You may never have heard the term "spillover event."
But you've seen people across the world spending a lot of time and energy on the most recent major spillover event — the coronavirus disease 2019, or COVID-19.
The virus, which actually causes COVID-19, has been named SARS-CoV-2. “Spillover event” is the term used to describe when a virus has overcome the many naturally occurring barriers necessary to “spill over” from one species to another.
These events tend to gain a lot of public attention when the viral cup runneth over into the human species, such as with SARS-CoV, MERS-CoV and now SARS-CoV-2. COVID-19 is causing immense suffering to the people whom it infects directly and to those whom it affects indirectly through quarantines that disrupt lives and separate families, economic costs of which we still don’t know the full extent, and the constant worry about the safety of loved ones, both near and far.
SARS-CoV2 is spilling, and the whole world is watching. We must ensure that global attention is focused with an eye to how this happened and how events like this can be prevented from happening again.
Members of the Infectious Diseases Institute at Ohio State — including Drs. Scott Kenney, Linda Saif, Qiuhong Wang and Anastasia Vlasova — are leading the effort to better understand spillover events and how to limit their impact.
We asked them some questions about coronavirus spillover events and how their current research aims to clean up future coronavirus spills. Here are their collective answers.
When viruses jump species
First, a little background on the steps and factors that lead to a viral spillover event. Kenney, an assistant professor in Ohio State's Department of Veterinary Preventive Medicine, says viral spillover events typically follow a five-step process:
- Reservoir. The virus replicates inside a primary host species, such as bats, without compromising the viability of the species, turning the species into a reservoir for the virus.
- Exposure. The secondary species host is exposed to the virus through close contact with the reservoir species.
- Breakthrough. The virus overcomes natural barriers such as species incompatibility and immune response in the new host.
- Transmission. The virus spreads efficiently from one new host to another.
- Tipping point. The virus achieves an increased disease incidence — new hosts get infected at an increasing rate.
The family of viruses known as “coronaviruses” is particularly poised to cause these spillover events, because they seem to have already overcome many of the natural hurdles that usually limit a virus to a single species.
Coronaviruses are zoonotic, meaning they are transmitted between animals and people. Coronaviruses are also RNA viruses, so they can jump these hurdles via evolution more quickly than other virus families with their ability to recombine and acquire point mutations.
While scholars may never be able to accurately predict the timeframe of a viral spill, experts have identified a number of factors that can predict areas on the geographic table where these viruses are closer to the edge. These factors include the type of virus, population (both animal and human), urbanization, demand for animal protein, travel and connectivity between population centers, habitat loss, climate change and increased interactions between people and animals.
Developing countries, with their growing human populations, decreasing animal habitats and often lack of access to adequate medical care are therefore considered to have the highest likelihood of a spillover events occurring.
The threat to humans — and animals
But it's not just human populations that are at risk. Animal populations are just as at risk if not more so than humans. Kenney cites the influenza virus (the flu) as one of the best examples of this fact.
Though it doesn’t attract quite the same level of media attention as COVID-19 when it arrives each year despite its much higher mortality rate, the flu is essentially an annual spillover event that occurs when migratory birds, acting as a reservoir species, fly the flu across the world and spread the disease among ducks, chickens, pigs and humans. The virus strains recombine as they move from species to species, picking up new abilities to leap the barriers between hosts along the way.
An even less covered yet still threatening example of a spillover event impacting domestic animals is porcine epidemic diarrhea virus (PEDV), which is also thought to have originated from a bat reservoir. PEDV had mortality rates ranging from 10% to 100% in suckling piglets. Despite the devastation PEDV caused to the worldwide pork industry, the global news gave it little attention since it impacted only pigs.
Though these animal spillover events create threats to human health through both the possibility of direct transmission and the impact they have on our food supplies, far fewer tools and less funding are available to study them. However, researchers such as Kenney think using a ground-up approach starting with animal spillover events may be the key to actually stopping animal virus threats in the future.