In infectious disease ecology and epidemiology, a natural reservoir, also known as a disease reservoir or a reservoir of infection, is the population of organisms or the specific environment in which an infectious pathogen naturally lives, persists, and reproduces, or upon which the pathogen primarily depends for survival. A reservoir is usually a living host of a particular species, such as an animal, human, or plant, within which the pathogen survives, often without causing severe disease in the reservoir itself. By some definitions, a reservoir may also be an environment external to an organism, such as contaminated water, soil, or air, if the pathogen can persist there long enough to contribute to transmission. Because of the diversity of infectious microorganisms, their host ranges, and the different ecological systems in which they circulate, definitions of a natural reservoir are numerous and sometimes conflicting. In general, the reservoir concept applies most clearly to pathogens capable of infecting more than one host population and is defined relative to a target population, meaning the population in which disease occurrence is being studied or controlled. The reservoir is any population of organisms, or in some cases any environment, in which the pathogen is maintained and from which infection is transmitted to the target population. Reservoirs may involve one or more species, may be the same as or different from the target population, and in broad usage may overlap with vector based transmission systems, although vectors are generally considered distinct from reservoirs. Identifying the natural reservoirs of infectious pathogens is important for preventing and controlling outbreaks in humans and domestic animals, particularly for zoonotic diseases and for diseases without effective vaccines. In some settings, reservoir directed interventions such as animal vaccination, reduction of human exposure, environmental remediation, or vector control may reduce transmission. However, identifying a true reservoir can be difficult, and for some pathogens, including the ebolaviruses, the presumed reservoir remains uncertain.
Definition and terminology The diversity of pathogens, hosts, and transmission systems has resulted in multiple definitions of the term natural reservoir, some of which emphasize a host species and others an ecological system. In a 2002 conceptual analysis published in Emerging Infectious Diseases, a natural reservoir was defined as "one or more epidemiologically connected populations or environments in which the pathogen can be permanently maintained and from which infection is transmitted to the defined target population". The target population is the population or species in which the pathogen causes disease and in which epidemiologists are attempting to measure, predict, or prevent transmission. In many medical contexts, humans are the target population, but in veterinary and wildlife epidemiology the target may be livestock, companion animals, or wildlife. A population may therefore function as a reservoir in one epidemiological context but not in another. Some definitions distinguish reservoirs from non reservoirs by the extent to which infected hosts develop clinical disease. Under this narrower interpretation, a reservoir host may harbor and transmit a pathogen while experiencing few or no symptoms. However, lack of symptoms is not required in all definitions, and some recognized reservoir hosts may still develop illness under certain conditions. A pathogen capable of being maintained in more than one host population may have multiple natural reservoirs.
Reservoir, source, and vector The terms reservoir, source of infection, and vector are related but not synonymous. The reservoir is the ecological habitat in which a pathogen normally lives, persists, and reproduces. The source of infection is the person, animal, object, or substance from which a host actually acquires infection. In some cases the reservoir and the source are the same, but in others they differ. For example, contaminated water or food may serve as the immediate source of infection even when the long term reservoir is environmental or animal based. A vector is a living organism, commonly an arthropod such as a mosquito, flea, or tick, that transmits a pathogen from one host to another. Vectors may play an essential role in transmission but are not automatically reservoirs. In some disease systems, the pathogen multiplies within the vector, whereas in others the vector mainly serves as a carrier. Distinguishing among reservoir, source, and vector is important in epidemiology because each implies different control strategies.
Types of reservoirs Natural reservoirs are commonly divided into three major categories: human, animal, and environmental.
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