Natural reservoir

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." 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).{{Cite journal|date=2002-12-01|title=Identifying Reservoirs of Infection: A Conceptual and Practical Challenge|journal=Emerging Infectious Diseases|volume=8|issue=12|pages=1468–1473|doi=10.3201/eid0812.010317|issn=1080-6040|pmc=2738515|pmid=12498665|last1=Haydon|first1=D. T.|last2=Cleaveland|first2=S.|last3=Taylor|first3=L. H.|last4=Laurenson|first4=M. K.}}

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.{{Cite journal|last1=Wallace|first1=Ryan M.|last2=Gilbert|first2=Amy|last3=Slate|first3=Dennis|last4=Chipman|first4=Richard|last5=Singh|first5=Amber|last6=Cassie Wedd|last7=Blanton|first7=Jesse D.|date=2014-10-08|title=Right Place, Wrong Species: A 20-Year Review of Rabies Virus Cross Species Transmission among Terrestrial Mammals in the United States|journal=PLOS ONE|language=en|volume=9|issue=10|pages=e107539|doi=10.1371/journal.pone.0107539|issn=1932-6203|pmc=4189788|pmid=25295750|bibcode=2014PLoSO...9j7539W |doi-access=free}} 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.{{cn|date=May 2023}}

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.{{cn|date=May 2023}}

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

Human reservoirs are human beings infected by pathogens that exist on or within the human body. Infections like poliomyelitis and smallpox, which exist exclusively within a human reservoir, are sometimes known as anthroponoses.{{Cite journal|last=Noble, Jr.|first=John|year=1970|title=A Study of New and Old World Monkeys to Determine the Likelihood of a Simian Reservoir|journal=Bulletin of the World Health Organization|volume=42|pages=509–514}}{{Citation |last=von Csefalvay |first=Chris |title=Host-vector and multihost systems |date=2023 |url=https://linkinghub.elsevier.com/retrieve/pii/B978032395389400013X |work=Computational Modeling of Infectious Disease |pages=121–149 |publisher=Elsevier |language=en |doi=10.1016/b978-0-32-395389-4.00013-x |isbn=978-0-323-95389-4 |access-date=2023-03-02|url-access=subscription }} Humans can act as reservoirs for sexually transmitted diseases, measles, mumps, streptococcal infection, various respiratory pathogens, and the smallpox virus.

File:Bushmeat - Buschfleisch Ghana.JPG 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.{{Cite web|url=https://www.independent.co.uk/life-style/health-and-families/health-news/african-monkey-meat-that-could-be-behind-the-next-hiv-7786152.html|title=African monkey meat that could be behind the next HIV|date=May 26, 2012|website=The Independent}}]]

Animal (non-human) reservoirs consist of domesticated and wild animals infected by pathogens. 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.{{cite book|title=Parasitic helminths and zoonoses in Africa|last1=Macpherson|first1=C.N.L.|last2=Craig|first2=P.S.|chapter=Animal reservoirs of schistosomiasis |date=1991|pages=224–236 |publisher=Springer|isbn=978-94-010-5358-7|editor1-last=Macpherson|editor1-first=C.N.L.|location=Dordrecht|editor2-last=Craig|editor2-first=P.S.|doi=10.1007/978-94-011-3054-7_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.{{Cite web|url=https://www.cdc.gov/vhf/ebola/about.html|title=About Ebola Hemorrhagic Fever{{!}} Ebola Hemorrhagic Fever {{!}} CDC|website=cdc.gov|language=en-us|access-date=2017-12-03}} Other zoonotic diseases that have been transmitted from animals to humans include: rabies, blastomycosis, psittacosis, trichinosis, cat-scratch disease, histoplasmosis, coccidioidomycosis, and salmonella.{{Cite web|url=http://www.health.state.mn.us/divs/idepc/dtopics/zoo/|title=Zoonotic Diseases: Disease Transmitted from Animals to Humans - Minnesota Dept. of Health|website=health.state.mn.us|language=en|access-date=2018-11-10}}

Common animal reservoirs include: bats, rodents, cows, pigs, sheep, swine, rabbits, raccoons, dogs, and other mammals.

{{More|Bat-borne virus}}

Numerous zoonotic diseases have been traced back to bats.{{Cite magazine|url=https://www.wired.com/2014/10/bats-ebola-disease-reservoir-hosts/|title=Why Bats Are Such Good Hosts for Ebola and Other Deadly Diseases|magazine=WIRED|access-date=2018-10-23|language=en-US}} 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. 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.{{Cite journal|author1-link=Charles Calisher|last1=Calisher|first1=Charles H.|last2=Childs|first2=James E.|last3=Field|first3=Hume E.|last4=Holmes|first4=Kathryn V.|last5=Schountz|first5=Tony|date=2006-07-01|title=Bats: Important Reservoir Hosts of Emerging Viruses|journal=Clinical Microbiology Reviews|language=en|volume=19|issue=3|pages=531–545|doi=10.1128/CMR.00017-06|issn=0893-8512|pmid=16847084|pmc=1539106}} 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. Fruit bats in particular serve as the reservoir host for Nipah virus (NiV).{{Cite book|url=https://www.ncbi.nlm.nih.gov/books/NBK114486/|title=TRANSMISSION OF HUMAN INFECTION WITH NIPAH VIRUS|last1=Luby|first1=Stephen P.|last2=Gurley|first2=Emily S.|last3=Hossain|first3=M. Jahangir|date=2012|publisher=National Academies Press (US)|language=en}}

Rats are known to be the reservoir hosts for several zoonotic diseases. Norway rats were found to be infested with the Lyme disease spirochetes.{{Cite journal|last1=Smith|first1=R. P.|last2=Rand|first2=P. W.|last3=Lacombe|first3=E. H.|last4=Telford|first4=S. R.|last5=Rich|first5=S. M.|last6=Piesman|first6=J.|last7=Spielman|first7=A.|date=September 1993|title=Norway rats as reservoir hosts for Lyme disease spirochetes on Monhegan Island, Maine|journal=The Journal of Infectious Diseases|volume=168|issue=3|pages=687–691|issn=0022-1899|pmid=8354910|doi=10.1093/infdis/168.3.687}} In Mexico rats are known carriers of Trypanosoma cruzi, which causes Chagas disease.{{Cite journal|date=2018-03-01|title=Chagas disease: Importance of rats as reservoir hosts of Trypanosoma cruzi (Chagas, 1909) in western Mexico|journal=Journal of Infection and Public Health|language=en|volume=11|issue=2|pages=230–233|doi=10.1016/j.jiph.2017.07.017|pmid=28774654|issn=1876-0341|last1=Rosal|first1=Gumercindo G.|last2=Nogueda-Torres|first2=Benjamín|last3=Villagrán|first3=María E.|last4=De Diego-Cabrera|first4=José A.|last5=Montañez-Valdez|first5=Oziel D.|last6=Martínez-Ibarra|first6=José A.|doi-access=free}}

White-footed mice (Peromyscus leucopus) are one of the most important animal reservoirs for the Lyme disease spirochete (Borrelia burgdorferi).{{Cite journal|last1=MATHER|first1=THOMAS N.|last2=WILSON|first2=MARK L.|last3=MOORE|first3=SEAN I.|last4=RIBEIRO|first4=JOSE M. C|last5=SPIELMAN|first5=ANDREW|title=Comparing the Relative Potential of Rodents as Reservoirs of the Lyme Disease Spirochete (Borreliaburgdorferi)|date=July 1989|journal=American Journal of Epidemiology|language=en|volume=130|issue=1|pages=143–150|doi=10.1093/oxfordjournals.aje.a115306|pmid=2787105|issn=1476-6256}} Deer mice serve as reservoir hosts for Sin Nombre virus, which causes hantavirus pulmonary syndrome (HPS).{{Cite web|url=https://www.cdc.gov/hantavirus/technical/hps/ecology.html|title=Ecology {{!}} Hantavirus {{!}} DHCPP |website=cdc.gov|language=en-us|access-date=2018-11-17}}

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.{{Cite web|url=http://agencia.fapesp.br/discovery-of-zika-virus-in-monkeys-suggests-disease-may-also-have-wild-cycle-/29081/|title=Discovery of Zika virus in monkeys suggests disease may also have wild cycle|website=AGÊNCIA FAPESP|language=en|access-date=2018-11-17}}

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. Pathogens 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.{{Cite journal|last1=Nelson|first1=Eric J.|last2=Harris|first2=Jason B.|last3=Morris|first3=J. Glenn|last4=Calderwood|first4=Stephen B.|last5=Camilli|first5=Andrew|date=2009|title=Cholera transmission: the host, pathogen and bacteriophage dynamic|journal=Nature Reviews Microbiology|volume=7|issue=10|pages=693–702|doi=10.1038/nrmicro2204|pmid=19756008|pmc=3842031}}

A disease reservoir acts as a transmission point between a pathogen and a susceptible host. 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. 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.{{cn|date=May 2023}}

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

Indirect transmission can occur by airborne transmission, by vehicles (including fomites), and by vectors.{{cn|date=May 2023}}

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).{{cn|date=May 2023}}

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.{{cn|date=May 2023}}

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 a human host by biological vector transmission.{{cn|date=May 2023}}

LH Taylor found that 61% of all human pathogens are classified as zoonotic.{{Cite journal|last1=Taylor|first1=L H|last2=Latham|first2=S M|last3=Woolhouse|first3=M E|date=2001-07-29|title=Risk factors for human disease emergence.|journal=Philosophical Transactions of the Royal Society B: Biological Sciences|volume=356|issue=1411|pages=983–989|doi=10.1098/rstb.2001.0888|issn=0962-8436|pmc=1088493|pmid=11516376}} Thus, the identification of the natural reservoirs of pathogens before 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.{{Cite news|url=https://phys.org/news/2018-11-machine-tool-viral-reservoirs-animal.html|title=Machine learning tool can predict viral reservoirs in the animal kingdom|access-date=2018-11-10}} 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."{{Cite web|url=https://www.usaid.gov/news-information/fact-sheets/emerging-pandemic-threats-program|archive-url=https://archive.today/20200414001954/https://www.usaid.gov/news-information/fact-sheets/emerging-pandemic-threats-program|url-status=dead|archive-date=April 14, 2020|title=Emerging Pandemic Threats {{!}} Fact Sheet|website=usaid.gov|language=en|access-date=2018-10-23|date=2016-05-24}} 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."

• Fomite
• Host (biology)
• Refuge (ecology)
• Vector (epidemiology)
• Zoonosis

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