A systematic review of animal models and sex as a variable in itch research

Introduction: Pruritus (or itch) research has gained momentum in the last decades and use of animal models to study itch behavior are a vital part of the research. Recent studies have found that many fields using animal models, including neuroscience, are predisposed toward using male animals in preclinical research. To address sex bias in animal research, the National Institutes of Health (NIH) began requiring researchers to include sex as a variable beginning in June 2015. Here, we test whether researchers studying itch are biased toward using males in preclinical research. Methods: The NIH’s PubMed database was searched for primary research articles written between August 2007 and December 2018 using the words “Itch” and “Pruritus.” The following information was extracted from articles fitting our inclusion criteria: type of itch (acute or chronic), the animal model and the sex of the animals used, and whether researchers considered sex as a variable. z-Tests, binomial tests, and the Cochran-Armitage test for trend were used to explore relationships between animal models and the usage of both sexes. Results: We found 5.3%±1.2% of papers in a given year used 1 of our 4 animal models. Mice were the most frequently used animal model, followed by rats, nonhuman primates, and dogs. Overall, researchers used male animals regardless of the animal model used. In preclinical research conducted on both male and female animals, sex was not considered a variable in a majority of these studies. Finally, since 2015, there has not been a change in the usage of male or female mice. Briefly, the incidence of papers utilizing both sexes has not changed. Discussion: We have found that itch researchers have a bias towards males in animal research. This bias has not changed since the NIH’s mandate to include sex as a variable in preclinical research.