24 | Animal Models and Animal Welfare Rodents, rabbits, and primates may first come to mind, but horses, roundworms, fruit flies, zebra fish, songbirds, and many, many others model some aspect of human biology Woodchucks, for instance, are susceptible to a virus similar to human hepatitis-B Leprosy can be produced in nine-banded armadillos but in few other animals Labrador retrievers develop a hip dysplasia that resembles human osteoarthritis Squid nerve axons transmit nervous signals much as human nerves These are but a few of the thousands of ways in which animals are used as models for normal and diseased human biology Cell and tissue culture have made many uses of animals obsolete After all, why use an animal as a model when actual human cells can be grown in the lab and studied? Typically, scientists use tissueculture systems to study events at the cellular and subcellular levels For example, tissue culture is used to study which types of cells HIV, the human immunodeficiency virus, is capable of infecting, and what events occur in the cell that eventually kill it But when research requires studying the interaction of many different cells and tissues, such as how the immune system fails to protect the brain from the effects of the AIDS virus, or how medications will affect this, then scientists may turn to whole-animal models Many factors influence the choice of model Animal welfare requires scientists to consider using less-sentient species when possible (such as fruit flies instead of mice or monkeys) Cost considerations push scientists to choose smaller animals with shorter life cycles for many studies Data may be most easily obtained and analyzed from smaller, simpler organisms than from larger ones; thus, zebra fish are chosen for studies of organ development, because the embryo is largely transparent and develops in an egg outside of the mother’s body On the other hand, larger size is sometimes required, such as when surgeons develop new techniques by using pigs To best interpret data in light of what is already known, scientists will often choose the animal models most common to their fields, whether that original association was somewhat arbitrary (such as use of rats rather than hamsters in psychology experiments) or based on unique biological attributes (such as studies of vitamin C in guinea pigs, one of the few non-primate mammals to require vitamin C the way humans do) Increasingly, research requires knowing an animal’s genetic makeup, so well-studied and easily modified species, especially mice, zebra fish, and fruit flies, have become more widely used It is controversial just how useful animal models are Certainly no drug is marketed in the United States without having been studied in animals Is this because there is always a biological need to use animal data to develop drugs, or simply because the law requires animal safety data to be submitted before a drug can be licensed? Models may be classified in many ways This essay looks at three broad categories of animal models: testing for product development, skills development, and induced and spontaneous models of disease Animal Models in Safety Testing Using animals to test the safety of drugs, cosmetics, and environmental chemicals is what most people think of— and criticize—first The crudest version of safety testing is to apply a compound to an animal—either acutely at high doses