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 year ago is now among the top concern of infectious disease specialists across the Western Hemisphere. Zika virus, transmitted primarily by the bites of Aedes mosquitoes, has been found across parts Africa and Asia for decades. But with its “explosive” spread across the Americas starting in 2015, Zika has become the most recent example of an emerging virus – viruses that are rapidly changing their geographic distribution and/or their incidence.
Although it is not a new disease, Zika virus did not begin spreading widely in the Western Hemisphere until last spring, when a major outbreak started in Brazil. Since May, as many as 1.5 million people have been infected in Brazil alone, and the World Health Organization recently estimated that there could be up to 4 million Zika infections in the Americas over the next year. Earlier this month, the US confirmed its first case of locally-transmitted Zika infection in a Texas patient who acquired the virus through sexual activity.
This follows the emergence of chikungunya — a close relative of Zika that causes an incapacitating fever — in the US in 2014. Dengue fever, another mosquito-borne virus related to Zika, appeared just a few years earlier, with the country’s first outbreak documented in Florida in 2009-2010.
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 Zika that are spread by arthropods (insects and arachnids, like ticks) are known as arboviruses (from arthropod-borne), a class of viral diseases that are strongly affected by environmental conditions. Research has shown that climate change and global warming can directly facilitate the emergence of these viruses through a variety of different mechanisms. For example, warmer temperatures and changing rainfall patterns have extended both the length of the mating season and the geographical range of some arthropods, which has resulted in more disease vectors spreading across more places and coming into contact with new, vulnerable species.
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 brand new tick-borne diseases 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. And just recently, scientists discovered a new species of bacteria that causes a more severe form of Lyme disease.and it may bring even worse symptoms than its more familiar predecessor. So far, the bacteria, called B. mayonii, has only been found in the upper Midwest. However, scientists think it’s only a matter of time before the newly-discovered bacteria takes hold in other parts of the country. “While the distribution of B. mayonii is currently said to be limited, I don’t expect that to last,” infectious disease specialist Dr. Judy Stone wrote about the newly-discovered pathogen.“The deer ticks have greatly increased their range and will likely carry this new bacteria with them.”
Likewise, the spread of Zika virus in the US is expected to increase in response to climate change-induced trends in vector distribution and activity. With warmer spring temperatures coming soon, experts believe local transmission within the US is likely, particularly along the Gulf Coast. Warmer temperatures help Zika by increasing the habitat for its mosquito vector, while speeding up the biological processes that help the virus replicate. “With higher temperatures you have more mosquitoes feeding more frequently and having a greater chance of acquiring infection,” Dr. Bill Reisen, an entomologist at UC Davis, told the Associated Press. “And then the virus replicates faster because it’s hotter, therefore the mosquitoes can transmit earlier in their life.”
Arboviruses like Zika, chikungunya, and dengue fever are so strongly influenced by climate change-related effects that scientists have been able to develop accurate models to predict the emergence and spread of disease based on climate data. However, 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 played a significant role in driving the 2014-2015 Ebola outbreak, which occurred 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 Zika 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 recently-discovered Middle-eastern respiratory syndrome (MERS)-CoV.
A constant worry is that an emerging virus may evolve to transmit more efficiently within the human population but we do have means to prevent virus emergence. Intense monitoring of changes in virus distribution, infection and symptom patterns, treatment response, and novel human/animal infections lies at the heart of our strategy to combat emerging viruses. Included in this strategy is the ongoing monitoring of immunization coverage to prevent the resurgence of vaccine-preventable viral infections such as measles and rubella.
For Zika virus 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. No treatments or vaccines are available for Zika virus, but President Obama recently called for U.S. scientists to accelerate efforts to develop better diagnostic tests, vaccinations, and therapeutics. Since Zika is a close relative of dengue fever, yellow fever, Japanese encephalitis and West Nile, scientists hope to build on their existing knowledge of these diseases to speed up progress towards a Zika vaccine. However, researchers say it will likely take another decade before a vaccine could be made available to the public.
Besides the long process of drug development and clinical trials, another challenge lies in predicting which viruses will pose the greatest future threat 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.