Inter-Species Travel

Dog walking on Mars, courtesy of NASA

Last week’s post Health has no boundaries made passing mention to Covid infections in mink populations. An article in the New York Times Magazine this week provides much more context for those mink farm infections: “Animals That Infect Humans Are Scary. It’s Worse When We Infect Them Back.” (by Sonia Shah, published 19 January 2022). Since minks aren’t getting headlines in our Covid news feeds, here’s a startling fact:

“By June 2021, scientists estimated, the virus had infected as many as seven million mink on more than 400 farms in Europe and North America, killing more than 700,000 of the animals, a death toll orders of magnitude greater than that borne by any other nonhuman species.”

Sonia Shah

Spillover & Spillback

The article explains the terms being used for transmission of microbes to and from humans: spillover (from non-humans to humans) and spillback (from humans to non-human animals). As the title of the article explains, spillbacks can be even more dangerous than spillovers, which get most attention since humans are the target.

“A spillback can ignite epidemics in wild species, including endangered ones, ravaging whole ecosystems. It can establish new wildlife reservoirs that shift the pathogens’ evolutionary trajectory, unleashing novel variants that can fuel new, dangerous waves of disease in humans.”

Sonia Shah

An example of this is morbillivirus, a family of viruses that are among the most deadly and infectious. These viruses kill up to 95 percent of those infected for the first time. In humans, this virus is known as measles, but it was known as rinderpest or cattle plague in the 10th century. It likely spilled back from humans to dogs, and is now known as distemper. This deadly virus affects many non-human animals, including dolphins and mink.

Fortunately, we have had measles vaccines since 1963. The US declared measles eliminated in 2000, thanks to both vaccinations and better control of the disease in the Americas region (since viruses know no borders). If you share your home with a dog, you probably vaccinated your pup to protect it from distemper. So far so good. Now consider that there are many populations of animals susceptible to distemper. We can’t do anything to mitigate the effects of the virus in those animals, some of which come into contact with dogs and humans. The vaccines, so far, are breaking the cycle of continuum of sharing the virus family morbillivirus.

Omicron – a possible origin story

Shah’s article explains that some scientists think the Omicron variant might have evolved in a non-human species after spilling back from humans, later spilling over to humans in its more aggressive form.

Omicron’s sheer number of mutations are stunning. Although it’s possible that the coronavirus relied solely on human incubation to mutate into the Omicron variant, it’s also possible that animals infected by humans carried coronavirus to other non-human hosts or became the new hosts themselves. Rodents, for instance, could have carried the virus from humans to another species, where the virus incubated.

In this scenario, the non-human biology of the new hosts encouraged a proliferation of variations that became Omicron. These mutations may have been necessary to overcome the defenses of the non-human host organism, so the variant could have needed lots of mutations to become successful. When the Omicron variant spilled over to humans, many of those mutations made it even more successful in human transmission, hitting us very hard.

The transmission continuum

It’s limiting to conceptualize viral transmission in an us/them dynamic: ‘jumping’ from one species to another or spilling over/back. These terms imply a separation that just isn’t there in the natural world.

“… [W]e are animals among animals, sharing a planet roiled by microbes. For many pathogens, the borders between species are as permeable as a sponge”

Sonia Shah
CDC One Health

When trained dogs smell the scent of VOCs emanating from an infected human, they are part of the continuum illustrated above not as hosts but as collaborators. Extending this thought, it’s reasonable that they could also smell the scent of these VOCs dispersed from infected animals that come into contact with humans regularly – for instance, with deer in suburban areas.

Backyard visitors

One study using samples from white-tailed deer in Illinois, Michigan, New York and Pennsylvania found antibodies to the coronavirus in 40% of the samples. A study from Penn State reported finding viral RNA in a third of the samples taken from white-tailed deer in Iowa.

Although we think of Covid spreading through indoor exposure, these findings demonstrate that the virus spreads just fine outdoors too. Although I live in a well-populated suburb of an urban center, I regularly see deer in our backyard and in parks. Those deer are susceptible to viral exposure from us, as we are from them.

Potential solution

We currently have limited means to monitor how Covid is being spread amongst humans. We have no means to monitor Covid transmissions amongst species, and yet inter-species sharing could prolong the pandemic. Creating the infrastructure to monitor transmissions across all borders – spatial or species — would hasten our control over this virus and its eventual eradication.

Identifying individuals with Covid infections is key to monitoring viral infections. Utilizing dogs’ ability to smell the presence of infection could be a part of a monitoring solution that is species agnostic. Trained dogs could be smelling not just humans for possible infection, but also the parks, subways, subdivisions, and fields where humans work, live and play.

Were we able to identify not just the humans in an area who were infected, but also those animals who share the space with humans, we could start to create a comprehensive view of the virus’ incubation world. Best of all, we could do this in time to take action.

One thought on “Inter-Species Travel

Leave a Reply

Your email address will not be published. Required fields are marked *