Where did the Zika Virus come from?
Deep in an African jungle, a monkey sits high in the canopy of a lush forest. This is a special monkey; he is called Rhesus 766. Every so often, people come to visit him. They give him food and water, clean his aerial pen, and check that he is healthy.
One day, Rhesus 766 comes down with a fever. His visitors, concerned, bring him back to a building near the forest to run some tests. What they find is striking; a brand new, unknown virus is causing Rhesus 766’s symptoms.
Although this sounds like something from the movies Outbreak or Contagion, it’s actually a true story. The year was 1947, and the people caring for Rhesus 766 were researchers from the Uganda Virus Research Institute (UVRI), an institute set up a few years earlier by the Rockefeller Foundation to combat yellow fever. Over time, UVRI branched out and set up a monitoring program to detect what other viruses were circulating within the jungle.
This is where Rhesus 766 came into the picture. He was a sentinel monkey, and by studying what viruses he picked up from his environment, researchers could keep tabs on what viruses were circulating in the area. There were probably many monkeys like him, but Rhesus 766 lived in the Zika forest, hence the name of the new virus.
A year after the Zika virus was found in Rhesus 766, researchers at UVRI found it in a mosquito as well. The rest is history: mosquitoes provided a way for the virus to get from monkey to man. Within four years, antibodies against the Zika virus were found in the blood of several people in nearby areas, indicating that they had been infected with the new Zika virus, too.
For more than 50 years, the Zika virus quietly remained in sparsely populated African areas, never causing any real problems. That all changed, though, when modern aviation allowed vast numbers of people to travel around the world. Suddenly, the Zika virus began showing up in large population centers where people had never been exposed to the disease before and had no protection against it. Where it had previously just been humming quietly in the background, it now was infecting a large enough number of people to really show its devastating effects.
Where do new viruses come from?
The fact that the Zika virus originated in a monkey isn’t unique; most new viruses originally come from other non-human animals. Like the Zika virus, many of these diseases originate in tropical areas. Warm, humid weather often combines with a large abundance and diversity of both mosquitoes and mammals to create a perfect storm of factors favorable to creating new viruses. A greater number of animals gives viruses more opportunities to evolve, and as soon as one does, mosquitoes quickly carry it to other animals.
Another important source of new viruses is agriculture. In these settings, people and animals are often living together in crowded, unsanitary conditions, perfect for viruses to circulate. One example of this is the rinderpest virus (RPV), which caused rinderpest in cattle and was eradicated in 2011, but not before it had evolved to cause a new human disease: measles.
How do new viruses cross from animals into people?
Your body is constantly being bombarded with all kinds of viruses. Luckily, a virus will only be able to infect you if it can get into your cells, and it’s not a simple matter of just floating up to any cell and busting its way in like the Kool-Aid man. The virus needs to find a specific door to the inside of the cell – called a receptor – and latch onto it before it can enter the cell.
If a virus can’t attach to the receptor and get into the cell, it can’t infect the person. This is why some people are immune to certain diseases. HIV, for example, relies on finding CD4/CCR5 receptors, which are normally used by certain white blood cells to communicate with other cells. Some people have mutations that cause these receptors to stay hidden within their cells; the HIV can’t find the doorway to get in, thus making them immune to HIV.
Cells have lots of different types of receptors, and they’re usually species-specific. For example, if a virus is adapted to find specific receptors in a bird, and makes its way into a human, it probably won’t find the right receptors in human cells and thus cannot infect the person. This is why you can’t give the flu to your dog or cat.
This isn’t always the case. Some receptors are the same in other animals as they are in people, and the more closely related two species are, the more similar their cell receptors are likely to be. The more similar that the two receptors are, the easier it will be for the viruses to evolve to take advantage of the new receptor type. This is why many human diseases come from animals like monkeys.
Are more viruses evolving now?
Viruses have existed for as long as their hosts have and will continue to evolve and change in the future. Recently, though, it seems like there are many more new viruses in the news – everything from avian influenza to the Zika virus.
Part of the reason why there are so many new viruses is because the environment is changing at a rapid pace through climate change, deforestation, and many other factors. As a result, there are new opportunities and niches for viruses to exploit. For example, as the climate becomes warmer, disease-bearing mosquitoes thrive and expand their ranges, bringing viruses like dengue and chikungunya with them. Who knows how these viruses will evolve once they make their way into new animals and possibly human populations?
So, the next time you spend time outside in the summer, make sure you bring your bug spray!