As a veterinarian who works in infectious diseases and public health, I am constantly asked for my views on COVID-19 in animals. Last month, a team of researchers released study results that suggest COVID-19 is widespread in the white-tailed deer population in the United States. Media coverage of these results prompted the largest number of queries I have received on COVID-19 in animals since the start of the pandemic when the focus was cat and dog COVID-19 susceptibility and infectiousness. My reaction: this is an interesting study that shows, beyond the theoretical threat of COVID in the white-tailed deer population to public health, that we should be much more concerned about the general threat of animal diseases to human health. 

In the study, the researchers analyzed deer immune tissues that had already been collected from across the state of Iowa for other disease surveillance purposes and found COVID-19 genetic fragments (suggesting active infections) in 94 out of 283 samples (33%). In some cases, the specific strains of COVID-19 found matched the most common strains in the human population. In addition, different COVID-19 strains were sometimes found in theoretically independent deer groups in samples collected at the same time. This suggests that there were multiple independent cases of human-to-deer transmission. The possibilities for transmission include direct human contact during deer feeding or deer being exposed to potentially infectious material like human urine. 

On the whole, at this point in the pandemic, finding that COVID-19 crossed over from humans to an animal population is unsurprising. The cell membrane component (ACE-2) that COVID-19 uses to enter cells is well conserved across animal species, and thus many species could be susceptible to COVID-19 infection. According to the Centers for Disease Control and Prevention (CDC), animals that have been infected so far include cats, dogs, ferrets, zoo animals such as otters and primates, and mink. Just this month, hippos with COVID-19 were found in a Belgian zoo. Additional species have likely been infected and not identified yet due to mild or nonexistent symptoms and a lack of monitoring.

The more troubling result from the deer research is that the high prevalence of the virus in deer across the state suggests, for the first time, extensive COVID-19 spread due to animal-to-animal transmission in a wild animal population. This raises as many questions as it answers. Will the virus persist in the deer population or die out as it spreads and the deer gain herd immunity? How often is the virus mutating as it spreads through the deer population? Could a more mutated and deadly strain cross back into humans? Mutations in COVID-19 in minks that spread back to humans led Denmark to cull its entire herd of 17 million minks. Some scientists speculate that the Omicron variant evolved in an animal host that was initially infected by humans sometime in 2020. Finally, are hunters at an increased risk of COVID-19 when they dress deer they have killed?

Scientists estimate that 3 out of every 4 new or emerging infectious diseases in people are zoonotic.

It is too early to determine if specific COVID-19 management strategies in the deer population (and extreme precautions for hunters) are necessary. Regardless, as concerning as these new developments are, the presence in a wild animal population of a viral disease that can potentially pass to humans is far from a novel discovery. For example, wild birds are known to carry the influenza virus. Consequently, the United States Department of Agriculture (USDA) puts out guidance for hunters on avian influenza in the wild bird population, and poultry regulators and the industry are always looking for incursions of avian influenza into domestic flocks.  

COVID-19, like its fellow coronaviruses SARS and MERS, likely originated in animals. Scientists estimate that 3 out of every 4 new or emerging infectious diseases in people are zoonotic, meaning they cross over from animals to humans.

Unfortunately, all too often attention and funding materialize only when these diseases make the jump into the human population, which is too late. Even then, there is a huge disparity in funds available for studying and managing zoonotic diseases in the human population compared to in animal populations. COVID-19 is only the latest illustration of this: hundreds of billions of dollars have been spent on the COVID-19 human health response by the federal government. In contrast, the primary federal agency that deals with animal populations, USDA’s Animal and Plant Health Inspection Service, received only a one-time $300 million funding boost to monitor and surveil COVID-19 in animals. In a typical year, APHIS’s entire zoonotic disease management budget is a paltry $20 million.

More needs to be done to surveil and manage zoonotic diseases in animal populations, with the ultimate goals of protecting animal health and preventing these diseases from crossing over into the human population. This pertains especially to zoonotic diseases in our food system animal populations, in which viruses and bacteria such as Salmonella can spread rapidly through concentrated animal populations and affect public health through environmental spread, infecting food system workers, or infecting consumers through contaminated food.  

The Center for Science in the Public Interest has recently advocated for more action to be taken to effectively manage zoonotic diseases in animals. This includes forming a coalition of animal industry, scientists, and consumer advocates to push the USDA to develop regulations to manage harmful Salmonella and Campylobacter bacteria on poultry farms before animals are sent to slaughterhouses. Expect much more from us at the Center for Science in the Public Interest, and ideally the entire public health sector, in the coming years on the need to adequately address zoonotic diseases in animal populations. COVID-19 and now its appearance in the omnipresent deer shows that there is no time to wait.