COLUMNS

DiPentima: SARS-CoV-2 and the animal connection

Rich DiPentima
Guest Columnist

One of the more interesting and worrisome news emerging from the ongoing SARS-CoV-2 (COVID-19) pandemic is the number of wild and domesticated animals that have been shown to be able to become infected with the virus. To date, this includes big cats, otters, mink, non-human primates and white tail deer. The virus can also infect many domesticated animals including dogs and cats. The number of species with SARS-CoV-2 continues to rise as the pandemic continues, and more species are tested for the virus.  None of this emerging news is a surprise to those studying and planning for pandemics.  In fact much of it was predicted and explained in the excellent 2012 book, "Spillover Animal Infections and the next Human Pandemic," by David Quammen.

Rich DiPentima

Why is this a problem, and what does it mean for the control of the SARS-CoV-2 pandemic?  As the virus finds more animal species in which to thrive, the number of biological reservoirs increases.  A biological reservoir for an infectious agent, such as a virus, bacteria, or a parasite, is the habitat in which the infectious agent normally lives, grows, and multiples.  Reservoirs can include humans, animals and the environment. Animal or human reservoirs may or may not become ill as a result of being infected with the pathogen, however in either case, they are able to spread the infectious agent to other susceptible animals or humans. As the number of reservoirs for an infectious agent grows, so does the difficulty in controlling the spread of that infectious agent.

An excellent example of an infectious agent with a limited reservoir is that of smallpox and its eradication. Smallpox was the first infectious agent/pathogen ever to be eradicated, only Polio has come as close to being fully eradicated. There were two very important factors that led to the eradication of smallpox; First there was a very effective vaccine that could be used to prevent humans from becoming infected and spreading the virus to other humans.  Second, was that the only reservoir for wild smallpox was humans. There was no animal or environmental reservoir from which humans could become infected with the smallpox virus.  As such, with a massive global effort to vaccinate as many people as possible against smallpox, coupled with a heroic global surveillance system searching for every case of smallpox, once the last case of human smallpox was identified and resolved and any previously unvaccinated contacts vaccinated, there was no remaining wild smallpox virus on the planet. There could no longer be any new cases of smallpox since there was no longer a natural reservoir for the smallpox virus. There does however still remain at least two artificial reservoirs for the smallpox virus, stored in freezers at the Centers for Disease Control and Prevention (CDC) and in freezers in labs in Russia.  As long as those artificial reservoirs remain secure there is no risk of smallpox disease re emerging. There is some concern however that as a result of global warming and the melting of the permafrost, some DNA viruses like smallpox could find their way back to humans. This is possible and needs to be explored further, but it is a very remote possibility.  

An example of a virus that has multiple animal reservoirs and has proven very difficult to control or even consider eradication is influenza. Influenza viruses can be found in many species including bats, birds, pigs, horses and cats. This creates a number of serious problems with regard to the infectiousness, severity, prevention and development of novel or unique strains of the virus. 

For example, if a pig is infected with a swine influenza virus, and comes in contact with a duck or goose that is infected with an avian strain of the influenza virus, there is a good chance that these two viruses will exchange genetic material and create a totally new or novel influenza virus. If that strain is able to be transmitted to humans, and easily spread from person to person, the stage has been set to initiate another influenza pandemic. In fact, that may be just what triggered the great "Spanish Flu" pandemic in 1918. The fact that influenza viruses mutate frequently adds to the problem of control, and the reason we receive a different combination of influenza virus strains in our flu shots every year. Other diseases that have animal origins that are spread to humans, known as zoonotic diseases, include, HIV/AIDS, SARS, plague, rabies, and many more. The reservoir for HIV was bats, which was then spread to chimpanzees and then to humans. Bats are also the reservoir for SARS-1 which was spread to civet cats and then to humans.

This brings us back to the current problem with SARS-CoV-2 and the growing number of animal reservoirs for this virus. How can we protect humans from becoming infected with Sars-CoV-2 from animals as well as from other humans?  Will new variants of the SARS-CoV-2 virus develop in animals as well as humans, and what will be the implications with regard to transmissibility, virulence, and vaccine efficacy? We have already seen more than a dozen SARS-CoV-2 variants develop over the past year and a half, some like the Delta variant have caused a significant burden of illness and death.  Fortunately, to date, none of the variants have impacted vaccine efficacy. 

We have already seen outbreaks of SARS-CoV-2 in mink farms where infected minks passed the virus back to humans.  As such we must rely on the only two effective means available to us to help manage this threat. In order to reduce the number of people susceptible to the SARS-CoV-2 virus, we must greatly increase the number of people around the globe who are fully vaccinated, and receive booster shots when needed. Second, We must develop  a very robust global surveillance system to monitor emerging SARSCoV-2 variants both in humans and various animal species.  Since it would be impossible to vaccinate animals against SARS-CoV-2, even if such a vaccine existed, a sample of animals, both wild and domestic, across the globe must be tested for the virus with a particular emphasis on identifying new virus variants. The quicker new variants can be identified the quicker public health measures can be put in place to limit the spread of the new variant.  Lastly, vaccines must be adjusted as necessary to meet the challenges of any new variants discovered, as well as the development of new antiviral drugs that could be effectively used to treat infected individuals.

If we have learned nothing else over the past almost two years of this pandemic, it should be that things will change, and we must be able to adjust quickly to those changes.  We also need to understand that our best chance of defeating this enemy is through science, a sense of community and a willingness to change and adapt as necessary. We are fighting a virus that is using every tool in its biological toolbox to survive and thrive. This virus is operating under a mandate from nature to do what viral pathogens do, find susceptible hosts, use that host to make more virus, and move it from place to place, and create new variants to thwart any vaccines or treatments,  And it will continue to do this until there are no longer sufficient susceptible hosts to infect.  If we intend to defeat this virus we must fight it with every tool we have in our scientific and public health toolbox, not selective actions which are too weak and ineffective.

Rich DiPentima of Portsmouth spent more than 30 years as a public health official and epidemiologist, including service as deputy public health director in Manchester and chief of communicable disease epidemiology at the New Hampshire Division of Public Health.