|A dog with rabies.|
A zoonosis (plural zoonoses, or zoonotic diseases) is an infectious disease caused by a pathogen (an infectious agent, such as a bacterium, virus, parasite or prion) that has jumped from an animal (usually a vertebrate) to a human. Typically, the first infected human transmits the infectious agent to at least one other human, who, in turn, infects others.
Major modern diseases such as Ebola virus disease and salmonellosis are zoonoses. HIV was a zoonotic disease transmitted to humans in the early part of the 20th century, though it has now mutated to a separate human-only disease. Most strains of influenza that infect humans are human diseases, although many strains of bird flu and swine flu are zoonoses; these viruses occasionally recombine with human strains of the flu and can cause pandemics such as the 1918 Spanish flu or the 2009 swine flu. Taenia solium infection is one of the neglected tropical diseases with public health and veterinary concern in endemic regions. Zoonoses can be caused by a range of disease pathogens such as emergent viruses, bacteria, fungi and parasites; of 1,415 pathogens known to infect humans, 61% were zoonotic. Most human diseases originated animals; however, only diseases that routinely involve non-human to human transmission, such as rabies, are considered direct zoonosis.
Zoonoses have different modes of transmission. In direct zoonosis the disease is directly transmitted from animals to humans through media such as air (influenza) or through bites and saliva (rabies). In contrast, transmission can also occur via an intermediate species (referred to as a vector), which carry the disease pathogen without getting sick. When humans infect animals, it is called reverse zoonosis or anthroponosis. The term is from Greek: ζῷον zoon "animal" and νόσος nosos "sickness".
Host genetics plays an important role in determining which animal viruses will be able to make copies of themselves in the human body. Dangerous animal viruses are those that require few mutations to begin replicating themselves in human cells. These viruses are dangerous since the required combinations of mutations might randomly arise in the natural reservoir.
Recently, there has been a rise in frequency of appearance of new zoonotic diseases. According to a report from the United Nations Environment Programme and International Livestock Research Institute named: "Preventing the next pandemic – Zoonotic diseases and how to break the chain of transmission" the causes are mostly environmental.
Zoonotic transmission can occur in any context in which there is contact with or consumption of animals, animal products, or animal derivatives. This can occur in a companionistic (pets), economic (farming, trade, butchering, etc.), predatory (hunting, butchering or consuming wild game) or research context.
In 2006 a conference held in Berlin focused on the issue of zoonotic pathogen effects on food safety, urging government intervention and public vigilance against the risks of catching food-borne diseases from farm-to-table dining.
Many food outbreaks can be linked to zoonotic pathogens. Many different types of food that have an animal origin can become contaminated. Some common foods linked to zoonotic contaminations include eggs, seafood, meat, dairy, and even some vegetables. Outbreaks involving contaminated food should be handled in preparedness plans to prevent widespread outbreaks and to efficiently and effectively contain outbreaks.
Contact with farm animals can lead to disease in farmers or others that come into contact with infected farm animals. Glanders primarily affects those who work closely with horses and donkeys. Close contact with cattle can lead to cutaneous anthrax infection, whereas inhalation anthrax infection is more common for workers in slaughterhouses, tanneries and wool mills. Close contact with sheep who have recently given birth can lead to clamydiosis, or enzootic abortion, in pregnant women, as well as an increased risk of Q fever, toxoplasmosis, and listeriosis in pregnant or the otherwise immunocompromised. Echinococcosis is caused by a tapeworm which can be spread from infected sheep by food or water contaminated with feces or wool. Bird flu is common in chickens. While rare in humans, the main public health worry is that a strain of bird flu will recombine with a human flu virus and cause a pandemic like the 1918 Spanish flu. In 2017, free range chickens in the UK were temporarily ordered to remain inside due to the threat of bird flu. Cattle are an important reservoir of cryptosporidiosis and mainly affects the immunocompromised. Recent reports have shown Minks can also get infected.
Veterinarians are exposed to unique occupational hazards and zoonotic diseases. In the US, studies have highlighted an increased risk to injuries and a lack of veterinary awareness for these hazards. Research has proved the importance for continued clinical veterinarian education on occupational risks associated with musculoskeletal injuries, animal bites, needle-sticks, and cuts.
A July 2020 report by the United Nations Environment Programme stated that the increase in zoonotic pandemics is directly attributable to anthropogenic destruction of nature and the increased global demand for meat, and that the industrial farming of pigs and chickens in particular will be a primary risk factor for the spillover of zoonotic diseases in the future.
Pets can transmit a number of diseases. Dogs and cats are routinely vaccinated against rabies. Pets can also transmit ringworm and Giardia, which are endemic in both animal and human populations. Toxoplasmosis is a common infection of cats; in humans it is a mild disease although it can be dangerous to pregnant women. Dirofilariasis is caused by Dirofilaria immitis through mosquitoes infected by mammals like dogs and cats. Cat-scratch disease is caused by Bartonella henselae and Bartonella quintana from fleas which are endemic in cats. Toxocariasis is infection of humans of any of species of roundworm, including species specific to the dog (Toxocara canis) or the cat (Toxocara cati). Cryptosporidiosis can be spread to humans from pet lizards, such as the leopard gecko. Encephalitozoon cuniculi is a microsporidial parasite carried by many mammals, including rabbits, and is an important opportunistic pathogen in people immunocompromised by HIV/AIDS, organ transplantation, or CD4+ T-lymphocyte deficiency.
Outbreaks of zoonoses have been traced to human interaction with and exposure to other animals at fairs, live animal markets, petting zoos, and other settings. In 2005, the Centers for Disease Control and Prevention (CDC) issued an updated list of recommendations for preventing zoonosis transmission in public settings. The recommendations, developed in conjunction with the National Association of State Public Health Veterinarians, include educational responsibilities of venue operators, limiting public animal contact, and animal care and management.
Kate Jones, chair of ecology and biodiversity at University College London, says zoonotic diseases are increasingly linked to environmental change and human behaviour. The disruption of pristine forests driven by logging, mining, road building through remote places, rapid urbanisation and population growth is bringing people into closer contact with animal species they may never have been near before. The resulting transmission of disease from wildlife to humans, she says, is now "a hidden cost of human economic development". In a guest article published by IPBES, Peter Daszak and three co-chairs of the 2019 Global Assessment Report on Biodiversity and Ecosystem Services, Josef Settele, Sandra Díaz and Eduardo Brondizio, write that "rampant deforestation, uncontrolled expansion of agriculture, intensive farming, mining and infrastructure development, as well as the exploitation of wild species have created a ‘perfect storm’ for the spillover of diseases from wildlife to people."
An April 2020 study published in the Proceedings of the Royal Society Part B found that increased virus spillover events from animals to humans can be linked to biodiversity loss and environmental degradation, as humans further encroach on wildlands to engage in agriculture, hunting and resource extraction they become exposed to pathogens which normally would remain in these areas. Such spillover events have been tripling every decade since 1980. An August 2020 study published in Nature concludes that the anthropogenic destruction of ecosystems for the purpose of expanding agriculture and human settlements reduces biodiversity and allows for smaller animals such as bats and rats, who are more adaptable to human pressures and also carry the most zoonotic diseases, to proliferate. This in turn can result in more pandemics.
In October 2020, the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services published its report on the 'era of pandemics' by 22 experts in a variety of fields, and concluded that anthropogenic destruction of biodiversity is paving the way to the pandemic era, and could result in as many as 850,000 viruses being transmitted from animals – in particular birds and mammals – to humans. The increased pressure on ecosystems is being driven by the "exponential rise" in consumption and trade of commodities such as meat, palm oil, and metals, largely facilitated by developed nations, and by a growing human population. According to Peter Daszak, the chair of the group who produced the report, "there is no great mystery about the cause of the Covid-19 pandemic, or of any modern pandemic. The same human activities that drive climate change and biodiversity loss also drive pandemic risk through their impacts on our environment."
According to a report from the United Nations Environment Programme and International Livestock Research Institute named: "Preventing the next pandemic – Zoonotic diseases and how to break the chain of transmission" climate change is one of the 7 human – related causes of increase in the number of zoonotic diseases.
This section needs expansion. You can help by adding to it. (August 2020)
|Disease||Pathogen(s)||Animals involved||Mode of transmission||Emergence|
|African sleeping sickness||Trypanosoma brucei rhodesiense||range of wild animals and domestic livestock||transmitted by the bite of the tsetse fly||'present in Africa for thousands of years' – major outbreak 1900–1920, cases continue (sub-Saharan Africa, 2020)|
|Angiostrongyliasis||Angiostrongylus cantonensis, Angiostrongylus costaricensis||rats, cotton rats||consuming raw or undercooked snails, slugs, other mollusks, crustaceans, contaminated water, and unwashed vegetables contaminated with larvae|
|Anisakiasis||Anisakis||whales, dolphins, seals, sea lions, other marine animals||eating raw or undercooked fish and squid contaminated with eggs|
|Anthrax||Bacillus anthracis||commonly – grazing herbivores such as cattle, sheep, goats, camels, horses, and pigs||by ingestion, inhalation or skin contact of spores|
|Babesiosis||Babesia spp.||mice, other animals||tick bite|
|Baylisascariasis||Baylisascaris procyonis||raccoons||ingestion of eggs in feces|
|Barmah Forest fever||Barmah Forest virus||kangaroos, wallabies, opossums||mosquito bite|
|Bird flu||Influenza A virus subtype H5N1||wild birds, domesticated birds such as chickens||close contact||2003–19 Avian Influenza in Southeast Asia and Egypt|
|Bovine spongiform encephalopathy||Prions||cattle||eating infected meat||isolated similar cases reported in ancient history; in recent UK history probable start in the 1970s|
|Brucellosis||Brucella spp.||cattle, goats, pigs, sheep||infected milk or meat||historically widespread in Mediterranean region; identified early 20th century|
|Bubonic plague, Pneumonic plague, Septicemic plague, Sylvatic plague||Yersinia pestis||rabbits, hares, rodents, ferrets, goats, sheep, camels||flea bite||Epidemics like Black Death in Europe around 1347–53 during the Late Middle Age, Third Plague Pandemic in China-Qing dynasty and India alone|
|Capillariasis||Capillaria spp.||rodents, birds, foxes||eating raw or undercooked fish, ingesting embryonated eggs in fecal-contaminated food, water, or soil|
|Cat-scratch disease||Bartonella henselae||cats||bites or scratches from infected cats|
|Chagas disease||Trypanosoma cruzi||armadillos, Triatominae (kissing bug)||Contact of mucosae or wounds with feces of kissing bugs. Accidental ingestion of parasites in food contaminated by bugs or infected mammal excretae.|
|Clamydiosis / Enzootic abortion||Chlamydophila abortus||domestic livestock, particularly sheep||close contact with postpartum ewes|
|COVID-19||severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2)||suspected: bats, felines, minks||respiratory transmission||COVID-19 pandemic; 2019–present; Ongoing pandemic|
|Creutzfeldt-Jacob disease||PrPvCJD||cattle||eating meat from animals with Bovine spongiform encephalopathy (BSE)||1996–2001: United Kingdom|
|Crimean–Congo hemorrhagic fever||Crimean-Congo hemorrhagic fever orthonairovirus||cattle, goats, sheep, birds, multimammate rats, hares||tick bite, contact with bodily fluids|
|Cryptococcosis||Cryptococcus neoformans||commonly – birds like pigeons||inhaling fungi|
|Cryptosporidiosis||Cryptosporidium spp.||cattle, dogs, cats, mice, pigs, horses, deer, sheep, goats, rabbits, leopard geckos, birds||ingesting cysts from water contaminated with feces|
|Cysticercosis and taeniasis||Taenia solium, Taenia asiatica, Taenia saginata||commonly – pigs and cattle||consuming water, soil or food contaminated with the tapeworm eggs (cysticercosis) or raw or undercooked pork contaminated with the cysticerci (taeniasis)|
|Dirofilariasis||Dirofilaria spp.||dogs, wolves, coyotes, foxes, jackals, cats, monkeys, raccoons, bears, muskrats, rabbits, leopards, seals, sea lions, beavers, ferrets, reptiles||mosquito bite|
|Eastern equine encephalitis, Venezuelan equine encephalitis, Western equine encephalitis||Eastern equine encephalitis virus, Venezuelan equine encephalitis virus, Western equine encephalitis virus||horses, donkeys, zebras, birds||mosquito bite|
|Ebola virus disease (a haemorrhagic fever)||Ebolavirus spp.||chimpanzees, gorillas, orangutans, fruit bats, monkeys, shrews, forest antelope and porcupines||through body fluids and organs||2013–16; possible in Africa|
|Other haemorrhagic fevers (Crimean-Congo haemorrhagic fever, Dengue fever, Lassa fever, Marburg viral haemorrhagic fever, Rift Valley fever)||Varies – commonly viruses||varies (sometimes unknown) – commonly camels, rabbits, hares, hedgehogs, cattle, sheep, goats, horses and swine||infection usually occurs through direct contact with infected animals||2019–20 dengue fever|
|Echinococcosis||Echinococcus spp.||commonly – dogs, foxes, jackals, wolves, coyotes, sheep, pigs, rodents||ingestion of infective eggs from contaminated food or water with feces of an infected, definitive host or fur|
|Fasciolosis||Fasciola hepatica, Fasciola gigantica||sheep, cattle, buffaloes||ingesting contaminated plants|
|Foodborne illnesses (commonly diarrheal diseases)||Campylobacter spp., Escherichia coli, Salmonella spp., Listeria spp., Shigella spp. and Trichinella spp.||animals domesticated for food production (cattle, poultry)||raw or undercooked food made from animals and unwashed vegetables contaminated with feces|
|Giardiasis||Giardia lamblia||beavers, other rodents, raccoons, deer, cattle, goats, sheep, dogs, cats||ingesting spores and cysts in food and water contaminated with feces|
|Glanders||Burkholderia mallei.||horses, donkeys||direct contact|
|Gnathostomiasis||Gnathostoma spp.||dogs, minks, opossums, cats, lions, tigers, leopards, raccoons, poultry, other birds, frogs||raw or undercooked fish or meat|
|Hantavirus||Hantavirus spp.||deer mice, cotton rats and other rodents||exposure to feces, urine, saliva or bodily fluids|
|Henipavirus||Henipavirus spp.||horses, bats||exposure to feces, urine, saliva or contact with sick horses|
|Histoplasmosis||Histoplasma capsulatum||birds, bats||inhaling fungi in guano|
|HIV||SIV Simian immunodeficiency virus||Non-human primates||Blood||Immunodeficiency resembling human AIDS was reported in captive monkeys in the United States beginning in 1983. SIV was isolated in 1985 from some of these animals, captive rhesus macaques suffering from simian AIDS (SAIDS). The discovery of SIV was made shortly after HIV-1 had been isolated as the cause of AIDS and led to the discovery of HIV-2 strains in West Africa. HIV-2 was more similar to the then-known SIV strains than to HIV-1, suggesting for the first time the simian origin of HIV. Further studies indicated that HIV-2 is derived from the SIVsmm strain found in sooty mangabeys, whereas HIV-1, the predominant virus found in humans, is derived from SIV strains infecting chimpanzees (SIVcpz)|
|Japanese encephalitis||Japanese encephalitis virus||pigs, water birds||mosquito bite|
|Kyasanur Forest disease||Kyasanur Forest disease virus||rodents, shrews, bats, monkeys||tick bite|
|La Crosse encephalitis||La Crosse virus||chipmunks, tree squirrels||mosquito bite|
|Leishmaniasis||Leishmania spp.||dogs, rodents, other animals||sandfly bite||2004 Afghanistan|
|Leprosy||Mycobacterium leprae, Mycobacterium lepromatosis||armadillos, monkeys, rabbits, mice||direct contact, including meat consumption. However, scientists believe most infections are spread human to human.|
|Leptospirosis||Leptospira interrogans||rats, mice, pigs, horses, goats, sheep, cattle, buffaloes, opossums, raccoons, mongooses, foxes, dogs||direct or indirect contact with urine of infected animals||1616–20 New England infection: Present day in the United States–Native Americans; Killed around 90–95% of (Native America)|
|Lassa fever||Lassa fever virus||rodents||exposure to rodents|
|Lyme disease||Borrelia burgdorferi||deer, wolves, dogs, birds, rodents, rabbits, hares, reptiles||tick bite|
|Lymphocytic choriomeningitis||Lymphocytic choriomeningitis virus||rodents||exposure to urine, feces, or saliva|
|Melioidosis||Burkholderia pseudomallei||various animals||direct contact with contaminated soil and surface water|
|Microsporidiosis||Encephalitozoon cuniculi||Rabbits, dogs, mice, and other mammals||ingestion of spores|
|Middle East respiratory syndrome||MERS coronavirus||bats, camels||close contact||2012–present: Saudi Arabia|
|Monkeypox||Monkeypox virus||rodents, primates||contact with infected rodents, primates, or contaminated materials|
|Nipah virus infection||Nipah virus (NiV)||bats, pigs||direct contact with infected bats, infected pigs|
|Orf||Orf virus||goats, sheep||close contact|
|Psittacosis||Chlamydophila psittaci||macaws, cockatiels, budgerigars, pigeons, sparrows, ducks, hens, gulls and many other bird species||contact with bird droplets|
|Q fever||Coxiella burnetii||livestock and other domestic animals such as dogs and cats||inhalation of spores, contact with bodily fluid or faeces|
|Rabies||Rabies virus||commonly – dogs, bats, monkeys, raccoons, foxes, skunks, cattle, goats, sheep, wolves, coyotes, groundhogs, horses, mongooses and cats||through saliva by biting, or through scratches from an infected animal||Variety of places like Oceanic, South America, Europe; Year is unknown|
|Rat-bite fever||Streptobacillus moniliformis, Spirillum minus||rats, mice||bites of rats but also urine and mucus secretions|
|Rift Valley fever||Phlebovirus||livestock, buffaloes, camels||mosquito bite, contact with bodily fluids, blood, tissues, breathing around butchered animals or raw milk||2006–07 East Africa outbreak|
|Rocky Mountain spotted fever||Rickettsia rickettsii||dogs, rodents||tick bite|
|Ross River fever||Ross River virus||kangaroos, wallabies, horses, opossums, birds, flying foxes||mosquito bite|
|Saint Louis encephalitis||Saint Louis encephalitis virus||birds||mosquito bite|
|Severe acute respiratory syndrome||SARS coronavirus||bats, civets||close contact, respiratory droplets||2002–04 SARS outbreak; started in China|
|Smallpox||Variola virus||Possible Monkeys or horses||Spread to person to person quickly||The last cases was in 1977; WHO certified to Eraticated (for the world) in December 1979 or 1980.|
|Swine influenza||A new strain of the influenza virus endemic in pigs (excludes H1N1 swine flu, which is a human virus).||pigs||close contact||2009–10; 2009 swine flu pandemic; The outbreak began in Mexico.|
|Taenia crassiceps infection||Taenia crassiceps||wolves, coyotes, jackals, foxes||contact with soil contaminated with feces|
|Toxocariasis||Toxocara canis, Toxocara cati||dogs, foxes, cats||ingestion of eggs in soil, fresh or unwashed vegetables or undercooked meat|
|Toxoplasmosis||Toxoplasma gondii||cats, livestock, poultry||exposure to cat feces, organ transplantation, blood transfusion, contaminated soil, water, grass, unwashed vegetables, unpasteurized dairy products and undercooked meat|
|Trichinosis||Trichinella spp.||rodents, pigs, horses, bears, walruses, dogs, foxes, crocodiles, birds||eating undercooked meat|
|Tuberculosis||Mycobacterium bovis||infected cattle, deer, llamas, pigs, domestic cats, wild carnivores (foxes, coyotes) and omnivores (possums, mustelids and rodents)||milk, exhaled air, sputum, urine, faeces and pus from infected animals|
|Tularemia||Francisella tularensis||lagomorphs (type A), rodents (type B), birds||ticks, deer flies, and other insects including mosquitoes|
|West Nile fever||Flavivirus||birds, horses||mosquito bite|
|Zika fever||Zika virus||chimpanzees, gorillas, orangutans, monkeys, baboons||mosquito bite, sexual intercourse, blood transfusion and sometimes bites of monkeys||2015–16 epidemic in the Americas and Oceanic|
During most of human prehistory groups of hunter-gatherers were probably very small. Such groups probably made contact with other such bands only rarely. Such isolation would have caused epidemic diseases to be restricted to any given local population, because propagation and expansion of epidemics depend on frequent contact with other individuals who have not yet developed an adequate immune response. To persist in such a population, a pathogen either had to be a chronic infection, staying present and potentially infectious in the infected host for long periods, or it had to have other additional species as reservoir where it can maintain itself until further susceptible hosts are contacted and infected. In fact, for many 'human' diseases, the human is actually better viewed as an accidental or incidental victim and a dead-end host. Examples include rabies, anthrax, tularemia and West Nile virus. Thus, much of human exposure to infectious disease has been zoonotic.
Through religious scripture, different civilizations as early as 500 years B.C.E had dietary laws that prohibit or allow the consumption of certain animals. Christian and Hebrew religions have reflected these traditions in the Book of Leviticus, while Islamic religions spread the laws throughout the Quran, referring to these rules as Haram and Halal. Some consider these dietary rules evolved, among other reasons, to reduce the risk of contracting diseases from animals.
Many modern diseases, even epidemic diseases, started out as zoonotic diseases. It is hard to establish with certainty which diseases jumped from other animals to humans, but there is increasing evidence from DNA and RNA sequencing, that measles, smallpox, influenza, HIV, and diphtheria came to humans this way. Various forms of the common cold and tuberculosis also are adaptations of strains originating in other species. Some experts have suggested that all human viral infections were originally zoonotic.
Zoonoses are of interest because they are often previously unrecognized diseases or have increased virulence in populations lacking immunity. The West Nile virus appeared in the United States in 1999 in the New York City area, and moved through the country in the summer of 2002, causing much distress. Bubonic plague is a zoonotic disease, as are salmonellosis, Rocky Mountain spotted fever, and Lyme disease.
A major factor contributing to the appearance of new zoonotic pathogens in human populations is increased contact between humans and wildlife. This can be caused either by encroachment of human activity into wilderness areas or by movement of wild animals into areas of human activity. An example of this is the outbreak of Nipah virus in peninsular Malaysia in 1999, when intensive pig farming began on the habitat of infected fruit bats. Unidentified infection of the pigs amplified the force of infection, eventually transmitting the virus to farmers and causing 105 human deaths.
Similarly, in recent times avian influenza and West Nile virus have spilled over into human populations probably due to interactions between the carrier host and domestic animals. Highly mobile animals such as bats and birds may present a greater risk of zoonotic transmission than other animals due to the ease with which they can move into areas of human habitation.
Because they depend on the human host for part of their life-cycle, diseases such as African schistosomiasis, river blindness, and elephantiasis are not defined as zoonotic, even though they may depend on transmission by insects or other vectors.
The first vaccine against smallpox by Edward Jenner in 1800 was by infection of a zoonotic bovine virus which caused a disease called cowpox. Jenner had noticed that milkmaids were resistant to smallpox. Milkmaids contracted a milder version of the disease from infected cows that conferred cross immunity to the human disease. Jenner abstracted an infectious preparation of 'cowpox' and subsequently used it to inoculate persons against smallpox. As a result, smallpox has been eradicated globally, and mass vaccination against this disease ceased in 1981.
Zoonoses are infectious diseases which jump from an animal host or reservoir into humans.
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