Zika virus, Ebola, Middle East Respiratory Syndrome (MERS) coronavirus, Severe Acute Respiratory Syndrome (SARS) coronavirus, Nipah virus, Hendra virus, bird flu, swine flu -- these viruses have all grabbed international attention in recent years. In the past few decades the world has witnessed an alarming surge in emerging infectious diseases (EIDs). Since 1980, new pathogens have emerged in the human population at a rate of about three each year.
Why are we seeing such a surge in new pathogens? One could argue that some of the pathogens may not be new at all; they could have circulated among humans for centuries and are just being identified for the first time because of increased surveillance and reporting. While this is true in a small number of cases, a study found that even after controlling for increased surveillance, there has still been a surge in EIDs in recent times.
In other words, the rise of new pathogens is very real.
So let's look at the major reasons why we are seeing this rise:
First: human overpopulation. I think we need to change the phrase "breed like rabbits" to "breed like humans," as no other species on this planet even comes close to the human reproduction rate. As our population grows, available land shrinks and more and more people are forced to live in crowded, urbanized environments, a situation ripe for the easy spread and emergence of infectious agents.
Second: increased travel. Our travels significantly increase our chances of catching a pathogen in one area and unwittingly transporting the infectious agent to another area, where it was never before seen and where little or no immunity exists.
Third: climate change. Vector-borne diseases are those that are spread through insects like mosquitoes, ticks, and spiders. A vector's life cycle greatly depends on climatic factors. And climate change is helping certain vectors, like mosquitoes, to thrive. Warmer temperatures shorten the incubation time of viruses carried by mosquitoes, accelerate the maturation of mosquito larvae, and increase the feeding frequency of adult mosquitoes. All of these factors increase our chances of catching a virus, like Zika, carried by mosquitoes.
Fourth: deforestation and natural habitat loss. Cleared land collects rainwater better than rainforests, providing more suitable breeding grounds for mosquitoes. Lyme disease is on the rise because of our encroachment upon and fragmentation of woodland habitats in the U.S. The Nipah virus is a newly discovered pathogen that is causing considerable public health concern because of its ability to infect a broad range of animals and its high lethality among humans. It was first detected in Malaysia. Habitat loss caused a mass exodus of Nipah virus-carrying fruit bats as they searched for food. This led the bats to cultivated fruit farms that were planted next to pig farms. Unfortunately, the pigs were highly susceptible to the Nipah virus. In turn, these pigs passed the virus on to humans.
While these four factors do indeed contribute to the rise in EIDs, a fifth is rapidly gaining in importance and may be paramount: the global trade in wildlife and production of animals for food. About two-thirds of the emerging pathogens come from other animals--and that's no accident. As our demand for animals for food, skins, and entertainment increases, so do our risks of infectious diseases.
The U.S. is one of the largest importers -- and exporters -- of animals. These animals are caught from the wild or bred in captivity and then sold for use as exotic pets, entertainment (for circuses and zoos), food, fur, skins, and for experiments. Animals such as foxes, cats, bats, rats, snakes, birds, bears, monkeys -- you name it -- are fair game for the wildlife trade.
As we delve deeper into the forests and jungles to seize new animals for the trade, we risk exposure to rarely encountered animals who may carry viruses that are entirely new to humans. We likely got HIV and Ebola through the bushmeat trade. And though we do not know exactly how humans first contracted Zika, we do know that Zika was first discovered in a rhesus monkey.
To add insult to injury, the animals in the wildlife trade suffer tremendously as they are passed from one dealer to another, shipped overseas, or killed on site in profoundly painful ways. The trade creates and brings together lots of very distressed and, as a result, very sick animals -- ideal conditions for pathogens to pass on from one animal to another and ultimately to humans.
This is how humans contracted SARS. It is believed that the SARS virus originated in fruit bats. At some point in the wildlife trade, fruit bats were brought into contact with civet cats (both animals captured for the wildlife trade) in the Guangdong Province of China. The SARS virus likely passed from fruit bats to civets and then on to humans.
By creating distressed and sick animals, we are also harming humankind. Nowhere is this more apparent than in the massive production of animals for meat, eggs, and dairy. If viruses, bacteria and parasites could tell us about their ideal environments, we would hear them describing animal agriculture among their top choices. For every human on this planet, there are about ten land animals raised and killed for food at any one time. As I have discussed before, the intensive confinement of animals for food is directly responsible for the explosion of deadly new strains of bird and swine flus.
Besides directly propelling the formation of new viruses, animal agriculture also contributes indirectly. Animal agriculture is a major driver of deforestation and climate change, which exacerbate the problems I described earlier.
Each time a new pathogen causes an outbreak, we follow a familiar pattern: we panic, ask ourselves how can this be, scramble to create new drugs, sigh with relief when the outbreak ends, and then continue the same behaviors that caused the pathogen to emerge in the first place. We need to stop being reactive. Medicines and vaccines only provide a temporary fix, at best. Even worse, vaccines can actually spur the evolution of viruses, creating resistant strains.
Unless we take a hard look at the choices we make in life, such as eating animals and reproducing at such a high rate, new pathogens will show themselves at an ever-increasing rate. As Dr. Dan Bausch from Tulane School of Public Health and Tropical Medicine once stated (1):
For every virus that we know about, there are hundreds that we don't know anything about. There are a lot more pathogens out there.
We can try to play catch up each time a new pathogen makes itself known or we can prevent them from emerging in the first pace. The choice is in our hands.
Stay tuned for my next post on the wildlife trade: a hug gap in our public health armor.
1. Calvert S, Kohn D. 'Out of Africa: A baffling variety of diseases' The Baltimore Sun May 15, 2005.