Requirements and selection of an animal model.

There are two broad classes of models: those based on analogy (similar structures imply similar functions), and those based on homology (structures derived from the same evolutionary precursor have the same or similar functions). There are four main categories of animal models: 1) induced or experimental models, that attempt to reproduce conditions found in the original species, 2) spontaneous or natural models, that are recognized as being similar to some condition in the original species, 3) negative or nonreactive models, that are the normal counterparts of a disease model, and 4) orphan models, that are animal diseases for which no human or animal counterpart is known. The selection of any model, but particularly animal models, for research should be based on the following considerations: 1) appropriateness as an analog, 2) transferability of information, 3) genetic uniformity of organisms, where applicable, 4) background knowledge of biological properties, 5) cost and availability, 6) generalizability of the results, 7) ease of and adaptability to experimental manipulation, 8) ecological consequences, and 9) ethical implications. The criteria for selection or rejection of particular animal models also include customary practice within a particular discipline, the existence of diseases or conditions that might complicate results, the existing body of knowledge on the problem under consideration, and special features of the animal, such as unique responses or microflora, that may make a particular species useful.