Natural reservoir

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. 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. Humans can also act as reservoirs for sexually transmitted diseases, measles, mumps, streptococcal infection, and various respiratory pathogens.. These microbial populations may function as reservoirs for not only pathogens but also for genetic elements that enable resistance, collectively referred to as the resistome. Horizontal gene transfer mechanisms allow resistance genes to move between commensal and pathogenic bacteria within the same host increasing the potential for the emergence of drug-resistant infections. As a result, the human microbiome plays an important role in shaping transmission dynamics, particularly for antimicrobial resistance.

Understanding human reservoirs therefore requires not only identifying infected individuals, but also accounting for colonisation, microbial community structure and the movement of resistance genes within and between hosts. This perspective has become increasingly important in epidemiology as antimicrobial resistance is now recognised as a global public health challenge.

Animal reservoirs

Animal reservoirs include domesticated and wild animals infected with pathogens that can be maintained in animal populations and transmitted to humans or other susceptible hosts. Viruses of the taxon Ebolavirus, which cause Ebola virus disease, are thought to have a natural reservoir in bats or related wildlife species, although the exact reservoir system remains incompletely defined.

Other zoonotic diseases transmitted from animals to humans include rabies, blastomycosis, psittacosis, trichinosis, cat-scratch disease, histoplasmosis, coccidioidomycosis, and salmonella.

Common animal reservoirs include bats, rodents, cattle, pigs, sheep, rabbits, raccoons, dogs, and other mammals. Their species diversity, social behavior, migration patterns, and long lifespan have all been proposed as reasons they are important hosts in the ecology of emerging viruses. Lyssaviruses, including the Rabies virus, henipaviruses, Menangle virus, Tioman virus, SARS related coronaviruses, and filoviruses have all been linked to different bat species.

Rodents

Rodents are important reservoirs for several zoonotic pathogens. White footed mice (Peromyscus leucopus) are among the most important reservoirs for the Lyme disease spirochete Borrelia burgdorferi. Deer mice serve as reservoir hosts for Sin Nombre virus, which causes hantavirus pulmonary syndrome. Rats have also been implicated in the ecology of diseases such as Chagas disease and other rodent associated infections.

Environmental reservoirs

Environmental reservoirs include living and non living reservoirs that harbor pathogens outside the bodies of animals. These reservoirs may occur in soil, water, plants, built environments, or air. Environmental reservoirs are especially important in diseases influenced by sanitation, water quality, and contamination of built environments.

Transmission from reservoirs

A disease reservoir functions as the ecological setting from which a pathogen reaches a susceptible host. Because many emerging infectious diseases originate in animal reservoirs before spilling over into humans, identifying reservoir hosts and the conditions that facilitate transmission is an important part of outbreak prevention and pandemic preparedness.

The One Health approach emphasizes collaboration across medicine, veterinary science, wildlife biology, and environmental health. In practice, this includes surveillance of animal reservoir populations alongside human disease monitoring, laboratory capacity building, outbreak reporting systems, and community based strategies to reduce high risk exposure at the human animal interface.

Climate change and shifting reservoirs

Climate change is increasingly recognized as a factor influencing the distribution, abundance, and behavior of reservoir species and vectors, with implications for the emergence and re emergence of infectious diseases. Rising temperatures, changing precipitation patterns, and extreme weather events can alter the geographic range of host and vector populations, creating new opportunities for pathogens to encounter susceptible hosts.

Environmental disruption linked to climate change, deforestation, agricultural expansion, and urbanization can also increase contact among wildlife, livestock, and humans, thereby increasing opportunities for pathogen spillover.

Public health significance

A substantial proportion of human pathogens are zoonotic, which makes reservoir identification an important part of public health preparedness, surveillance, and prevention. Identifying which populations or environments maintain a pathogen can guide targeted interventions such as animal vaccination, reduction of wildlife exposure, vector control, sanitation measures, and environmental decontamination.

Reservoir surveillance has also become an important part of efforts to predict and prevent future outbreaks. Programs such as the PREDICT Project and other One Health initiatives have sought to improve detection of zoonotic threats at the wildlife human interface and strengthen preparedness for emerging infectious diseases.