Animal testing on rodents

Rodents have been employed in biomedical experimentation from the 1650s. Rodent studies up to the early 19th century were mainly physiological or toxicological. The first rodent behavioral study was carried out in 1822, a purely observational study, while quantitative rodent behavioral testing began in the late 19th century. Currently, rodents are commonly used in animal testing for physiological, pathological and behavioral scientific studies, particularly mice and rats, but also guinea pigs, hamsters, gerbils and others. Mice are the most commonly used vertebrate species, due to their availability, size, low cost, ease of handling, and fast reproduction rate.

In the UK in 2015, there were 3.33 million procedures on rodents (80% of total procedures that year). The most common species used were mice (3.03 million procedures, or 73% of the total) and rats (268,522, or 6.5%). Other rodents species included guinea pigs (21,831 / 0.7%), hamsters (1,500 / 0.04%), and gerbils (278 / 0.01%).

In the U.S., the numbers of rats and mice used are not reported, but estimates range from around 11 million to approximately 100 million. In 2000, the Federal Research Division, Library of Congress, published the results of an analysis of its Rats/Mice/and Birds Database: Researchers, Breeders, Transporters, and Exhibitors.

Over 2,000 research organizations are listed in the database, of which approximately 500 were researched and of these, 100 were contacted directly by FRD staff. These organizations include hospitals, government organizations, private companies (pharmaceutical companies, etc.), universities/colleges, a few secondary schools, and research institutes. Of these 2,000, approximately 960 are regulated by USDA; 349 by NIH; and 560 accredited by AALAC. Approximately 50 percent of the organizations contacted revealed a specific or approximated number of animals in their laboratories. The total number of animals for those organizations is: 250,000–1,000,000 rats; 400,000–2,000,000 mice; and 130,000–900,000 birds.

Mice are the most commonly used vertebrate species, popular because of their availability, size, low cost, ease of handling, and fast reproduction rate. Mice are quick to reach sexual maturity, as well as quick to gestate, where labs can have a new generation every three weeks as well as a relatively short lifespan of two years.

They are widely considered to be the prime model of inherited human disease and share 99% of their genes with humans. With the advent of genetic engineering technology, genetically modified mice can be generated to order and can cost hundreds of dollars each.

Transgenic animal production consists of injecting each construct into 300–350 eggs, typically representing three days' work. Twenty to fifty mice will normally be born from this number of injected eggs. These animals are screened for the presence of the transgene by a polymerase chain reaction genotyping assay. The number of transgenic animals typically varies from two to eight.

Chimeric mouse production consists of injecting embryonic stem cells provided by the investigator into 150–175 blastocysts, representing three days of work. Thirty to fifty live mice are normally born from this number of injected blastocysts. Normally, the skin color of the mice from which the host blastocysts are derived is different from that of the strain used to produce the embryonic stem cells. Typically two to six mice will have skin and hair with greater than seventy percent ES cell contribution, indicating a good chance for embryonic stem cell contribution to the germline.