Research institutions registered under the AWA are required to submit an annual report to APHIS that details, among other specifications, a listing of the common names and the numbers of reportable species used. In 2010, there were 1,104 research institutions in the USA registered under the requirements of the AWA. These institutions reported the total use of 1,134,693 regulated animals. The most abundant species used were guinea pigs (213,629), rabbits (210,172), and the combination of several species of hamsters (145,895) of which >90% were estimated to be of the Syrian variety. Other animals used included nonhuman primates (71,317), dogs (64,930), pigs (53,260), domestic cats (21,578), sheep (13,271), and a combination of marine mammals, other farm animals, and other species not identified (341,214). There is no federal reporting requirement for mice, rats, birds, amphibians, or fish, making precise numbers difficult to determine. Research subjects of these species, for the most part, are obtained via a combination of in-house breeding colonies and from commercial vendors, with production and sales data from the latter being proprietary and not disclosed publicly. Wild-caught birds and aquatic species sometimes may be captured for experimental purposes. Since 2000, the estimated total number of mice and rats used for research purposes annually in the USA has ranged fairly consistently from 20 to 30 million. There are millions of zebra fish and thousands of amphibians, likewise, used annually.
The domestic mouse, Mus musculus and related subspecies (see Mice and Rats as Laboratory Animals), is popular as a mammalian research model because of its small size, adaptability, docility, low husbandry costs, fecundity, well-defined health and genetic backgrounds, and relative ease of genetic manipulation. The development of genetic engineering techniques of inserting foreign genes (transgenes) into the mouse genome and the ability to delete genes, leading to what are known as “knockout” mice, profoundly increased the use of mice as research subjects. Since these advances, countless mutant genotypes of mice have been developed, ranging from subtle defects in immune function to full-fledged, inherited diseases virtually homologous with those of higher mammals.
Among other rodents, the domestic Norway rat (Rattus norvegicus) is second only to the mouse as a research subject. Rats (see Mice and Rats as Laboratory Animals) share many of the attributes of mice that make them attractive for use in research, but because they are larger than mice, they are suited for a greater variety of manipulations. Numerous mutant and inbred strains and outbred stocks of rats are used in a broad array of studies, including topics such as aging, cancer, reproductive physiology, drug effects, behavior, addiction, alcoholism/cirrhosis, arthritis, brain and nerve injury, hypertension, embryology, teratology, endocrine diseases, neurophysiology, infectious disease, stroke, organ transplantation, and surgically induced disease. The genetic engineering of mutant rat types, however, has lagged behind mice and is only beginning to show effect.
Excluding mice and rats, guinea pigs and rabbits are the most common mammals used in research, although their numbers have declined from peaks in the 1980s. Although the guinea pig (Cavia porcellus) was among the first animals to be used in medical research, its popularity has diminished relative to that of mice and rats because of its long gestation period (59–72 days), small litter size (2–5), poor vascular access, and difficulties in anesthesia. Guinea pigs are still used in notable ways in immunology, in vaccine and infectious disease research, and as hearing models. Rabbits are used most often in product safety testing; for polyclonal antibody production; and in studies of vision, orthopedics, and cardiology.
In addition to the Syrian hamster (Mesocricetus auratus), a few other species of hamsters are used in research, including the Armenian, Siberian (Djungarian), Chinese, European, and Turkish species. Hamsters are readily available, reproduce easily, and are relatively free of spontaneous diseases but susceptible to many induced viral diseases. They are used for studies of obesity, induced carcinogenesis, prostatic disease, toxicity, infectious diseases (including slow viruses), dental caries, chronic bronchitis, and teratogenesis.
Other rodent species used in research include gerbils, deer mice, chinchillas, cotton rats, rice rats, multimammate rats, Egyptian spiny mice, degus, voles, and woodchucks, among others.
While rodent models are unarguably the most common for scientific use, larger animal models provide unique opportunities for biomedical research. Dogs are used in studies of cardiology, endocrinology, orthopedics, prosthetic devices, surgical techniques, pharmacokinetics, and product safety. Since use of dogs began declining in 1984, livestock have been used more frequently. This has been a consequence of regulatory and public pressure related to the use of dogs, but it is also due to attributes of comparative anatomy and physiology that make livestock more conducive to particular investigations. For example, swine are used for cardiovascular research (particularly atherosclerosis), in studies of digestive physiology, as surgical models, and for xenotransplantation. Sheep are used for studies of neonatal development, human vaccine improvement, asthma pathogenesis and treatment, drug delivery, circadian rhythms, and surgical techniques.
Nonhuman primates remain critical in studies of vision, the neurosciences, infectious diseases, vaccines, and product safety testing. In recent years, they have become increasingly valuable as models of immunodeficiency virus infection and the neurodegenerative diseases associated with aging.
Although the absolute number of cats used in research has been in steady decline since 1980, cats are still important models in the neurosciences and in the study of infectious diseases.
The most important aquatic species used in research are the zebra fish and the African clawed frog.
Other species used in scientific research include goats, calves, horses, ferrets, armadillos, opossums, domestic and wild birds, reptiles, amphibians, other species of fish, and invertebrates.