On reflection, I shouldn’t have been surprised. I can’t count all the times I have cut myself while chopping onions. The difference is that onions aren’t closely related to us humans, and an onion virus has far less chance of taking hold in us than does a monkey virus.
The statistics are telling. Researchers like Mark Woolhouse, professor of infectious disease epidemiology at the University of Edinburgh in Scotland, have found at least 868 human pathogens that infect both animals and humans, although some are not as fearsome as they seem.
Overhyped microbes include anthrax (famous for the U.S. mail attacks in 2000), the Ebola and Marburg viruses (which can cause dramatic bleeding and high fever in their victims), and the prion agent of mad cow disease (otherwise known as bovine spongiform encephalopathy, or BSE), which kills people by making their nervous systems degenerate. These bugs arouse terror because they kill so many of their victims. For example, in the 2000 Ebola outbreak, which struck the Gulu district of Uganda, 53 percent of the 425 people who contracted the disease died. The case fatality rate for BSE is 100 percent.
Although spectacularly lethal, these pathogens generally kill just a few hundred people at a time and then burn themselves out. They transmit from human to human too inefficiently to spread very widely; 100 percent of a small number of victims is still a small number of fatalities.
There are many reasons why an agent leaping from animals to humans might not affect more individuals. For example, humans do not normally bite, scratch, hunt, or eat each other. This surely contributes to the rarity or nonexistence of human-to-human transmission of rabies (acquired by the bite of an infected dog or bat); cat-scratch disease (which causes skin lesions and swollen lymph nodes); tularemia (a disease, often acquired when hunting and cutting up an infected rabbit, that can cause skin ulcers, swollen lymph nodes, and fever); and BSE (probably acquired by eating the nervous system tissue of infected cows).
Some outbreaks, once recognized, are relatively easy to control. Anthrax is treatable with antibiotics; after an initial malaria-like stage, the rapid onset and severity of Ebola and Marburg symptoms have made identification and containment straightforward.
In fact, within the last 40 years, only HIV (derived from chimpanzees) has taken off to cause a pandemic.
Back to the Future
If not anthrax or Ebola, which pathogens might spawn the next deadly pandemic in our midst?
New pandemics are most likely to be triggered by mutant strains of familiar microbe species, especially those that have caused plagues by churning out mutant strains in the past. For example, the highest known epidemic death toll in history was caused by a new strain of influenza virus that killed more than 20 million people in 1918 and 1919. Unfairly named Spanish influenza, it apparently emerged in Kansas during World War I, was carried by American troops to Europe, and then spread around the world in three waves before ebbing in outbreaks of declining virulence in the 1920s. Mutant strains of influenza or cholera remain prime candidates for another deadly outbreak. Both can persist in animal reservoirs or the environment, and both are adept at spawning new strains. Both pathogens also transmit efficiently, and it is possible that these two important diseases of the past could become important diseases of the future.
A future pandemic could also come from tuberculosis. New mutants have already arisen through the mechanism of drug resistance. And the disease lives on in the human population, especially among those with weakened immunity, including patients with HIV.
Also of concern are emerging sexually transmitted diseases, which, once introduced, may be difficult to control because it is hard to persuade humans to change sexual behavior or to abstain from sex. HIV offers a grim warning: Despite its huge global impact, the AIDS epidemic would have been far worse if the sexual transmissibility of HIV (which is actually rather modest) had equaled that of some other sexually transmitted agents, such as human papilloma virus (HPV). While the probability of HIV transmission varies with the stage of the disease and the type of sexual contact, it appears to pass from infected to uninfected individuals in less than 1 percent of acts of unprotected heterosexual intercourse, while the corresponding probability of HPV transmission is thought to be higher than 5 percent—probably much higher.
Similarly, it could be difficult to control emerging pathogens transmitted by pets, which increasingly include exotic species along with traditional domestic animals like dogs and cats. Already we are at risk of catching rabies from our dogs, toxoplasmosis and cat-scratch disease from our cats, and psittacosis from our parrots. Most people now accept the need to cull millions of farmyard animals in the face of epidemics like mad cow disease, but it is hard to imagine killing beloved puppies, bunnies, and kittens, even if those pets do turn out to offer a pathway for a dangerous new disease.
Have Plague, Will Travel
Once a killer disease has emerged, modern societies offer new ways for it to flourish and spread. Global travel, the close quarters of the urban environment, climate change, the evolution of drug-resistant microbes, and increasing numbers of the elderly or antibiotic-treated immunosuppressed could all aid the next great plague.
For example, rapid urbanization in Africa could transform yellow fever, chikungunya fever (which causes severe joint pain and fever), and other rural African arboviruses (viruses, including yellow fever, spread by bloodsucking insects) into plagues of African cities, as has already happened with dengue hemorrhagic fever. One of us (Wolfe) theorizes that this might follow increasing demand in those cities for bush meat. Like urban people everywhere, urban Africans love to eat the foods enjoyed by their village-dwelling ancestors, and in tropical Africa this means bush meat. In that respect it’s similar to the smoked fish and bagels that I eat in the United States, which give me some comforting memory of my Eastern European roots. But there’s an important difference: The wild game that I see served in fancy restaurants in the capital of Cameroon is much more likely to transmit a dangerous virus to the person who hunted and butchered it, or to the cook who prepared it, or to the restaurant patron who ate the meat undercooked, than is my brunch of smoked fish and bagels.
By connecting distant places, meanwhile, globalization permits the long-distance transfer of microbes along with their insect vectors and their human victims, as evidenced not only by the spread of HIV around the world, but also by North American cases of cholera and SARS brought by infected passengers on jet flights from South America and Asia, respectively. Indeed, when a flight from Buenos Aires to Los Angeles stopped in Lima in 1992, it picked up some seafood infected with the cholera then making the rounds in Peru. As a result, dozens of passengers who arrived in Los Angeles, some of whom then changed planes and flew on to Nevada and even as far as Japan, found that they had contracted cholera. Within days that single airplane spread cholera 10,000 miles around the whole rim of the Pacific Basin.
Consider as well those diseases thought of as “just” tropical because they are transmitted by tropical vectors: malaria transmitted by mosquitoes, sleeping sickness spread by tsetse flies, and Chagas’ disease (associated with edema, fever, and heart disease) spread by kissing bugs. How will we feel about those tropical diseases if global warming enables their vectors to spread into temperate zones? While microbe and vector movement can be difficult to detect, modeling suggests that global warming will expand the reach of malaria to higher latitudes and into tropical mountain regions.
The transmission of emerging diseases has also been enhanced by a host of modern practices and technologies. The commercial bush meat trade has introduced retroviruses into human populations. Ecotourism has exposed first-world tourists to cutaneous leishmaniasis and other third-world diseases. Underequipped rural hospitals have facilitated Ebola virus outbreaks in Africa. Air conditioners and water circulation systems have spread Legionnaires’ disease. Industrial food production was responsible in Europe for the spread of BSE. And intravenous drug use and blood transfusion have both spread HIV and hepatitis B and C.
All this shows that disease prevention and treatment need to be supplemented by a new effort: disease forecasting. This refers to the early detection of potential pandemics at a stage when we might still be able to localize them, before they have had the opportunity to infect a high percentage of the local population and thereby spread around the world, as happened with HIV. Already one of us (Wolfe) is working through a new initiative, the Global Viral Forecasting Initiative (GVFI), to do just that. GVFI works in countries throughout the world to monitor the entry and movement of new agents before they become pandemics. By studying emerging agents at the interface between humans and animals, GVFI hopes to stop new epidemics before they explode. Monitoring for the emergence of both new sexually transmitted diseases and pet-associated diseases would be good investments.
The predictions here are admittedly educated guesses—but they are educated by some of the best science available. The time to act is now. If we don’t, then we will continue to be like the cardiologists of the 1950s, waiting for their patients’ heart attacks and doing little to prevent them. If we do act, we have the potential to avert the next HIV, saving millions of lives and billions of dollars. The choice seems obvious.