Although animal shelters have existed for many years in the United States, it has only been recently acknowledged that it requires special expertise to provide appropriate advice on how to care for animals housed in these facilities. Until the 1990s, shelters often utilized euthanasia as the primary method to control population numbers and disease, and there was limited need for veterinarians to provide their expertise on a regular basis. That has changed as animals are held long term for both legal and ethical reasons. Shelter medicine is specialized herd health medicine for small animals. It is very different from providing care for animals in kennels, pet shops and research facilities and is not covered in the curriculum of most veterinary colleges. The focus of shelter medicine is on disease prevention, not treatment. Private practitioners often underestimate the degree of knowledge required to successfully manage a disease prevention program in a shelter. Animals are relinquished to shelters for a variety of reasons, but regardless of whether they are sick, injured, abandoned or simply unwanted, the input of the veterinary community is necessary to implement appropriate standards of care. In addition to just providing shelter, food, water and basic veterinary care, shelters are expected to implement programs that address the overall physical and emotional health and well being of the animals. A comprehensive shelter medicine health care program must consider shelter design, stress reduction and sanitation in addition to preventive health care measures in order to successfully control disease transmission. The value of small animals in shelters is often measured by emotion not dollars, so it may be difficult to apply large animal herd health principles that were developed with a different agricultural production goal in mind. Most small animal practitioners are unaccustomed to this approach to small animal medicine. Some veterinary colleges like Cornell, UC Davis, Penn, Illinois, Colorado, Wisconsin, Tennessee, etc are offering courses and residencies in shelter medicine. There is ongoing and growing support and interest at the university level for developing special training programs in shelter medicine for students and practicing veterinarians. Continuing education courses are being offered by veterinary conferences, on the Internet through VIN and at various other venues. The Association of Shelter Veterinarians or ASV ( http://www.sheltervet.org/) was formed in 2001 and has over 600 members. It is in the early stages of seeking specialty board status from the American Veterinary Medical Association (AVMA).

There are numerous reasons why disease transmission is so difficult to control in animal shelters; the high number and turnover of animals entering with unknown diseases, the variety of ways that diseases are spread, incubation periods and carrier states that make disease difficult to detect, the ability of certain disease agents such as parvo and calici virus to survive long term in the environment, stress, poor staff and volunteer training, high turnover of staff, aging, poorly designed facilities that do not have adequate ventilation or isolation areas for effective disease management, inadequate healthcare program, lack of data/poor data management, lack of financial and human resources, lack of defined, proven protocols of disease control in herd situations, open access to the public, lack of epidemiological data and so on.

In addition, some disease is inevitable in shelters because the interaction between agent, environmental and host factors affects the animal's ability to resist disease. Some host factors include immune status, stress, nutritional state, age, state of health, etc. Environmental factors include overcrowding, poor facility design and air quality, etc. Agent factors include the virulence, shedding pattern, carrier state, route of infection of the pathogen, etc. An effective disease control program must employ management techniques that manipulate environmental factors, exploit the weaknesses of the agent and enhance the strengths of the host.