A zoonosis (; : zoonoses) or zoonotic disease is an infectious disease of humans caused by a pathogen (an infectious agent, such as a virus, bacterium, parasite, fungus or prion) which is transmitted from a non-human animal to a human. When humans infect non-humans, it is called reverse zoonosis or anthroponosis.
Major modern diseases such as Ebola 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 into a separate human-only disease. Human infection with animal influenza viruses is rare, as they do not transmit easily to or among humans. However, avian and swine influenza viruses in particular possess high zoonotic potential, and these occasionally recombine with human strains of the flu and can cause pandemics such as the 2009 swine flu. 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 in non-humans; however, only diseases that routinely involve non-human to human transmission, such as rabies, are considered direct zoonoses.
Zoonoses have different modes of transmission. In direct zoonosis the disease is directly transmitted between non-humans and humans through the air (influenza), bites and saliva (rabies).
Host genetics plays an important role in determining which non-human viruses will be able to make copies of themselves in the human body. Dangerous non-human viruses are those that require few mutations to begin replicating themselves in human cells. These viruses are potentially dangerous since the required combinations of mutations might randomly arise in the natural reservoir.
Epidemiology Zoonotic diseases account for a substantial proportion of infectious diseases affecting humans. It is estimated that approximately 60% of known human infectious diseases and up to 75% of emerging infectious diseases are zoonotic in origin.Jones, K. E., Patel, N. G., Levy, M. A., Storeygard, A., Balk, D., Gittleman, J. L., & Daszak, P. (2008). Global trends in emerging infectious diseases. Nature, 451(7181), 990–993. https://doi.org/10.1038/nature06536World Health Organization. (2022). Zoonoses. https://www.who.int/news-room/fact-sheets/detail/zoonoses
Zoonotic diseases frequently emerge through spillover events, in which pathogens are transmitted from animal reservoirs to human populations.Karesh, W. B., Dobson, A., Lloyd-Smith, J. O., Lubroth, J., Dixon, M. A., Bennett, M., et al. (2012). Ecology of zoonoses: Natural and unnatural histories. The Lancet, 380(9857), 1936–1945. https://doi.org/10.1016/S0140-6736(12)61678-X The likelihood of spillover is influenced by environmental and anthropogenic factors, including deforestation, land-use change, and increased human encroachment into wildlife habitats.Karesh et al., 2012
Disease emergence hotspots are often located in tropical regions where high biodiversity coincides with rapid human population growth and environmental disruption.Jones et al., 2008 Climate change further contributes by altering the distribution of vectors such as mosquitoes and ticks, thereby influencing transmission dynamics.World Health Organization, 2022
Examples of zoonotic diseases include Ebola virus disease, associated with wildlife reservoirs, and Rift Valley fever, which demonstrates climate-sensitive vector transmission patterns.
The transmission dynamics of zoonotic diseases vary widely. Some infections result in limited human-to-human transmission, while others may sustain outbreaks depending on factors such as the pathogen's basic reproduction number (R0), population density, and the effectiveness of public health interventions.
Causes Urban wildlife and disease transmission Urban wildlife includes animals that live in cities and suburbs, such as rodents, pigeons, bats, and raccoons. Urbanization can increase contact between humans, domestic animals, and wildlife, creating opportunities for zoonotic disease transmission.
Rodents are among the most important urban reservoirs for zoonotic pathogens. They can spread diseases to humans through bites, scratches, contaminated food, or contact with urine, saliva, or droppings. Diseases associated with rodents include leptospirosis, hantavirus infection, salmonellosis, and rat-bite fever.
Urban birds may also contribute to disease exposure in some settings. Pigeon droppings are associated with fungal infections such as histoplasmosis and cryptococcosis, typically through inhalation of contaminated dust.
Wild mammals adapted to urban environments can also contribute to zoonotic risk. Rabies is a zoonotic viral disease affecting mammals, including wildlife such as bats and raccoons.
A One Health approach is used to address zoonotic disease risks by integrating human, animal, and environmental health. Prevention strategies include improving sanitation, waste management, controlling wildlife populations, and strengthening public health surveillance systems. The emergence of zoonotic diseases originated with the domestication of animals. 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.
Recently, there has been a rise in frequency of appearance of new zoonotic diseases. "Approximately 1.67 million undescribed viruses are thought to exist in mammals and birds, up to half of which are estimated to have the potential to spill over into humans", says a study led by researchers at the University of California, Davis. According to a report from the United Nations Environment Programme and International Livestock Research Institute a large part of the causes are environmental like climate change, unsustainable agriculture, exploitation of wildlife, and land use change. Others are linked to changes in human society such as an increase in mobility. The organizations propose a set of measures to stop the rise.
Contamination of food or water supply Foodborne zoonotic diseases are caused by a variety of pathogens that can affect both humans and animals. The most significant zoonotic pathogens causing foodborne diseases are:
Bacterial pathogens Escherichia coli O157:H7, Campylobacter, Caliciviridae, and Salmonella.
Viral pathogens
Hepatitis E: Hepatitis E virus (HEV) is primarily transmitted through pork products, especially in developing countries with limited sanitation. The infection can lead to acute liver disease and is particularly dangerous for pregnant women. Norovirus: Often found in contaminated shellfish and fresh produce, norovirus is a leading cause of foodborne illness globally. It spreads easily and causes symptoms like vomiting, diarrhea, and stomach pain.
Parasitic pathogens
Toxoplasma gondii: This parasite is commonly found in undercooked meat, especially pork and lamb, and can cause toxoplasmosis. While typically mild, toxoplasmosis can be severe in immunocompromised individuals and pregnant women, potentially leading to complications. Trichinella spp. is transmitted through undercooked pork and wild game, causing trichinellosis. Symptoms range from mild gastrointestinal distress to severe muscle pain and, in rare cases, can be fatal.
Farming, ranching and animal husbandry
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 infection with the bacterium Chlamydia psittaci, causing chlamydiosis (and enzootic abortion in pregnant women), as well as increase the risk of Q fever, toxoplasmosis, and listeriosis, in the pregnant or otherwise immunocompromised. Echinococcosis is caused by a tapeworm, which can spread from infected sheep by food or water contaminated by feces or wool. Avian influenza is common in chickens, and, while it is rare in humans, the main public health worry is that a strain of avian influenza will recombine with a human influenza 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 avian influenza. Cattle are an important reservoir of cryptosporidiosis, which mainly affects the immunocompromised. Reports have shown mink can also become infected. In Western countries, hepatitis E burden is largely dependent on exposure to animal products, and pork is a significant source of infection, in this respect. Similarly, the human coronavirus OC43, the main cause of the common cold, can use the pig as a zoonotic reservoir, constantly reinfecting the human population.
Veterinarians are exposed to unique occupational hazards when it comes to zoonotic disease. In the US, studies have highlighted an increased risk of injuries and lack of veterinary awareness of 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. Habitat loss of viral reservoir species has been identified as a significant source in at least one spillover event.
Wildlife trade or animal attacks The wildlife trade may increase spillover risk because it directly increases the number of interactions across animal species, sometimes in small spaces. The origin of the COVID-19 pandemic is traced to the wet markets in China.
Zoonotic disease emergence is demonstrably linked to the consumption of wildlife meat, exacerbated by human encroachment into natural habitats and amplified by the unsanitary conditions of wildlife markets. These markets, where diverse species converge, facilitate the mixing and transmission of pathogens, including those responsible for outbreaks of HIV-1, Ebola, and mpox, and potentially even the COVID-19 pandemic. Notably, small mammals often harbor a vast array of zoonotic bacteria and viruses, yet endemic bacterial transmission among wildlife remains largely unexplored.
Insect vectors African sleeping sickness Dirofilariasis Eastern equine encephalitis Japanese encephalitis Saint Louis encephalitis Scrub typhus Tularemia Venezuelan equine encephalitis West Nile fever Western equine encephalitis Zika fever
Pets