Beyond the frenzied efforts to contain the spread of Ebola lies a scary problem: No one knows exactly where the virus comes from or how to stop it from initiating new outbreaks.
Zaire ebolavirus has caused two dozen outbreaks in Africa since it first emerged in 1976. It is coming from somewhere — probably bats — but experts haven’t yet reached a consensus regarding its origins in nature. If researchers can figure out which animal carries Ebola, and how the virus made the leap to people in West Africa, they may be able to suggest strategies to prevent or contain future outbreaks.
But that has had to wait until they can tame the current outbreak, which has claimed more than 1,100 lives in four countries — the worst toll from Ebola in history.
“First and foremost get the outbreak under control. Once that piece is resolved, then go back and find what the source is,” said Jonathan Towner, a scientist who helped find the bat source of another Ebola-like disease called Marburg. Towner works for the U.S. Centers for Disease Control and Prevention.
Throughout history, some of the biggest wins against infectious diseases have involved not just limiting person-to-person spread but also finding and controlling the sources in nature fueling new cases. Plague was halted after the germ was tied to rat-riding fleas. With the respiratory disease SARS, civet cats played a role. With typhus it was lice, and with bird flu, live poultry markets. Efforts to control MERS, a virus causing sporadic outbreaks in the Middle East, include exploring the role of camels.
In the case of Ebola, health experts think the initial cases in each outbreak get it from eating or handling infected animals. They think the virus may come from certain bats, and in parts of Africa, bats are considered a delicacy.
Scientists believe that fruit bats like the one pictured above may be a host for the Ebola virus, but the evidence remains inconclusive at this point.
Some of the evidence implicating bats comes from a study in the April edition of the journal Viruses by Kevin Olival and David Hayman of Massey University in Palmerston North, New Zealand. The researchers mapped the ranges of fruit bat species that might carry Ebola or related viruses. Some of the bats’ ranges include both West African and Central African countries such as Democratic Republic of the Congo, where Ebola has appeared in the past. The finding opens the possibility that the virus could have traversed vast distances via bats.
Still, experts say there is insufficient evidence to attribute West Africa’s outbreak to bats — or even to conclude that bats are the primary carrier of the virus. While three species of bats have been found to carry antibodies to Ebola, no one has found evidence that the bats actually carry the live virus. Besides bats, the World Health Organization lists chimpanzees, gorillas, monkeys, forest antelope and porcupines as possibly playing a role. Even pig farms may amplify infection because of fruit bats on the farms, the WHO says.
Virus may have lurked in West Africa for years before outbreak
Part of the puzzle is how long the virus has been in West Africa. Previous outbreaks have been in the east and central regions of the continent. The current outbreak began in rural Guinea, and the first suspected case was a 2-year-old child who died in Gueckedou prefecture in December, researchers wrote in the New England Journal of Medicine. They did not speculate on how the child may have become infected.
But some scientists think the virus has been lurking in the area for years. They point to the case of a lone scientist who got sick in 1994 after doing an autopsy on a wild chimpanzee in Ivory Coast and to new research that explores the possibility that past Ebola cases in the region have gone undiagnosed.
With Ebola appearing for the first time in West Africa, scientists are working to determine how — and why — the virus emerged in the region.
In a recent investigation, scientists in the United States and Sierra Leone looked back at hundreds of blood samples that were sent to a testing laboratory in eastern Sierra Leone from 2006 through 2008. The samples initially were checked only for Lassa fever, which is common in West Africa. But when the scientists recently went back and tested for other infections, they found nearly 9 percent was Ebola.
One or more types of Ebola virus have “probably been there in the mix” for some time but for some reason didn’t explode into a widespread epidemic in West Africa until this year, said Stephen Morse, a Columbia University infectious disease expert. “In this part of Africa, although the virus has probably been there in nature for quite some time, some change in environmental conditions or some accidental contact with one of the animals that might be carrying Ebola, probably a bat, somebody got infected, and people are still learning how to handle it,” Dr. Morse added.
Ebola’s jump from animals to people is thought to be rare. Experts say there may be a large degree of bad luck in becoming infected — in a cave associated with an outbreak of Marburg virus, scientists found the virus in only 3 percent of the bats they tested. Even if an animal source is clearly identified and people are warned, “there is always likely to be an occasional exposure — someone who drives off the highway, in essence,” Morse said.
But with other diseases, control measures have paid off.
In 2003, when civet cats were tied to SARS, live animal markets where the cats were sold and butchered for food were identified as hub for the virus. Control of those markets in southern China helped limit the outbreak, though experts admit that it can be a tough feat to pull off. That’s particularly true in rural areas of West Africa, where many people rely on inexpensive bush meat as their primary source of protein.
Bush meat, which is widely consumed throughout Africa, is thought to be a primary route of animal-to-human transmission of the Ebola virus.
Bats, great apes, other primates and antelopes known as duikers are commonly eaten but also are among the animals most likely to be infected with Ebola, experts say. Some local governments banned eating bush meat as the outbreak grew in size, but by that time, the disease had already taken hold in the human population.
Emergence of disease affected by climate change
While we don’t know exactly what factors precipitated the current outbreak, it is becoming increasingly clear that human activity plays an important role in the emergence and reemergence of zoonotic diseases—those that jump between animal and human—like Ebola. Humans are venturing farther and farther into forests, adding to the burden on local ecosystems through small-scale gold and diamond mining, deforestation, and conflict.
West African countries are already feeling the effects of climate change, the International Food Policy Research Institute noted in a report published last year. There are now more “seasonal droughts, strong winds, thunderstorms, landslides, heat waves, floods, and changed rainfall patterns.” All that is changing the forests where Ebola first begins to take hold.
In the remote villages of West Africa, where human populations meet the forests, people are having increased interactions with animals, and that, combined with traditional hunting practices, is driving up the risk of a “spillover” occurring, where Ebola can leap across species.
Other experts point to extreme weather as a factor behind the emergence of Ebola in West Africa, citing meteorological data showing that previous outbreaks typically occur in clusters after sudden weather changes. The increasing rate of global warming, which has caused rainfall patterns to change and extreme weather to become more intense and frequent, has disrupted agriculture and food sources across the world, driving some farmers in affected areas of West Africa to abandon their crops and venture into the forest for food.
Climate change is driving outbreaks of diseases like Ebola through multiple mechanisms, including forcing farmers to abandon their crops and seek food further in the forest, increasing their chances of coming into contact with disease-carrying animals.
Changes in precipitation also have a major impact on the rate at which trees shed their branches and buds, which in turn directly affects the Ebola-carrying bats that feed on the trees’ fruit. Not only do rainfall patterns influence the size of the bat population, but they also drive migration patterns — if food is scarce one year, the bats may have to move to another area to find food.
“Increasingly, spillover of viruses from bats and other wildlife occurs due to increased human activities that bring people into closer contact with wildlife, such as land-use change and agricultural practices,” said Dr. Jonathan Epstein, Associate Vice President at EcoHealth Alliance, an international organization of scientists working to understand the dynamics of emerging diseases and the ecology of associated wildlife reservoirs in an effort to prevent and better control potentially pandemic outbreaks.
Perhaps most unsettling of all is that Ebola is just one of many, many diseases on the rise thanks to human activity — a phenomenon that is well documented in the scientific literature.
In a 2008 study, researchers analyzed data on 335 emerging diseases from 1940 to 2004 in an effort to uncover global temporal and spatial trends in emergence. After plotting the data on maps that correlated disease with human population density, rainfall and wildlife biodiversity, the team found that disease emergence events have roughly quadrupled over the past 50 years.
Nearly 60 percent of the new pathogens were zoonotic infections. And, alarmingly, a good percentage of the infections were caused by antiobiotic resistant bacteria.
The global map of emerging disease also revealed that the hotspots for disease span sub-Saharan Africa, India and China, with smaller spots in Europe, and North and South America. They tend not to be in the richer countries in the North that can afford surveillance but in relatively impoverished areas such as the tropics, a region rich in wildlife species and under increasing human pressure, and in dense human populations, where one would expect them to take hold more easily.
Two years later, the same team of researchers published a new analysis showing that species losses in ecosystems spur rises in pathogens as we intrude into areas of high biodiversity. This is significant because current extinction rates are estimated at 100 to 1,000 times higher than in past epochs, and are projected to increase at least a thousand times more in the next 50 years. In fact, some scientists believe we are on the verge of another mass extinction event — however, unlike the five previous mass extinction events in Earth’s history, this one is directly attributable to human activity and worldwide surges in population.
Globalization and the spread of disease
Public health officials in the U.S. have long waited and prepared for the day Ebola arrives in this country. While infectious diseases have historically been confined to the areas surrounding the geographic epicenter of an outbreak, the landscape of disease transmission has undergone dramatic changes in more recent years.
With air travel reaching all corners of the globe, infectious diseases are no longer contained to a geographic region, making any outbreak an international issue.
“Diseases don’t respect borders,” Daniel Feikin, CDC Epidemiologist, said in May. “In this day and age, we need to be prepared for diseases coming to the United States.”
The unprecedented volume and speed of human mobility are perhaps the most conspicuous manifestations of the present era of globalization. From international tourists to war-displaced refugees, more people are on the move than ever before. They are also traveling faster and are regularly visiting what used to be very remote parts of the world. Air travel, in particular, has completely changed the landscape of disease transmission.
A century and a half ago, it took about 365 days to circumnavigate the globe by ship; today it takes less than 36 hours. One of the particularly threatening aspects of this compression of time is that people can now cross continents in periods of time shorter than the incubation periods of most diseases. This means that, in some cases, travelers can depart from their point of origin, arrive at their destination, and begin infecting people without even knowing that they are sick.
“Now that we have a billion people traveling each year, a disease in one country becomes a problem for everyone else,” said William Karesh, the executive vice-president for health and policy at the EcoHealth Alliance.
The effects of these changes are striking: According to the World Health Report 2007, worldwide infectious diseases are currently spreading faster and emerging quicker than ever before. “Since the 1970s, new diseases have been identified at the unprecedented rate of one or more per year,” the report warns. These effects compound those of climate change, which is also facilitating the spread of diseases to regions where they were previously nonexistent.
The large-scale movement of goods and people around the globe increases the probability of vectors (often insects, but also animals) and nonhuman carriers of disease being introduced into areas where neither previously existed – frequently with fatal results.
The reintroduction of cholera to South America in the 1990s is thought to have resulted from a freighter discharging ballast water from China into Peruvian coastal waters. The water carried the cholera vibrio which flourished in algal blooms enriched with nitrogen and phosphorous from sewage and fertilizers. Algae are filtered and eaten by molluscs, crustaceans and fish that are, in turn, eaten by people. Once it entered Latin America, the infection spread rapidly, encouraged by rapid urbanization and IMF- and World Bank-imposed cutbacks in sanitation and public health programs. By December 1994, millions of Latin Americans had become ill, and thousands had died. Reported cases are thought to be only a fraction of those infected.
Current development policies have also contributed to the spread of disease by undermining local livelihoods and forcing people to migrate in search of work. The resurgence of malaria, for example, has been greatly exacerbated not only by the building of irrigation schemes, creating drainage problems that increase the opportunities for vector mosquitoes to breed, but also by migrant workers bringing the pathogen into areas where it previously did not exist. Non-immune migrants entering endemic areas may fuel another kind of outbreak. Political and economic oppression has exacerbated the problem, as more and more people are forced to move within and between countries. The effect is that diseases once limited to small regions of the globe are no longer confined.
And although new global markets have created unprecedented economic opportunities and growth, the benefits have not been equally distributed, and the risks–especially the health risks–of our increasingly interconnected and fast-paced world continue to grow. While the burden is greatest for the developing world, infectious diseases are a growing threat to all nations. However, the same globalizing forces that create such rampant opportunity for pathogens also can provide mechanisms for innovative, global efforts to control infectious diseases.
Global inequities in health care contribute greatly to the emergence of new diseases. In West Africa, a lack of medical supplies and protective equipment has driven the spread of Ebola.
Unfortunately, the international community is often slow to respond to public health crises in developing nations. The global response to West Africa’s Ebola outbreak, which was officially declared by the WHO in March, is just now picking up — and even with the increase in international aid, treatment centers don’t have enough beds to keep up with all of the new cases, and a lack of basic cleaning supplies and medical equipment continues to put health care workers at high risk of infection.
As Jim Yong Kim, President of the World Bank Group, and Nkosazana Dlamini Zuma, Chairperson of the African Union, explained in a column last week:
The international community is now starting to come to the aid of Guinea, Liberia, and Sierra Leone. We must acknowledge, though, that this tragedy is a wake-up call. We all knew that in these three countries, and in many others in the developing world, the health systems were extremely weak and could not effectively contain an infectious disease outbreak such as this Ebola epidemic. We must also strengthen regional institutional capacity in Africa for disease prevention and control.
Now we are witnessing the results of our acceptance of the status quo. We will be able to stop Ebola in the coming weeks and months. But that is not the end of the story. Will we also build a strong enough health system to stop the next outbreak? We believe that it is a moral and economic imperative to do so, and all of us must work toward that goal.