The caregiver placebo effect in pet treatments

The placebo effect of treatments in humans is well known. This is when a benefit from a treatment is seen that is not due to the treatment itself, but instead is due to the expectations of the person receiving it. This is the reason why drug trials include a group of people taking placebo, so that the placebo effect can be accounted for.

But what about animals? Surely there cannot be a placebo effect, because animals can’t have any expectations about their treatments. Well, researchers have found that there can be a placebo effect in osteoarthritis treatment of dogs – not from the animals, but from the people giving the treatment. 

Clinical trials designed to assess the effects of osteoarthritis treatments often include subjective measures, such as lameness scores and owner questionnaires. These are useful because they can give an idea of how much a patient’s joint hurts or how much the disease affects the patient’s quality of life. But subjective measures can also be a source of bias in the form of a caregiver (owner or vet) placebo effect, which can make a treatment appear better than it really is.

Researchers set out to measure the placebo effect in caregivers when assessing a treatment for osteoarthritis in dogs. They did this by comparing the results of subjective measures of lameness with objective measures. The study included 58 dogs that were in the placebo arm of a large placebo-controlled trial designed to evaluate the safety and efficacy of an oral anti-inflammatory drug. Both owners and vets were unaware of which dogs were receiving the test drug and which were receiving placebo. The subjective outcome measures (owner questions about lameness and veterinary examination to assess lameness at walk, lameness at trot, and signs of pain on palpation) were compared with the objective outcome measure (ground reaction forces). A dog’s gait was considered improved if the ground reaction forces increased by at least 5% of its body weight and deteriorated if the measure decreased by at least 5%. Otherwise, gait was considered unchanged.

When measured objectively over the 42-day evaluation period, limb function improved in 12.1% and worsened in 8.6%, but did not change in most dogs. By contrast, caregivers (both owners and vets) reported improvements in lameness from the start, with the reported improvements increasing with time. The caregiver placebo effect appeared to be around 57% for owners and 40–45% for vets and was statistically significant at all assessment time points.

The lesson from this is that both objective and subjective outcome measures are needed in clinical trials of osteoarthritis treatments because they measure different aspects of treatment. It is also important to include a control group in clinical trials to understand the contribution of the caregiver placebo effect on subjective measures. The study’s authors concluded that vets should also consider placebo effects when interpreting owner responses and veterinary examination findings, when assessing treatment effects in clinical practice.

Reference

Conzemius MG, Evans RB. Caregiver placebo effect for dogs with lameness from osteoarthritis. JAVMA; 2012: 1314–9.

Andrea Tarr, Founder of Veterinary Prescriber, an independent reference and learning resource on veterinary medicines. www.veterinaryprescriber.org.