COVID-19 Risks With Animals

Now that Prime Minister Morrison has clarified that the aim of Australia’s aggressive suppression strategy is zero community transmission, we need to begin to reposition to be more proactive rather than reactive towards COVID-19.

Australia has long been a world leader in biosecurity policy and implementation, and together with our natural advantages from being a relatively remote island, we have managed to remain free from many of the important animal and plant diseases and pests.

Think about it, on your overseas travels, what other countries so jealously guard the good health of primary industries and important native plants and wildlife?

The biosecurity operations that travellers into Australia witness at airports and cruise terminals is only the tip of the iceberg.

But this is an article about the COVID-19 pandemic, the most serious disease challenge to confront humanity in a century. Why are we talking about animal diseases?

When an infectious disease is first discovered a key aspect of management is to understand the host range of the pathogen, whether it be a virus, bacteria or other organism that causes the disease. 

If we are discussing a virus, the host range refers to all the organisms which may be infected by that virus.

This is important because in order to manage disease outbreaks we need to study the epidemiology – the way the pathogen spreads – in all its potential hosts.

Some pathogens infect only a small number of organisms, and thus have a narrow host range, while others have a broad host range.

The broader the host range the more challenging will be efforts to prevent its introduction or reintroduction, and the management of outbreaks.

The more animal species that may be infected by a particular pathogen, the greater the routes for introduction of the pathogen, and once introduced the greater the potential for populations of animals to act as reservoirs from which disease outbreaks can be re-initiated. 

Unfortunately, research-based predictions suggested that the novel coronavirus, named SARS-CoV-2, has a broad host range, and real-world research and observations through the first six months of the COVID-19 pandemic is confirming this to be the case.

Initial studies were done with the aim of determining from where the virus originated, or what was the original host of SARS-CoV-2 and whether the virus went through any other hosts – called intermediate hosts – from which people in Wuhan first became infected.

Bats were suspected as the original host because a former colleague of mine, Dr Shi Zhengli, found that they were the original host for the SARS virus. Based in Wuhan, Shi also led the group which discovered that a novel coronavirus was the cause of COVID-19.

These studies are laborious necessitating surveying and taking samples from farmed and wild animals for signs of infection or prior infections. The original work by Shi and her group necessitated climbing through caves knee-deep in pungent guano to collect samples from bats.

In the COVID-19 pandemic time is of the essence so scientists have utilised what is known of the way that the novel coronavirus enters human cells to predict its potential host range. This is a critical aspect because viruses cannot reproduce outside of cells; they must enter cells to multiply and spread infection within the body and then to be shed to spread infection to other hosts.

Initially these studies were performed to narrow down the search for a potential intermediate host. That initial source and intermediate host(s) for the novel coronavirus (I will use this description hereon for SARS-CoV-2) have not yet been identified with real world investigations. 

Those predictive studies found that, unsurprisingly, the closely related primates are the most likely to also be susceptible to infection by the novel coronavirus. Consistently predicted to be susceptible to infection included a broad range of commercially or culturally significant mammals including: cattle, horses, goats, sheep, bison, water buffalo, hamsters, cats (domestic and wild), rabbits, ferrets and some rodent species, while predictions on camels and pigs varied.

Other interesting findings from these studies showed that cetaceans, marine mammals, were found to be highly likely to be susceptible to infection, but that raises the issue of pathways for and likelihood of exposure. 

Of particular interest in Australia, the one monotreme (egg-laying mammal) and four marsupial species studied were predicted to have a very low likelihood of being susceptible to infection.

Perhaps the most curious finding was that bats and pangolins, considered the most likely original and intermediate hosts for the novel coronavirus, were less likely to be infected via this channel. In other words, it seems likely that the novel coronavirus can utilise more than one mechanism to enter host cells, which suggests that the host range may be even broader than these studies predicted.

While the predictive studies suggesting a broad range were concerning, interactions between potential hosts and viruses are extraordinarily complex so observations with real animals are vital.

These studies take the form of transmission trials to establish infections by unnatural routes (injection) or natural routes (swabbing virus onto membranes or cohabitation with infected animals), along with analysis of samples from sick or surveyed farmed or wild animals.

Actual transmission of important pathogens under experimental conditions requires biosecure facilities which are rare. These trials are important because conditions can be strictly managed and replicated to confirm research findings, and animals are exposed to a known amount of the pathogen (under conditions strictly audited for ethical considerations). 

This approach allows the development of animal models to study infections with pathogens which infect people, but it is obviously time consuming and resource intensive. 

In transmission trials by swabbing the novel coronavirus onto nasal membranes, infection could not be established in pigs, chickens or ducks. However, cats, bats and ferrets were infected, and these species passed on the infection to in-contact animals of the same species making it clear that they are susceptible to the virus and will transmit infection. Transmission studies suggested dogs have low susceptibility to infection, but some did show signs of infection.

While transmission trials with SARS-CoV-2 are ongoing, real world developments are progressing faster as the virus spreading rapidly and widely amongst people has led to a broader range of animals being exposed. 

We are learning that indeed the novel coronavirus can infect a broad range of other mammals.

Initially there were reports of companion pets such as cats and dogs found to be infected with novel coronavirus. In April lions and tigers at a zoo in New York were found to be infected by the novel coronavirus, and the most likely source was an infected zoo employee.

More recently there have been reports of outbreaks of disease amongst farm animals infected by the novel coronavirus. Mink in at least 25 farms in Spain, the Netherlands and Denmark have been found to be infected with the novel coronavirus, and it is suspected that the virus was transmitted between the mink and the workers in both directions in this environment.

In one outbreak in Spain 87% of mink were found to be infected. Detection of the novel coronavirus in animals necessitates the culling of the entire stock of mink.

This highlights some profoundly serious considerations.

It raises the issue of epidemiological surveillance for the virus in farmed populations of susceptible or potentially susceptible animals. Also, the risks of some infected material entering the processing, or pet or human food, chain must be addressed.

Meat processing facilities have been the site of serious outbreaks of COVID-19 amongst workers globally. Attention has focused on the environmental conditions within these facilities and the general living conditions for workers which favour contagion.

These host range studies suggest the potential for the novel coronavirus to be introduced into the meat processing plants is not just with the workers but with the animals that are being processed.

The novel coronavirus has been shown to maintain viability, without any loss of infectivity, for 3 weeks on the surface (i.e. simulating contamination) of fish, chicken and pork stored at normal refrigeration temperature of +4ºC and normal commercial freezer temperature of -20ºC. Moreover, a similar coronavirus has been shown to remain viable in meat frozen for 2 years.

This highlights the potential for the spread of the virus geographically and over time within meat. Virus actually within tissues, due to infection of the animal prior to slaughter, is likely to be present at much higher titres (quantities) and is likely to be more persistent (survive for longer) than virus lying on the surface of meat due to contamination. Thus meat from an infected animal would represent a higher risk than meat that was contaminated with the novel coronavirus.

As for the risks associated with transmission of COVID-19 with contaminated meat, the risks of transmission to people from animals infected by the novel coronavirus are being downplayed by authorities in the US and even in Australia.

In the US the argument is similar to that discussed in the companion post to this, On The Risk of Transmission of SARS-CoV-2 With Contaminated Meat, that with their limited resources they need to concentrate their efforts to slow spread of their very severe pandemic with other more direct strategies such as testing of people.

As discussed previously, Australia together with New Zealand and other countries that have mounted aggressive responses that have succeeded in eliminating or very strongly suppressing the pandemic, the situation is very different. In these countries much greater attention must be turned to proactive biosecurity measures to limit the probability of re-introduction of the novel coronavirus.

These countries have much to gain from that proactivity in terms of protection of human life and also their important domesticated and native animals.

There are clear implications here for Australian primary industries which trade on an image of animals being raised in pristine and natural environments free from major infectious disease. If that is maintained through the COVID-19 pandemic that will provide significant economic benefits to rural Australia.

On the other hand, if the novel coronavirus is found in populations of commercially significant animals then culling will be required to curtail its spread.

In the companion article to this one I mentioned that the new anti-food wastage campaign initiated very recently in China may be in part due to concerns about food security (obtaining sufficient food) if food safety was called into question during the COVID-19 pandemic. It may be that this aspect, the potential for important food animals to carry infection, may be what is driving that apparent anxiety. It would also be acutely concerning for the World Health Organisation with the remit to “promote health, keep the world safe, and guard the vulnerable”.

The more the virus spreads within people in Australia the greater the probability companion, domesticated and wild animals will be exposed to the virus, and thus for it to escape into the wild and become endemic. This will create reservoirs of the virus to cause future outbreaks in people and other animals.

At this stage only a few native Australian species of mammals have been the subject of predictive studies on whether they may be susceptible to infection by the novel coronavirus, and no transmission trials have been reported. From these limited studies it appears that the unique native Australian mammalian fauna may be less susceptible to infection.

This evolutionary distinction may work in their favour, by being refractory (not susceptible to infection), or their naivety may increase their susceptibility to infection and severe disease. That is difficult to predict when it is suspected that the novel coronavirus utilises multiple mechanisms to establish infection.

There are three key takeaways for Australia from this emerging knowledge around the broad host range for the novel coronavirus that causes COVID-19:

1) if Australia’s aim is zero community transmission, we need to be extremely aggressive in minimising outbreaks because the longer or more intense the outbreak the greater the opportunity for other susceptible animals to be exposed and become a reservoir for the virus to re-establish infections in people;

2) Australia needs to be proactive and utilise its biosecurity expertise to determine what pathways exist for the virus to enter Australia other than being carried by people, for example in animal products or with live animal “passengers” on cargo ships; and

3) To protect primary industries, and as custodians of special and precarious native fauna, Australia needs to be acutely aware of the need to assess and manage the risks posed by the introduction of the novel coronavirus into native, domesticated and feral animal populations, and should commence surveillance for the novel coronavirus in domesticated and wild animals starting in areas that have been identified as hotspots for infection in people.

Drafted entirely by Brett Edgerton on 28 July, with minor revisions and additions on 26 August immediately prior to publishing.

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© Copyright Brett Edgerton 2020

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