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Viruses are viruses and they are all the same, true? Well, no, that is not true. There are many different kinds of viruses in the world – those that infect people, those that infect animals, and those that infect plants, for example. The viruses, because there are so many of them, are grouped into different kinds or categories based on a classification system. The classification system has many levels, a few of which are class, order, family, genus, and species. This same classification system, the Linnaean system, is used to group all living things in the world, like animals, plants, bacteria, and yes, viruses. We bring this up, not to bore you with details, but to make the point that even though an apple and an orange are both fruits, in the Class Magnoliopsida, they are very different. Hence that common phrase, “It’s like comparing apples to oranges,” something often said when someone tries to compare two things that are so distinct like, well, an apple and an orange. While it is slightly more appropriate to compare an orange to a lemon, both fruits in the Family Rutaceae, Genus Citrus, even a child can tell that an orange is quite different from a lemon. So too are coronaviruses of pigs and people quite different (Fehr & Perlman, 2015). We tell the story of PEDv, an alphacoronavirus, to show you how a different virus, SARS-CoV-2, a betacoronavirus and the cause of COVID-19, can spread globally. Most importantly, we share what we can all learn to improve the health of the world’s human and animal populations.
The year was 2013. A coronavirus entered the US pig population and devastated the US swine industry. That virus was an alphacoronavirus called porcine epidemic diarrhea virus (PEDv) and it was first detected in April. In nine months, it had spread to most of the pig farms in the United States (Alvarez et al 2016). A very similar strain of PEDv then spread globally, quite rapidly, affecting many of the pig producing countries within a year. In the US alone, PEDv affected more than 50% of the breeding herds (MSHMP report), reduced the number of pigs slaughtered by more than 5 million (3%); yet, paradoxically, producers had net returns above what was expected before the outbreak hit (Schulz et al., 2015). More importantly, it changed how we looked at new disease introduction into the US. It opened the eyes of many to the vulnerabilities of imports and the dependence on global production chains. It was a cruel wake-up call that made us realize how unprepared we were for the introduction of a novel disease into our naïve pig population.
Fast forward to 2020 and we can draw some parallels to the new human coronavirus that emerged in 2019 and that now is spreading globally resulting in a pandemic, disrupting global distribution chains and raising financial market alarms. In just three months, COVID-19 disease, caused by SARS-CoV-2 - a betacoronavirus, with wild animal origins that was picked up by people - has spread to more than 147 countries. When it comes to transmission, COVID-19 has parallels to some of the pig coronaviruses we know, and also to influenza viruses, which we all know too well.
COVID-19 spreads rapidly among people, mostly through the respiratory route, resembling the spread of influenza. To a lesser degree, COVID-19 can also be shed in feces although it is unclear how much this route of transmission is contributing to the spread of COVID-19. Direct close contact, aerosol spread through droplets and contaminated fomites are considered the main routes of transmission. It is estimated that one infectious person will infect two or more susceptible individuals resulting in major outbreaks most of the time. To date there are no documented cases of COVID-19 virus affecting pigs.
Some similarities also exist with influenza. Influenza is a zoonotic disease transmitted by direct contact of respiratory secretions, aerosols and fomites. Transmission is rapid in susceptible populations. The median reproduction number for the 2009 H1N1 influenza pandemic was 1.46 (Biggerstaff et al., 2014), meaning that between 1 and 2 susceptible people will get infected if they come in contact with an infected individual. When that H1N1 influenza virus emerged in 2009, a pandemic was declared in less than a month, resulting in the first pandemic of the 21st century. Pandemic influenza spread rapidly during summer 2009 given the lack of immunity present in the population. In every other year in the 21st century including 2020, influenza spreads commonly and seasonally with higher incidence during the cooler seasons of the year. Influenza in pigs is also seasonal with infection peaks in the cooler seasons, such as winter and spring in the United States, although many farms have endemic year-round influenza in their pigs. Similarly, pig coronaviruses, like PEDv and TGEv (transmissible gastroenteritis virus), are considered seasonal with higher incidence in the fall and winter in the United States, although they can also remain endemic in immune populations year-round (think endemic TGE in the 20th century).
In general, influenza viruses and coronaviruses are susceptible to hot temperatures and don’t transmit as well during summer. Massive measures are being implemented to curve the transmission of COVID-19 in people, such as limiting travel and movement of people from one place to another. It is still unknown whether or when these measures will be able to contain or eliminate the virus. We have many examples where we have eliminated coronaviruses in pigs but that requires discipline, hard work and stopping the movement and introduction of susceptible individuals into infected populations. Easier said than done when it comes to people. Our veterinary experience and our public-private partnerships as One Health professionals throughout the world certainly can help think through containment and prevention protocols. If COVID-19 resembles influenza in its ability to cause infections season to season, then it will become yet another endemic human coronavirus in people, just like the common cold is endemic. Hopefully, this will not be the case for COVID-19 but if our knowledge of endemic pig coronaviruses and influenza applies here, it reminds us that the viruses don’t go away easily and may come back in the fall.
Even though COVID-19 is a human disease problem right now and our focus remains on protecting the health and safety of our family, friends, and co-workers, it serves as a reminder of how important it is to keep stringent biosecurity measures in place, and that we remain prepared for any new disease threat. If we learned anything from the PEDv introduction into the United States in 2013, it was that the industry was unprepared and not ready to deal with a devastating new disease. Since then, thankfully, significant efforts have been put in place to prepare our industry to prevent the introduction of diseases such as African Swine Fever, but more needs to be done. It seems similar lessons will be learned for COVID-19 and for those of us who care for pigs, it will serve as a reminder on how important people and pig movements are when trying to control disease spread.
