Auburn veterinary professor discusses zoonotic diseases in connection with COVID-19

With more questions than answers currently circulating about the current global pandemic of the COVID-19 virus, Dr. Bruce Smith of Auburn University’s College of Veterinary Medicine discusses zoonotic diseases, their origins and how humans can better deal with them.

Define zoonotic diseases. Are there examples besides COVID-19?  

Zoonotic diseases, or zoonoses (zoo-oh-no-sees), are diseases that can be transmitted from animals to humans. That is, they cross species boundaries. Of course, if a disease can move from animals to humans, that same disease can likely move in the opposite direction as well. There are actually quite a few zoonoses that are known. The CDC estimates more than 60 percent of known infectious diseases in people and 75 percent of emerging diseases come from animals. Examples of zoonotic diseases include rabies, salmonella, West Nile and influenza.

Why are zoonotic diseases so dangerous? 

It’s not so much that the diseases themselves are any more dangerous. Some, like rabies, are nearly 100 percent fatal and some, like West Nile, are mild. Rather, the potential for emerging disease and disease reservoirs are what gets our attention with zoonoses. The problem with an emerging disease is that we’ve never seen it before, so we don’t know what it is. As shown with COVID-19, it takes some time to recognize that this is a new disease and identify the agent. Compounding the problem is the lack of effective treatments and vaccines. Zoonotic disease reservoirs are also a problem. These reservoirs are in animals in which the disease circulates, making it nearly impossible to eradicate. An excellent example right here in the U.S. is rabies, which exists in wild populations of raccoons, skunks, foxes and bats (and in Puerto Rico, in the mongoose). Because of aggressive vaccination of domestic dogs and cats, rabies is very rare in these animals in the U.S., but in other countries, dogs and cats can represent a major rabies reservoir.

Why is it often so difficult to trace the specific source of a zoonotic disease outbreak?

Today, we can use the tools of modern molecular biology in addition to all the other tools of epidemiology to try to trace the source of an outbreak. With these tools, we can work backwards toward what we think is the first case to try to identify where a disease outbreak started. Often, that means simply looking at medical records until we don’t see anything that looks like the disease being reported. Unfortunately, sometimes disease goes unreported. The latest reports on COVID-19 show there was virus in the sewage system of Milan, Italy, in December 2019. That’s well before the first case was identified in Italy on Feb. 21, 2020, and even before China reported its first case. We can also play detective at the genome level, by comparing sequences of the virus from patients and possible sources. That is where animals like the bat and the pangolin come in. By comparing virus RNA (coronaviruses have RNA, not DNA genomes) changes, we can tell how closely related two viruses are. The more recent the evolution of the virus, the fewer changes. The closest animal virus to COVID-19 that has been found so far has been in bats in the Wuhan area of China. However, that virus is not identical to COVID-19, meaning it probably passed through another animal, possibly the pangolin, mutated slightly and then passed to humans.

Does a virus have to mutate in order to make the jump between an animal species and humans?

The simple answer is no. The bottom line is that some viruses can infect multiple species without mutating. This is true for many of the current zoonotic viruses. However, these viruses had to come from somewhere, so in that sense, they have mutated over time and, at some point, evolved the ability to infect both humans and other species. The need for mutation in order to infect humans is higher when we talk about new or emerging diseases. Not that it is required, though. For example, if there is a virus circulating in a population of animals that never comes into contact with humans, even if that virus has the ability to infect humans, we’d never see the disease. On the other hand, mutations can allow a virus that previously could not infect people to begin to do so.

Are some animal species—bats for instance—more likely to be a source of zoonotic diseases than others?

I hate to blame bats in particular, because there is nothing special about bats that would lead to increased risks of diseases jumping from them to humans other than that they often live in large colonies, with makes transmission between bats easier. The risk of transmission between species increases with the relatedness of those species. For example, humans and apes pretty much share most diseases. We don’t think of this as a risk to humans, for the most part, because the population of apes is small. In fact, we humans are a major infectious disease risk to the remaining apes. Bats, being mammals, are relatively close relatives to humans when considering the greater breadth of the animal kingdom, and so there is the potential for infectious organisms to be able to infect both bats and humans.

You mentioned zoonotic diseases can transfer both ways—animal to human or human to animal. What about one animal species to another animal species?

If a virus can infect and replicate in two different species, for example humans and cats, then there is no reason to believe the transmission shouldn’t be able to occur in either direction. That is, if humans can get the disease from cats, then they would also be able to give the disease to cats. And yes, there is nothing special about humans with respect to this. There are lots of examples of diseases passing between different animal species. Canine parvovirus is a very interesting example. Prior to 1978, canine parvovirus did not exist. It emerged in that year and within two years had spread worldwide, with some people estimating that it killed more than one million dogs over the first five years. Sequencing of the genome of parvo showed that it only differed in two places from that of feline panleukopenia, a parvovirus of cats. Interestingly, in this case, the feline virus does not infect dogs, and the canine virus does not infect cats. So just two small changes completely changed the target of the virus.

What—if anything—can be done to better protect humans and animals from zoonotic diseases?  

The realist in me says it is impossible to completely protect humans and animals from zoonotic diseases. However, there are some simple things that can go a long way to help. The first is active surveillance. We need to be continually on the alert for emerging disease and be ready to act quickly when it is identified. The next key is to address habitat reduction and destruction for wild species. The more intense these pressures get with wild animals, the more frequent their encounters with humans, and the more likely disease transmission will occur. Intrusion of human farming into wild habitats also means there is more potential of wild and domestic animals interacting and allowing disease organisms to jump from wild animal, to domestic animal, to humans.

Finally, we need to be smart in our interactions with animals. We need to make sure that domestic animals are vaccinated appropriately. Since vaccination is difficult with wild animals, we need to be careful around them to reduce possible transmission. If possible, don’t interact directly (touch, kiss, etc.) with wild animals. And if you see a wild animal behaving strangely, contact animal control. Last, but not least, take the simple precautions that have become part of our daily lives. Wash your hands frequently, avoid, if possible, vectors of zoonoses like mosquitoes, handle food properly and ensure a safe source of drinking water for everyone. Many of these issues occur in countries outside the U.S., so we need to have a global perspective to deal with the problem of zoonotic and emerging disease.

About Dr. Bruce Smith:

Dr. Bruce Smith is director of the Auburn University Research Initiative in Cancer, a professor in the Department of Pathobiology and a scientist in the Scott-Ritchey Research Center in the Auburn University College of Veterinary Medicine. Over his career, Dr. Smith has made significant discoveries in the identification of canine models of Duchenne Muscular Dystrophy (DMD), nucleic acid immunization, CpG based immunomodulation and Oncolytic virus therapy for cancer. Dr. Smith’s current work focuses on understanding the molecular cascades involved in DMD and developing next-generation oncolytic viral vectors for a variety of cancers.