The US is on the brink of a new virus epidemic; a virus that wasn’t there ten years ago but which is now worrying officials. Chikungunya, which causes an incapacitating fever, is spread via Aedes mosquitoes and usually found across Africa and Eurasia. But it is now the most recent example of an emerging virus – viruses that are rapidly changing their geographic distribution and/or their incidence.
Other emerging viruses such as the Ebolaviruses – which go on to cause ebola haemorrhagic fever – and severe acute respiratory syndrome coronavirus (SARS-CoV), are less common while others like mumps virus, are re-emerging after a period of relative absence in the western hemisphere. These viruses arise, often unexpectedly, amid some level of mystery about where they come from and why they are spreading. Their origins are more complex than they might appear.
Arboviruses affected by climate
Viruses like chikungunya that are spread by arthropods (insects and arachnids, like ticks) are known as arboviruses (from arthropod borne) and are affected by climate change and global warming, which directly facilitates their emergence. Global warming affects the distribution of arthropods, which act as vectors for the virus and increase the capacity for the viruses to grow within them.
An outbreak of Bluetongue virus – an infection of sheep and cattle that is spread by Culicoides midges – began in northern Europe in 2006, where it had never been seen before, and infected more animals than previously recorded.
Now, Chikungunya virus would appear set to spread across the US, much as West Nile virus did after it appeared in New York in 1999 – and which is still appearing. But not all emerging viruses are as predictable as the arboviruses.
A significant proportion of emerging viruses are zoonotic viruses, which spread from animals. These viruses are the most unpredictable, meaning that interaction between animals and humans is critical to their “spillover” into humans. The domestication of livestock has allowed multiple species – each with their own viruses - to come into close contact, which has created the right conditions for zoonosis.
Poultry and pigs are well known for a generation of new novel influenza viruses. However, it was also pig farms that ultimately resulted in the first cases of Nipah virus in Malaysia in 1999. Though harboured by flying foxes, the virus spread to pigs and then to humans causing around 100 deaths.
Human encroachment into new environments and the disruption of wildlife can also lead to humans being exposed to animals and their viruses. Outbreaks of ebolavirus haemorrhagic fever in African villages are often associated with the bushmeat trade.
A reproductive number
The myriad examples of virus emergence can be understood using the concept of the basic reproductive number, otherwise known as R0, which is a measure of the average number of new infections a virus produced from one single infection. An R0 of one means that an average of one new infection will arise from another, while a virus with an R0 of more than one will spread efficiently throughout a population. If a virus has an R0 of less than one it may eventually die out, as it fails to generate enough new infections over time – unless it is continuously re-introduced.
Processes that influence this number affect emergence. So while emerging viruses with an R0 of less than one may fail to efficiently infect and transmit within a new population, climate change and human behaviour could influence a virus’ R0 score in a given geographical area. Also important are virus-host interactions at the level of cells, which is a process governed by evolution. What makes viruses like chikungunya so worrying is that they require no further evolution to infect humans.
A suitable host
Viruses, as obligate, intracellular parasites that need hosts to spread, are composed of a protein or lipid coat that protects the viral genome, which encodes the instructions to make the viral proteins needed for infection. These proteins must allow entry of the virus into the host cell; make new copies of themselves; spread to more cells and evade your immune system. Differences in the efficiencies of these steps can all influence R0.
A virus’ genome can influence the fit between viral and host proteins; a virus with a better fit may be selected for and increase in frequency – which we can see as emergence.
Some viruses adapt and transmit easily, such as SARS-CoV and influenza (until we put a stop to them), while others fail to change their transmission, such as ebolavirus and the recent Middle-eastern respiratory syndrome (MERS)-CoV.
A constant worry is that an emerging virus may evolve to transmit efficiently within the human population but we do have means to prevent virus emergence. Intense monitoring of changes in virus distribution and novel human/animal infections lies at the heart of our strategy to combat emerging viruses.
For chikungunya and its relatives, targeting the mosquitoes that help it to spread and reducing the burden of climate change on at-risk areas may contain spread into new regions. The development of effective antiviral drugs and vaccines could also secure virus control. However, a challenge lies in predicting which viruses are most important and difficult in a global arena of continuing complexity and uncertainty.
The reality is that we have lived through this before with HIV/Aids and the spectre of once emerging but now-established viruses. This should continue to pique our interest in dealing with new ones that appear.