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 evolved to a separate human-only disease. Most strains of influenza that infect humans are human diseases, although many strains of swine and bird 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 viruses, bacteria, fungi and parasites; of 1,415 pathogens known to infect humans, 61% were zoonotic. Most human diseases originated in animals; however, only diseases that routinely involve animal to human transmission, like 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 infected. When humans infect animals, it is called reverse zoonosis or anthroponosis. The term is from Greek: ζῷον zoon "animal" and νόσος nosos "sickness".
Zoonotic transmission can occur in any context in which there is companionistic (pets), economic (farming, etc.), predatory (hunting, butchering or consuming wild game) or research contact with or consumption of animals, animal products, or animal derivatives (vaccines, etc.).
In 2006, a conference held in Berlin was focusing on the issue of zoonotic pathogen effects on food safety, urging governments to intervene, and the public to be vigilant towards the risks of catching food-borne diseases from farm-to-dining table.
Many food outbreaks can be linked to zoonotic pathogens. Many different types of food can be contaminated that have an animal origin. Some common foods linked to zoonotic contaminations include eggs, seafood, meat, dairy, and even some vegetables. Food outbreaks 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 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.
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.
Outbreaks of zoonoses have been traced to human interaction with and exposure to animals at fairs, 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 and animal contact, and animal care and management.
|Disease||Pathogen(s)||Animals involved||Mode of transmission|
|African sleeping sickness||Trypanosoma brucei rhodesiense||range of wild animals and domestic livestock||transmitted by the bite of the tsetse fly|
|Angiostrongyliasis||Angiostrongylus cantonensis, Angiostrongylus costaricensis||rats, cotton rats||consuming raw and/or undercooked snails, slugs, other mollusks, crustaceans, monitor lizards, frogs and unwashed vegetables contaminated with larvae|
|Anisakiasis||Anisakis||whales, dolphins, seals, sea lions, other marine animals||eating raw and/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|
|Baylisascariasis||Baylisascaris spp.||raccoons, badgers, skunks, bears, pandas, fishers, martens, kinkajous, marmots||ingestion of eggs in feces|
|Bird flu||Influenza A virus subtype H5N1||wild birds, domesticated birds such as chickens||close contact|
|Bovine spongiform encephalopathy||Prions||cattle||eating infected meat|
|Brucellosis||Brucella spp.||cattle, goats||infected milk or meat|
|Bubonic plague, Pneumonic plague, Septicemic plague||Yersinia pestis||rabbits, hares, rodents, ferrets, goats, sheep, camels||flea bite|
|Cat-scratch disease||Bartonella henselae, Bartonella quintana||cats||bites or scratches from infected cats|
|Chagas disease||Trypanosoma cruzi||armadillos, Triatominae (kissing bug)||bite|
|Clamydiosis / Enzootic abortion||Chlamydophila abortus||domestic livestock, particularly sheep||close contact with postpartum ewes|
|Variant Creutzfeldt–Jakob disease||PrPvCJD||cattle||eating meat from animals with bovine spongiform encephalopathy (BSE)|
|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, sea lions, beavers, ferrets, reptiles||mosquito bite|
|Eastern equine encephalitis, Venezuelan equine encephalitis, and 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, fruit bats, monkeys, shrews, forest antelope and porcupines||through body fluids, organs and ticks|
|Other haemorrhagic fevers (Marburg viral haemorrhagic fever, Lassa fever, Crimean-Congo 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|
|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 and/or fur|
|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 and/or undercooked food made from animals and unwashed vegetables contaminated with feces|
|Gnathostomiasis||Gnathostoma spp.||dogs, minks, opossums, cats, lions, tigers, leopards, raccoons, poultry, other birds, frogs||raw and/or undercooked fish or meat|
|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|
|Hantavirus||Hantavirus spp.||deer mice, cotton rats and other rodents||exposure to feces, urine, saliva or bodily fluids|
|Histoplasmosis||Histoplasma capsulatum||birds, bats||inhaling fungi in guano|
|Influenza||Influenza A virus||horses, pigs, domestic and wild birds, wild aquatic mammals such as seals and whales, minks and farmed carnivores||droplets transmitted through air|
|Japanese encephalitis||Japanese encephalitis virus||pigs, water birds||mosquito bite|
|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|
|Lyme disease||Borrelia burgdorferi||deer, wolves, dogs, birds, rodents, rabbits, hares, reptiles||tick bite|
|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, opossums, mongooses and cats||through saliva by biting, or through scratches from an infected animal|
|Rat-bite fever||Streptobacillus moniliformis, Spirillum minus||rats||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 and/or raw milk|
|Saint Louis encephalitis||Saint Louis encephalitis virus||birds||mosquito bite|
|Swine influenza||any strain of the influenza virus endemic in pigs (excludes H1N1 swine flu, which is a human virus)||pigs||close contact|
|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 and/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, 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) and rodents (type B)||ticks, deer flies, and other insects including mosquitoes|
|West Nile fever||Flavivirus||birds, horses||mosquito bite|
|Zika fever||Zika virus||chimpanzees, monkeys, apes, baboons||mosquito bite, sexual intercourse, blood transfusion and sometimes bites of monkeys|
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.
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.
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.
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