, healthier and happier lives than any human beings ever.

There's always been resistance to some of these ideas, and that resistance tends to come and go in cycles or in waves for a variety of very complicated reasons. I'm not a historian or sociologist, so my understanding of that is somewhat limited. But I believe since the mid-'90s, we've really been in a period where we're seeing significant resistance to a lot of those Enlightenment ideals and to the notion that science gives us meaningful truth about nature.

One of the early manifestations of that was the anti-vaccine movement, where we saw a significant decline in the use of vaccines for preventable diseases and an increase or resurgence in things like whooping cough and measles that we had largely defeated in the developed world.

My grandmother — who spent 18 months in a sanatorium with tuberculosis expecting to die because we didn't have antibiotics, and who wouldn't let her daughter go out and play in the summer because we didn't have a polio vaccine — would have just been mystified by the idea that you wouldn't grab onto these tools eagerly as soon as they were available.

But my point in bringing it up is that it's about beliefs and attitudes as much as it is about the method itself, or the actual tools that we have, or the technologies we use. I think as veterinarians, we're not primarily scientists or researchers but we are scientifically literate, and we work in a science-informed profession, and I think that gives us some responsibility to advocate for these ideas. The Age of Endarkenment is just a dramatic way of saying we're moving away from some of these Enlightenment values that made the modern world possible, and that's something that we can respond to and participate in to try to help bring us back to a more balanced view of the role of science in our day-to-day lives.

I wonder, do you think these trends worsened during the pandemic?

My feeling at the beginning of it all was, "OK, here we go. This is an imminent, obvious threat to health and welfare. We're going to see immediately how important it is that we pay attention to the scientific understanding, that we use the tools science provides us." I thought it was really going to change the dialogue in a positive way.

What we saw was kind of the opposite. Certainly, the vast majority of people still had the vaccines and listened to the scientists. But there is a substantial minority, in some cases 40%, 50% of people, who have significant doubt about the mainstream, well-supported, scientific case for vaccinations and masking and things like that. We're seeing resistance to this at the highest levels in government. We're seeing policies in place that have quite literally led to millions of unnecessary deaths around the world.

I think the pandemic has made it worse in the sense that we can no longer think of this kind of anti-scientific thinking as a quaint bunch of people who think the Earth is flat that we can safely ignore. I think this is a deep-enough problem that it presents a real, present danger to public health and well-being. In that sense, it has, if not made things worse, certainly made things much more visible.

How have these attitudes filtered into veterinary medicine?

We tend to always experience the trickle-down from whatever's going on in human medicine. Most of the therapies we use, conventional and alternative, are in one way or another first instituted in human medicine and then brought over and adapted for us in some way — the grain-free diet, which is just an extension of the gluten-phobia in humans, and resistance to vaccines, which largely followed the anti-vaccine movement in children. I think that we're participating in the larger problem.

Do you think that there's something about veterinary medicine that makes it particularly susceptible to pseudoscience, or is it the same or worse in human medicine, as far as you can tell?

I think it's a little bit of both. I do have some involvement in this on the human side. I've written occasionally for a blog called Science-Based Medicine, which is sort of the flagship for the fight against misinformation and pseudoscience on the human side. I think that the general problems are similar, the kinds of things that we see people thinking or saying are very similar, and many of the same actors are involved.

[Veterinary medicine does] have a couple of unique features. One is that our evidence base is woefully lacking compared with that in human medicine. There's often less good-quality scientific evidence upon which to base our conclusions. That both makes it harder to disabuse people of misconceptions and makes veterinarians a lot more comfortable with working on the basis of pretty limited and unreliable evidence. We rely on our personal experiences a lot more heavily because we have to, because no one is out there doing clinical studies in 10,000 dogs for 15 years to give us the right answer. That makes us a little more friendly to the arguments that these therapies should be used on the basis of personal experience or history and anecdote.

If there was one product or idea that you could erase from veterinary medicine, what would it be?

It would be the idea that your personal observations of cause-and-effect relationships are reliable. The number one objection I get when I present a solid, robust evidence-based case against anything, conventional or alternative, is, "Yeah, but I tried it, and it worked." [It's] the post hoc, ergo propter hoc fallacy, which just means if one thing happens before a second thing, the first thing must absolutely be the cause for the second thing. You know: Every time I wash my car, it rains because, clearly, washing my car makes it rain.

Our brains are literally built around narratives, around storytelling and around watching these cause-effect relationships. Those kinds of arguments and experiences are far more compelling than tables of statistical data. We haven't evolved to find graphs and tables emotionally compelling. So, if I could take something away, it would be this automatic confidence in those kinds of personal experiences because that just is the biggest barrier we face to changing our minds about stuff.

I wonder about pet owners, who are bombarded with misinformation. How are they supposed to weigh that against what they are being told by veterinarians if veterinarians aren't always using the best evidence?

We have an interesting problem here because I think we all accept that the paternalistic medical model, where you shut up and do what the doctor tells you, is not an appropriate model and is subject to all kinds of abuse, particularly because doctors are working with exactly the same kind of brain you have and are subject to many of the same errors.

We're asking people, on the one hand, to be scientifically literate and to be thoughtful consumers of health care. But at the same time, we do want you to recognize that being an informed consumer is not the same thing as being a medical expert. There are things that you don't know. It's being able to recognize that you aren't just a blind follower of your veterinarian's advice, but you're also not equivalent to your veterinarian in terms of your understanding of these issues.

The hope that I have for the future is coming from what I see in my daughter's education. Because a lot of kids in the recent millennial and early Gen Z generations are getting a lot of education in school about how to identify reliable information versus unreliable information online, because that's a huge problem.

You've recently shifted your focus more to the science of canine aging. Why?

There are a couple of things at play here. One is that I started the blog in 2009, and I've been writing about these things for a long time, and my interest in things does wane over time. When I've spent 15 years developing my understanding of an area and having the same conversations repeatedly, that becomes a little less satisfying. In 2019, I published a book, which was called Placebos for Pets? The Truth about Alternative Medicine in Animals. It was essentially a summary of all that I had learned and was a way of wrapping up that chapter. 

At the same time, I've been in clinical practice for 23 years and I was getting a little bored with day-to-day clinical medicine, and the pandemic and some ownership changes had some impact on our practice that changed my practice environment in a way that made it a little less satisfying. About that time, along comes Dr. Frances Chen, who was a PhD DVM at Loyal [a veterinary medicine startup in the San Francisco Bay Area]. They were beginning to work on developing medications to extend lifespan and delay age-associated disease in dogs. [Chen] called me up initially to see if I would participate in one of their clinical studies. I'm an associate, not an owner, and I couldn't make that happen at my practice. But I thought, "Wow, that sounds really cool. That sounds like the ultimate in preventative medicine, right? We're going to treat aging primarily before all the bad stuff happens instead of playing Whac-A-Mole at the end. What else can I do for you?"

I got hired as a consultant because they didn't have anybody at the time working for them who had direct clinical experience. They had mostly research veterinarians working for them. It's useful for me to say to the researchers, "Yeah, you're not going to take a biopsy every time the dog comes in because nobody's gonna let you do that." They needed some of that pragmatic perspective. For me, it was a whole new area of science to explore and to learn about, which was really satisfying.

You changed your focus, but I see from your blog that you find yourself writing about some familiar misinformation bugaboos.

Exactly, because all of the things that lead to pseudoscience and misinformation are not bugs, they're features. They're fundamental aspects of how human reasoning and the brain works.

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