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One small leap to man – how diseases cross the species barrier

By Mindo - 31st Aug 2020

Zoonotic diseases are a major global challenge. David Lynch examines how the progress of Covid-19 has led to renewed calls for a ‘One Health’ approach to the pandemic

According to the World Health Organisation (WHO), all available evidence on Covid-19 suggests that SARS-CoV-2 has a zoonotic source.

One of the most common hypotheses is that it emerged from a bat-borne virus with perhaps an intermediate animal reservoir, such as a pangolin, before reaching humans.

But as the search for the specific origin of the current pandemic continues, the overall impact of zoonotic disease is well established. According to the US Centres for Disease Control and Prevention (CDC), more than six out of every 10 known global infectious diseases in people can be spread from animals, and three out of every four new or emerging infectious diseases in people come from animals.

The ‘One Health’ concept asserts that the health of people is connected to the health of animals and the environment. The WHO, the World Organisation for Animal Health (OIE) and the Food and Agriculture Organisation (FAO) have all recognised that adopting a One Health approach is “key to meeting grand challenges to our health such as climate change and antimicrobial resistance”.

The Centre for One Health (COH), which is part of the Ryan Institute at NUI Galway, held a virtual seminar on Covid-19 at the end of July.


“There are those who say that if wet market regulations had been more stringent in China, we may not be where we are,” Prof Máire Connolly, Adjunct Professor of International Health and Development, NUI Galway, told the seminar. “But I think the reality is that there are probably thousands, if not hundreds of thousands, of microbes out there in the animal world that pose a threat to humans. We have a global interest that the One Health approach is driven by the public.”

In Ireland, the Department of Agriculture, Food and the Marine (DAFM) said the main route of transmission of SARS-CoV-2 worldwide remains “human-to-human spread”.

To date, no animals have tested positive for SARS-CoV-2 virus in Ireland

“Although, research to date has found that many species of animals are susceptible to infection with this virus including, for example, cats and dogs, the numbers of animals worldwide infected with SARS-CoV-2 is considered to be very low and there is no evidence that companion animals are playing an epidemiological role in the spread of Covid-19 in humans,” the DAFM spokesperson told the Medical Independent (MI).

The spokesperson added that there is “no evidence indicating that livestock can be infected with SARS-CoV-2 and preliminary findings from studies suggest that poultry and pigs are not susceptible to the virus”.

“Apart from the initial animal to human transmission in Wuhan, China, which is suspected to have been caused by bats, the only other few cases in humans, which are believed to have been infected with SARS-CoV-2 virus from animals have involved mink farm personnel in mainland Europe.”

On current and future testing of animals, the spokesperson said resources and equipment are prioritised for human Covid-19 cases and animal testing would only take place where there were public health considerations or a scientific rationale for doing so.

“To date, no animals have tested positive for SARS-CoV-2 virus in Ireland.”
Dr Alan Rossiter, MVB, Senior Veterinary Surgeon, Blacklion Pet Hospital and past-President of Veterinary Ireland, told MI that a Covid-19 test for animals is available from Idexx Veterinary Laboratories, based in the UK. He said that Veterinary Ireland had asked whether Idexx had received any samples from Ireland and if so what the results have been, but as yet they had received no reply.

Dr Alan Rossiter

“We are working on very limited data, with very few peer reviewed papers being available, but it seems that mink, ferrets*, cats and, to a lesser extent, dogs can be infected by SARS-CoV-2 and there is evidence that they can go on to show clinical signs and replicate and disseminate the virus,” Dr Rossiter told MI.

“Clinical cases seem to be self-limiting and resolve without treatment. The fatality rate due to SARS-CoV-2 in animals is not available to us but it is likely to be extremely low to negligible.

“All this is said with a health warning, that we are working off very limited data and therefore this information may change in time as more data emerges.”


Dr Rossiter added: “So far it appears that susceptible animals have only been infected by humans -reverse zoonosis.”

“Whilst there has so far been no documented cases of real-world zoonotic transmission, ie, animal-to-human, it is likely that these could occur – animal-to-animal transmission (cat-to-cat specifically) has been demonstrated in laboratory settings and it is almost certain mink [to] mink infection occurred in a mink farm in Denmark. It is believed the mink were initially infected by humans working on the farm.

“However, looking at our realistic scenarios in Ireland, the risk of a dog or cat causing infection to spread from one household to another is extremely low – the infected animal would have to move from one house to the other house and this does not readily happen in the absence of a human moving with them – in which case it is far more likely the human will spread the virus.”

Speaking at the recent Centre for One Health online seminar, Prof Roberto La Ragione, Professor of Veterinary Microbiology and Pathology at the University of Surrey, UK, addressed the question of the risk of a human being infected with Covid-19 from animals.

Prof Roberto La Ragione

“Well what we have seen so far is that a number of different species can be infected or carry the virus,” he said.

“And right at the beginning of the pandemic, wet markets were cited as a potential vehicle for where the virus may have originated from.

Subsequently many wild species have been shown to potentially have the virus…however, there is currently no evidence that pets play a significant role in spreading Covid-19 to people.

Veterinarians have extensive academic and also real-world experience of investigation, tracing, control and suppression of infectious disease outbreaks in animals

“And this may be the case. But there needs to be [a] significant amount more research into this, and targeted surveillance studies to understand whether animals are exposed, whether the virus can cause disease and how it might be spread from one to another, and even could they be reservoirs for the virus.”

He added that experts did not know for sure whether animals were “a neglected transmission route” for the virus, but he added that “there are some very good studies underway”.


On whether there had been enough veterinary input into the Irish public health response to the pandemic, Dr Rossiter said there had been “limited involvement”.

He added that “more could be learned by NPHET [national public health emergency team] and other bodies by involving the veterinary profession”.
“Veterinarians have extensive academic and also real-world experience of investigation, tracing, control and suppression of infectious disease outbreaks in animals,” Dr Rossiter said.

“The principles are the same whether it be human medicine or veterinary medicine and it may well be that our human medical and epidemiological colleagues may learn from the experience of veterinary medical and epidemiological expertise.

“At the very least there is nothing to be lost by having veterinary medical expertise input. Examples of the veterinary profession’s experience in dealing with epidemics include avian influenza, foot and mouth disease, African swine fever, rabies, etc. We have successfully eradicated certain animal diseases such as rinderpest.

“So yes, there should be more openness to listen to the views of veterinary expertise as we have ‘been there, done that’.”

Dr Rossiter added that “in addition vets have an ‘inbuilt skillset’ to perform tracing and other epidemiological exercise with minimal extra training involved; to this end some colleagues in UCD were involved in the leading [of] the UCD tracing team”.

“Finally, in the very worst case scenario of absolute overload of the healthcare system, vets have expertise to assist their human colleagues if required – for example in the UK vets were trained, but not called on, to monitor ventilated patients.”

In response to our queries regarding veterinary medicine’s input, a Department of Health spokesperson told MI that the Irish epidemiological modelling advisory group, a NPHET subgroup, “includes members from a veterinary epidemiological background.”

In addition, the DAFM said the One Health approach is “an integral part of the Department’s strategy to deal with new and emerging diseases with zoonotic potential”.

“For example, the Centre for Veterinary Epidemiology and Risk Analysis (CVERA) in UCD were involved in the development of the inputs for the models developed by NPHET’s modelling advisory group,” the DAFM’s spokesperson told MI.

CVERA is a DAFM-funded research centre within UCD. “The CVERA team was augmented by epidemiology staff from the Department and UCD veterinary school, to assist in gathering the data for model inputs,” according to the DAFM.

The spokesperson also said DAFM veterinary laboratory facilities in Backweston were made available for Covid-19 testing and vets from the Department have assisted and supported HSE local outbreak control teams dealing with Covid-19 outbreaks in meat plants.

On the issue of modern industrialised livestock agriculture for production, Prof La Ragione told the COH virtual seminar that the current “intensification of production, just the number of animals and our proximity to them certainly contributes to our exposure or potential exposure to emerging infectious diseases”.

“I think in addition to that, in many countries we are seeing a change in the environment whether that is deforestation or just more human beings entering the wild environment and then being in contact with animals who have new and emerging viruses which could cross the species barrier.

“I think the way some of these animals are raised, can lead to closer interaction with large numbers of human beings travelling from different types of livestock farms, which can result in subsequent spread.

“It is an important area not just on how we improve production but in how we improve the bio-security both for the animals, but also the [threat of] transmission to humans.”

Pet Procedure

Although the risk of a dog or cat causing infection to spread from one household to another is believed to be low, some guidelines have been formulated.

Dr Alan Rossiter, past-President of Veterinary Ireland, told MI that “irrespective of this low risk the advice of Veterinary Ireland is that if there is a confirmed or suspect case of Covid-19 in a household then cats must be kept indoors and dogs must only be out on very short lead walks for toileting, with no contact to be permitted between that household’s dog and other dogs or humans from another household”.

“Accordingly if a person called a vet saying they suspect their cat or dog has Covid-19 we would have to question them as to their own health status and advise that if they suspect their animal has Covid-19 then the only likely source is from a person in their household, and thus they should call their GP for advice regarding being referred for testing.”

Like GPs, vets have had to introduce some element of remote consultations since the beginning of the pandemic.

“If there is a suspect case of Covid-19 in a household and an owner calls looking for a veterinary consultation for their animal, the approach, as with human GPs, is that a video or telephone consultation would take place initially,” said Dr Rossiter.

“If the vet was of the opinion based on the remote consultation that the animal required a physical exam then this can take place, taking all necessary measures regarding PPE etc.

“If after that examination – and if then the vet had a suspicion that the animal was suffering from Covid-19 – then yes, we feel it would be appropriate to test this animal. We need to find out if the animal can be infectious to other humans and also we need to build a database of information as to the extent of animals’ susceptibility to SARS-CoV-2.

“In this case Veterinary Ireland feels this service should be provided by, or paid for by, the State as without this owners may not agree to the test due to financial constraints.”

Vets have continued to provide a service for the animals under their care and were thus categorised as ‘essential workers’ at the outset of the pandemic.

“Measures were very rapidly put in place to minimise risk of disease transmission at veterinary practices and on farms… Overall the public have accepted these measures without complaint and practices have adapted well. This way of working, whilst safe, is much more inefficient so practices have had to increase staff numbers to be able to continue to provide a full service.”

One for all?

At the recent Centre for One Health (COH) seminar on Covid-19, Dr David Nabarro, WHO Special Envoy on Covid-19, said that medical training institutions need to help undergraduates and postgraduates understand that a “One Health approach that explicitly works across disciplines, sectors and institutions is worthwhile from a societal as well as an economic perspective”.

“There are going to be challenges in aligning the values of people working in veterinary health and people working for human health, and people working for environmental health,” said Dr Nabarro. He said those working in public health should encourage sustained dialogue between the different groups.

“Let’s do what we can to show that if the disciplines work together they can add value to whatever happens in the livestock sector, to whatever happens in infectious disease management, to whatever happens in the environmental health arena.”

Also speaking at the conference, Prof Martin Cormican, Professor of Bacteriology at NUI Galway, said that “one of the things that strikes me about this, is that we might call this (Covid-19) a zoonotic disease, and we talk about viruses jumping the ‘species barrier’, but those terms tend to emphasise compartmentalisation… The One Health concept is a much better concept in emphasising that continuity”.

“A vulnerable host is not just limited to non-immune people. We have also seen reports of infection of other species including for example mink in some countries.”

Dr Alan Rossiter said “one aspect of the One Health concept recognises that there is only one ‘practice of medicine’ albeit with two subsets – human medicine dealing with one species, and veterinary medicine dealing with all other species”.

“The principles of the practice of medicine are the same irrespective of the species being treated,” he told MI.

“Specifically in this case [the current pandemic] the principles of dealing with infectious disease of all species, and transmission of infectious disease between species, be that human-to-human, non-human-to-human or indeed human-to-non-human, is for all intents and purposes identical and there is no doubt that the experience and knowledge of practitioners of animal medicine would and can aid the practitioners of human medicine.

“Veterinarians can do, and indeed do, just about everything that human doctors can do. A simple example is a dog I saw yesterday is undergoing an MRI today and will probably have spinal surgery tomorrow for a prolapsed disc at L3-4 causing pelvic limbs paresis.

“Most human doctors, if told this, would be surprised. Our only surprise as veterinarians is that human doctors are surprised at this. Veterinarians practise medicine on all species bar one and, apart from the bit in our brain that allows us to know we are human, mammalian bodies work much the same, go wrong in much the same way and respond to much the same treatments, irrespective of species.”

Is the pandemic changing animal behaviour?

Have the pandemic, international lockdowns, and reduced human mobility had an impact on animal behaviour?

This interesting question was discussed in a paper titled ‘Covid-19 lockdown allows researchers to quantify the effects of human activity on wildlife’, published in the July edition of the Nature, Ecology and Evolution journal.
The paper looked at the concept of the ‘anthropause’ and its impact on animal life. The authors referred to this as the “considerable global slowing of modern human activities, notably travel”.

Asked about this subject at the recent COH virtual seminar on Covid-19, Prof Roberto La Ragione, Professor of Veterinary Microbiology and Pathology at the University of Surrey, UK, agreed it is “a really interesting area, one where you need to study much more”.

“Lockdown has brought a lot of people into the countryside…it brings people in closer proximity to wildlife, it has probably made wildlife retreat in some areas.

“However, the other side of this is that we have seen less cars on the road, less air traffic, less ocean noise and pollution and that most certainly would have resulted in movement of animals.

“I think what will be interesting is the tracking of what happened with things like roadkill, because that will be very telling if there has been any change in the animal behaviour; if we all of sudden see an increase or decrease in that roadkill [figures], but I think it could work both ways.
“There could be an increase or decrease and it probably depends on the area and the type of animal in that particular area.”

*UPDATE: Dr Rossiter has pointed out that in recent weeks a new non-peer-reviewed study from Cummings School of Veterinary Medicine at Tufts University (USA) revealed that ferrets are not susceptible to COVID even if they are directly exposed for a prolonged time.

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