ON APRIL 18, 1947, a rhesus monkey being studied for yellow fever research in Uganda's Zika Forest became ill with a fever of 103° F (39.44° C). Scientists took blood samples and conducted tests. Rhesus monkey #766 had been stricken by something unknown. In time, the emerging virus would be named after the place where it was first discovered. In early 1948, scientists also isolated the Zika virus from Aedes africanus mosquitoes trapped in the same forest. Even though the Zika virus would generate only sporadic and limited scientific attention in the next few decades, a new microbial threat to human health had emerged.1
What's an emerging disease?
We're now familiar with severe acute respiratory syndrome (SARS), Ebola virus disease (EVD), and human immunodeficiency virus (HIV) infection, but these were once considered emerging infectious diseases. According to the CDC, an emerging infectious disease is one whose incidence has increased in the past 20 years and could increase in the near future.2 Interestingly, the literature tells us that of the diseases now considered “emerging,” 75% are zoonotic, meaning they originated in animals.3
No single factor drives disease emergence. Humans and microbes are at the center of a continually evolving, complex set of elements that make up this perfect storm. The Convergence Model developed by the Institute of Medicine is a helpful way to visualize disease emergence. (See Visualizing emerging disease with the Convergence Model.)
How humans and microbes interact
Many microbes are helpful, such as Lactobacillus acidophilus in our gut, or fairly harmless, such as rhinovirus (the common cold). However, we occasionally discover a pathogenic microbe that can do us great harm or may even threaten our existence. Even with technology, we can identify only a small portion of the microbes that inhabit our earth and our human biosphere. There's still a lot we don't understand about the invisible world of microbes and their effects on the human body.
Microbes are the most abundant life form and have thrived for millions of years. During this long lifespan, they've become quite ingenious at adaptation to avoid annihilation by the human immune system.4 Unfortunately, humans are unwittingly aiding them in their efforts to prevail. We've been so generous in our use of antibiotics that microbes have learned how to create resistant versions of themselves and spread that resistance to other microbes. By creating environments for these “super” microbes to proliferate, humans are taking away the power of the best weapons we have in our arsenal to fight dangerous bacterial infections.5
Travel is a powerful force in disease emergence and transmission. People can now travel to the far side of our planet in less time than it takes for infected people to manifest signs and symptoms of a disease, potentially infecting anyone they might come in contact with along their route.4 One example of the effect of travel on disease transmission was described in 2006, when researchers published empirical data suggesting that the spread of influenza was delayed in the U.S. in 2001 because of the decrease in air travel after the September 11 attacks.6
The effect of travel on Zika virus transmission following the 2016 Olympic Games in Brazil is a similar example. Zika's estimated incubation period—the time from exposure to symptoms—is anywhere from a 2 to 14 days.7,8 This left plenty of time for an unsuspecting infected Olympic athlete or spectator traveling back from Rio to spread contagion.
While many people travel for business or pleasure, others are forced to leave their homes to escape war, famine, oppression, or poverty. This type of migration brings with it even more elements that can contribute to the emergence of infectious diseases: close living quarters, unsanitary living conditions and water sources, and unsafe food acquisition and preparation practices.4
Millions of people in villages and logging camps in Eastern and Southern Africa lack enough domesticated animals for meat consumption and must rely on wildlife hunting to meet this need. Approximately 1 million metric tons of bushmeat are consumed each year in Central Africa. Also alarming, thousands of pounds of meat from primates, antelope, and other wild animals are smuggled into the United States and Europe every year for cultural events.9 Hunting, preparation, and consumption are linked with several pandemics and epidemics, most notably HIV, EVD, and SARS.10 While the enhanced risk of disease transmission due to international trade isn't well understood, it could potentially be a danger to people.
Changes in how humans use the land and environment are frequent drivers of disease emergence. (See Drivers of disease emergence in humans.) Land development for housing or agricultural use and the building of reservoirs and dams to help create water sources for agricultural and public use in previously undeveloped geographical areas increase the potential for humans to come into contact with mosquitos, ticks, rodents, and other animals that may be harboring an infectious disease.4 Human encroachment into these previously undisturbed areas increases humans' access to remote areas and introduces more vectors and reservoirs of infection to new hosts.10
In reaction to each emerging disease threat, we typically pour global health resources into finding vaccines to circumvent the capability of those microbes to do widespread harm. Vaccines have prevented an estimated 100 million deaths in the U.S. alone and continue to save 2.5 million lives worldwide every year.12
One exciting recent breakthrough in vaccine development is completion of Phase 3 clinical trials for RTS,S/AS01, a malaria vaccine that will be piloted in selected sub-Saharan countries where deaths from malaria in 2015 alone climbed to over 390,000 people.13 Work is moving quickly forward on vaccines for other ongoing threats such as EVD, dengue, and Zika.
Vaccines are a potent weapon against a known disease threat, but wouldn't it be amazing if we had the technology to locate and prevent these emerging diseases before they had the chance to wreak havoc? This isn't yet a reality, but groups such as the CDC's Global Disease Detection Program (GDD) and WHO's Global Outbreak Alert and Response Network are tirelessly watching over the planet's human-microbe interaction, developing tracking technology and responding to reports of escalating illness and other health emergencies.14
Each of us can do our part to diminish the threat of infectious diseases by pulling ourselves out of our domestic silo and adopting a global mindset. We need to see the reality of our interconnectedness and be aware that what's happening in other parts of our world may find a way to our hometown. We can make ourselves less vulnerable to disease threats (known and unknown) by understanding basic facts about how diseases are transmitted and the importance of hand hygiene and staying up-to-date with age-appropriate vaccinations. Emerging infectious diseases threaten everyone on the planet, and everyone potentially has a part to play—either in slowing transmission or prevention. After all, the next emerging disease might be just a plane ride away.
Visualizing emerging disease with the Convergence Model4
This illustration depicts how the human-microbe interaction can be influenced by diverse and continually evolving factors in our world. According to this model, the emergence of a microbial threat to human health originates from the convergence of four domains:
- genetic and biological factors
- physical environmental factors
- ecologic factors
- social, political, and economic factors.
At the intersection of humans and microbes is a box representing the convergence of factors leading to the emergence of an infectious disease. The interior of this box is a gradient moving inward from white to black. The outer white edges represent what we know about the factors in emergence. The black center represents the unknown. Like the “black box” on an airplane that we search for after a crash to help us understand why the crash occurred, researchers are continually working to shed light into the “black box” of emergence. This knowledge could help us prevent the next global microbial threat.
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Copyright © 2018 Wolters Kluwer Health, Inc. All rights reserved.
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. New York, NY: St. Martin's Press; 2011.