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 and reproduces, or upon which the pathogen primarily depends for its survival. A reservoir is usually a living host of a certain species, such as an animal or a plant, inside of which a pathogen survives, often (though not always) without causing disease for the reservoir itself. By some definitions a reservoir may also be an environment external to an organism, such as a volume of contaminated air or water.^[1][2]

Because of the enormous variety of infectious

Identifying the natural reservoirs of infectious pathogens has proven useful in treating and preventing large outbreaks of disease in humans and domestic animals, especially those diseases for which no vaccine exists. In principle, zoonotic diseases can be controlled by isolating or destroying the pathogen's reservoirs of infection. The mass culling of animals confirmed or suspected as reservoirs for human pathogens, such as birds that harbor avian influenza, has been effective at containing possible epidemics in many parts of the world; for other pathogens, such as the ebolaviruses, the identity of the presumed natural reservoir remains obscure.

The great diversity of infectious pathogens, their possible hosts, and the ways in which their hosts respond to infection has resulted in multiple definitions for "natural reservoir", many of which are conflicting or incomplete. In a 2002 conceptual exploration published in the CDC's Emerging Infectious Diseases, the natural reservoir of a given pathogen is 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."^[2] The target population is the population or species in which the pathogen causes disease; it is the population of interest because it has disease when infected by the pathogen (for example, humans are the target population in most medical epidemiological studies).^[3]

A common criterion in other definitions distinguishes reservoirs from non-reservoirs by the degree to which the infected host shows symptoms of disease. By these definitions, a reservoir is a host that does not experience the symptoms of disease when infected by the pathogen, whereas non-reservoirs show symptoms of the disease.^[4] The pathogen still feeds, grows, and reproduces inside a reservoir host, but otherwise does not significantly affect its health; the relationship between pathogen and reservoir is more or less commensal, whereas in susceptible hosts that do develop disease caused by the pathogen, the pathogen is considered parasitic.^{[citation needed]}

What further defines a reservoir for a specific pathogen is where it can be maintained and from where it can be transmitted. A "multi-host" organism is capable of having more than one natural reservoir.^{[citation needed]}

Natural reservoirs can be divided into three main types: human, animal (non-human), and environmental.^[1]

Human reservoirs are human beings infected by pathogens that exist on or within the human body.

Animal (non-human) reservoirs consist of domesticated and wild animals infected by pathogens.

Common animal reservoirs include: bats, rodents, cows, pigs, sheep, swine, rabbits, raccoons, dogs, and other mammals.[1]^[6]

Numerous zoonotic diseases have been traced back to bats.

White-footed mice (Peromyscus leucopus) are one of the most important animal reservoirs for the Lyme disease spirochete (Borrelia burgdorferi).^[16] Deer mice serve as reservoir hosts for Sin Nombre virus, which causes hantavirus pulmonary syndrome (HPS).^[17]

The Zika virus originated from monkeys in Africa. In São José do Rio Preto and Belo Horizonte, Brazil the Zika virus has been found in dead monkeys. Genome sequencing has revealed the virus to be very similar to the type that infects humans.^[18]

Environmental reservoirs include living and non-living reservoirs that harbor infectious pathogens outside the bodies of animals. These reservoirs may exist on land (plants and soil), in water, or the air.

A disease reservoir acts as a transmission point between a pathogen and a susceptible host.[1] Transmission can occur directly or indirectly.

Direct transmission can occur from direct contact or direct droplet spread. Direct contact transmission between two people can happen through skin contact, kissing, and sexual contact. Humans serving as disease reservoirs can be symptomatic (showing illness) or asymptomatic (not showing illness), act as disease carriers, and often spread illness unknowingly. Human carriers commonly transmit disease because they do not realize they are infected and take no special precautions to prevent transmission. Symptomatic persons aware of their illness are not as likely to transmit infection because they take precautions to reduce possible transmission of the disease and/or seek out treatment to prevent the spread of the disease.^[1] Direct droplet spread is due to solid particles or liquid droplets suspended in the air for some time. Droplet spread is considered the transmission of the pathogen to a susceptible host within a meter of distance; said droplet spread can occur from coughing, sneezing, and/or just talking.^{[citation needed]}

• Neisseria gonorrhoeae (Gonorrhea) is transmitted by sexual contact involving the penis, vagina, mouth, and anus through direct contact transmission.
• Pertussis
  ) is transmitted by cough from the human reservoir to the susceptible host through direct droplet spread.