I’ve previously written to give a brief general overview of the Zika virus, but I haven’t really told you much about Zika’s story: where did it come from and why was there such an explosive spread of the virus in 2015-16?
Zika – that’s a funny name? Where does it come from?
The virus is named after the Zika forest in Uganda where it was originally discovered in 1947.
Before this big outbreak in 2015-16 it seemed Zika was mostly confined to Africa where it was almost exclusively a disease of monkeys. Mosquitos would transmit the virus between populations of monkeys and humans seemed to only be affected very infrequently. In fact, there were only 13 cases reported in humans in the first 57 years after its discovery.
In 2007, there was an outbreak on the Island of Yap, part of the Federated State of Micronesia. Almost the entirety of the island’s population became infected (5000 of the island’s 6700 people), but the virus caused only a mild illness and across the world, no-one paid any attention at all to the outbreak of this benign virus on such a tiny Pacific island.
A subsequent outbreak in 2013-14 in French Polynesia was estimated to have affected 32,000 people, and it was here that a spike in the number of Guillain-Barré cases was first noted. Other outbreaks then occurred in other Pacific islands, including New Caladonia (2014), Easter Island (2014), Cook Islands (2014), Samoa (2015), and American Samoa (2016). Prior to this, there had been only a handful of cases of the virus outside of Africa (in Pakistan, Malaysia, the Philippines and Indonesia).
How did Zika first reach Latin America?
There are a couple of different theories as to how the virus crossed over to Latin America from the Pacific islands. The first is that it was brought over during the 2014 Football World Cup in Brazil. However, there weren’t actually that many visitors to Brazil from the Pacific Islands during the World Cup. Another theory is that a world championship canoeing race, which was attended by several teams from the Pacific Islands helped first bring the virus to Brazil.
A contributing factor seems to have been the 2015 El Nino event. This is a meteorlogical phenomenom whereby, every few years, due to a shift in oceanic currents, temperatures around the Eastern Pacific Ocean and Latin America are higher than normal. A modelling study has suggested that this provided favourable conditions for mosquito breeding during 2015 and may have contributed to Zika’s rapid spread in the region.
It seems we’ll never know for sure, exactly how Zika first reached Latin America. What we do know however, is that our world is increasingly becoming a place that is ‘ripe’ for the spread of infectious diseases. Future pandemics that spread rapidly across the globe are an inevitability.
Why was there such a sudden ‘explosion’ of Zika in 2015-16?
We’ll probably never know the exact reason that Zika took off in this way in 2015-16. However, there have been lots of factors at play over the last few decades, which have made our world increasingly susceptible to the spread of infectious diseases:
1) Growing populations and urbanisation mean people live in closer contact, which facilitates the spread of infections
2) Global travel has increased exponentially. In the 1950s there were 25 million international arrivals at airports, now there are 1.2 billion a year. With greater numbers moving around the world comes more opportunities for infections to hop a ride.
3) Global warming is helping mosquitos to survive in areas where they previously couldn’t and their overall numbers are increasing
4)The global tire trade is a particular culprit in contributing to the spread of mosquito-borne diseases such as Zika. Small pools of water in tires make an ideal breeding ground for mosquitos, which lay their eggs there. There’s a huge international market for second-hand tires and when these tires are transported overseas to be sold, the mosquitos, along with the diseases they transmit, are spread as well.
There’s also a theory that a mutation in the Zika virus increased its infectivity and ability to spread rapidly around the world. Some researchers have suggested that the genetic differences between the African strain of Zika and the Asian strain (which is the one that reached Latin America) are what made the virus capable of causing birth defects and neurological complications for the first time.
Next up, I’ll be writing about how the Zika virus outbreak looks right now and what we expect to happen over the coming years.