Sequencing of the human genome has created considerable interest in the parallel sequencing of rodent genes. In particular, mouse and rat genes are now being cloned at an ever-increasing rate, providing a genetic framework for animal models of many human diseases. Although mouse and rat are the animals of choice for many pathological processes, less cited animals such as ferret, pig, guinea pig, and rabbit have also been used for a variety of experimental conditions. Another animal that is gaining in popularity for use as an experimental model is the cotton rat. So named for its habit of making a nest out of cotton, this animal was first used in polio research in the 1930’s. It has proven to be an excellent model for general viral research. Based on cost, handling, and pathophysiology, the cotton rat could be considered the model of choice for measles (paramyxovirus), herpes simplex (oral and ophthalmic), influenza (orthomyxovirus), HIV-1, and RSV (respiratory synctial virus) investigation. Cotton rat is also an excellent model for adenovirus or upper respiratory infection. It is the animal of choice for adenovirus-based gene replacement therapy research, allowing optimal combinations of viral genes for minimal immunoreactivity accompanied by maximal gene expression.
The cotton rat is technically known as Sigmodon hispidis (i.e. Sigmodon is Greek meaning "tooth", in reference to the ridges of enamel on its molars, and hispidis is Latin meaning bristles or spines, in reference to its coat). There are seven Sigmodon species and 25 subspecies of the hispidis variety. Although it is a rodent, S. hispidis is neither a rat nor a mouse; its closest relatives are lemmings, voles, and muskrats. The cotton rat is the most common rodent in the southeastern United States as well as Mexico and Central America. It is typically 10-16 inches in length with a 3-6 inch tail, and weighs anywhere from one-quarter to one-half pound. Its fur coat is multicolored, with black or brown interspersed with buff or grey. It usually lives two years in captivity, a value that drops to 6 months in the wild. The cotton rat’s diet is based on perennial grasses, and it is generally considered a solitary species. Notably, the cotton rat in the wild is now recognized as a primary reservoir of hantavirus in the southeastern United States, and thus it has acquired considerable standing in the epidemiological community.
Until recently, there were several issues that prevented the use of S. hispidis as an experimental model. It lacked in-breeding, there was no commercial source for its purchase, a comprehensive bibliography on the animal’s biology was unavailable, and reagents specific for cotton rat had yet to be developed. Through the collaborative efforts of NIH and Virion Systems, Inc. of Rockville, MD, however, many of these issues have now been addressed. Dr. Carl Hanson of NIH has successfully in-bred S. hispidis and S. fulviventer, and Virion Systems now offers commercial cotton rats, and authors a cotton rat bibliography. For more information on the cotton rat, refer to the overview authored by Dr. Gregory Prince at www.nal.usda.gov/awic/newsletters/v5n2/5n2princ.htm.