Major modern diseases such asEbola andsalmonellosis 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.[5][6][7] Human infection with animalinfluenza viruses is rare, as they do not transmit easily to or among humans.[8] However,avian andswine influenza viruses in particular possess high zoonotic potential,[9] and these occasionally recombine with human strains of the flu and can causepandemics such as the2009 swine flu.[10] Zoonoses can be caused by a range of disease pathogens such asemergent viruses, bacteria, fungi and parasites; of 1,415 pathogens known to infect humans, 61% were zoonotic.[11] Most human diseases originated in non-humans; however, only diseases that routinely involve non-human to human transmission, such asrabies, are considered direct zoonoses.[12]
Zoonoses have different modes of transmission. In direct zoonosis the disease is directly transmitted from non-humans to humans through media such as air (influenza) or bites and saliva (rabies).[13] In contrast, transmission can also occur via an intermediate species (referred to as avector), which carry the disease pathogen without getting sick. The term is fromAncient Greek: ζῷονzoon "animal" and νόσοςnosos "sickness".
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 dangerous since the required combinations of mutations might randomly arise in thenatural reservoir.[14]
The emergence of zoonotic diseases originated with thedomestication of animals.[15] 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.[16][17]
Recently, there has been a rise in frequency of appearance of new zoonotic diseases. "Approximately 1.67 million undescribed viruses are thought to exist inmammals and birds, up to half of which are estimated to have the potential to spill over into humans", says a study[18] led by researchers at theUniversity of California, Davis. According to a report from theUnited Nations Environment Programme andInternational Livestock Research Institute a large part of the causes are environmental likeclimate change, unsustainable agriculture, exploitation of wildlife, andland 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.[19][20]
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:
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.[24]
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.[25]
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.[26]
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.[27]
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 tocutaneous anthrax infection, whereasinhalation anthrax infection is more common for workers inslaughterhouses,tanneries, andwool mills.[28] Close contact with sheep who have recently given birth can lead to infection with the bacteriumChlamydia psittaci, causing chlamydiosis (andenzootic abortion in pregnant women), as well as increase the risk ofQ fever,toxoplasmosis, andlisteriosis, in the pregnant or otherwiseimmunocompromised.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 the1918 Spanish flu.[29] In 2017,free-range chickens in the UK were temporarily ordered to remain inside due to the threat of avian influenza.[30] Cattle are an important reservoir ofcryptosporidiosis,[31] which mainly affects the immunocompromised. Reports have shownmink can also become infected.[32] 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.[24] Similarly, thehuman coronavirus OC43, the main cause of the common cold, can use the pig as a zoonotic reservoir,[33] constantly reinfecting the human population.
Veterinarians are exposed to uniqueoccupational 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 withmusculoskeletal injuries, animal bites, needle-sticks, and cuts.[34]
A July 2020 report by theUnited Nations Environment Programme stated that the increase in zoonotic pandemics is directly attributable toanthropogenic destruction of nature and the increased global demand for meat and that theindustrial farming of pigs and chickens in particular will be a primary risk factor for the spillover of zoonotic diseases in the future.[35] Habitat loss of viral reservoir species has been identified as a significant source in at least onespillover event.[36]
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.[44] These markets, where diverse species converge, facilitate the mixing and transmission of pathogens, including those responsible for outbreaks of HIV-1,[45] Ebola,[46] andmpox,[47] and potentially even the COVID-19 pandemic.[48] Notably, small mammals often harbor a vast array of zoonotic bacteria and viruses,[49] yet endemic bacterial transmission among wildlife remains largely unexplored. Therefore, accurately determining the pathogenic landscape of traded wildlife is crucial for guiding effective measures to combat zoonotic diseases and documenting the societal and environmental costs associated with this practice.
Pets may also serve as a reservoir of viral disease and contribute to the chronic presence of certain viral diseases in the human population. For instance, approximately 20% of domestic dogs, cats, and horses carry anti-hepatitis E virusantibodies and thus these animals probably contribute to human hepatitis E burden as well.[52] For non-vulnerable populations (e.g., people who are not immunocompromised) the associated disease burden is, however, small.[53][54] Furthermore, the trade of non domestic animals such as wild animals as pets can also increase the risk of zoonosis spread.[55][56]
Hunting involves humans tracking, chasing, and capturing wild animals, primarily for food or materials like fur. However, other reasons like pest control or managing wildlife populations can also exist. Transmission of zoonotic diseases, those leaping from animals to humans, can occur through various routes: direct physical contact, airborne droplets or particles, bites or vector transport by insects, oral ingestion, or even contact with contaminated environments.[60] Wildlife activities like hunting and trade bring humans closer to dangerous zoonotic pathogens, threatening global health.[61]
According to the Center for Diseases Control and Prevention (CDC) hunting and consuming wild animal meat ("bushmeat") in regions like Africa can expose people to infectious diseases due to the types of animals involved, like bats and primates. Unfortunately, common preservation methods like smoking or drying aren't enough to eliminate these risks.[62] Although bushmeat provides protein and income for many, the practice is intricately linked to numerous emerging infectious diseases like Ebola, HIV, andSARS, raising critical public health concerns.[61]
A review published in 2022 found evidence that zoonotic spillover linked to wildmeat consumption has been reported across all continents.[63]
Deforestation, biodiversity loss and environmental degradation
Kate Jones, Chair of Ecology and Biodiversity atUniversity College London, says zoonotic diseases are increasingly linked to environmental change and human behavior. The disruption of pristine forests driven by logging, mining, road building through remote places, rapid urbanization, 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".[64] In a guest article, published byIPBES, President of theEcoHealth Alliance and zoologistPeter Daszak, along with three co-chairs of the 2019Global Assessment Report on Biodiversity and Ecosystem Services, Josef Settele,Sandra Díaz, and Eduardo Brondizio, wrote 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."[65]
Joshua Moon, Clare Wenham, and Sophie Harman said that there is evidence that decreased biodiversity has an effect on the diversity of hosts and frequency of human-animal interactions with potential for pathogenic spillover.[66]
An April 2020 study, published in theProceedings of the Royal Society'sPart B journal, found that increased virus spillover events from animals to humans can be linked tobiodiversity loss andenvironmental 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.[67] An August 2020 study, published inNature, concludes that theanthropogenic destruction of ecosystems for the purpose of expanding agriculture and human settlements reduces biodiversity and allows for smaller animals such as bats and rats, which are more adaptable to human pressures and also carry the most zoonotic diseases, to proliferate. This in turn can result in more pandemics.[68]
In October 2020, theIntergovernmental 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 ofbiodiversity 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 agrowing 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 driveclimate change and biodiversity loss also drive pandemic risk through their impacts on our environment."[69][70][71]
According to a report from theUnited Nations Environment Programme andInternational Livestock Research Institute, entitled "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 the increase in the number of zoonotic diseases.[19][20] The University of Sydney issued a study, in March 2021, that examines factors increasing the likelihood ofepidemics and pandemics like the COVID-19 pandemic. The researchers found that "pressure on ecosystems, climate change and economic development are key factors" in doing so. More zoonotic diseases were found inhigh-income countries.[72]
A 2022 study dedicated to the link between climate change and zoonosis found a strong link between climate change and the epidemic emergence in the last 15 years, as it caused a massive migration of species to new areas, and consequently contact between species which do not normally come in contact with one another. Even in a scenario with weak climatic changes, there will be 15,000 spillover of viruses to new hosts in the next decades. The areas with the most possibilities for spillover are the mountainous tropical regions of Africa and southeast Asia. Southeast Asia is especially vulnerable as it has a large number of bat species that generally do not mix, but could easily if climate change forced them to begin migrating.[73]
A 2021 study found possible links between climate change and transmission of COVID-19 through bats. The authors suggest that climate-driven changes in the distribution and robustness of bat species harboring coronaviruses may have occurred in eastern Asian hotspots (southern China, Myanmar, and Laos), constituting a driver behind the evolution and spread of the virus.[74][75]
Zoonotic diseases contribute significantly to the burdened public health system as vulnerable groups such the elderly, children, childbearing women and immune-compromised individuals are at risk.[citation needed] According to theWorld Health Organization (WHO), any disease or infection that is primarily ‘naturally’ transmissible from vertebrate animals to humans or from humans to animals is classified as a zoonosis.[76] Factors such as climate change, urbanization, animal migration and trade, travel and tourism, vector biology, anthropogenic factors, and natural factors have greatly influenced the emergence, re-emergence, distribution, and patterns of zoonoses.[76]
Zoonotic diseases generally refer to diseases of animal origin in which direct or vector mediated animal-to-human transmission is the usual source of human infection. Animal populations are the principal reservoir of the pathogen and horizontal infection in humans is rare. A few examples in this category include lyssavirus infections, Lyme borreliosis, plague, tularemia,leptospirosis, ehrlichiosis, Nipah virus, West Nile virus (WNV) and hantavirus infections.[77] Secondary transmission encompasses a category of diseases of animal origin in which the actual transmission to humans is a rare event but, once it has occurred, human-to-human transmission maintains the infection cycle for some period of time. Some examples includehuman immunodeficiency virus (HIV)/acquired immune deficiency syndrome (AIDS), certain influenza A strains,Ebola virus and severe acute respiratory syndrome (SARS).[77]
One example is Ebola which is spread by direct transmission to humans from handlingbushmeat (wild animals hunted for food) and contact with infected bats or close contact with infected animals, including chimpanzees, fruit bats, and forest antelope. Secondary transmission also occurs fromhuman to human by direct contact with blood, bodily fluids, or skin of patients with or who died of Ebola virus disease.[78] Some examples of pathogens with this pattern of secondary transmission are human immunodeficiency virus/acquired immune deficiency syndrome, influenza A, Ebola virus and severe acute respiratory syndrome. Recent infections of theseemerging and re-emerging zoonotic infections have occurred as a results of many ecological and sociological changes globally.[77]
During most of humanprehistory groups ofhunter-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 adequateimmune response.[79] To persist in such a population, a pathogen either had to be achronic infection, staying present and potentially infectious in the infected host for long periods, or it had to have other additional species asreservoir where it can maintain itself until further susceptible hosts are contacted and infected.[80][81] In fact, for many "human" diseases, the human is actually better viewed as an accidental or incidental victim and adead-end host. Examples include rabies, anthrax, tularemia, and West Nile fever. Thus, much of human exposure to infectious disease has been zoonotic.[82]
Possibilities for zoonotic disease transmissions
Many diseases, even epidemic ones, have zoonotic origin andmeasles,smallpox,influenza, HIV, anddiphtheria are particular examples.[83][84] Various forms of thecommon cold andtuberculosis also are adaptations of strains originating in other species.[85][86] Some experts have suggested that all human viral infections were originally zoonotic.[87]
A major factor contributing to the appearance of new zoonotic pathogens in human populations is increased contact between humans and wildlife.[89] 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 ofNipah virus in peninsular Malaysia, in 1999, whenintensive pig farming began within the habitat of infected fruit bats.[90] The unidentified infection of these pigs amplified the force of infection, transmitting the virus to farmers, and eventually causing 105 human deaths.[91]
Similarly, in recent times avian influenza and West Nile virus havespilled over into human populations probably due to interactions between the carrier host and domestic animals.[92] 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.
The first vaccine against smallpox byEdward Jenner in 1800 was by infection of a zoonotic bovine virus which caused a disease calledcowpox.[94] 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 of vaccination, smallpox has been eradicated globally, and mass inoculation against this disease ceased in 1981.[95] There are a variety of vaccine types, including traditional inactivated pathogen vaccines,subunit vaccines,live attenuated vaccines. There are also new vaccine technologies such as viral vector vaccines andDNA/RNA vaccines, which include many of theCOVID-19 vaccines.[96]
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