A virus pandemic has struck the world in 2020. Coronavirus disease 2019, or Covid-19, is an infectious respiratory disease firstly identified in Wuhan, China in December 2019 and has since spread to every corner of the world, killing many hundreds of thousands of people. A lot of research is still necessary to understand the disease and the virus that causes it, yet scientists believe it was originated in animals. But what do we know so far about the role of animals in the emergence of this disease and which animal is the most likely to have transmitted the virus to humans? Can animals also be affected by Covid-19 and how can they help us in fighting the disease? Discover some answers for these and other questions in the following article…
By Ricardo Ribas
Coronaviruses are a large family of viruses that have existed for over 10,000 years. The name derives from the latin word “corona” that means crown due to the presence of crown-like spikes in the virus surface. Coronaviruses are often responsible for respiratory and intestinal diseases in mammals and birds but symptoms can vary according to the host as well as with the type of virus involved. For instance, in chickens, coronaviruses tend to attack predominantly the respiratory and urogenital tracts, whilst in dogs, cats, ferrets, rabbits, turkeys, pigs and cows it affects predominantly the intestines and digestive systems leading to diarrhoea.
Humans can also be infected by coronaviruses. So far there are seven types of coronaviruses known to infect humans, four of which are responsible for mild respiratory infections and three that lead to serious and sometimes lethal respiratory infections, such as pneumonia. These include the Severe Acute Respiratory Syndrome (SARS), a disease initially discovered in China in 2003; the Middle East Respiratory Syndrome (MERS), first found in Saudi Arabia in 2012; and the most recent outbreak that started in China in 2019-2020 (Covid-19), all of which believed to have been transmitted from animals to humans.
All the three coronaviruses responsible for serious infections (SARS, MERS and Covid-19) appear to be originated in bats, that is believed to be the natural reservoir. Bats are often reservoir organisms due to their tolerant immune systems, what allows the viruses to live and reproduce in their bodies without causing any disease. But to be able to reach and infect humans, these viruses generally need an intermediate host animal. In the case of SARS, it is believed that the disease has jumped to humans through the consumption of civet cats previously infected by bats, whilst in the MERS, it is though that the transmission has occurred through dromedary camel’s meat. Scientists are still trying to understand the intermediary specie for the Covid-19 virus but it appears to have happened in a wildlife market probably during consumption of animal products. To try to identify the intermediate animal, researchers have compared the genomic sequence of the human coronavirus responsible for Covid-19 and other coronaviruses that infect several animals and they have discovered large similarities with the virus that infect pangolins, indicating that this is the most likely intermediate host.
But, can animals also get sick from Covid-19?
A lot of research is still necessary to better understand this virus, but the good news is that it looks like animals are not playing a significant role in the spread of the human disease and the risks of animals spreading Covid-19 to humans is low. To date, only a few but very exceptional cases have been reported of humans contaminating animals and one case in which animals have transmitted the virus back to humans. The Netherlands government reported in April 2020 that several minks suffering from respiratory symptoms tested positive for Covid-19 in four different farms possibly infected by humans. More recently, it was announced that two farm workers have now been infected from one of those animals, in what appears to be the first European case of transmission from animals to human. Similarly, several tigers and lions in the New York Zoo also tested positive for the disease after showing symptoms of a cough and loss of appetite. Scientists believe these animals may have been infected by a member of staff carrying the virus, however investigations are still ongoing. Similarly, there have been a small number of cases of confirmed Covid-19 reported in dogs and cats suffering from diarrhoea, vomiting and shortness of breath after being in close contact with people infected by the virus.
Several studies are underway in laboratories to understand which species can or cannot be infected by the disease. Preliminary data in a small number of animals have shown that cats, ferrets and hamsters can be easily infected by the virus and transmit it to other animals of the same species, but that dogs appear to be less sensitive to infection, whilst pigs, chickens and ducks failed to become infected.
As we learn more about this disease and the way it can affect animals, governments around the world are recommending to treat pets in the same way as we would treat other members of our household, to wash the hands frequently before and after handling animals and their food, and to contact the vet if have any questions.
How can animals help scientists fighting and understanding the disease?
Many efforts are ongoing to develop new methods of diagnosis and to discover new treatments and vaccines. To achieve that, scientists are taking advantage of animals to help them answer some of these questions.
It is already known that some dogs can efficiently detect the odour of many human diseases such as cancer, Parkinson’s diseases, diabetes and malaria. Scientists in the UK (at the London School of Hygiene and Tropical Medicine and Durham University) and around the world are now trying to find out if trained sniffer dogs can also be able to reliably detect Covid-19 infections in people even before symptoms appear. So far, preliminary studies seem promising and if successful, this could unleash a new non-invasive tool and a potent early warning measure to detect coronavirus and to help control the spread of the disease.
Some other studies and trials are underway to learn if llamas can help protect humans against this virus. Humans produce antibodies, proteins that recognise the virus and attach to their surface to help in destroying it, but scientists have found that camels, llamas and alpacas produce a small version of these antibodies, called nanobodies. Due of their smaller size, nanobodies are more stable and capable of attaching and sneak into certain pockets of the virus surface, that otherwise human antibodies would struggle. Studies in Belgium have previously shown that nanobodies derived from llamas are efficient in fighting SARS and MERS and scientists are now working towards clinical trials to understand if this is also possible against Covid-19.
Apart from dogs and llamas, scientists also take advantage of other species to help study the disease and to develop vaccines and new treatments. Mice are often used to test the safety of vaccines and therapies before testing in humans. Due to its close proximity with humans, non-human primates such as monkeys are used to help understand how the virus work inside the body, as well as to test the safety and the efficacy of new treatments or if people can be repeatedly infected by the virus. Finally, researchers are taking advantage of other species, such as ferrets, hamsters and cats, because they are good models of the disease or because they can be easily genetically modified to become susceptible to Covid-19 infection.
But, why are animal-derived diseases getting more frequent nowadays?
In the last century, we have witnessed a rapid emergence of new infectious diseases transmitted from animals to humans (zoonosis), such as Ebola, HIV, Swine-Flu, Bird-Flu and several coronavirus infections like SARS, MERS and now Covid-19. But why are these outbreaks becoming more frequent? The fast-growing world population and the development of large metropolis are pushing animals away from their natural habitat, allowing humans to intermingle with wildlife and encouraging the trade of isolated groups of animals often carrying microorganisms to which humans have never been in contact with. Additionally, environmental and climatic changes are causing alterations in temperature and rainfalls therefore shifting animal habitats and ecosystems, altering life-styles, geographic locations and eating habits (who eat whom). The increasing consumption of animal meat is another important factor contributing to the rise of zoonosis as a result of the increased consumption of wild animals, the intensification of greenhouse gas emissions and the deforestation of massive areas for farming and the animal food industry. Finally, and to worsen the situation, the escalation of international travel further contributes to the rapid spread of these diseases.
History has repeated itself and humans are once again suffering the consequences of altering Earth’s habitats and environments. Hopefully lessons will be learned from this pandemic, helping reshape human’s future decisions.
By Ricardo Ribas, Veterinary Doctor, doctorate in veterinary sciences and researcher in the area of oncology in London
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The material and ideas discussed in this article are for informational purposes only. For more information consult a vet or a professional in the area. Whilst every effort is made to make sure the article is accurate at the time of publication, I take no liability for any new developments on the subject as well as any errors or omissions.