Natural reservoir

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Cows are natural reservoirs of African trypanosomiasis Cow horned portrait.jpg
Cows are natural reservoirs of African trypanosomiasis

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]

Contents

Because of the enormous variety of infectious microorganisms capable of causing disease, precise definitions for what constitutes a natural reservoir are numerous, various, and often conflicting. The reservoir concept applies only for pathogens capable of infecting more than one host population and only with respect to a defined target population – the population of organisms in which the pathogen causes disease. The reservoir is any population of organisms (or any environment) which harbors the pathogen and transmits it to the target population. Reservoirs may comprise one or more different species, may be the same or a different species as the target, and, in the broadest sense, may include vector species, [2] which are otherwise distinct from natural reservoirs. Significantly, species considered reservoirs for a given pathogen may not experience symptoms of disease when infected by the pathogen.

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.

Definition and terminology

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 ]

Types of reservoirs

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

Human reservoirs

Human reservoirs are human beings infected by pathogens that exist on or within the human body. [1] Infections like poliomyelitis and smallpox, which exist exclusively within a human reservoir, are sometimes known as anthroponoses. [5] [6] Humans can act as reservoirs for sexually transmitted diseases, measles, mumps, streptococcal infection, various respiratory pathogens, and the smallpox virus. [1]

Bushmeat being prepared for cooking in Ghana, 2013. Human consumption of animals as bushmeat in equatorial Africa has caused the transmission of diseases, including Ebola, to people. Bushmeat - Buschfleisch Ghana.JPG
Bushmeat being prepared for cooking in Ghana, 2013. Human consumption of animals as bushmeat in equatorial Africa has caused the transmission of diseases, including Ebola, to people.

Animal reservoirs

Animal (non-human) reservoirs consist of domesticated and wild animals infected by pathogens. [1] [2] For example, the bacterium Vibrio cholerae , which causes cholera in humans, has natural reservoirs in copepods, zooplankton, and shellfish. Parasitic blood-flukes of the genus Schistosoma , responsible for schistosomiasis, spend part of their lives inside freshwater snails before completing their life cycles in vertebrate hosts. [8] Viruses of the taxon Ebolavirus , which causes Ebola virus disease, are thought to have a natural reservoir in bats or other animals exposed to the virus. [9] Other zoonotic diseases that have been transmitted from animals to humans include: rabies, blastomycosis, psittacosis, trichinosis, cat-scratch disease, histoplasmosis, coccidiomycosis and salmonella. [10]

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

Common animal reservoirs

Bats

Numerous zoonotic diseases have been traced back to bats. [11] There are a couple of theories that serve as possible explanations as to why bats carry so many viruses. One proposed theory is that there exist so many bat-borne illnesses because there exist a large number of bat species and individuals. The second possibility is that something about bats' physiology makes them especially good reservoir hosts. [11] Perhaps bats' "food choices, population structure, ability to fly, seasonal migration and daily movement patterns, torpor and hibernation, life span, and roosting behaviors" are responsible for making them especially suitable reservoir hosts. [12] Lyssaviruses (including the Rabies virus), Henipaviruses, Menangle and Tioman viruses, SARS-CoV-Like Viruses, and Ebola viruses have all been traced back to different species of bats. [12] Fruit bats in particular serve as the reservoir host for Nipah virus (NiV). [13]

Rats

Rats are known to be the reservoir hosts for a number of zoonotic diseases. Norway rats were found to be infested with the Lyme disease spirochetes. [14] In Mexico rats are known carriers of Trypanosoma cruzi , which causes Chagas disease. [15]

Mice

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]

Monkeys

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

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 in the air. [1] Pathogens found in these reservoirs are sometimes free-living. The bacteria Legionella pneumophila , a facultative intracellular parasite which causes Legionnaires' disease, and Vibrio cholerae , which causes cholera, can both exist as free-living parasites in certain water sources as well as in invertebrate animal hosts. [1] [19]

Disease transmission

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

Direct transmission

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 consequently take no special precautions to prevent transmission. Symptomatic persons who are 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 droplet suspended in 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 by talking.[ citation needed ]

Indirect transmission

Indirect transmission can occur by airborne transmission, by vehicles (including fomites), and by vectors.[ citation needed ]

Airborne transmission is different from direct droplet spread as it is defined as disease transmission that takes place over a distance larger than a meter. Pathogens that can be transmitted through airborne sources are carried by particles such as dust or dried residue (referred to as droplet nuclei).[ citation needed ]

Vehicles such as food, water, blood and fomites can act as passive transmission points between reservoirs and susceptible hosts. Fomites are inanimate objects (doorknobs, medical equipment, etc.) that become contaminated by a reservoir source or someone/something that is a carrier. A vehicle, like a reservoir, may also be a favorable environment for the growth of an infectious agent, as coming into contact with a vehicle leads to its transmission.[ citation needed ]

Vector transmission occurs most often from insect bites from mosquitoes, flies, fleas, and ticks. There are two sub-categories of vectors: mechanical (an insect transmits the pathogen to a host without the insect itself being affected) and biological (reproduction of the pathogen occurs within the vector before the pathogen is transmitted to a host). To give a few examples, Morbillivirus (measles) is transmitted from an infected human host to a susceptible host as they are transmitted by respiration through airborne transmission. Campylobacter (campylobacteriosis) is a common bacterial infection that is spread from human or non-human reservoirs by vehicles such as contaminated food and water. Plasmodium falciparum (malaria) can be transmitted from an infected mosquito, an animal (non-human) reservoir, to human host by biological vector transmission.[ citation needed ]

Implications for public health

LH Taylor found that 61% of all human pathogens are classified as zoonotic. [20] Thus, the identification of the natural reservoirs of pathogens prior to zoonosis would be incredibly useful from a public health standpoint. Preventive measures can be taken to lessen the frequency of outbreaks, such as vaccinating the animal sources of disease or preventing contact with reservoir host animals. [21] In an effort to predict and prevent future outbreaks of zoonotic diseases, the U.S. Agency for International Development started the Emerging Pandemic Threats initiative in 2009. In alliance with University of California-Davis, EcoHealth Alliance, Metabiota Inc., Smithsonian Institution, and Wildlife Conservation Society with support from Columbia and Harvard universities, the members of the PREDICT project are focusing on the "detection and discovery of zoonotic diseases at the wildlife-human interface." [22] There are numerous other organizations around the world experimenting with different methods to predict and identify reservoir hosts. Researchers at the University of Glasgow created a machine learning algorithm that is designed to use "viral genome sequences to predict the likely natural host for a broad spectrum of RNA viruses, the viral group that most often jumps from animals to humans." [21]

See also

Related Research Articles

A human pathogen is a pathogen that causes disease in humans.

<span class="mw-page-title-main">Zoonosis</span> Disease that can be transmitted from other species to humans

A zoonosis or zoonotic disease is an infectious disease of humans caused by a pathogen that can jump from a non-human to a human and vice versa.

<span class="mw-page-title-main">Infection</span> Invasion of an organisms body by pathogenic agents

An infection is the invasion of tissues by pathogens, their multiplication, and the reaction of host tissues to the infectious agent and the toxins they produce. An infectious disease, also known as a transmissible disease or communicable disease, is an illness resulting from an infection.

<span class="mw-page-title-main">Epidemic</span> Rapid spread of disease affecting a large number of people in a short time

An epidemic is the rapid spread of disease to a large number of hosts in a given population within a short period of time. For example, in meningococcal infections, an attack rate in excess of 15 cases per 100,000 people for two consecutive weeks is considered an epidemic.

In medicine, public health, and biology, transmission is the passing of a pathogen causing communicable disease from an infected host individual or group to a particular individual or group, regardless of whether the other individual was previously infected. The term strictly refers to the transmission of microorganisms directly from one individual to another by one or more of the following means:

An emergent virus is a virus that is either newly appeared, notably increasing in incidence/geographic range or has the potential to increase in the near future. Emergent viruses are a leading cause of emerging infectious diseases and raise public health challenges globally, given their potential to cause outbreaks of disease which can lead to epidemics and pandemics. As well as causing disease, emergent viruses can also have severe economic implications. Recent examples include the SARS-related coronaviruses, which have caused the 2002-2004 outbreak of SARS (SARS-CoV-1) and the 2019–21 pandemic of COVID-19 (SARS-CoV-2). Other examples include the human immunodeficiency virus which causes HIV/AIDS; the viruses responsible for Ebola; the H5N1 influenza virus responsible for avian flu; and H1N1/09, which caused the 2009 swine flu pandemic. Viral emergence in humans is often a consequence of zoonosis, which involves a cross-species jump of a viral disease into humans from other animals. As zoonotic viruses exist in animal reservoirs, they are much more difficult to eradicate and can therefore establish persistent infections in human populations.

A reverse zoonosis, also known as a zooanthroponosis or anthroponosis, is a pathogen reservoired in humans that is capable of being transmitted to non-human animals.

<span class="mw-page-title-main">Disease vector</span> Agent that carries and transmits an infectious pathogen into another living organism

In epidemiology, a disease vector is any living agent that carries and transmits an infectious pathogen to another living organism; agents regarded as vectors are organisms, such as parasites or microbes. The first major discovery of a disease vector came from Ronald Ross in 1897, who discovered the malaria pathogen when he dissected a mosquito.

Ehrlichia chaffeensis is an obligate intracellular, Gram-negative species of Rickettsiales bacteria. It is a zoonotic pathogen transmitted to humans by the lone star tick. It is the causative agent of human monocytic ehrlichiosis.

A robovirus is a zoonotic virus that is transmitted by a rodent vector.

A fomite or fomes is any inanimate object that, when contaminated with or exposed to infectious agents, can transfer disease to a new host.

Spillover infection, also known as pathogen spillover and spillover event, occurs when a reservoir population with a high pathogen prevalence comes into contact with a novel host population. The pathogen is transmitted from the reservoir population and may or may not be transmitted within the host population. Due to climate change and land use expansion, the risk of viral spillover is predicted to significantly increase.

Cross-species transmission (CST), also called interspecies transmission, host jump, or spillover, is the transmission of an infectious pathogen, such as a virus, between hosts belonging to different species. Once introduced into an individual of a new host species, the pathogen may cause disease for the new host and/or acquire the ability to infect other individuals of the same species, allowing it to spread through the new host population. The phenomenon is most commonly studied in virology, but cross-species transmission may also occur with bacterial pathogens or other types of microorganisms.

<span class="mw-page-title-main">Bat virome</span> Group of viruses associated with bats

The bat virome is the group of viruses associated with bats. Bats host a diverse array of viruses, including all seven types described by the Baltimore classification system: (I) double-stranded DNA viruses; (II) single-stranded DNA viruses; (III) double-stranded RNA viruses; (IV) positive-sense single-stranded RNA viruses; (V) negative-sense single-stranded RNA viruses; (VI) positive-sense single-stranded RNA viruses that replicate through a DNA intermediate; and (VII) double-stranded DNA viruses that replicate through a single-stranded RNA intermediate. The greatest share of bat-associated viruses identified as of 2020 are of type IV, in the family Coronaviridae.

Disease ecology is a sub-discipline of ecology concerned with the mechanisms, patterns, and effects of host-pathogen interactions, particularly those of infectious diseases. For example, it examines how parasites spread through and influence wildlife populations and communities. By studying the flow of diseases within the natural environment, scientists seek to better understand how changes within our environment can shape how pathogens, and other diseases, travel. Therefore, diseases ecology seeks to understand the links between ecological interactions and disease evolution. New emerging and re-emerging infectious diseases are increasing at unprecedented rates which can have lasting impacts on public health, ecosystem health, and biodiversity.

<span class="mw-page-title-main">Wildlife trade and zoonoses</span> Health risks associated with the trade in exotic wildlife

Wildlife trafficking practices have resulted in the emergence of zoonotic diseases. Exotic wildlife trafficking is a multi-billion dollar industry that involves the removal and shipment of mammals, reptiles, amphibians, invertebrates, and fish all over the world. Traded wild animals are used for bushmeat consumption, unconventional exotic pets, animal skin clothing accessories, home trophy decorations, privately owned zoos, and for traditional medicine practices. Dating back centuries, people from Africa, Asia, Latin America, the Middle East, and Europe have used animal bones, horns, or organs for their believed healing effects on the human body. Wild tigers, rhinos, elephants, pangolins, and certain reptile species are acquired through legal and illegal trade operations in order to continue these historic cultural healing practices. Within the last decade nearly 975 different wild animal taxa groups have been legally and illegally exported out of Africa and imported into areas like China, Japan, Indonesia, the United States, Russia, Europe, and South America.

<span class="mw-page-title-main">Climate change and infectious diseases</span> Overview of the relationship between climate change and infectious diseases

Global climate change has resulted in a wide range of impacts on the spread of infectious diseases. Like other climate change impacts on human health, climate change exacerbates existing inequalities and challenges in managing infectious disease. Infectious diseases whose transmission can be impacted by climate change include dengue fever, malaria, tick-borne diseases, leishmaniasis and Ebola virus disease. For example, climate change is altering the geographic range and seasonality of the mosquito that can carry dengue.

Human-to-human transmission (HHT) is an epidemiologic vector, especially in case the disease is borne by individuals known as superspreaders. In these cases, the basic reproduction number of the virus, which is the average number of additional people that a single case will infect without any preventative measures, can be as high as 203.9. Interhuman transmission is a synonym for HHT.

<span class="mw-page-title-main">Transmission of COVID-19</span> Mechanisms that spread coronavirus disease 2019

The transmission of COVID-19 is the passing of coronavirus disease 2019 from person to person. COVID-19 is mainly transmitted when people breathe in air contaminated by droplets/aerosols and small airborne particles containing the virus. Infected people exhale those particles as they breathe, talk, cough, sneeze, or sing. Transmission is more likely the more physically close people are. However, infection can occur over longer distances, particularly indoors.

<span class="mw-page-title-main">David Hayman (disease ecologist)</span> New Zealand epizootic epidemiologist

David Hayman is a New Zealand-based epizootic epidemiologist and disease ecologist whose general multi-disciplinary work focuses on the maintenance of infectious diseases within their hosts and the process of emergence and transmission to humans specifically related to bats. He has gathered data on the relationship between ecological degradation due to anthropogenic actions, and increased pathogen emergence in humans and animals. During COVID-19 he was involved as an expert in several international collaborations, some convened by the World Health Organization, and was a regular commentator in the New Zealand media about the country's response to the pandemic. He has had lead roles in research organisations at Massey University and Te Pūnaha Matatini and was the recipient of the 2017 Rutherford Discovery Fellowship Award. Since 2014 Hayman has been a professor at Massey University.

References

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  11. 1 2 "Why Bats Are Such Good Hosts for Ebola and Other Deadly Diseases". WIRED. Retrieved 2018-10-23.
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  14. ↑ Smith, R. P.; Rand, P. W.; Lacombe, E. H.; Telford, S. R.; Rich, S. M.; Piesman, J.; Spielman, A. (September 1993). "Norway rats as reservoir hosts for Lyme disease spirochetes on Monhegan Island, Maine". The Journal of Infectious Diseases. 168 (3): 687–691. doi:10.1093/infdis/168.3.687. ISSN   0022-1899. PMID   8354910.
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  21. 1 2 "Machine learning tool can predict viral reservoirs in the animal kingdom" . Retrieved 2018-11-10.
  22. ↑ "Emerging Pandemic Threats | Fact Sheet | U.S. Agency for International Development". usaid.gov . 2016-05-24. Retrieved 2018-10-23.
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