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The science of Spillover viruses [1]

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Date: 2023-02-15  

NPR has an interesting article this morning on new tools and ways to find, track and combat spillover viruses and hopefully prevent the next pandemic. There are obviously scientists who are always looking for spillover viruses like COVID-19 and for a long time the theory has been that they are rare and a virus really has to hit the jackpot to crossover from animals to humans or even other animals.

(Spillovers) are extremely rare. Animal viruses tend to stay in their animal host. Scientists have described it is that a virus spilling over is, in a way, winning the lottery: The virus is in the right place at the right time, and on top of that, it has special, rare characteristics that allow it to infect people. For all these events to coincide is remarkably rare, the thinking went. This theory has shaped how scientists look for new deadly pathogens — or try to predict which ones could cause future pandemics. In particular, it led scientists to focus on searching for new viruses in wild animals.

However as scientists study different cases they have come to realize this isn’t necessarily the case. At the time Coronavirus-19 was discovered scientists knew of only 6 coronaviruses that could jump from animals to humans, but there are likely over 1,200 of them in the wild and scientists needed better tools to detect them. Leshan Xiu — a postdoctoral researcher managed to develop a sensitive array and it can be used to help figure out where unknown viruses originate.

In the case described in the NPR article a baby’s raspatory virus was tracked back to dogs, though it appears to have also been seen in pigs and cats based on it’s genetic composition. They also managed to track several other infections back to various animals. Most of them infected only a few people or a classroom or two at a school and never managed to go widespread like COVID did. Along the way they found several cases in different areas that all were from the same spillover virus and had been discovered by other doctors treating different patients in different parts of the world — sometimes 1000’s of miles apart.

Spillovers turned out to be fairly common. When they analyzed samples from various places where unknown pathogens were discovered they found multiple instances where people had been infected by the same local spillover virus at different times. In fact, they found that 75% of the people who handle camels regularly showed signs of having previously been infected with MERS, but since most of them were younger and in good health they didn’t develop severe cases and no large outbreak occurred. This led to the conclusion that spillovers happen regularly, even several times a day around the world.

First off, the vast majority of these spillovers don't harm anyone, (Marco) Salemi says. Most people's immune systems fight off the pathogen without having symptoms at all. When a virus does trigger symptoms, the illness masquerades as a cold, flu or stomach bug. On top of that, the virus rarely spreads to another person, or only to a few people. Outbreaks are small. "The virus jumps into humans, infects a few people, and then the pathogen essentially does not have the capacity to really infect a large number of people," Salemi says. That's because the animal viruses, in the vast majority of cases, aren't adapted to live in humans or jump between us, he says.

The good news is that as we learn more we gain tools we can use to help prevent the next pandemic. If virus spillovers are common and people have already been infected they can be studied and hopefully treatments can be developed. At minimum it’s a much more effective way to find these viruses than trapping and testing random wild animals. The better news is that even when spillovers do occur they rarely lead to major outbreaks or cause widespread health issues, so go ahead and hug your pet. It’s likely not deadly to do so.

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[1] Url: https://www.dailykos.com/stories/2023/2/15/2153111/-The-science-of-Spillover-viruses

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