Placebo effect observed in double-blinded food challenges for dogs with atopic dermatitis

A randomized, double-blinded, placebo-controlled trial evaluated the placebo effect during food challenges in 12 dogs diagnosed with atopic dermatitis and adverse food reactions. The intervention involved serial 1-week challenges with eight different food items (beef, chicken, codfish, corn flour, cow's milk, hen's egg, lamb, wheat), each administered at 40 g/day mixed with an elimination diet and water. The comparator was a placebo challenge consisting of the elimination diet mixed with water alone. Relapse was defined using a composite endpoint of owner global assessment, CADESI-04 score, and pruritus Visual Analog Scale.

The main results showed that 11 of 12 dogs (91.7%) had positive reactions to between one and six of the food item challenges. Notably, half of the dogs that reacted to food items also had a false positive reaction during the placebo challenge. The number of positive food challenges did not differ between dogs with positive versus negative placebo reactions. The study did not report specific effect sizes, confidence intervals, or p-values for these comparisons.

Safety and tolerability data were not reported. The study's key limitation is its very small sample size of 12 dogs, which limits generalizability. The authors note that the observed placebo effect creates doubts about the accuracy of results from food challenges, both in this study and in previous studies that used open (non-blinded) challenge designs. Funding sources and conflicts of interest were not reported.

For practice, this study highlights that placebo responses can occur during blinded food elimination trials in dogs with atopic dermatitis. Clinicians should interpret positive food challenge results cautiously, especially when challenges are not placebo-controlled. The findings suggest that open food challenges may overestimate true food sensitivities due to unmeasured placebo or expectation effects.