The US looks to be on the brink of a new viral epidemic, as a virus that did not even exist in the region a decade ago is now among the top concern of infectious disease specialists across the country. Chikungunya, a severe viral infection that causes an incapacitating fever, is spread via Aedes mosquitoes and usually found across Africa and Eurasia. But now, with its recent emergence in the US, chikungunya is 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 hemorrhagic fever – and severe acute respiratory syndrome coronavirus (SARS-CoV), are less common while others like mumps virus, are reemerging 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 facilitate 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. In 2011, researchers developed an algorithm to model the effects of climate change on the emergence of Bluetongue virus and found that warmer temperatures in the UK are driving the spread of the virus, which is expected to increase in prevalence by 17 percent by the year 2050.
The emergence of tick-borne viral illnesses — like Rocky Mountain Spotted Fever — in new geographical regions and in much larger numbers has been attributed to climate change-related effects on the tick population. We’ve also seen the emergence of a brand new tick-borne disease in the US: scientists in 2012 documented the first cases of the Heartland Virus, a viral infection similar to ehrlichiosis (another tick-borne infection) but for which there is no known treatment.
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 harbored by flying foxes, the virus spread to pigs and then to humans causing around 100 deaths. Now, the WHO says there is evidence of human-to-human transmission of Nipah virus, which can cause symptoms ranging from mild flu-like symptoms to acute respiratory syndrome and fatal encephalitis.
Human encroachment into new environments and the disruption of wildlife can also lead to humans being exposed to animals and their viruses. Outbreaks of Ebola virus hemorrhagic fever in African villages are often associated with the bush meat trade, and some experts say human activity and climate change are driving the current Ebola outbreak, which is located in a region of West Africa where the disease had never been seen before.
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 behavior could influence a virus’ R0 score in a given geographical area. That’s what makes climate change such a major public health concern, as the effects are unpredictable and thus difficult to prepare for.
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 the Ebola virus 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. Just this week, researchers announced that a new vaccine for chikungunya has shown ‘highly impressive’ results in humans, though it remains years away from approval.
Besides the long process of drug development and clinical trials — which can take over a decade in some circumstances — another challenge lies in predicting which viruses are most important and difficult in a global arena of continuing complexity and uncertainty. Globalization has only made the situation more complex, giving pathogens a direct pathway to all corners of the earth.
Having said that, the reality is that we have lived through this before with HIV/AIDS and the specter of once emerging but now-established viruses. This should continue to pique our interest in dealing with new ones that appear — the more we learn about them, the better equipped we are to face even bigger challenges in the future.