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Emerging zoonoses are defined as “zoonotic diseases caused either by apparently new etiological agents or by previously known agents appearing in places or in species in which the disease was previously unknown”, but shows an increase in incidence or expansion in geographical, host or vector range.
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Emergence and re-emergence of zoonotic diseases may be any one of the following categories
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a known agent appearing in a new geographic area.
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a known agent appearing in a unsusceptible species.
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a previously unknown agent detected for the first time.
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New animal diseases with an unknown host spectrum are also emerging zoonotic diseases.
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Natural animal reservoirs are more frequently playing role as a source of new agents than the sudden appearance of a completely new agent in the human population are considered as new and emerging zoonotic diseases.
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Emerging zoonoses are usually complex, involving mechanisms at the molecular level, such as antigenic drift or shift, and changes in the immunological status.
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Social and ecological conditions influencing population growth and movement, food habits, the environment and many other factors may play a more important role very frequently in the emergence of zoonotic diseases in human population.
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Examples: Avian influenza, Bovine Spongiform Encephalitis (BSE), Rift Valley fever, Alveolar Echinococcosis, Severe acute respiratory syndrome (SARS), Monkeypox, Nipah virus, Orphan viruses such as Enteroviruses, Myxoviruses, Herpesviruses, Nontyphoid strains of Salmonella, Borrelia burgdorferi (Lyme borreliosis), Ehrlichia and Anaplasma spp. (Ehrlichiosis), Mycobacteria paratuberculosis (Johne’s/Chronne’s disease), Francisella tularensis (Tularemia), Bartonella spp. (Bartonellosis), Enterohemorrhagic Escherichia coli ( Bloody diarrhoea), Campylobacter spp.
Figure: Factors influencing emergence of zoonotic diseases
Factors determining disease emergence
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There is no way to predict when or where the next important new zoonotic pathogen will emerge.
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There are several factors which contribute to the emergence of a new zoonotic disease, such as:
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Agent determinants (mutation, natural selection and evolutionary progression – antigenic drif or shift)
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Host population determinants (host behavioral characteristics, societal, transport, commercial and iatrogenic factors)
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Human and animal demography and changes in farming practice.
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Social and cultural factors such as food habits and religious beliefs play a role.
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Urbanization, globalization, population movement (refugees) and increased air travel.
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Microbial adaptation such as anti-microbial resistance and breakdown in public health or control measures as well as breakdown in the host’s defenses.
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Increasing global human and livestock animal populations results in close contact between animals and human population.
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Increasing bioterroristic activities.
Example 1: Yellow fever
Example 2: Japanese encephalitis
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Ecologic factors pertaining to uncontrolled urbanization and environmental pollution are contributing to many emergent disease episodes. Arthropod vectors breeding in accumulations of water (e.g., tin cans, old tires) and sewage-laden water are a problem worldwide. Environmental chemical toxicants (herbicides, pesticides, residues) can also affect vector-virus relationships directly or indirectly. Ecologic factors related to expanding primitive irrigation systems are becoming important in virus disease emergence, as exemplified by the emergence of Japanese encephalitis in newly developed rice-growing areas of southern Asia.
Example 4: Bovine spongiform encephalopathy epidemic in cattle and new-variant Creutzfeldt-Jakob disease in humans
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Bovine spongiform encephalopathy (BSE) in the United Kingdom may provide more lessons than any other recent emergent zoonotic disease episode. The disease was first diagnosed in the United Kingdom in 1986. More than 1,70,000 cattle had been reported as infected, roughly half of which entered the human food chain in the United Kingdom. In 1995, the BSE agent was reported to be the cause of a new human zoonotic disease, new-variant Creutzfeldt-Jakob disease.
Table: Factors associated with the emergence of zoonotic diseases
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Determinants
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Examples of zoonoses
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Ecological factors: Construction of dams, changes in water ecosystems, deforestation/reforestation, agriculture,flood/drought, famine, climate change
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Rift Valley fever (dams, irrigation), Argentine haemorrhagic fever, Hanta or Korean haemorrhagic fever (agriculture)
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Human demography: Urbanization, globalization, migration, war or civil conflict, economic impoverishment, commercial sex trade, intravenous drug use, outdoor recreation, use of childcare facilities
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Spread of human immunodeficiency virus and other sexually transmitted diseases, Dengue fever(urbanisation)
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International travel by air and ship: Worldwide movement of goods and people
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Dissemination of human immunodeficiency virus, mosquito vectors such as Aedes albopictus (Asian tiger mosquito), rat-borne hantaviruses, introduction of cholera into South America
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Technology and industry: Food production and processing, globalisation of food supplies, changes in food processing and packaging
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Escherichia coli strains from cattle contaminating meat and other food products), bovine spongiform encephalopathy, Nipah virus (pigs), avian influenza
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Health care: New medical devices, organ or tissue transplantation, immunosuppressive drugs, widespread use of antibiotics
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Ebola and human immunodeficiency virus (health care and medical technology, contaminated injection equipment), Creutzfeldt-Jakob disease from contaminated batches of human growth hormone
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Microbial adaptation and evolution
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‘Antigenic drift’ in influenza virus, development of multi-drug-resistant tuberculosis, chloroquine-resistant malaria
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Breakdown of the host’s defenses: Immunosuppression and imunnodieficiency
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Emergence of M.bovis and L. monocytogenes in humans
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Lack of public health awareness or control measures
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Resurgence of tuberculosis, cholera, diphtheria
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