Pests and Pestilence

Why humans are more vulnerable than ever to animal-borne diseases.

In October 1347, Genoese galleys bound from Crimea docked in the Sicilian port of Messina. The ships’ crew had "sickness clinging to their very bones." Soon the whole of Messina had it; then Italy; then the rest of Europe. A disease carried by Asian black rats had infected humans and triggered what came to be known as the Black Death. By 1350, about a third of Europe’s population was dead, not to mention huge losses in India, Mesopotamia, and elsewhere.

On the face of it, there should be less risk of humans contracting animal-borne diseases today. Most of us rarely encounter creatures more exotic than our domestic pets. Back then, however, cattle and pigs shared our ancestors’ houses, rats had the run of ships, and rabid wolves and malarial mosquitoes were common across Europe. And yet the World Health Organization (WHO) recently reported a rise in cases of what are known as zoonotic diseases, which humans can contract from animals either through direct transmission (such as catching rabies from a dog bite), by eating infected meat (such as meat products from animals with mad cow disease), or through an intermediary (such as mosquitoes that carry West Nile fever from birds to humans). Two thirds of the 30 infectious diseases that have emerged in the last two decades are known to have come from animals. Severe acute respiratory syndrome (SARS) is just the latest: Public health officials believe SARS first infected a human when a mutated bird virus set up home in human lungs somewhere in southern China last year. Meanwhile, old diseases harbored by animals — such as malaria, dengue fever, Ebola, and even the plague — are making a comeback and claiming more lives every year.

Why the upsurge? The frightening pace at which SARS hopscotched from Asia to North America shows how cheap and easy international travel can transform a local outbreak into a global pandemic in a matter of weeks. But globalization doesn’t just break down the barriers between nations; it also breaks down the barriers between species. In several parts of the world, humans mix with animals much more frequently and intimately than in the sanitized, segregated West. Many southern Chinese markets in particular are a cornucopia of imported birds, snakes, turtles, and various mammals, all waiting to be killed for the table. And overcrowded factory farms act as an evolutionary pressure cooker for new strains of disease, as in 1998, when a large Malaysian pig farm incubated a flulike virus picked up from a local colony of fruit bats before passing it on to humans.

Environmental degradation and climatic change also help unleash zoonotic diseases. Just as rats launched the Black Death by fleeing from flooded fields in Asia, so too have animals exposed humans to diseases such as Lassa fever and the Marburg virus by being forced to flee their shrinking forest habitats. Another unintended legacy of deforestation has been the rise of the bushmeat trade in Africa. As roads built by logging companies have penetrated into previously inaccessible areas of the rain forests, commercial hunters are finding it easier to kill apes and to sell the meat in African cities. Some scientists believe HIV/AIDS — which is probably destined to be the most catastrophic zoonotic disease since the Black Death — may have reached humans about 40 years ago in infected ape or monkey meat. Recent, recurrent outbreaks in central Africa of the Ebola virus, one of the most virulent diseases known to exist, likely started the same way.

Despite the widely acknowledged threat of zoonotic diseases, public health officials have been slow to adapt. Even as viruses routinely leap the species barrier, doctors and veterinarians keep their disciplines separate. What we have here is a failure to communicate, often with catastrophic results. Shortly after West Nile fever showed up in New York in 1999, veterinary researchers warned that migratory birds would quickly carry the virus across the United States. But medical experts failed to heed that warning, and the disease spread to the West Coast in just three years. Likewise, vets in Britain failed to change slaughterhouse practices to prevent the spread of mad cow disease from cattle herds to humans. As a result, British epidemiologists estimate as many as 7,000 people could eventually contract the illness.

But some signs indicate the doctor-vet divide will narrow in the future. Last December, prominent veterinary researchers met with doctors at the U.S. Centers for Disease Control (CDC) to discuss ways to improve joint disease surveillance systems. The participants at the meeting agreed that vets needed to play a larger role in the Laboratory Response Network, which was created by the CDC, the Federal Bureau of Investigation, and the Association of Public Health Laboratories to counter the threat of bioterrorism through the fast identification of new diseases. Animal diseases such as the plague, anthrax, and botulism make up four-fifths of the contagions classified as posing the greatest risk to U.S. national security because they can easily be cultivated by terrorists and used against humans.

The United Nations has been taking similar steps. Last year, a WHO study group argued for more basic research into diseases that afflict wildlife and called upon international organizations to help fund veterinary research facilities throughout the developing world. But the recent experience of SARS shows where the fault lines still lie. Although Chinese doctors swiftly spotted the virulent new virus last autumn, WHO investigators did not figure out until March of this year that most of the earliest sufferers were food handlers — a discovery that also might have been made sooner with better cooperation from the Chinese government.

Public health officials may succeed in limiting the spread of SARS before it becomes the next global pandemic. But will they be ready when the next plague ship sails into port?

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