In recent decades, the interspecies transmission of RNA viruses such as SARS and avian flu has sparked widespread media attention. The final scene of Contagion documents ground-zero for a deadly virus, when an infected bat drops contaminated food into the feeding trough of a pig farm. A restaurant chef butchers the swine and shakes Gwyneth Paltrow’s hands, cueing ‘Day 1’ of the pandemic. This cinematic depiction isn’t too far off. The SARS virus spread between an eclectic range of hosts, including Chinese horseshoe bats, Himalayan palm civets, raccoon dogs, and Chinese ferret badgers, before coming into contact with humans. Why is the distribution of RNA viruses among various species so deadly?
The answer lies partly in how easily the viruses incubate in flying animals. Bats and birds roost in dense groups and form an optimal environment for the spread of RNA viruses. They are able to fly long distances and represent a relatively large number of species in the animal kingdom. Bats have a long lifespan, giving RNA viruses time to mutate and spread between individuals. Aquatic waterfowl and shorebirds are the origin for most avian influenza viruses. They feed on submerged plants in wet landscapes, sometimes contaminated with the excrement from other species. These RNA viruses can spread undetected among asymptomatic hosts. Woo et al charted an evolutionary timeline of coronoviruses, the strains responsible for SARS and avian flu. Their model has bats and birds as the original incubators of these viruses.
The plastic structure of RNA viruses allows for rapid mutation and spread between hosts. Coronoviruses have a large genetic makeup, allowing genetic material to be switched out without compromise to evolutionary fitness. Different species of viruses can co-exist in the same individual and swap material, creating a novel, deadly strain. In the case of avian influenza, a single pig can have strains of Swine H1N1, Avian H1N1, and Human-like H2N3. The mutation rate of coronoviruses is relatively high among viruses, speeding up the selection process.
Shirogane et al propose that unique strains of RNA viruses can cooperate with one another and form new combinations of genetic material. This ‘quasispecies’ theory states that RNA viruses are a cloud of genetic variants that acts as a single unit of selection, rather than each variant acting as a unit by itself. This means that scientists have to study viruses on the population level when looking at an infected host.
The animal markets of South China and Hong Kong have since enacted biosecurity measures that reduce the likelihood of viral spread among hosts. They segregate species so they don’t come into contact with each other, and depopulate the farms to reduce the group’s density. To combat the spread of coronoviruses, we must treat the viral genome as a dynamic network among hosts of different species. The viruses are invisible in asymptomatic hosts, but can elicit a fatal response when transferred to a human being.