Animal sentinels, or sentinel species, are animals used to detect risks to humans by providing advanced warning of a danger. The terms primarily apply in the context of environmental hazards rather than those from other sources. Some animals can act as sentinels because they may be more susceptible or have greater exposure to a particular hazard than humans in the same environment. People have long observed animals for signs of impending hazards or evidence of environmental threats. Plants and other living organisms have also been used for these purposes.
Historical examples 
There are countless examples of environmental effects on animals that later manifested in humans. The classic example is the "canary in the coal mine". Well into the 20th century, coal miners brought canaries into coal mines as an early-warning signal for toxic gases, primarily 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.
In Minamata Bay, Japan, cats developed "dancing cat fever" before humans were affected due to eating mercury-contaminated fish. Dogs were recognized as early as 1939 to be more susceptible to tonsil cancer if they were kept in crowded urban environments. Studies similarly found higher disease rates in animals exposed to tobacco smoke.
Animal sentinels must have measurable responses to the hazard in question, whether that is due to the animal's death, disappearance, or some other determinable aspect.:34 Many of these species are ideally unendangered and easy to handle. It is important that the species' range overlap with the range being studied. Often the ideal species is determined by the characteristics of the hazard.
For example, honey bees are susceptible to air pollution.:35 Similarly both bats and swallows have been used to monitor pesticide contamination due to their diet of insects that may have been affected by the chemicals.:35 By the same token, aquatic animals, or their direct predators, are used as sentinel species to monitor water pollution.
Some species may show effects of a contaminant before humans due to their size, their reproductive rate, or their increased exposure to the contaminant.
Specific applications 
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Toxic gases 
Canaries were iconically used in coal mines to detect the presence of carbon monoxide. The bird's rapid breathing rate, small size, and high metabolism, compared to the miners, led birds in dangerous mines to succumb before the miners, thereby giving them time to take action.
Air and water pollution 
Infectious diseases 
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.
Household toxins 
Bio terrorism events 
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.
"One Health" 
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 that 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.
See also 
- National Research Council (U.S.). Committee on Animals as Monitors of Environmental Hazards, "Animals as Sentinels of Environmental Health Hazards: Committee on Animals as Monitors of Environmental Hazards," National Academy Press: 1991, ISBN 0309040469.
- David A. Bengston, Diane S. Henshel, "Environmental Toxicology and Risk Assessment: Biomarkers and Risk Assessment", ASTM International, 1996, ISBN 0803120311, p 220.
- Stephen J. Withrow, David M. Vail, Withrow and MacEwen's Small Animal Clinical Oncology, Elsevier: 2007, ISBN 0721605583, p. 73-4.
- Arthur D. Bloom, Frederick de Serres, Ecotoxicity and Human Health: A Biological Approach to Environmental Remediation, CRC Press: 1995, ISBN 1566701414, page 76.
- van der Schalie WH, Gardner HS Jr, Bantle JA, De Rosa CT, Finch RA, Reif JS, Reuter RH, Backer LC, Burger J, Folmar LC, Stokes WS. (Apr 1999). "Animals as sentinels of human health hazards of environmental chemicals". Environ Health Perspect 107 (4): 309–315.
- O'Brien DJ, Kaneene JB, Poppenga RH (Mar 1993). "The use of mammals as sentinels for human exposure to toxic contaminants in the environment". Environ Health Perspect 99: 351–368. PMID 8319652.
- Backer LC, Grindem CB, Corbett WT, Cullins L, Hunter JL (2001-07-02). "Pet dogs as sentinels for environmental contamination". Sci Total Environ 274 (1–3): 161–169. doi:10.1016/S0048-9697(01)00740-9.
- Rabinowitz P, Gordon Z, Chudnov D, Wilcox M, Odofin L, Liu A, Dein J. (Apr 2006). "Animals as sentinels of bioterrorism agents". Emerg Infect Dis 12 (4): 647–652. PMID 16704814.
- Meselson M, Guillemin J, Hugh-Jones M, Langmuir A, Popova I, Shelokov A, Yampolskaya O (1994-11-18). "The Sverdlovsk anthrax outbreak of 1979". Science 266 (5188): 1202–1208. doi:10.1126/science.7973702. PMID 7973702.
- Kahn LH. (Apr 2006). "Confronting zoonoses, linking human and veterinary medicine". Emerg Infect Dis 12 (4): 556–561.