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In
infectious
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 dis ...
disease ecology and
epidemiology
Epidemiology is the study and analysis of the distribution (who, when, and where), patterns and determinants of health and disease conditions in a defined population.
It is a cornerstone of public health, and shapes policy decisions and evide ...
, a natural reservoir, also known as a disease reservoir or a reservoir of infection, is the
population
Population typically refers to the number of people in a single area, whether it be a city or town, region, country, continent, or the world. Governments typically quantify the size of the resident population within their jurisdiction using a ...
of organisms or the specific environment in which an infectious
pathogen
In biology, a pathogen ( el, πάθος, "suffering", "passion" and , "producer of") in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a germ ...
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.
Because of the enormous variety of infectious
microorganism
A microorganism, or microbe,, ''mikros'', "small") and ''organism'' from the el, ὀργανισμός, ''organismós'', "organism"). It is usually written as a single word but is sometimes hyphenated (''micro-organism''), especially in olde ...
s 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,
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
A vaccine is a biological preparation that provides active acquired immunity to a particular infectious or malignant disease. The safety and effectiveness of vaccines has been widely studied and verified. exists. In principle,
zoonotic diseases
A zoonosis (; plural zoonoses) or zoonotic disease is an infectious disease of humans caused by a pathogen (an infectious agent, such as a bacterium, virus, parasite or prion) that has jumped from a non-human (usually a vertebrate) to a human ...
can be controlled by isolating or destroying the pathogen's reservoirs of infection. The mass
culling
In biology, culling is the process of segregating organisms from a group according to desired or undesired characteristics. In animal breeding, it is the process of removing or segregating animals from a breeding stock based on a specific tr ...
of animals confirmed or suspected as reservoirs for human pathogens, such as birds that harbor
avian influenza
Avian influenza, known informally as avian flu or bird flu, is a variety of influenza caused by viruses adapted to birds. , has been effective at containing possible
epidemic
An epidemic (from Greek ἐπί ''epi'' "upon or above" and δῆμος ''demos'' "people") is the rapid spread of disease to a large number of patients among a given population within an area in a short period of time.
Epidemics of infectious ...
s in many parts of the world; for other pathogens, such as the
ebolavirus
The genus ''Ebolavirus'' (- or ; - or ) is a virological taxon included in the family '' Filoviridae'' (filament-shaped viruses), order ''Mononegavirales''. The members of this genus are called ebolaviruses, and encode their genome in the for ...
es, 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
The Centers for Disease Control and Prevention (CDC) is the national public health agency of the United States. It is a United States federal agency, under the Department of Health and Human Services, and is headquartered in Atlanta, Georgi ...
's ''
Emerging Infectious Diseases
''Emerging Infectious Diseases'' (EID) is an open-access, peer-reviewed journal published by the Centers for Disease Control and Prevention (CDC). EID is a public domain journal and covers global instances of new and reemerging infectious diseas ...
'', 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).
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. 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
Commensalism is a long-term biological interaction (symbiosis) in which members of one species gain benefits while those of the other species neither benefit nor are harmed. This is in contrast with mutualism, in which both organisms benefit fro ...
, whereas in susceptible hosts that do develop disease caused by the pathogen, the pathogen is considered
parasitic
Parasitism is a close relationship between species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The entomologist E. O. Wilson ha ...
.
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.
Types of reservoirs
Natural reservoirs can be divided into three main types: human, animal (non-human), and environmental.
Human reservoirs
''Human reservoirs'' are human beings infected by pathogens that exist on or within the human body.
Poliomyelitis and
smallpox
Smallpox was an infectious disease caused by variola virus (often called smallpox virus) which belongs to the genus Orthopoxvirus. The last naturally occurring case was diagnosed in October 1977, and the World Health Organization (WHO) c ...
exist exclusively within a human reservoir. Humans can act as reservoirs for
sexually transmitted disease
Sexually transmitted infections (STIs), also referred to as sexually transmitted diseases (STDs) and the older term venereal diseases, are infections that are spread by sexual activity, especially vaginal intercourse, anal sex, and oral ...
s,
measles,
mumps, streptococcal infection, various respiratory pathogens, and the smallpox virus.
Animal reservoirs
Animal (non-human) reservoirs consist of domesticated and wild animals infected by pathogens.
For example, the bacterium ''
Vibrio cholerae
''Vibrio cholerae'' is a species of Gram-negative, facultative anaerobe and comma-shaped bacteria. The bacteria naturally live in brackish or saltwater where they attach themselves easily to the chitin-containing shells of crabs, shrimps, and oth ...
'', which causes
cholera in humans, has natural reservoirs in
copepods,
zooplankton
Zooplankton are the animal component of the planktonic community ("zoo" comes from the Greek word for ''animal''). Plankton are aquatic organisms that are unable to swim effectively against currents, and consequently drift or are carried along by ...
, and
shellfish. Parasitic blood-flukes of the genus ''
Schistosoma'', responsible for
schistosomiasis, spend part of their lives inside freshwater
snail
A snail is, in loose terms, a shelled gastropod. The name is most often applied to land snails, terrestrial pulmonate gastropod molluscs. However, the common name ''snail'' is also used for most of the members of the molluscan class G ...
s before completing their life cycles in vertebrate hosts. Viruses of the taxon ''
Ebolavirus
The genus ''Ebolavirus'' (- or ; - or ) is a virological taxon included in the family '' Filoviridae'' (filament-shaped viruses), order ''Mononegavirales''. The members of this genus are called ebolaviruses, and encode their genome in the for ...
'', which causes
Ebola virus disease, are thought to have a natural reservoir in bats or other animals exposed to the virus. Other zoonotic diseases that have been transmitted from animals to humans include:
rabies
Rabies is a viral disease that causes encephalitis in humans and other mammals. Early symptoms can include fever and tingling at the site of exposure. These symptoms are followed by one or more of the following symptoms: nausea, vomiting, ...
,
blastomycosis
Blastomycosis or blasto is a fungal infection caused by inhaling spores of a ''Blastomyces'' fungus. Only about half of people with the disease have symptoms, which can include fever, cough, night sweats, muscle pains, weight loss, chest pain, a ...
,
psittacosis,
trichinosis
Trichinosis, also known as trichinellosis, is a parasitic disease caused by roundworms of the '' Trichinella'' type. During the initial infection, invasion of the intestines can result in diarrhea, abdominal pain, and vomiting. Migration of ...
,
cat-scratch disease,
histoplasmosis
Histoplasmosis is a fungal infection caused by '' Histoplasma capsulatum''. Symptoms of this infection vary greatly, but the disease affects primarily the lungs. Occasionally, other organs are affected; called disseminated histoplasmosis, it can ...
,
coccidiomycosis
Coccidioidomycosis (, ), commonly known as cocci, Valley fever, as well as California fever, desert rheumatism, or San Joaquin Valley fever, is a mammalian fungal disease caused by ''Coccidioides immitis'' or ''Coccidioides posadasii''. Coccidio ...
and
salmonella.
Common animal reservoirs include: bats, rodents, cows, pigs, sheep, swine, rabbits, raccoons, dogs, other mammals.
Common animal reservoirs
= Bats
=
Numerous zoonotic diseases have been traced back to bats.
There are a couple 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 amount of bat species and individuals. The second possibility is that something about bats' physiology make 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.
Lyssavirus
''Lyssavirus'' (from the Ancient Greek, Greek ''lyssa'' "rage, fury, rabies" and the Latin ''Virus#Etymology, vīrus'') is a genus of RNA viruses in the family ''Rhabdoviridae'', order ''Mononegavirales''. Mammals, including humans, can serve ...
es (including the
Rabies virus
Rabies virus, scientific name ''Rabies lyssavirus'', is a neurotropic virus that causes rabies in humans and animals. Rabies transmission can occur through the saliva of animals and less commonly through contact with human saliva. ''Rabies lys ...
),
Henipavirus
''Henipavirus'' is a genus of negative-strand RNA viruses in the family ''Paramyxoviridae'', order ''Mononegavirales'' containing six established species, and numerous others still under study. Henipaviruses are naturally harboured by several sp ...
es,
Menangle and
Tioman
Tioman Island ( ms, Pulau Tioman) is a mukim and an island in Rompin District, Pahang, Malaysia. It is located off the east coast of the state, and is some long and wide. It has seven villages, the largest and most populous being Kampung Te ...
viruses,
SARS-CoV
Severe acute respiratory syndrome coronavirus 1 (SARS-CoV-1; or Severe acute respiratory syndrome coronavirus, SARS-CoV) is a strain of coronavirus that causes severe acute respiratory syndrome (SARS), the respiratory illness responsible for ...
-Like Viruses, and
Ebola viruses have all been traced back to different species of bats.
Fruit bats
In botany, a fruit is the seed-bearing structure in flowering plants that is formed from the ovary after flowering.
Fruits are the means by which flowering plants (also known as angiosperms) disseminate their seeds. Edible fruits in partic ...
in particular serve as the reservoir host for
Nipah virus
Nipah virus, scientific name ''Nipah henipavirus'', is a bat-borne virus that causes Nipah virus infection in humans and other animals, a disease with a high mortality rate. Numerous disease outbreaks caused by Nipah virus have occurred in Sou ...
(NiV).
= Rats
=
Rats are known to be the reservoir hosts for a number of
zoonotic diseases
A zoonosis (; plural zoonoses) or zoonotic disease is an infectious disease of humans caused by a pathogen (an infectious agent, such as a bacterium, virus, parasite or prion) that has jumped from a non-human (usually a vertebrate) to a human ...
. Norway rats were found to be infested with the
Lyme disease
Lyme disease, also known as Lyme borreliosis, is a vector-borne disease caused by the ''Borrelia'' bacterium, which is spread by ticks in the genus ''Ixodes''. The most common sign of infection is an expanding red rash, known as erythema migran ...
spirochetes. In Mexico rats are known carriers of ''
Trypanosoma cruzi
''Trypanosoma cruzi'' is a species of parasitic euglenoids. Among the protozoa, the trypanosomes characteristically bore tissue in another organism and feed on blood (primarily) and also lymph. This behaviour causes disease or the likelihood o ...
'', which causes
Chagas disease.
= Mice
=
White-footed mice (''
Peromyscus leucopus
The white-footed mouse (''Peromyscus leucopus'') is a rodent native to North America from Ontario, Quebec, Labrador, and the Maritime Provinces (excluding the island of Newfoundland) to the southwestern United States and Mexico. In the Maritimes, ...
'') are one of the most important animal reservoirs for the Lyme disease spirochete (''
Borrelia burgdorferi
''Borrelia burgdorferi'' is a bacterial species of the spirochete class in the genus '' Borrelia'', and is one of the causative agents of Lyme disease in humans. Along with a few similar genospecies, some of which also cause Lyme disease, it mak ...
''). Deer mice serve as reservoir hosts for
Sin Nombre virus
''Sin Nombre orthohantavirus'' (SNV) (from Spanish, meaning "without a name") is the prototypical etiologic agent of hantavirus cardiopulmonary syndrome (HCPS).
Discovered in 1993 near the Cañon de la Muerte on the Navajo Reservation, it was ...
, which causes
hantavirus pulmonary syndrome
Hantavirus pulmonary syndrome (HPS) is one of two potentially fatal syndromes of zoonotic origin caused by species of hantavirus. These include
Black Creek Canal virus (BCCV), New York orthohantavirus (NYV), Monongahela virus (MGLV), ''Sin Nomb ...
(HPS).
= Monkeys
=
The
Zika virus
''Zika virus'' (ZIKV; pronounced or ) is a member of the virus family ''Flaviviridae''. It is spread by daytime-active '' Aedes'' mosquitoes, such as '' A. aegypti'' and '' A. albopictus''. Its name comes from the Ziika Forest of Uganda, w ...
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.
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.
Pathogens found in these reservoirs are sometimes free-living. The bacteria ''
Legionella pneumophila'', a
facultative intracellular parasite
Intracellular parasites are microparasites that are capable of growing and reproducing inside the cells of a host.
Types of parasites
There are two main types of intracellular parasites: Facultative and Obligate.
Facultative intracellular pa ...
which causes
Legionnaires' disease, and ''
Vibrio cholerae
''Vibrio cholerae'' is a species of Gram-negative, facultative anaerobe and comma-shaped bacteria. The bacteria naturally live in brackish or saltwater where they attach themselves easily to the chitin-containing shells of crabs, shrimps, and oth ...
'', which causes
cholera, can both exist as free-living parasites in certain water sources as well as in invertebrate animal hosts.
Disease transmission
A disease reservoir acts as a transmission point between a pathogen and a susceptible host.
Transmission can occur directly or indirectly.
Direct transmission
Direct transmission can occur from direct contact or direct
droplet
A drop or droplet is a small column of liquid, bounded completely or almost completely by free surfaces. A drop may form when liquid accumulates at the lower end of a tube or other surface boundary, producing a hanging drop called a pendant ...
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.
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.
* ''
Neisseria gonorrhoeae
''Neisseria gonorrhoeae'', also known as ''gonococcus'' (singular), or ''gonococci'' (plural), is a species of Gram-negative diplococci bacteria isolated by Albert Ludwig Sigesmund Neisser, Albert Neisser in 1879. It causes the sexually transmit ...
'' (
Gonorrhea
Gonorrhea, colloquially known as the clap, is a sexually transmitted infection (STI) caused by the bacterium '' Neisseria gonorrhoeae''. Infection may involve the genitals, mouth, or rectum. Infected men may experience pain or burning with ...
) is transmitted by sexual contact involving the penis, vagina, mouth, and anus through direct contact transmission.
* ''
Bordetella pertussis
''Bordetella pertussis'' is a Gram-negative, aerobic, pathogenic, encapsulated coccobacillus of the genus ''Bordetella'', and the causative agent of pertussis or whooping cough. Like '' B. bronchiseptica'', ''B. pertussis'' is motile and expres ...
'' (
Pertussis
Whooping cough, also known as pertussis or the 100-day cough, is a highly contagious bacterial disease. Initial symptoms are usually similar to those of the common cold with a runny nose, fever, and mild cough, but these are followed by two or t ...
) is transmitted by cough from human reservoir to susceptible host through direct droplet spread.
*
Indirect transmission
Indirect transmission can occur by airborne transmission, by vehicles (including fomites), and by vectors.
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).
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.
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
''Morbillivirus'' is a genus of viruses in the order '' Mononegavirales'', in the family '' Paramyxoviridae''. Humans, dogs, cats, cattle, seals, and cetaceans serve as natural hosts. This genus includes seven species. Diseases in humans associa ...
'' (
measles) is transmitted from an infected human host to a susceptible host as they are transmitted by respiration through airborne transmission. ''
Campylobacter'' (
campylobacteriosis
Campylobacteriosis is an infection by the ''Campylobacter'' bacterium, most commonly '' C. jejuni''. It is among the most common bacterial infections of humans, often a foodborne illness. It produces an inflammatory, sometimes bloody, diarrhea or ...
) is a common bacterial infection that is spread from human or non-human reservoirs by vehicles such as contaminated food and water. ''
Plasmodium falciparum
''Plasmodium falciparum'' is a Unicellular organism, unicellular protozoan parasite of humans, and the deadliest species of ''Plasmodium'' that causes malaria in humans. The parasite is transmitted through the bite of a female ''Anopheles'' mosqu ...
'' (
malaria
Malaria is a mosquito-borne infectious disease that affects humans and other animals. Malaria causes symptoms that typically include fever, tiredness, vomiting, and headaches. In severe cases, it can cause jaundice, seizures, coma, or death. S ...
) can be transmitted from an infected mosquito, an animal (non-human) reservoir, to human host by biological vector transmission.
Implications for public health
LH Taylor found that 61% of all human pathogens are classified as zoonotic. 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.
In an effort to predict and prevent future outbreaks of zoonotic diseases, the
U.S. Agency for International Development
The United States Agency for International Development (USAID) is an independent agency of the U.S. federal government that is primarily responsible for administering civilian foreign aid and development assistance. With a budget of over $27 ...
started the Emerging Pandemic Threats initiative in 2009. In alliance with
University of California-Davis,
EcoHealth Alliance
EcoHealth Alliance is a US-based non-governmental organization with a stated mission of protecting people, animals, and the environment from emerging infectious diseases. The nonprofit is focused on research that aims to prevent pandemics and p ...
,
Metabiota Inc.,
Smithsonian Institution
The Smithsonian Institution ( ), or simply the Smithsonian, is a group of museums and education and research centers, the largest such complex in the world, created by the U.S. government "for the increase and diffusion of knowledge". Founded ...
, and
Wildlife Conservation Society
The Wildlife Conservation Society (WCS) is a non-governmental organization headquartered at the Bronx Zoo in New York City, that aims to conserve the world's largest wild places in 14 priority regions. Founded in 1895 as the New York Zoological ...
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."
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
Machine learning (ML) is a field of inquiry devoted to understanding and building methods that 'learn', that is, methods that leverage data to improve performance on some set of tasks. It is seen as a part of artificial intelligence.
Machine ...
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."
See also
*
Fomite
A fomite () or fomes () is any inanimate object that, when contaminated with or exposed to infectious agents (such as pathogenic bacteria, viruses or fungi), can transfer disease to a new host.
Transfer of pathogens by fomites
A fomite is any ina ...
*
Host (biology)
In biology and medicine, a host is a larger organism that harbours a smaller organism; whether a parasite, parasitic, a mutualism (biology), mutualistic, or a commensalism, commensalist ''guest'' (symbiont). The guest is typically provided with ...
*
Refuge (ecology)
A refuge is a concept in ecology, in which an organism obtains protection from predation by hiding in an area where it is inaccessible or cannot easily be found. Due to population dynamics, when refuges are available, populations of both predator ...
*
Vector (epidemiology)
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 vec ...
*
Zoonosis
A zoonosis (; plural zoonoses) or zoonotic disease is an infectious disease of humans caused by a pathogen (an infectious agent, such as a bacterium, virus, parasite or prion) that has jumped from a non-human (usually a vertebrate) to a human. ...
References
{{Concepts in infectious disease
Disease ecology
Epidemiology