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 2006 there were 1,072 research institutions in the USA registered under the requirements of the AWA. These institutions reported the total use of 1,012,713 regulated animals. The most abundant species used were rabbits (239,720), guinea pigs (204,809), and the combination of several species of hamsters (167,571) of which ~90% were estimated to be of the Syrian variety. Other animals used included dogs (66,314), nonhuman primates (62,315), and pigs (57,671). Among the least commonly used of the reportable species were domestic cats (21,637) and sheep (13,577).

There is no federal reporting requirement for mice, rats, birds, amphibians, or fish, making precise numbers used difficult to estimate. Mice are generally considered to constitute >90% of the mammals used in research globally. By some projections, >25 million mice were raised worldwide for scientific use in 2000, representing approximately twice the number used in 1990.

The domestic mouse, Mus musculus and related subspecies (see Rodents: Mice and Rats as Laboratory Animals), is popular as a research model because of its small size, adaptability, docility, low husbandry costs, fecundity, and well-defined health and genetic backgrounds. The development of the means of inserting foreign genes (transgenes) into the mouse genome and the ability to delete genes, leading to what are known as “knockout” mice, also increased the utility of mice as research subjects. Since these advances, >1,500 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 Rodents: 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.

While the guinea pig (Cavia porcellus) was among the first animals to be used in medical research, its popularity has diminished due to 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, vaccine and infectious disease research, and as hearing models.

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 dog use 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 making 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 they have been in steady decline in absolute numbers used in research since 1980, cats are still important models in the neurosciences and in the study of infectious diseases.

Excluding mice and rats, rabbits are the most common mammal used in research, although the number used has declined since 1987. Rabbits are used most often in product safety testing, for polyclonal antibody production, and in studies of vision, orthopedics, and cardiology.

Other species used in scientific research include goats, calves, horses, ferrets, armadillos, opossums, domestic and wild birds, reptiles, amphibians, fish, and invertebrates.

Last full review/revision July 2011 by Michael J. Huerkamp, DVM, DACLAM