I have written many times about the misuse of diagnostic tests and the risks of misdiagnosis and overdiagnosis associated with improper use of screening tests. Recently, I condensed my rants on these topics into an article I hope will be useful to veterinarians.

McKenzie, B.A. Rational use of diagnostic and screening tests. Journal of Small Animal Practice (2021), 1–6. DOI: 10.1111/jsap.13393

 Veterinarians have a vast and ever-expanding array of diagnostic tests available to them. However, this abundance can be an embarrassment of riches that confounds diagnosis and undermines patient care if we do not make critical and informed decisions about the selection and interpretation of the tests we employ. Effective use of diagnostic tests requires a deliberate and informed approach. We must consider the strengths and weaknesses of the tests themselves and the clinical context, and we must be wary of the many biases that skew our use and interpretation of diagnostic tests. Understanding sensitivity and specificity, likelihood, prevalence and predictive value, the basic principles of Bayesian reasoning, and the cognitive biases that drive inappropriate testing are all critical to ensuring our use of imaging and laboratory testing improves patient outcomes.

It is really never a good sign when a miraculous new breakthrough in veterinary medicine is announced on the internet. That isn’t where true scientific breakthroughs show up. Legitimate science is a slow, detailed process where even brilliant, revolutionary ideas have to go through a long, rigorous process of critique and exploration before they are accepted. Public relations press announcements are about making money and generating hype, not about exploring good science.

It is even less inspiring of confidence when the supposed breakthrough is offered by a person or institution with a track record of promoting pseudoscience. 

In the case of OncoK9 Liquid Biopsy, such red flags abound. Press announcements with hyperbolic language like “pioneering,” “revolution,” and “breakthrough innovation” are not only meaningless but outright misleading when the only evidence offered to support them is a company study that supposedly “will be submitted for publication in a leading peer-reviewed journal” someday. Legitimate science does not start with press releases and then move to possiblepublication in the scientific literature.

It is also not inspiring of confidence that the product is being initially offered for sale by Petco, a company which has a sketchy history when it comes to pet health topics. Selling raw diets, grain-free foods and bogus nutritional supplements while running a vapid and unscientific public relations campaign lauding its ban on “artificial” ingredients in the foods sold at PetCo stores does not establish the chain as a reliable source of information on pet health. 

So what is OncoK9? According to the company that developed the product, it is “a multi-cancer early detection (MCED) test for the detection and characterization of cancer-associated genomic alterations in DNA isolated from canine whole blood samples, using next-generation sequencing (NGS) technology.” Sounds fancy, eh? The idea is plausible. Cancer cells may, depending on the type of cancer and many other factors, circulate in the blood. If such cells are present, it is potentially possible to detect them by identifying genetic markers associated with such abnormal cells. If there are then other tests available to identify and locate the cancer, and if there are therapies that might be more effective in treating the disease when caught at an earlier stage, such a test could improve health and lifespan in dogs.

Notice all the “ifs” and caveats here? The idea is sound, but the devil is in the details, and this company has not even come close to proving that all the steps in this process will work in a way that actually benefits dogs. The test was developed by mixing DNA from cancer cells and healthy canine cells and showing that the test could detect the abnormal DNA. Apparently, the test was then applied to samples from dogs know to have and thought not to have cancer (this is the study that has not yet been submitted for publication but from which the company is reporting results in its marketing materials). That’s a good start, but a long way from proving that regular clinical use of this test will benefit canine patients.

The company recommends using the test as a screening tool, “as an annual screening test for dogs that are 8 years and older and/or belong to breeds that are highly predisposed to cancer.” Screening is a particularly tricky kind of testing because you are looking at dogs without any signs or symptoms of disease. Most apparently healthy dogs are actually healthy, so the risk of false positive results, finding disease that isn’t actually there, is higher than when testing sick patients. 

The risk of overdiagnosis is also high. This is where a test finds a disease that is truly there but isn’t likely to ever cause any symptoms. Many kinds of screening tests in humans, based on the same idea that early detection of asymptomatic disease is better, have been scaled back because it turns out they do more harm than good. Finding a type of cancer that isn’t ever going to progress or make someone sick isn’t in their best interests if subsequent testing and treatment cause harm without any benefit in terms of health or increased lifespan. 

Assuming the methods and controls for error and bias were appropriate and the data is accurate (all of which we have to blindly assume in the absence of any scientific publication), the company study suggests an overall sensitivity of 48% for the test. This means that 48%, less than half, of dogs with cancer will correctly test positive. This is not, on the face of it, a very sensitive test. However, if we are looking for cancer in dogs without any clinical symptoms, that might actually be a good thing. We would rather miss a case in a dog with cancer that is not causing any problems than mistakenly identify a dog as having cancer who doesn’t. 

The specificity, that is the proportion of dogs without cancer who correctly test negative, is reported to be 97%. This is the more important value in this case since we don’t want to falsely diagnose cancer and send a lot of dogs into unnecessary, costly, and potentially dangerous further testing or treatment they don’t need.

However, given that the company actively recommends the test as a screening tool, it is likely most of the dogs tested will be healthy. A rule for screening tests is that the less likely the patients you test are to actually have the disease you are looking for, the more wrong answers your test will give you. One way of looking at this is with the Positive Predictive Value (PPV) of a test.

The company advertising materials calculate the PPV for OncoK9 based on guessing that 10%-20% of dogs tested will have cancer. This seems high (which would make the test look better than guessing a lower rate of hidden cancers), and there aren’t good research data to tell us what the actually prevalence of asymptomatic cancer is in dogs of a specific age or breed. The best data we have are variable and not consistent between studies, so I will give the company the benefit of the doubt on these guesses for the time being. 

Using these numbers, then, the PPV for OncoK9 would be between 64% and 80%. At the low end, that means that 64% of the dogs with a positive test actually have cancer and 36% do not. It seems a bit concerning that we are going to tell over a third of the dogs we test that they might have cancer when they don’t. Presumably, this would be followed by x-rays, ultrasound, other tests, and potentially months to years of anxiety about this supposed hidden cancer, none of which would improve the health or longevity of these dogs. Even at the more optimistic figure of 80% PPV, 20% or 1/5 dogs testing positive would actually be healthy. This raises the potential for significant costs and harms from unnecessary testing and treatment for a lot of dogs.

And remember all of the other caveats about screening tests for cancer? What if the OncoK9 test is correct but we can’t find the cancer anywhere until it progresses? What if it is a type of cancer for which there is no effective treatment? What if it is a type of cancer that is non-progressive and is never going to make the dog sick or lead to death? What if treating the cancer earlier, and for a longer time, doesn’t actually work better than treating it when it causes symptoms that we can detect without this test? All of these are likely scenarios, and in none of these cases would the cost of the test or the cost, anxiety, and risks of further testing and treatment offer any benefits for the dogs. This test has significant potential to cause harm not only when it gets the wrong answer but even when it is correct!

Of course, having a test that reliably detects cancer at a stage where the dog is still healthy and can be effectively treated in a way that preserves health and prolongs life would be a great thing. I would welcome such a test once it was proven to work as intended. Unfortunately, there is no way to know if OncoK9 will lead to net benefit or harm to dogs based on the evidence currently available. Despite the undoubtedly good intentions of the people developing and promoting this test, it is disappointing, even irresponsible to market it without having first done the necessary work to show it will actually make life better for our canine companions. 

I will keep an eye out for the promised publications. I will even hold out a little bit of hope that eventually studies will be done to determine if the test as actually used in practice reduces disease and saves lives or not. Unfortunately, my fear is that once the test is on the market, the research will stop and continued use will be supported by heartwarming anecdotes about dogs “saved” by an early cancer diagnosis. We will likely never know how much misdiagnosis and overdiagnosis comes from this test since there will be no financial incentive to do the necessary research to figure this out. For now, I would urge dog owners to think very carefully about spending $400 for a test that involves big promises, little evidence, and a lot of uncertainty.

Veterinary medicine suffers from a chronic lack of scientific evidence to identify safe and effective treatments. We are authorized to used medicines approved for human or animal use on an off-label basis in other species or conditions because the Food and Drug Administration (FDA) recognizes that there are so few properly tested and approved medications for our patients. Without this off-label authority, our ability to treat serious health problems would be crippled.¹ Unfortunately, this means therapies are often widely used on the basis of low-quality or unreliable evidence, including in vitro or lab animal studies, extrapolation from human studies, clinical experience and anecdote, or weak clinical evidence. 

One of the most critical unmet needs in companion animal medicine is for oral analgesics other than non-steroidal anti-inflammatory drugs (NSAIDs). NSAIDs are the mainstay of our treatment for acute and chronic pain in dogs and cats. While they are safe and effective in many cases, sometimes they are contraindicated or the risks of NSAID use may outweigh the benefits.^2–6 There is also an inappropriate level of anxiety among pet owners and veterinarians about using these drugs, especially in cats, that further limits their utility.

Many alternative oral analgesics are recommended and commonly used, but these typically lack the robust evidence regarding safety and efficacy we have for the NSIADs.^7–11 In some cases, such therapies have been widely employed and then eventually found to be ineffective. Tramadol is perhaps the most well-known example of this.¹² Despite promising evidence from human and preclinical research, the extensive use of this drug as a treatment for acute and chronic pain in dogs likely resulted in significant undertreatment and unnecessary suffering. Though the evidence for CBD is slightly better, the ubiquity of this remedy as an analgesic is also out of proportion to the strength of this evidence, and it remains to be seen if we will come to regret rapid and widespread adoption of this as an oral analgesic.^13,14

The current leading non-NSAID oral analgesic appears to be gabapentin. I often encounter pet owners and veterinarians firmly convinced that this is a proven therapy for acute and chronic pain in dogs and cats. A recent survey of veterinarians found it the most popular prescription for chronic musculoskeletal pain in cats, ahead of much more extensively studied treatments such as meloxicam.⁸ Is this confidence justified? Let’s take a look at the evidence.

What is Gabapentin?
Gabapentin is a structurally similar to gamma-aminobutyric acid (GABA), though it is not a GABA receptor agonist.¹⁰It’s mechanisms of action are not well-understood, but it appears to operate by affecting pre-synaptic calcium channels in neurons.^10,11 It is licensed as an anti-seizure medication in humans and for treatment of herpesvirus-associated neuralgia.^10,11

Studies in Humans
Apart from its validated uses for seizures and herpetic neuralgia, gabapentin has been studies for many types of acute and chronic pain in humans. The evidence is mixed, low-quality, and inconclusive.^15–23 Studies of perioperative gabapentin use in patients undergoing knee replacement¹⁹, hip replacement¹⁷, Cesarian section²², breast cancer²³, and other surgical procedures^16,18,20 are inconsistent. Some show reduction of opioid use and various measures of pain and others do not. In many of these reviews, some assessment tools for pain or discomfort may show beneficial effects from gabapentin while others do not. Even studies examining neuropathic pain in humans, which is considered the most reliable indication for gabapentin, find that many patients experience no benefit.²¹ Nearly all literature reviews emphasize that the evidence is of poor quality and that better quality research is needed to support firm conclusions.

Veterinary Evidence
Basic pharmacology and pharmacokinetic research indicates gabapentin given orally can be absorbed and achieve plasma levels associated with analgesia in humans.¹⁰ Because the drug has a very short half-life, it likely needs to be given every eight hours to achieve these levels.¹⁰

A few preclinical studies have been done in dogs and cats. Gabapentin did not affect thermal nociceptive response when given orally in cats. It also did not lower the minimum alveolar concentration (MAC) of isoflurane when given intravenously in this species.^24,25 The MAC for isoflurane in dogs was decreased by oral gabapentin, though whether this reflects analgesia or some other effect, such as sedation, is unclear.²⁶

Some case series, and the usual mountain of less formal anecdotal evidence, have suggested analgesic benefits from oral gabapentin in dogs and cats.^27–30 However, these uncontrolled observations are at high risk of bias and error, and they frequently do not reflect the findings of better controlled clinical studies.

There are some clinical trial studies of oral gabapentin as an analgesic in dogs and cats. These have generally not been very encouraging, though they all have some methodological limitations, from ineffective blinding or other bias-control measures to administration of gabapentin every 12 hours, which is less than pharmacokinetic studies suggest is needed to achieve an effect. 

In dogs, adding gabapentin to opioid or NSAID analgesia provided no additional pain benefit by most measures in dogs undergoing intervertebral disk surgery,³¹ mastectomy³², and forelimb amputation³³. Studies involving dogs with neuropathic^34,35 pain have also failed to find robust evidence of any benefit. One study in cats undergoing ovariohysterectomy found no analgesic benefit of oral gabapentin added to buprenorphine or meloxicam compared with a placebo.³⁶ Reviews of the evidence uniformly conclude that the widespread use of gabapentin for acute and chronic pain in dogs and cats is not based on high-quality, robust scientific research.^9–11

Is it Safe?
The potential risks of gabapentin are based almost entirely on anecdotal reports. Sedation, ataxia, and vomiting or diarrhea are commonly listed as potential adverse effects and have been reported in clinical trials, but there is virtually no research specifically investigating the potential risks of this drug in dogs or cats.³⁷

Bottom Line
The widespread acceptance and use of gabapentin as a safe and effective treatment for acute and chronic pain in cats and dogs is not based on reliable scientific evidence. Extrapolation from preclinical research and the limited evidence available in humans is a common but poor foundation for clinical use of the drug in veterinary patients. The few studies so far published in dogs and cats have not been encouraging. It may turn out that gabapentin is a useful non-NSAID oral analgesic, but it seems equally likely that we will eventually realize we have been relying on yet another ineffective pain therapy. While we inevitably have to make the best effort we can to treat our patients in the information-poor context of veterinary medicine, we have ample reason to be wary of trusting therapies with such weak evidence behind them. Our patients deserve better than treatments we only hope will effectively treat their pain. Until better more definitive evidence is available, gabapentin should be regarded as no more than theoretically beneficial as an adjunctive treatment, and it should not be relied on as a sole therapy or an alternative to treatments with better evidence of real benefits, such as opioids and NSAIDs.

References

1.        American Veterinary Medical Association. Extralabel Drug Use and AMDUCA: FAQ. https://www.avma.org/extralabel-drug-use-and-amduca-faq. Published 2021. Accessed January 7, 2021.

2.        Monteiro B, Steagall P, Lascelles B, et al. Long-term use of non-steroidal anti-inflammatory drugs in cats with chronic kidney disease: from controversy to optimism. J Small Anim Pract. 2019;60(8). doi:10.1111/JSAP.13012

3.        Sparkes A, Heiene R, Lascelles B, et al. ISFM and AAFP consensus guidelines: long-term use of NSAIDs in cats. J Feline Med Surg. 2010;12(7). doi:10.1016/J.JFMS.2010.05.004

4.        Innes J, Clayton J, Lascelles B. Review of the safety and efficacy of long-term NSAID use in the treatment of canine osteoarthritis. Vet Rec. 2010;166(8). doi:10.1136/VR.C97

5.        Luna S, Basílio A, Steagall P, et al. Evaluation of adverse effects of long-term oral administration of carprofen, etodolac, flunixin meglumine, ketoprofen, and meloxicam in dogs. Am J Vet Res. 2007;68(3). doi:10.2460/AJVR.68.3.258

6.        Monteiro-Steagall B, Steagall P, Lascelles B. Systematic review of nonsteroidal anti-inflammatory drug-induced adverse effects in dogs. J Vet Intern Med. 2013;27(5). doi:10.1111/JVIM.12127

7.        Epstein ME, Rodan I, Griffenhagen G, et al. 2015 AAHA/AAFP Pain Management Guidelines for Dogs and Cats. J Feline Med Surg. 2015;17(3):251-272. doi:10.1177/1098612X15572062

8.        Adrian DE, Rishniw M, Scherk M, Lascelles BDX. Prescribing practices of veterinarians in the treatment of chronic musculoskeletal pain in cats. J Feline Med Surg. 2019;21(6):495-506. doi:10.1177/1098612X18787910

9.        Ruel H, Steagall P. Adjuvant Analgesics in Acute Pain Management. Vet Clin North Am Small Anim Pract. 2019;49(6). doi:10.1016/J.CVSM.2019.07.005

10.      KuKanich B. Outpatient oral analgesics in dogs and cats beyond nonsteroidal antiinflammatory drugs: an evidence-based approach. Vet Clin North Am Small Anim Pract. 2013;43(5). doi:10.1016/J.CVSM.2013.04.007

11.      Moore S. Managing Neuropathic Pain in Dogs. Front Vet Sci. 2016;3. doi:10.3389/FVETS.2016.00012

12.      McKenzie BA. Is tramadol an effective analgesic for dogs and cats? Vet Pract News. June 2018:32-33.

13.      McKenzie B. A conclusion on cannabis? Vet Pract News. July 2019:26-27.

14.      McKenzie BA. Cannabis-based remebdies: No reliable clinical research evidence. Vet Pract News. August 2017:38.

15.      Fabritius M, Wetterslev J, Mathiesen O, Dahl J. Dose-related beneficial and harmful effects of gabapentin in postoperative pain management – post hoc analyses from a systematic review with meta-analyses and trial sequential analyses. J Pain Res. 2017;10. doi:10.2147/JPR.S138519

16.      Fabritius M, Geisler A, Petersen P, Wetterslev J, Mathiesen O, Dahl J. Gabapentin in procedure-specific postoperative pain management – preplanned subgroup analyses from a systematic review with meta-analyses and trial sequential analyses. BMC Anesthesiol. 2017;17(1). doi:10.1186/S12871-017-0373-8

17.      Mao Y, Wu L, Ding W. The efficacy of preoperative administration of gabapentin/pregabalin in improving pain after total hip arthroplasty: a meta-analysis. BMC Musculoskelet Disord. 2016;17(1). doi:10.1186/S12891-016-1231-4

18.      Egunsola O, Wylie C, Chitty K, Buckley N. Systematic Review of the Efficacy and Safety of Gabapentin and Pregabalin for Pain in Children and Adolescents. Anesth Analg. 2019;128(4). doi:10.1213/ANE.0000000000003936

19.      Han C, Li X, Jiang H, Ma J, Ma X. The use of gabapentin in the management of postoperative pain after total knee arthroplasty: A PRISMA-compliant meta-analysis of randomized controlled trials. Medicine (Baltimore). 2016;95(23). doi:10.1097/MD.0000000000003883

20.      Fabritius M, Geisler A, Petersen P, et al. Gabapentin for post-operative pain management – a systematic review with meta-analyses and trial sequential analyses. Acta Anaesthesiol Scand. 2016;60(9). doi:10.1111/AAS.12766

21.      Wiffen P, Derry S, Bell R, et al. Gabapentin for chronic neuropathic pain in adults. Cochrane database Syst Rev. 2017;6(6). doi:10.1002/14651858.CD007938.PUB4

22.      Felder L, Saccone G, Scuotto S, et al. Perioperative gabapentin and post cesarean pain control: A systematic review and meta-analysis of randomized controlled trials. Eur J Obstet Gynecol Reprod Biol. 2019;233. doi:10.1016/J.EJOGRB.2018.11.026

23.      Rai A, Khan J, Dhaliwal J, et al. Preoperative pregabalin or gabapentin for acute and chronic postoperative pain among patients undergoing breast cancer surgery: A systematic review and meta-analysis of randomized controlled trials. J Plast Reconstr Aesthet Surg. 2017;70(10). doi:10.1016/J.BJPS.2017.05.054

24.      Pypendop B, Siao K, Lkiw J. Thermal antinociceptive effect of orally administered gabapentin in healthy cats. Am J Vet Res. 2010;71(9). doi:10.2460/AJVR.71.9.1027

25.      Reid P, Pypendop B, Ilkiw J. The effects of intravenous gabapentin administration on the minimum alveolar concentration of isoflurane in cats. Anesth Analg. 2010;111(3). doi:10.1213/ANE.0B013E3181E51245

26.      Johnson B, Aarnes T, Wanstrath A, et al. Effect of oral administration of gabapentin on the minimum alveolar concentration of isoflurane in dogs. Am J Vet Res. 2019;80(11). doi:10.2460/AJVR.80.11.1007

27.      Davis L V, Hellyer PW, Downing RA, Kogan LR. Retrospective Study of 240 Dogs Receiving Gabapentin for Chronic Pain Relief. J Vet Med Res. 2020;7(4):1194. https://www.jscimedcentral.com/VeterinaryMedicine/veterinarymedicine-7-1194.pdf. Accessed January 7, 2021.

28.      Steagall P, Monteiro-Steagall B. Multimodal analgesia for perioperative pain in three cats. J Feline Med Surg. 2013;15(8). doi:10.1177/1098612X13476033

29.      Vettorato E, Corletto F. Gabapentin as part of multi-modal analgesia in two cats suffering multiple injuries. Vet Anaesth Analg. 2011;38(5). doi:10.1111/J.1467-2995.2011.00638.X

30.      Lorenz N, Comerford E, Iff I. Long-term use of gabapentin for musculoskeletal disease and trauma in three cats. J Feline Med Surg. 2013;15(6). doi:10.1177/1098612X12470828

31.      Aghighi S, Tipold A, Piechotta M, Lewczuk P, Kästner S. Assessment of the effects of adjunctive gabapentin on postoperative pain after intervertebral disc surgery in dogs. Vet Anaesth Analg. 2012;39(6). doi:10.1111/J.1467-2995.2012.00769.X

32.      Crociolli G, Cassu R, Barbero R, Rocha T, Gomes D, Nicácio G. Gabapentin as an adjuvant for postoperative pain management in dogs undergoing mastectomy. J Vet Med Sci. 2015;77(8). doi:10.1292/JVMS.14-0602

33.      Wagner A, Mich P, Uhrig S, Hellyer P. Clinical evaluation of perioperative administration of gabapentin as an adjunct for postoperative analgesia in dogs undergoing amputation of a forelimb. J Am Vet Med Assoc. 2010;236(7). doi:10.2460/JAVMA.236.7.751

34.      Plessas I, Volk H, Rusbridge C, Vanhaesebrouck A, Jeffery N. Comparison of gabapentin versus topiramate on clinically affected dogs with Chiari-like malformation and syringomyelia. Vet Rec. 2015;177(11). doi:10.1136/VR.103234

35.      Ruel H, Watanabe R, Evangelista M, et al. Pain burden, sensory profile and inflammatory cytokines of dogs with naturally-occurring neuropathic pain treated with gabapentin alone or with meloxicam. PLoS One. 2020;15(11). doi:10.1371/JOURNAL.PONE.0237121

36.      Steagall P V, Benito J, Monteiro BP, Doodnaught GM, Beauchamp G, Evangelista MC. Analgesic effects of gabapentin and buprenorphine in cats undergoing ovariohysterectomy using two pain-scoring systems: a randomized clinical trial. J Feline Med Surg. 2018;20(8):741-748. doi:10.1177/1098612X17730173

37.      Peck C. The adverse effect profile of gabapentin in dogs – a retrospective questionnaire study. 2017.

I’ve written about stem cell therapies numerous times over the years, though not for quite a while. My conclusions were always along the lines of “promising but unproven.” This is one of those perfectly plausible therapies that I expect will be beneficial at some point but which has, like so many conventional and alternative treatments, has been rushed into use well before we have sufficient evidence to know what it can really do and what it can’t.

I have recently written an update on the topic, and not a great deal has changed since I first covered it over a decade ago. There is much more research evidence, but the variety of types of treatments tested and the different conditions and measures of effect used make it difficult to confidently asses the effectiveness of any specific stem cell treatment on the market for any specific problem. Overall, the evidence is still promising but unproven, though it has gotten at least a little better for the most common use, osteoarthritis. Despite this, there is a long way to go before we can recommend stem cell treatments without a lot of caveats about the uncertainty for their benefits and for long-term safety.

Leaping Before Looking
Science can be frustrating. There is always a gap between having a great idea and having a new tool to change things in the real world. Ideally, that gap is bridged by robust, rigorous scientific research which tells us whether the original idea is really as great as it seems and what it will actually let us accomplish. Even in the best case, filling the gap between inspiration and real-world change takes time. And all too often, seemingly good ideas crack under the pressure of scientific investigation and fail to live up to their promise. 

Our natural human tendency is to let our excitement over a new discovery or hypothesis to carry us away, and to start implementing our new ideas as quickly as possible. If we’re lucky, that lets us achieve the change we want to make in the world more quickly. Unfortunately, in most cases the result is less positive. Skipping past the process of interrogating new hypotheses scientifically commonly leaves us with tools that don’t work, or that have harmful effects we didn’t anticipate. Nature is inevitably more complex that our ideas about it, and enthusiasm can’t overcome the gap between what we think we understand and the reality of the natural world.

In veterinary medicine, we are always seeking new understandings and new tools to better care for our patients. Compared to our colleagues in human medicine, we are relatively unconstrained in trying out our ideas. As I discussed in my last essay, regulatory oversight of veterinary medicine is light, and we are accustomed to therapies with little supporting evidence and to leaping into new practices well before they have built the kind of supportive evidence required in human medicine. 

This is a necessary evil given the limited resources available to build better evidence in veterinary medicine. When our ideas turn out to be right, we have the advantage of getting effective treatments to our patients faster and with less cost than in human medicine. When we are wrong, of course, we end up exposing our patients to therapies that seem like they should work but are actually ineffective (e.g. tramadol, glucosamine, homeopathy) or harmful (e.g. high-dose steroids for spinal cord injury, treatment of asymptomatic bacteriuria with antibiotics).

One of the most exciting ideas I have watched develop over my twenty years as a veterinarian is the hypothesis that stem cell therapies can be used to treat a great variety of different health problems in veterinary patients. Stem cells have been a subject of intense enthusiasm, controversy, and research in human medicine for several decades, and this has spilled over into veterinary medicine, with the usual time delay and lower level of available evidence. In an example of the potential benefits from the looser regulatory burden in our field, stem cell therapies are being also explored in our patients as a springboard for more rapid development and approval of treatments for humans. 

As often happens, preliminary research in laboratory animals and in human medicine led to relatively rapid commercialization and clinical use of stem cell treatments in veterinary medicine well before robust clinical trial evidence in companion animals with natural disease had been developed. Fortunately, as better evidence has been slowly accumulated, it is looking more and more like we may have “guessed right” in this case, and that the risks to our patients are minimal (though not negligible) and there may well be meaningful benefits. Regulatory approval of commercial veterinary stem cell therapies is just beginning, and I am hopeful that despite our cart-before-the-horse approach to this treatment, therapies which are demonstrably safe and effective may be available soon. However, as a recent review of the science in this field concluded, “despite considerable advancements in veterinary regenerative

medicine in recent years, this field is still in its infancy and much more work is needed to resolve many questions before proven, standardized therapies [can] be offered to the clinical patients.”¹

What Are Stem Cells?
Stem cells possess the ability to differentiate into a variety of cell types.¹ The degree of differentiation present in a stem cell and the potential for further replication and differentiation varies among different kinds of stem cells. Pluripotent trophoblasts present in the early embryo can give rise to a complete organism, while stem cells present in adults have considerably less potential to differentiate.² Early stem cell research focused on embryonic cells derived from fetal tissue, but social and political controversy around this led to the predominance of mesenchymal stem cells (MSC) derived from adult tissues as the focus of research into potential stem-cell therapies.³ It is also possible to induce fully-differentiated adult cells to become pluripotent stem cells through genetic manipulation, though there are potential safety concerns to such methods which are still being investigated.^1,2,4

Early hypotheses about the medical benefits of stem cell use centered on the idea that these cells could be introduced into diseases tissue where they would differentiate into needed cell types. For example, if a dog has a damaged cranial cruciate ligament or degraded articular cartilage, intra-articular stem cell injection could produce new connective tissue or cartilage to replace the damaged tissues. This is now understood to be incorrect, and the mechanisms by which exogenous stem cells influence disease are more complex and indirect.

Stem cells have significant immunomodulatory effects, exerting paracrine influence on a variety of cells via cytokines and mediators of inflammation and growth. MSCs can also influence apoptosis of other cells, and they appear to be able to exchange extracellular vesicles and even mitochondria with endogenous cells.^1,2 There is a great deal still to be learned about how exogenous stem cells function and what physiologic effects they have when administered to veterinary patients. These details are critical to rational use of these cells as clinical treatments, and failing to appreciate this complexity is likely to lead to ineffective therapy.

Finally, stem cell therapies can be categorized based on their origins. As already mentioned, they may be embryonic, induced pluripotent adult cells, or mesenchymal stem cells, which are most commonly used in veterinary medicine. MSCs can also be taken from the patient we intend to treat (autologous), from another donor individual of the same species (allogenic), or even from a donor of a different species (xenogenic). Each of these sources has different potential risks and benefits. 

Autologous stem cells are most commonly used in commercial treatments available today. This is largely due to regulatory constraints. The Food and Drug Administration (FDA) has indicated that while the agency considers stem cell therapies to be animal drugs under the law, and therefore theoretically an FDA license is required for their use, they choose to employ discretion in enforcement. Specifically, the FDA allows use of autologous stem cell products under a set of specific conditions:

1. The product is minimally manipulated.
2. The product is for homologous use. 
3. The product is for use in nonfood-producing animals. 
4. The manufacture of the product does not involve the combination of the cells with another article, except for water, crystalloids, or a sterilizing, preserving, or storage agent, provided that the addition of water, crystalloids, or the sterilizing, preserving, or storage agent does not raise new safety concerns with respect to the product. 
5. The finished product is not combined with or modified by the addition of any component that is a drug or device.

Allogenic stem cells do not meet these criteria and so require FDA licensing for use, which is a significant burden in terms of time and cost, and this has encouraged regenerative medicine companies to focus on autologous products.

In theory, autologous stem cells should have  minimal risk since they are derived from the patient in which they are used. However, the complexity of stem cell behavior and the potential for changes in these cells with isolation and handling after harvesting means there is still some potential for adverse effects.¹

One disadvantage to autologous stem cell therapies is the need for harvesting of tissue (usually fat) from the patient under sedation or anesthesia. This is followed by isolation of MSCs and then administration of these cells to the patient, often with another procedure under sedation or anesthesia depending on the route of administration. Patients who are ill or cachectic or who cannot tolerate the delay between harvesting and administration of the therapy may not be good candidates for this process.

Commercial, ready-to-use allogenic stem cell therapies would obviate the need for harvesting MSC form the patient and allow faster, more standardized treatments. Because of their capacity to multiply, stem cells harvested from a single healthy donor could be used to produce therapies for many patients. The major impediments to this approach are the regulatory requirements for demonstrating safety and efficacy and the inherently greater potential for harm in giving tissue from one individual to another. However, the benefits of allogenic tissue donations can outweigh the risks, as is often the case for common practices such as blood transfusion. If this proves true for allogenic MSC therapies, there would be significant advantages compared to the currently more common autologous products. 

What Can Stem Cells Be Used For?
The potential benefits of stem cell therapies are many.^1,2,5 The most commonly studied involves  administering stem cells to support tissue healing and regeneration. Stem cells may be useful in reducing pain and disability associated with osteoarthritis, promoting healing of wounds, and even supporting development of nervous system tissues to restore function lost after nerve damage. Typically, MSCs are delivered directly to the site of injury to support healing and regeneration. However, our growing understanding of stem cell activities and effects has raised the possibility of less intuitive uses.

For example, MSCs have also been shown to exhibit homing, a chemotactic behavior in which they migrate to sites of tissue injury.¹ This opens the possibility of treating diseases through systemic administration of MSCs rather than local delivery. This is a significant advantage when local administration is not safe or practical, as in patients unable to tolerate anesthesia or in organs it may be difficult to administer stem cells to directly, such as the heart or kidneys. The paracrine and other effects of MSCs on endogenous cell activity may allow for treatment of inflammatory and autoimmune diseases and other indications beyond straightforward tissue regeneration. 

There are, of course, technical challenges to such applications, such as the difficulty ensuring MSCs get to the target tissue and the complex and the sometimes unpredictable interactions between such exogenous cells and the body of the recipient. It is also important to bear in mind that many specific uses for stem cell therapies are largely hypothetical, will little or no reliable evidence to validate them. 

What is the Evidence?
As I suggested earlier, stem cell therapies are an example of a promising idea that was turned into commercial products and rushed into clinical use in veterinary patients use well before adequate research evidence was available to establish the true risks and benefits. I have been quite critical of this field in the past, and while better supporting evidence is gradually accumulating, it is still disappointing that so many veterinary patients have received largely unproven stem cell therapies.

The International Society for Stem Cell Research (ISSCR) has warned human patients for many years that most stem cell therapies marketed to them are not scientifically validated. Apart from bone marrow transplantation and the use of stem cells in some tissue grafting procedures, there are no stem cell therapies approved for use in humans. While the FDA applies similar standards to regulation of veterinary and human stem cell therapies, including a relatively permissive attitude towards autologous treatments, it has warned manufacturers of stem cell therapies for both animal and human patients against marketing products or making claims that are not consistent with the regulatory limits the agency has set. Unapproved stem cell treatments have cause serious injury to human patients⁶, and while there are few reports of harm in veterinary patients,⁷ this is likely due as much to a lack of reporting and surveillance as to a lack of actual harm.

The veterinary stem cell industry has proven profitable, with estimates in the tens of millions of dollars and projections for growth into the hundreds of millions in the near future. One of the major companies in the field, VetStem, reports first using its stem cell therapy in dogs in 2004. By 2020, the company claimed 30,000 of its treatments had been given. FDA approval for this therapy, however, is still “expected” sometime in 2022. Though manufacturers of veterinary stem cell therapies and others have conducted some studies, it is difficult to justify such extensive use of multiple different stem cell treatments without even the minimum standard of FDA approval, much less a robust base of replicable clinical trial evidence showing the safety and efficacy of specific therapies. Only one veterinary stem cell therapy has received official regulatory licensure, a product for use in horses in the European Union.

The research literature concerning stem cell therapies is large and varied. There are in vitro and lab animal studies, studies in humans, and clinical trials in veterinary species, mostly horses and dogs. These studies cover many different types of both autologous and allogenic treatments for numerous medical conditions.^1,2,5 Each type of evidence, and every specific study, has particular strengths and limitations, and a comprehensive review is beyond the scope of this article. The most robust evidence, and the most common use for stem cell therapies in small animals, involves osteoarthritis in dogs, so I will focus on that subject. This is a fair representation of the best companion animal stem cell research has to offer as well as the still significant gaps in the evidence.

There have been some relatively good quality studies, with randomization, blinding, and control groups, for both autologous and allogenic stem cell therapies for arthritis in dogs.^8–10 The strongest of these compared intra-articular injection of an allogenic stem cell product and saline in 74 dogs using subjective measures of pain and function from both owners and veterinarians.⁸ The study found an expected placebo effect but also significantly greater improvement in most outcome measures for the treatment group.

A smaller study of 21 dogs compared an autologous adipose-derived product with saline injected into the hip joints of arthritic dogs.⁹ In general, blinded subjective assessments by veterinarians showed improvement in measures of pain and lameness for both groups, with the treatment group improving more than the control. Owner assessments also showed more improvement in treated dogs than in the placebo group, though the differences were not statistically significant. Another study by the same group using the same product in 14 dogs with elbow arthritis also reported improvement in both veterinary and owner assessments, but no control group or blinding was reported.¹¹

Other studies have had less robust methodology. One, for example, showed improvement with autologous MSC treatment in 8 dogs with severe hip arthritis on some objective measures of weight bearing, but the control dogs were healthy and were not treated with a placebo.¹⁰ Another study evaluated an allogenic stem cell therapy in over 200 dogs and reported dramatic improvements.¹² However, there was no control group or other control for bias in this study, which render the results potentially unreliable.

A study of an autologous adipose-derived MSC treatment in 10 dogs with osteoarthritis of the knee show some clinical improvement but no change in radiographic appearance or synovial fluid composition compared with a saline placebo.¹³However, there is no reported information about blinding or other important bias control methods and no statistical analysis, making the reliability of the findings difficult to assess.

A trial investigating allogenic adipose-derived MSCs given as an intra-articjular injection to 30 dogs with elbow arthritis followed the dogs for a year and reported significant improvements. Again, unfortunately, the results of an uncontrolled and unblinded study such as this cannot support confident statements about efficacy.¹⁴

In addition to placebos, stem cell therapies have been compared to other regenerative treatments such as platelet rich plasma (PRP, which I have discussed previously).¹⁵ In one study, 31 dogs were randomized to an adipose-derived autologous MSC treatment or PRP and evaluated on a number of measures of pain and function by veterinarians and owners. There were statistically significant improvements in both groups, though the changes were sometimes small in magnitude, and the MSC group appeared to improve more than the PRP-treated dogs. The lack of a placebo control is a limitation in this study.

Far less plausible uses for stem cell therapies in arthritic dogs have been studied, such as the injection of MSCs at acupuncture points to treat hip arthritis.¹⁶ Apart from the numerous problems with acupuncture in general, this study also failed to include blinding, placebo controls, or other methods necessary to produce reliable and clinically useful research evidence.

These studies illustrate that while there is some encouraging evidence for stem cell treatments in dogs with osteoarthritis, and even a few quite persuasive studies,^8,9 the research is often at high risk of bias and error and lacking the sample sizes and methodological rigor needed for confidence in therapeutic claims. The research evidence for other applications of this technology is no better, and often it is quite a bit weaker. The research in other companion animals, such as cats, is negligible, though the evidence in horses is a bit stronger than in dogs.

Stem Cell Safety
Limited evidence for efficacy almost always means limited evidence for safety as well, since we can only know the risks of a given treatment through the same robust research studies needed to show it works. As mentioned earlier, there are reports of serious adverse effects in humans from unapproved stem cell therapies. There are few such reports in companion animal patients, but it is not clear that anyone is really looking for them.

The limited clinical studies we do have, and the somewhat larger collection of lab animal studies done in dogs, suggests that the risks of stem cell therapies are low, particularly for autologous treatments.^1,2 This is reassuring, but we must bear in mind the lack of targeted and long-term safety studies and the biologic complexity of stem cell therapies. There is certainly much we have yet to understand about these treatments, and it may well be that greater knowledge identifies not only more potential benefits but also unanticipated risks.

Bottom Line
Stem cell therapies have always presented a dilemma for me. On the one hand, they are based on plausible hypotheses founded in extensive basic science research. If these hypotheses prove true, then stem cells have the potential to provide dramatic health benefits to veterinary patients, including treatments for diseases we cannot currently treat safely and effectively. Who wouldn’t be excited by that!

On the other hand, stem cell therapies are a model for the backwards process of developing therapies in veterinary medicine. Insufficiently tested treatments based on very limited evidence are marketed and given to thousands of patients over many years with remarkably little oversight or incentive for manufacturers to conduct the kind of rigorous investigation we ought to have to ensure the welfare of patients. This is a dangerous way to develop novel therapies, and it can easily do harm. 

I am cautiously optimistic that in the next decade we will reach the point of having several safe and effective stem cell products that have been adequately tested, and beyond that we may even be able to realize some of the more dramatic possibilities of these therapies, such as regeneration of lost nerve function or safer and more effective treatment for chronic degenerative and inflammatory diseases. 

It is easy to get impatient with the progress of science. If it turns out that speculations about the potential of stem cell therapies were correct, this will likely encourage some to see shortcutting the scientific process as justifiable. I will be happy to have the treatments for my patients that stem cell therapies may offer once they are adequately validated, but I remain convinced that we need to be more patient, and more dedicated to testing such therapies thoroughly before we put them into widespread use.

References

1.        Voga, M., Adamic, N., Vengust, M. & Majdic, G. Stem Cells in Veterinary Medicine—Current State and Treatment Options. Front. Vet. Sci. 7, 278 (2020).

2.        MB, G., A, A. & GT, S. Mesenchymal stem cell basic research and applications in dog medicine. J. Cell. Physiol.234, (2019).

3.        Lo, B. & Parham, L. Ethical issues in stem cell research. Endocr. Rev. 30, 204–13 (2009).

4.        Kimura, K. et al. Efficient Reprogramming of Canine Peripheral Blood Mononuclear Cells into Induced Pluripotent Stem Cells. Stem Cells Dev. 30, 79–90 (2021).

5.        Hoffman, A. M. & Dow, S. W. Concise Review: Stem Cell Trials Using Companion Animal Disease Models. doi:10.1002/stem.2377

6.        Bauer, G., Elsallab, M. & Abou?El?Enein, M. Concise Review: A Comprehensive Analysis of Reported Adverse Events in Patients Receiving Unproven Stem Cell?Based Interventions. Stem Cells Transl. Med. 7, 676 (2018).

7.        Kang, M. H. & Park, H. M. Evaluation of adverse reactions in dogs following intravenous mesenchymal stem cell transplantation. Acta Vet. Scand. 56, 16 (2014).

8.        Harman, R. et al. A Prospective, Randomized, Masked, and Placebo-Controlled Efficacy Study of Intraarticular Allogeneic Adipose Stem Cells for the Treatment of Osteoarthritis in Dogs. Front. Vet. Sci. 3, 81 (2016).

9.        Black, L. L., Gaynor, J., Dean Gahring, D. & Cheryl Adams, D. Effect of Adipose-Derived Mesenchymal Stem and Regenerative Cells on Lameness in Dogs with Chronic Osteoarthritis of the Coxofemoral Joints: A Randomized, Double-Blinded, Multicenter, Controlled Trial*. Vet. Ther. 8, (2007).

10.      Vilar, J. M. et al. Controlled, blinded force platform analysis of the effect of intraarticular injection of autologous adipose-derived mesenchymal stem cells associated to PRGF-Endoret in osteoarthritic dogs. BMC Vet. Res. 9, 131 (2013).

11.      Black, L. L., Gaynor, J., Adams, C., Dhupa, S. & Sams, A. E. Effect of Intraarticular Injection of Autologous Adipose-Derived Mesenchymal Stem and Regenerative Cells on Clinical Signs of Chronic Osteoarthritis of the Elbow Joint in Dogs*. Vet. Ther. 9, (2008).

12.      Shah, K. et al. Outcome of Allogeneic Adult Stem Cell Therapy in Dogs Suffering from Osteoarthritis and Other Joint Defects. Stem Cells Int. 2018, 7309201 (2018).

13.      Mohoric, L., Zorko, B., Ceh, K. & Majdic, G. Blinded placebo study of bilateral osteoarthritis treatment using adipose derived mesenchymal stem cells. Slov. Vet. Res. 53, 167–74 (2016).

14.      Kriston-Pál, É. et al. Characterization and therapeutic application of canine adipose mesenchymal stem cells to treat elbow osteoarthritis. Can. J. Vet. Res. 81, 73–78 (2017).

15.      Cuervo, B. et al. Hip osteoarthritis in dogs: a randomized study using mesenchymal stem cells from adipose tissue and plasma rich in growth factors. Int. J. Mol. Sci. 15, 13437–60 (2014).

16.      Marx, C. et al. Acupoint injection of autologous stromal vascular fraction and allogeneic adipose-derived stem cells to treat hip dysplasia in dogs. Stem Cells Int. 2014, 391274 (2014).