Zoonosis may be a word with an ominous undertone, but the interspecies transmission of viruses is commonplace in the microbial world. An animal-borne, or zoonotic, infection means that a virus or bacterium hops from animals to humans or vice versa. Alarming as that may sound, the vast majority of all the infectious diseases that can affect humans are in fact zoonoses, including Covid-19.
“A new virus has to come from somewhere. Practically the only alternative to a zoonosis is bioterrorism – the deliberate production and release of viruses – but there is no indication that the new coronavirus was bioengineered,” says Pekka Nuorti, professor of epidemiology at Tampere University.
For a virus to pass from an animal to a human, a person must first be bitten or otherwise exposed to the secretions or bodily fluids of an infected animal. The virus must also be capable of replicating in the human body. The odds of a virus leaping successfully from an animal to a human are slim.
“Transmission usually requires close or prolonged contact. The risk of a zoonotic infection increases if people handle animals without gloves or are in close or direct contact with animals. We do not yet have an exact understanding of how the coronavirus crossed over to people.”
Viruses are not living organisms. They are only tiny scraps of genetic code, either double-stranded DNA or single-stranded RNA. In order to reproduce, a virus needs a host cell and an animal that acts as a viral reservoir. The animal that carries the virus either develops an immunity or is able to coexist with the virus without becoming sick.
“The animal harbours the virus with few ill effects. From the virus’s perspective, the host dying could mean that the viral lineage goes extinct,” Nuorti points out.
While modern science has unravelled many mysteries about viruses, there are important questions that remain unanswered.
Some zoonotic viruses can hop over to humans directly from the host animal. Some of the most common zoonoses that are directly transmitted are the intestinal infections salmonellosis and campylobacteriosis. However, zoonoses are often transmitted via an intermediate species called a vector that carries the disease. Many of these vectors are insects but, for example, the SARS virus that caused an outbreak in 2004 likely jumped from bats to civet cats before spilling over to humans.
The new coronavirus, SARS-CoV-2, also originated in bats but the intermediate species is as yet unidentified. Based on the virus’s genome, a range of animals – from snakes to scaly anteaters – have been suggested. Viruses can mutate inside vectors, but the movement of vectors also allows them to spread to previously disease-free areas.
“The outbreak of new viral disease requires a germ, a person and a suitable environment. The vector is connected to all three.”
While modern science has unravelled many mysteries about viruses, there are important questions that remain unanswered. Bacteria were first viewed under a microscope in the 1670s, but it was not until 1939 that a virus was visualised using an electron microscope.
We now know that a litre of seawater contains roughly 10 billion viruses. While only a fraction are harmful to human health, a fraction of 10 billion is still a substantial number. The Global Virome Project estimates that there are 1.67 million yet-to-be-discovered viral species with the potential to infect humans in mammal and bird hosts.
We are already familiar with many of the viral mechanisms for manipulating host cells, but further research is needed to elucidate the links between viral activities and the living conditions of animals. Take bats: they are known reservoirs for dangerous viruses such as SARS and Ebola, but they appear to tolerate these infections with little evidence of disease.
The immune system of bats has become the subject of intense scientific interest worldwide. Studies have shown that under normal circumstances bats have a unique ability to thwart deadly diseases and coexist with viruses, but when they get stressed their immune systems deteriorate and their tolerance to viral infections weakens.
Bats are responding to stress from such things as habitat destruction – therefore having to fly longer distances to find their prey – and living in cages in East Asian wet markets that sell live animals.
“For viruses, the primary transmission mechanism is zoonotic. The results of human activity, such as climate change, environmental degradation, tropical deforestation and the clearing of land for new settlements, amplify the risk of the emergence of new zoonotic agents,” Nuorti says.
With humans destroying nature at an unprecedented rate, animals suffer and are exposed to diseases in ways they have not become used to previously.
The coronavirus, Ebola, bird flu, dengue fever, malaria, Lassa fever... the list of zoonotic diseases goes on. The coronavirus pandemic has hit home for us all, but outbreaks of deadly viral diseases frequently occur around the world. Nuorti points out that a large number of zoonotic diseases are also found in Finland.
“New viruses tend to dominate the media spotlight, but endemic zoonoses cause a severe disease burden in Finland on an annual basis,” Nuorti notes.
One of them is Pogosta disease, a mosquito-borne viral disease that has a natural reservoir in grouse, a group of game birds. The Puumala virus carried by the bank vole can cause thousands of cases of nephrophathia epidemica (a mild form of haemorrhagic fever with renal syndrome) during severe outbreaks. Ticks harbour a virus that causes tick-borne encephalitis (TBE).
We have known for decades that environmental problems and degradation exacerbate the transmission of infectious diseases. With humans destroying nature at an unprecedented rate, animals suffer and are exposed to diseases in ways they have not become used to previously.
“Influenza is one example. Influenza viruses can infect pigs and birds, and the cramped conditions of large-scale agricultural facilities allow viruses to infect again and again, increasing the frequency of mutations.”
These mutations can not only cause the virus to become harmful to humans but also impact its ability to spread – although it is possible for a mutation to reduce virulence. On a large scale, population growth, migration and the expansion of settlements increase the spread of infectious diseases. Modern life brings people into closer contact with wildlife.
“We are living in close proximity with animal species that we may never have been near before. Viruses have crossed from animals to humans in the past, but outbreaks were most likely largely confined to remote rural areas and eventually died out. Now we are living in a mobile world, and air travellers can introduce infectious diseases to any part of the world within 24 hours,” Nuorti notes.
Pathogenic viruses to which we have no immunity may be preserved in permafrost.
Our modern lifestyle alone does not drive or prevent the emergence of zoonotic diseases but creates more opportunities for viral spillover. Climate change, in itself the result of human activity, increases the risk of zoonotic diseases.
“For example, the endemic regions of vector-borne viral and bacterial diseases are moving northwards in Finland,” Nuorti says.
With the habitat of ticks expanding in the north, the incidence of tick-borne Lyme disease and TBE is increasing. The habitat of mosquitoes that carry malaria, which kills around 430,000 people worldwide each year, could expand from tropical regions to parts of the United States and Southern Europe.
Another health impact of climate change is related to water. Climate change increases the frequency of heavy rains and flooding, which can contaminate groundwater and the raw water sources of water treatment plants. For example, noroviruses and legionella bacteria thrive in warm waters.
Global warming is causing the Earth’s glaciers and polar ice sheets to gradually melt.
Some researchers are already warning of the consequences of the thawing permafrost. Pathogenic viruses to which we have no immunity may be preserved in glacial ice. Although unlikely, long-dormant viruses may be unearthed through mining and drilling, and something that once vanished may reappear.
Researchers have successfully revived worms that have been frozen in the Siberian permafrost for 40,000 years as well as a 30,000-year-old harmless virus. Still, a much more urgent threat is presented by the pathogens already among us and the bacteria emerging from the melting permafrost, which releases compounds that further accelerate climate change.
“The transmission of viruses is a tremendously complex process that requires several factors to align. The felling of tropical forests facilitates the spread of infectious diseases, but the primary drivers are poverty, wars and human mobility,” Nuorti says.
Viruses evolved long before humans, but we have created the ideal conditions for them to spread and mutate.
“Preventing, tackling and managing pandemics and epidemics requires a multidisciplinary global effort,” Nuorti says.