There is increasing recognition that wild, domestic, and companion animals may act as "sentinels" for environmental health hazards by providing early warning of human health hazards in the environment. Animals can act as sentinels because they may be more susceptible or have greater exposure to a particular hazard compared to humans living nearby.
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The classic example of animals serving as sentinels is the "canary in the coal mine". Well into the 20th century, coal miners in the United Kingdom and the United States brought canaries into coal mines as an early-warning signal for toxic gases including methane and carbon monoxide. The birds, being more sensitive, would become sick before the miners, who would then have a chance to escape or put on protective respirators.
The discovery of West Nile Virus in the western hemisphere was heralded by an outbreak of disease in crows and other wild birds. Other emerging diseases have demonstrated linkages between animal health events and human risk, including Monkeypox, SARS, and Avian Influenza.
Dogs may provide early warning of lead poisoning hazards in a home, and certain cancers in dogs and cats have been linked to household exposures to pesticides, cigarette smoke, and other carcinogens.
A study has shown that dogs, with often only relatively short training, can detect lung and breast cancer trace elements in the breath of people with these cancers, and clearly distinguish them from the breath of healthy subjects.[1]
Some speculate that animals could provide early warning of a terrorist attack using biological or chemical agents. Since most potential bioterrorism threats are zoonoses (infectious diseases of animal origin), animals could also be at risk from a terrorism attack and may be first to show signs of illness due to increased exposure or susceptibility. For example, when anthrax was inadvertently released from a Soviet weapons facility in Sverdlovsk, livestock died at a greater distance from the plant compared to human cases.
Recently, there has been a call for linkage of human and veterinary medicine in a "One Health" approach that recognizes disease events in non-human species may indicate human health risk. The "One Health" approach involves greater information sharing between human health and veterinary clinicians and public health professionals and cooperative efforts to identify and prevent diseases that act across species barriers in a way that is mutually beneficial.