An emergent virus (or emerging virus) is a

virus A virus is a wikt:submicroscopic, submicroscopic infectious agent that replicates only inside the living Cell (biology), cells of an organism. Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and ...

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 disease An emerging infectious disease (EID) is an infectious disease whose incidence has increased recently (in the past 20 years), and could increase in the near future. The minority that are capable of developing efficient transmission between human ...

s and raise

public health Public health is "the science and art of preventing disease, prolonging life and promoting health through the organized efforts and informed choices of society, organizations, public and private, communities and individuals". Analyzing the det ...

challenges globally, given their potential to cause

outbreak In epidemiology, an outbreak is a sudden increase in occurrences of a disease when cases are in excess of normal expectancy for the location or season. It may affect a small and localized group or impact upon thousands of people across an entire ...

s of disease which can lead to

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 d ...

s and

pandemic A pandemic () is an epidemic of an infectious disease that has spread across a large region, for instance multiple continents or worldwide, affecting a substantial number of individuals. A widespread endemic disease with a stable number of i ...

s. As well as causing

disease A disease is a particular abnormal condition that negatively affects the structure or function of all or part of an organism, and that is not immediately due to any external injury. Diseases are often known to be medical conditions that a ...

, emergent viruses can also have severe

economic An economy is an area of the production, distribution and trade, as well as consumption of goods and services. In general, it is defined as a social domain that emphasize the practices, discourses, and material expressions associated with t ...

implications. Recent examples include the

SARS-related coronavirus ''Severe acute respiratory syndrome–related coronavirus'' (SARSr-CoV or SARS-CoV)The terms ''SARSr-CoV'' and ''SARS-CoV'' are sometimes used interchangeably, especially prior to the discovery of SARS-CoV-2. This may cause confusion when some ...

es, which have caused the 2002-2004 outbreak of

SARS Severe acute respiratory syndrome (SARS) is a viral respiratory disease of zoonotic origin caused by the severe acute respiratory syndrome coronavirus (SARS-CoV or SARS-CoV-1), the first identified strain of the SARS coronavirus species, '' s ...

(

SARS-CoV-1 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 ...

) and the 2019–21 pandemic of

COVID-19 Coronavirus disease 2019 (COVID-19) is a contagious disease caused by a virus, the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). The first known case was identified in Wuhan, China, in December 2019. The disease quickl ...

(

SARS-CoV-2 Severe acute respiratory syndrome coronavirus 2 (SARS‑CoV‑2) is a strain of coronavirus that causes COVID-19 (coronavirus disease 2019), the respiratory illness responsible for the ongoing COVID-19 pandemic. The virus previously had a No ...

). Other examples include the

human immunodeficiency virus The human immunodeficiency viruses (HIV) are two species of ''Lentivirus'' (a subgroup of retrovirus) that infect humans. Over time, they cause acquired immunodeficiency syndrome (AIDS), a condition in which progressive failure of the immun ...

which causes

HIV/AIDS Human immunodeficiency virus infection and acquired immunodeficiency syndrome (HIV/AIDS) is a spectrum of conditions caused by infection with the human immunodeficiency virus (HIV), a retrovirus. Following initial infection an individual ...

; the viruses responsible for

Ebola Ebola, also known as Ebola virus disease (EVD) and Ebola hemorrhagic fever (EHF), is a viral hemorrhagic fever in humans and other primates, caused by ebolaviruses. Symptoms typically start anywhere between two days and three weeks after bec ...

; the

H5N1 Influenza A virus subtype H5N1 (A/H5N1) is a subtype of the influenza A virus which can cause illness in humans and many other animal species. A bird-adapted strain of H5N1, called HPAI A(H5N1) for highly pathogenic avian influenza virus of type ...

influenza virus responsible for

avian flu Avian influenza, known informally as avian flu or bird flu, is a variety of influenza caused by viruses adapted to birds. and

H1N1/09 The pandemic H1N1/09 virus is a swine origin influenza A virus subtype H1N1 strain that was responsible for the 2009 swine flu pandemic. This strain is often called swine flu by the public media. For other names, see the Nomenclature section ...

, which caused the

2009 swine flu The 2009 swine flu pandemic, caused by the H1N1 influenza virus and declared by the World Health Organization (WHO) from June 2009 to August 2010, is the third recent flu pandemic involving the H1N1 virus (the first being the 1918–1920 Span ...

pandemic (an earlier emergent strain of

H1N1 In virology, influenza A virus subtype H1N1 (A/H1N1) is a subtype of influenza A virus. Major outbreaks of H1N1 strains in humans include the Spanish flu, the 1977 Russian flu pandemic and the 2009 swine flu pandemic. It is an orthomyxoviru ...

caused the 1918

Spanish flu The 1918–1920 influenza pandemic, commonly known by the misnomer Spanish flu or as the Great Influenza epidemic, was an exceptionally deadly global influenza pandemic caused by the H1N1 influenza A virus. The earliest documented case wa ...

pandemic). Viral emergence in humans is often a consequence of

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. ...

, which involves a cross-species jump of a

viral disease A viral disease (or viral infection) occurs when an organism's body is invaded by pathogenic viruses, and infection, infectious virus particles (virions) attach to and enter susceptible cells. Structural Characteristics Basic structural charact ...

into humans from other animals. As zoonotic viruses exist in

animal 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 rep ...

s, they are much more difficult to eradicate and can therefore establish persistent infections in human populations. Emergent viruses should not be confused with re-emerging viruses or newly detected viruses. A re-emerging virus is generally considered to be a previously appeared virus that is experiencing a resurgence, for example measles. A newly detected virus is a previously unrecognized virus that had been circulating in the species as

endemic Endemism is the state of a species being found in a single defined geographic location, such as an island, state, nation, country or other defined zone; organisms that are indigenous to a place are not endemic to it if they are also found els ...

infections. Newly detected viruses may have escaped

classification Classification is a process related to categorization, the process in which ideas and objects are recognized, differentiated and understood. Classification is the grouping of related facts into classes. It may also refer to: Business, organizat ...

because they left no distinctive clues, and/or could not be isolated or propagated in cell culture. Examples include

human rhinovirus The rhinovirus (from the grc, ῥίς, rhis "nose", , romanized: "of the nose", and the la, vīrus) is the most common viral infectious agent in humans and is the predominant cause of the common cold. Rhinovirus infection proliferates in tem ...

(a leading cause of common colds which was first identified in 1956),

hepatitis C Hepatitis C is an infectious disease caused by the hepatitis C virus (HCV) that primarily affects the liver; it is a type of viral hepatitis. During the initial infection people often have mild or no symptoms. Occasionally a fever, dark urine, ...

(eventually identified in 1989), and

human metapneumovirus ''Human metapneumovirus'' (HMPV) is a negative-sense single-stranded RNA virus of the family '' Pneumoviridae'' and is closely related to the '' Avian metapneumovirus'' (AMPV) subgroup C. It was isolated for the first time in 2001 in the Netherla ...

(first described in 2001, but thought to have been circulating since the 19th century). As the detection of such viruses is technology driven, the number reported is likely to expand.

Zoonosis

Given the rarity of spontaneous development of new virus species, the most frequent cause of emergent viruses in humans is

. This phenomenon is estimated to account for 73% of all emerging or re-emerging pathogens, with viruses playing a disproportionately large role.

RNA virus An RNA virus is a virusother than a retrovirusthat has ribonucleic acid (RNA) as its genetic material. The nucleic acid is usually single-stranded RNA ( ssRNA) but it may be double-stranded (dsRNA). Notable human diseases caused by RNA viruse ...

es are particularly frequent, accounting for 37% of emerging and re-emerging pathogens. A broad range of animals - including wild birds, rodents and bats - are associated with zoonotic viruses. It is not possible to predict specific zoonotic events that may be associated with a particular animal reservoir at any given time. Zoonotic spillover can either result in self-limited 'dead-end' infections, in which no further human-human transmission occurs (as with 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 ...

), or in infectious cases, in which the zoonotic pathogen is able to sustain human-human transmission (as with the

Ebola virus ''Zaire ebolavirus'', more commonly known as Ebola virus (; EBOV), is one of six known species within the genus '' Ebolavirus''. Four of the six known ebolaviruses, including EBOV, cause a severe and often fatal hemorrhagic fever in humans and o ...

). If the zoonotic virus is able to maintain successful human-human transmission, an

may occur. Some spillover events can also result in the virus adapting exclusively for human infection (as occurred with the

HIV virus The human immunodeficiency viruses (HIV) are two species of ''Lentivirus'' (a subgroup of retrovirus) that infect humans. Over time, they cause acquired immunodeficiency syndrome (AIDS), a condition in which progressive failure of the immun ...

), in which case humans become a new reservoir for the pathogen. A successful zoonotic 'jump' depends on human contact with an animal harbouring a virus variant that is able to infect humans. In order to overcome host-range restrictions and sustain efficient human-human transmission, viruses originating from an animal reservoir will normally undergo

mutation In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA or viral replication, m ...

genetic recombination Genetic recombination (also known as genetic reshuffling) is the exchange of genetic material between different organisms which leads to production of offspring with combinations of traits that differ from those found in either parent. In eukary ...

and

reassortment Reassortment is the mixing of the genetic material of a species into new combinations in different individuals. Several different processes contribute to reassortment, including assortment of chromosomes, and chromosomal crossover. It is particu ...

. Due to their rapid replication and high mutation rates, RNA viruses are more likely to successfully adapt for invasion of a new host population.

Examples of animal sources

Bats

While bats are essential members of many ecosystems, they are also frequently implicated as frequent sources of emerging virus infections. Their

immune system The immune system is a network of biological processes that protects an organism from diseases. It detects and responds to a wide variety of pathogens, from viruses to parasitic worms, as well as Tumor immunology, cancer cells and objects such ...

s have evolved in such a way as to suppress any

inflammatory response Inflammation (from la, inflammatio) is part of the complex biological response of body tissues to harmful stimuli, such as pathogens, damaged cells, or irritants, and is a protective response involving immune cells, blood vessels, and molecu ...

to viral infections, thereby allowing them to become tolerant hosts for evolving viruses, and consequently provide major reservoirs of zoonotic viruses. They are associated with more zoonotic viruses per host species than any other mammal, and molecular studies have demonstrated that they are the natural hosts for several high-profile zoonotic viruses, including

severe acute respiratory syndrome-related coronavirus ''Severe acute respiratory syndrome–related coronavirus'' (SARSr-CoV or SARS-CoV)The terms ''SARSr-CoV'' and ''SARS-CoV'' are sometimes used interchangeably, especially prior to the discovery of SARS-CoV-2. This may cause confusion when some ...

es and

Marburg Marburg ( or ) is a university town in the German federal state (''Bundesland'') of Hesse, capital of the Marburg-Biedenkopf district (''Landkreis''). The town area spreads along the valley of the river Lahn and has a population of approx ...

hemorrhagic fever filoviruses. In terms of their potential for spillover events, bats have taken over the leading role previously assigned to rodents. Viruses can be transmitted from bats via several mechanisms, including bites, aerosolization of saliva (e.g. during echolocation), and faeces/urine. Due to their distinct

ecology Ecology () is the study of the relationships between living organisms, including humans, and their physical environment. Ecology considers organisms at the individual, population, community, ecosystem, and biosphere level. Ecology overl ...

/behaviour, bats are naturally more susceptible to viral infection and transmission. Several bat species (e.g. brown bats) aggregate in crowded roosts, which promotes intra- and interspecies viral transmission. Moreover, as bats are widespread in urban areas, humans occasionally encroach on their habitats which are contaminated with

guano Guano (Spanish from qu, wanu) is the accumulated excrement of Seabird, seabirds or bats. As a manure, guano is a highly effective fertilizer due to the high content of nitrogen, phosphate, and potassium, all key nutrients essential for plant ...

and urine. Their ability to fly and

migration patterns Animal migration is the relatively long-distance movement of individual animals, usually on a seasonal basis. It is the most common form of migration in ecology. It is found in all major animal groups, including birds, mammals, fish, reptiles, ...

also means that bats are able to spread disease over a large geographic area, while also acquiring new viruses. Additionally, bats experience persistent viral infections which, together with their extreme longevity (some bat species have lifespans of 35 years), helps to maintain viruses and transmit them to other species. Other bat characteristics which contribute to their potency as viral hosts include: their food choices,

torpor Torpor is a state of decreased physiological activity in an animal, usually marked by a reduced body temperature and metabolic rate. Torpor enables animals to survive periods of reduced food availability. The term "torpor" can refer to the tim ...

/ hibernation habits, and susceptibility to reinfection.

Drivers of viral emergence

Viral emergence is often a consequence of both nature and human activity. In particular, ecological changes can greatly facilitate the emergence and re-emergence of zoonotic viruses. Factors such as

deforestation Deforestation or forest clearance is the removal of a forest or stand of trees from land that is then land conversion, converted to non-forest use. Deforestation can involve conversion of forest land to farms, ranches, or urban area, urban ...

reforestation Reforestation (occasionally, reafforestation) is the natural or intentional restocking of existing forests and woodlands ( forestation) that have been depleted, usually through deforestation, but also after clearcutting. Management A debat ...

habitat fragmentation Habitat fragmentation describes the emergence of discontinuities (fragmentation) in an organism's preferred environment (habitat), causing population fragmentation and ecosystem decay. Causes of habitat fragmentation include geological proces ...

and

irrigation Irrigation (also referred to as watering) is the practice of applying controlled amounts of water to land to help grow crops, landscape plants, and lawns. Irrigation has been a key aspect of agriculture for over 5,000 years and has been dev ...

can all impact the ways in which humans come into contact with wild animal species, and consequently promote virus emergence. Additionally,

climate change In common usage, climate change describes global warming—the ongoing increase in global average temperature—and its effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes to ...

can affect

ecosystem An ecosystem (or ecological system) consists of all the organisms and the physical environment with which they interact. These biotic and abiotic components are linked together through nutrient cycles and energy flows. Energy enters the syst ...

s and

vector Vector most often refers to: *Euclidean vector, a quantity with a magnitude and a direction *Vector (epidemiology), an agent that carries and transmits an infectious pathogen into another living organism Vector may also refer to: Mathematic ...

distribution, which in turn can affect the emergence of vector-borne viruses. Other ecological changes - for example, species introduction and predator loss - can also affect virus emergence and prevalence. Some

agricultural Agriculture or farming is the practice of cultivating plants and livestock. Agriculture was the key development in the rise of sedentary human civilization, whereby farming of domesticated species created food surpluses that enabled peopl ...

practices, for example livestock intensification and inappropriate management/disposal of farm animal faeces, are also associated with an increased risk of zoonosis. Viruses may also emerge due to the establishment of human populations that are vulnerable to infection. For example, a virus may emerge following loss of cross-protective immunity, which may occur due to loss of a wild virus or termination of

vaccination Vaccination is the administration of a vaccine to help the immune system develop immunity from a disease. Vaccines contain a microorganism or virus in a weakened, live or killed state, or proteins or toxins from the organism. In stimulating ...

programmes. Well-developed countries also have higher proportions of aging citizens and obesity-related disease, thus meaning that their populations may be more immunosuppressed and therefore at risk of infection. Contrastingly, poorer nations may have immunocompromised populations due to

malnutrition Malnutrition occurs when an organism gets too few or too many nutrients, resulting in health problems. Specifically, it is "a deficiency, excess, or imbalance of energy, protein and other nutrients" which adversely affects the body's tissues ...

or chronic infection; these countries are also unlikely to have stable vaccination programmes. Additionally, changes in human

demographics Demography () is the statistical study of populations, especially human beings. Demographic analysis examines and measures the dimensions and dynamics of populations; it can cover whole societies or groups defined by criteria such as edu ...

– for example, the birth and/or migration of immunologically naïve individuals – can lead to the development of a susceptible population that enables large-scale virus infection. Other factors which can promote viral emergence include globalisation; in particular,

international trade International trade is the exchange of capital, goods, and services across international borders or territories because there is a need or want of goods or services. (see: World economy) In most countries, such trade represents a significan ...

and human travel/

migration Migration, migratory, or migrate may refer to: Human migration * Human migration, physical movement by humans from one region to another ** International migration, when peoples cross state boundaries and stay in the host state for some minimum l ...

can result in the introduction of viruses into new areas. Moreover, as densely populated cities promote rapid pathogen transmission, uncontrolled

urbanization Urbanization (or urbanisation) refers to the population shift from rural to urban areas, the corresponding decrease in the proportion of people living in rural areas, and the ways in which societies adapt to this change. It is predominantly t ...

(i.e. the increased movement and settling of individuals in

urban area An urban area, built-up area or urban agglomeration is a human settlement with a high population density and infrastructure of built environment. Urban areas are created through urbanization and are categorized by urban morphology as cities, ...

s) can promote viral emergence.

Animal migration Animal migration is the relatively long-distance movement of individual animals, usually on a seasonal basis. It is the most common form of migration in ecology. It is found in all major animal groups, including birds, mammals, fish, repti ...

can also lead to the emergence of viruses, as was the case for the

West Nile virus West Nile virus (WNV) is a single-stranded RNA virus that causes West Nile fever. It is a member of the family ''Flaviviridae'', from the genus ''Flavivirus'', which also contains the Zika virus, dengue virus, and yellow fever virus. The virus ...

which was spread by migrating bird populations. Additionally, human practices regarding food production and consumption can also contribute to the risk of viral emergence. In particular,

wet market A wet market (also called a public market or a traditional market) is a marketplace selling fresh foods such as meat, fish, produce and other consumption-oriented perishable goods in a non-supermarket setting, as distinguished from " dry mar ...

s (i.e. live animal markets) are an ideal environment for virus transfer, due to the high density of people and wild/farmed animals present. Consumption of

bush meat Bushmeat is meat from wildlife species that are hunted for human consumption, most often referring to the meat of game in Africa. Bushmeat represents a primary source of animal protein and a cash-earning commodity for inhabitants of humid trop ...

is also associated with pathogen emergence.

Prevention

Control and prevention of zoonotic diseases depends on appropriate global surveillance at various levels, including identification of novel pathogens,

public health surveillance Public health surveillance (also epidemiological surveillance, clinical surveillance or syndromic surveillance) is, according to the World Health Organization (WHO), "the continuous, systematic collection, analysis and interpretation of health-relat ...

(including

serological survey Serology is the scientific study of serum and other body fluids. In practice, the term usually refers to the diagnostic identification of antibodies in the serum. Such antibodies are typically formed in response to an infection (against a given mi ...

s), and analysis of the risks of transmission. The complexity of zoonotic events around the world predicates a multidisciplinary approach to prevention. The

One Health Model The concept of ''One Health'' is the unity of multiple practices that work together locally, nationally, and globally to help achieve optimal health for people, animals, and the environment. When the people, animals, and environment are put tog ...

has been proposed as a global strategy to help prevent the emergence of zoonotic diseases in humans, including novel viral diseases. The One Health concept aims to promote the health of animals, humans, and the environment, both locally and globally, by fostering understanding and collaboration between practitioners of different interrelated disciplines, including

wildlife biology A wildlife biologist studies animals and their behavior along with the role each animal plays in its natural habitat. The duties of a wildlife biologist can include: developing and conducting experiments/studies on animals in their natural habitats, ...

veterinary science Veterinary medicine is the branch of medicine that deals with the prevention, management, diagnosis, and treatment of disease, disorder, and injury in animals. Along with this, it deals with animal rearing, husbandry, breeding, research on nutri ...

medicine Medicine is the science and Praxis (process), practice of caring for a patient, managing the diagnosis, prognosis, Preventive medicine, prevention, therapy, treatment, Palliative care, palliation of their injury or disease, and Health promotion ...

agriculture Agriculture or farming is the practice of cultivating plants and livestock. Agriculture was the key development in the rise of sedentary human civilization, whereby farming of domesticated species created food surpluses that enabled peop ...

microbiology Microbiology () is the scientific study of microorganisms, those being unicellular (single cell), multicellular (cell colony), or acellular (lacking cells). Microbiology encompasses numerous sub-disciplines including virology, bacteriology, ...

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 ...

, and

biomedical engineering Biomedical engineering (BME) or medical engineering is the application of engineering principles and design concepts to medicine and biology for healthcare purposes (e.g., diagnostic or therapeutic). BME is also traditionally logical sciences ...

Virulence of emergent viruses

As hosts are immunologically naïve to pathogens they have not encountered before, emergent viruses are often extremely

virulent Virulence is a pathogen's or microorganism's ability to cause damage to a host. In most, especially in animal systems, virulence refers to the degree of damage caused by a microbe to its host. The pathogenicity of an organism—its ability to ...

in terms of their capacity to cause disease. Their high virulence is also due to a lack of adaptation to the new host; viruses normally exert strong

selection pressure Any cause that reduces or increases reproductive success in a portion of a population potentially exerts evolutionary pressure, selective pressure or selection pressure, driving natural selection. It is a quantitative description of the amount of ...

on the immune systems of their natural hosts, which in turn exerts a strong selection pressure on viruses. This

coevolution In biology, coevolution occurs when two or more species reciprocally affect each other's evolution through the process of natural selection. The term sometimes is used for two traits in the same species affecting each other's evolution, as well ...

means that the natural host is able to manage infection. However, when the virus jumps to a new host (e.g. humans), the new host is unable to deal with infection due to a lack of coevolution, which results in mismatch between host immunoeffectors and virus

immunomodulator Immunotherapy or biological therapy is the treatment of disease by activating or suppressing the immune system. Immunotherapies designed to elicit or amplify an immune response are classified as ''activation immunotherapies,'' while immunotherap ...

s. Additionally, in order to maximise transmission, viruses often naturally undergo attenuation (i.e.

virulence Virulence is a pathogen's or microorganism's ability to cause damage to a host. In most, especially in animal systems, virulence refers to the degree of damage caused by a microbe to its host. The pathogenicity of an organism—its ability to ...

is reduced) so that infected animals can survive long enough to infect other animals more efficiently. However, as attenuation takes time to achieve, new host populations will not initially benefit from this phenomenon. Moreover, as zoonotic viruses also naturally exist in animal reservoirs, their survival is not dependent on transmission between new hosts; this means that emergent viruses are even more unlikely to attenuate for the purpose of maximal transmission, and they remain virulent. Although emergent viruses are frequently highly virulent, they are limited by several host factors including:

innate immunity The innate, or nonspecific, immune system is one of the two main immunity strategies (the other being the adaptive immune system) in vertebrates. The innate immune system is an older evolutionary defense strategy, relatively speaking, and is th ...

, natural antibodies and receptor specificity. If the host has previously been infected by a pathogen that is similar to the emergent virus, the host may also benefit from cross-protective immunity.

Examples of emergent viruses

Influenza A

Influenza Influenza, commonly known as "the flu", is an infectious disease caused by influenza viruses. Symptoms range from mild to severe and often include fever, runny nose, sore throat, muscle pain, headache, coughing, and fatigue. These symptom ...

is a highly contagious respiratory infection, which affects approximately 9% of the global population and causes 300,000 to 500,000 deaths annually. Based on their core proteins, influenza viruses are classified into types A, B, C and D. While both influenza A and B can cause epidemics in humans, influenza A also has pandemic potential and a higher mutation rate, therefore is most significant to public health. Influenza A viruses are further classified into subtypes, based on the combinations of the surface

glycoprotein Glycoproteins are proteins which contain oligosaccharide chains covalently attached to amino acid side-chains. The carbohydrate is attached to the protein in a cotranslational or posttranslational modification. This process is known as g ...

hemagglutinin In molecular biology, hemagglutinins (or ''haemagglutinin'' in British English) (from the Greek , 'blood' + Latin , 'glue') are receptor-binding membrane fusion glycoproteins produced by viruses in the ''Paramyxoviridae'' family. Hemagglutinins ar ...

(HA) and

neuraminidase Exo-α-sialidase (EC 3.2.1.18, sialidase, neuraminidase; systematic name acetylneuraminyl hydrolase) is a glycoside hydrolase that cleaves the glycosidic linkages of neuraminic acids: : Hydrolysis of α-(2→3)-, α-(2→6)-, α-(2→8)- gly ...

(NA). The primary natural reservoir for most influenza A subtypes are wild aquatic birds; however, through a series of mutations, a small subset of these viruses have adapted for infection of humans (and other animals). A key determinant of whether a particular influenza A subtype can infect humans is its binding specificity. Avian influenza A preferentially binds to cell surface receptors with a terminal α2,3‐linked

sialic acid Sialic acids are a class of alpha-keto acid sugars with a nine-carbon backbone. The term "sialic acid" (from the Greek for saliva, - ''síalon'') was first introduced by Swedish biochemist Gunnar Blix in 1952. The most common member of this ...

, while human influenza A preferentially binds to cell surface receptors with a terminal α2,6‐linked sialic acid. Via mutation, some avian influenza A viruses have successfully altered their binding specificity from α2,3‐ to α2,6‐linked sialic acid. However, in order to emerge in humans, avian influenza A viruses must also adapt their

RNA polymerase In molecular biology, RNA polymerase (abbreviated RNAP or RNApol), or more specifically DNA-directed/dependent RNA polymerase (DdRP), is an enzyme that synthesizes RNA from a DNA template. Using the enzyme helicase, RNAP locally opens the ...

s for function in mammalian cells, as well as mutating for stability in the acidic respiratory tract of humans. Following adaptation and

host switch In parasitology and epidemiology, a host switch (or host shift) is an evolutionary change of the host specificity of a parasite or pathogen. For example, the human immunodeficiency virus used to infect and circulate in non-human primates in West- ...

, influenza A viruses have the potential to cause epidemics and pandemics in humans. Minor changes in HA and NA structure (

antigenic drift Antigenic drift is a kind of genetic variation in viruses, arising from the accumulation of mutations in the virus genes that code for virus-surface proteins that host antibodies recognize. This results in a new strain of virus particles that i ...

) occur frequently, which enables the virus to cause repetitive outbreaks (i.e.

seasonal influenza Flu season is an annually recurring time period characterized by the prevalence of an outbreak of influenza (flu). The season occurs during the cold half of the year in each hemisphere. It takes approximately two days to show symptoms. Influe ...

) by evading immune recognition. Major changes in HA and NA structure (

antigenic shift Antigenic shift is the process by which two or more different strains of a virus, or strains of two or more different viruses, combine to form a new subtype having a mixture of the surface antigens of the two or more original strains. The term is ...

), which are caused by genetic reassortment between different influenza A subtypes (e.g. between human and animal subtypes), can instead cause large regional/global

s. Due to the emergence of antigenically different influenza A strains in humans, four pandemics occurred in the 20th century alone. Additionally, although animal influenza A viruses (e.g.

swine influenza Swine influenza is an infection caused by any of several types of swine influenza viruses. Swine influenza virus (SIV) or swine-origin influenza virus (S-OIV) refers to any strain of the influenza family of viruses that is endemic in pigs. As ...

) are distinct from human influenza viruses, they can still cause zoonotic infection in humans. These infections are largely acquired following direct contact with infected animals or contaminated environments, but do not result in efficient human-human transmission; examples of this include H5N1 influenza and H7N9 influenza.

SARS-CoV

In 2002, a highly pathogenic

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 ...

(Severe Acute Respiratory Syndrome Coronavirus) strain emerged from a zoonotic reservoir; approximately 8000 people were infected worldwide, and mortality rates approached 50% or more in the elderly. As SARS-CoV is most contagious post-symptoms, the introduction of strict public health measures effectively halted the pandemic. The natural reservoir host for SARS-CoV is thought to be

horseshoe bat Horseshoe bats are bats in the family Rhinolophidae. In addition to the single living genus, ''Rhinolophus'', which has about 106 species, the extinct genus ''Palaeonycteris'' has been recognized. Horseshoe bats are closely related to the Old ...

s, although the virus has also been identified in several small carnivores (e.g. palm

civet A civet () is a small, lean, mostly nocturnal mammal native to tropical Asia and Africa, especially the tropical forests. The term civet applies to over a dozen different species, mostly from the family Viverridae. Most of the species diversit ...

s and

racoon dog ''Nyctereutes'' (Greek: ''nyx, nykt-'' "night" + ''ereutēs'' "wanderer") is a genus of canid which includes only two extant species both known as raccoon dogs; the common raccoon dog (''Nyctereutes procyonoides'') and the Japanese raccoon dog ( ...

s). The emergence of SARS-CoV is believed to have been facilitated by Chinese wet markets, in which civets positive for the virus acted as intermediate hosts and passed SARS-CoV onto humans (and other species). However, more recent analysis suggests that SARS-CoV may have directly jumped from bats to humans, with subsequent cross-transmission between humans and civets. In order to

infect 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 ...

cells, SARS-CoV uses the spike surface

to recognise and bind to host ACE-2, which it uses as a cellular entry receptor; the development of this characteristic was crucial in enabling SARS-CoV to ‘jump’ from bats to other species.

MERS-CoV

First reported in 2012,

MERS-CoV ''Middle East respiratory syndrome–related coronavirus'' (''MERS-CoV''), or EMC/2012 ( HCoV-EMC/2012), is the virus that causes Middle East respiratory syndrome (MERS). It is a species of coronavirus which infects humans, bats, and camels. Th ...

(Middle East Respiratory Syndrome Coronavirus) marks the second known introduction of a highly pathogenic coronavirus from a zoonotic reservoir into humans. The case mortality rate of this emergent virus is approximately 35%, with 80% of all cases reported by Saudi Arabia. Although MERS-CoV is likely to have originated in bats, dromedary camels have been implicated as probable intermediate hosts. MERS-CoV is believed to have been circulating in these mammals for over 20 years, and it is thought that novel camel farming practices drove the spillover of MERS-CoV into humans. Studies have shown that humans can be infected with MERS-CoV via direct or indirect contact within infected dromedary camels, while human-human transmission is limited. MERS-CoV gains cellular entry by using a spike surface protein to bind to the host DPP4 surface receptor; the core subdomain of this spike surface protein shares similarities with that of SARS-CoV, but its receptor binding subdomain (RBSD) significantly differs.

Bluetongue disease

Bluetongue disease Bluetongue disease is a noncontagious, insect-borne, viral disease of ruminants, mainly sheep and less frequently cattle, yaks, goats, buffalo, deer, dromedaries, and antelope. It is caused by ''Bluetongue virus'' (''BTV''). The virus is t ...

is a non-contagious vector-borne disease caused by bluetongue virus, which affects species of

ruminant Ruminants ( suborder Ruminantia) are hoofed herbivorous grazing or browsing mammals that are able to acquire nutrients from plant-based food by fermenting it in a specialized stomach prior to digestion, principally through microbial actions. Th ...

s (particularly

sheep Sheep or domestic sheep (''Ovis aries'') are domesticated, ruminant mammals typically kept as livestock. Although the term ''sheep'' can apply to other species in the genus ''Ovis'', in everyday usage it almost always refers to domesticated sh ...

). Climate change has been implicated in the emergence and global spread of this disease, due to its impact on vector distribution. The natural vector of the bluetongue virus is the African midge ''C. imicola'', which is normally limited to Africa and subtropical Asia. However, global warming has extended the geographic range of ''C. imicola'', so that it now overlaps with a different vector (''C. pulcaris'' or ''C. obsoletus'') with a much more northward geographic range. This change enabled the bluetongue virus to jump vector, thus causing the northward spread of bluetongue disease into Europe.

References

External links

* * {{Authority control Epidemiology Viruses Zoonoses Virology

Zoonosis

Examples of animal sources

Bats

Drivers of viral emergence

Prevention

Virulence of emergent viruses

Examples of emergent viruses

Influenza A

SARS-CoV

MERS-CoV

Bluetongue disease

See also

References

Further reading

External links