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Reducing the Number of Rabbits in Eye and Skin Irritancy Tests

Reduction in the number of rabbits used for acute testing of irritation in the eye and skin is a worthwhile goal, and some recent publications have supported such reduction from six to three animals. In anticipation of making such reductions in our laboratory, data from 185 eye and 179 skin irritation tests performed in-house were evaluated within the framework of several schemes of classification. Mean scores for irritation in all possible two, three, four, and five-rabbit subsets were compared to scores with all six rabbits. Parameters included mean weighted Draize scores (eye) and Occupational Safety and Health Administration (OSHA) primary irritation index (skin). Classification of irritants is often based on scales with two or more categories of relative severity (e.g., nonirritating, mild, moderate, severe). Therefore, irritation scores of each subset were grouped into two, three, four, or five categories of irritation. Overall agreement of rankings for the subsets was closer to rankings with six rabbits when fewer categories were used with greater numbers of animals in the subset. Mean agreement across all categories was >90% with three rabbits and four categories for both eye and skin irritation. However, agreement within a given category across subsets was best with nonirritants and least among irritants; eye irritants with mean weighted Draize scores of 16–35 had only ∼76% agreement with 3 rabbits/subset. Comparisons were also made between subsets of three rabbits and the original six rabbits using both the European Community (EC) system of mean values for eye irritation and the OSHA system based on the number of individuals with “positive” scores. Agreement was better with the EC system. To maintain accuracy within the framework of American guidelines, our laboratory will continue to use six rabbits for regulatory studies. Other investigators should carefully evaluate the effect on accuracy of results before reducing the numbers of animals used per acute study.