The Human-Animal Link

William B. Karesh and Robert A. Cook

From Foreign Affairs, July/August 2005

Summary:  Recent outbreaks of avian flu, SARS, the Ebola virus, and mad cow disease wreaked havoc on global trade and transport. They also all originated in animals. Humanity today is acutely vulnerable to diseases that start off in other species, yet our health care remains dangerously blinkered. It is time for a new, global approach.

William B. Karesh is Director of the Field Veterinary Program at the Wildlife Conservation Society and Co-chair of the World Conservation Union's Veterinary Specialist Group. Robert A. Cook is Vice President of and Chief Veterinarian at the Wildlife Conservation Society.

According to recent analysis, more than 60 percent of the 1,415 infectious diseases currently known to modern medicine are capable of infecting both animals and humans. Most of these diseases (such as anthrax, Rift Valley fever, bubonic plague, Lyme disease, and monkeypox) are "zoonotic," meaning that they originated in animals but have crossed the species barrier to infect people. The others, which receive less attention, are "anthropozoonotic," meaning they are typically found in humans but can and do infect animals as well (examples include the human herpes virus, tuberculosis, and measles). Dividing infectious agents into these two groups is convenient for teaching purposes. But it overlooks the critically important fact that all of them can move back and forth among species, mutating and changing their characteristics in the process. Avian influenza -- which started in birds but is now infecting humans as well -- has recently highlighted the need for a more holistic view of disease.

It is probably just luck that has so far allowed scientists to maintain these distinctions. One of the greatest medical success stories of the last century was the eradication of smallpox. But this achievement was largely due to the fact that smallpox survives in only one host species, namely humans. If even one more type of animal had been able to harbor the disease, there is a good chance that eradication would not have been accomplished, despite the Herculean global effort. When a pathogen can find refuge or a place to mutate in a range of hosts, controlling it becomes far more complex, requiring an integrated -- and much more difficult -- approach.

To get a sense of the breadth and the seriousness of the issue, consider HIV/AIDS, which most scientists now think arose in Africa as a result of the human consumption of primates that were infected with simian immunodeficiency viruses. Or consider the Ebola virus, which has a similar history. The disease first came to international attention in 1976, when it appeared around the Ebola River in what was then called Zaire. The virus infects people, gorillas, chimpanzees, and monkeys, causing severe internal and external hemorrhaging and leading to death in up to 90 percent of its human victims. Human infection spreads quickly, especially via caregivers and people who flee an area to escape the illness. Since the disease first appeared, successive human outbreaks have been recorded in Côte d'Ivoire, Gabon, Sudan, and Uganda. But humans have not been the only victims; lowland gorillas and chimpanzees in Gabon and Congo and chimpanzees in western equatorial Africa have been decimated by the sickness. Other forest animals, such as duikers (small antelopes) and bush pigs may also be affected. When subsistence hunters discover a sick or dead animal in the forest, they view it as good fortune and bring it home to feed their families or trade with their neighbors. The Ebola virus then easily infects those handling the meat, and a chain of contacts and infections ensue. Each of the human outbreaks in central Africa during the late 1990s and the first years of this century was traced to humans handling infected great apes.

SARS also arose from contact with wild animals. The illness first appeared in late 2002 in China's Guangdong Province, where people began complaining of high fever, cough, and diarrhea, and eventually developed severe pneumonia. The unknown disease was very contagious; within a matter of weeks, a visitor to Hong Kong helped spread it to five continents. By July of 2003, the WHO had tallied 8,437 cases and 813 deaths. Due mostly to a lack of understanding of the new disease, global travel and trade were disrupted as fear spread.

After four months, scientists eventually discovered that the mystery disease was caused by a coronavirus (a family of viruses found in many animal species). The virus, in turn, was traced back to a small mammal called the palm civet, which is farmed in the Guangdong region and sold for human consumption. Later, evidence of the virus was also found in raccoon dogs, ferrets, and badgers being sold in Guangdong's wildlife markets, as well as in domestic cats living in the city. Epidemiological studies confirmed that the first human infections had indeed come through animal contact, although the exact species responsible has not been definitively identified.

In the months after SARS first appeared, the Chinese government closed down its live wildlife markets. Within ten days of linking the disease to the wild animal trade, the government also confiscated close to a million animals, many of which had been brought into the area from other parts of the world and which hosted a variety of exotic viruses and bacteria. But the damage had already been done. Prior to the government action, the animals were often housed together, exposed to one another's waste, and sometimes even fed to one another. For a virus or bacteria capable of jumping between species, the markets had provided the perfect place to reproduce.

THE WORLD'S NOT FLAT, IT'S A MIXING BOWL

China, however, is far from the only country where people risk infection from animal-borne diseases. The West is also in danger, as was discovered in late May 2003, when the first cases of a mysterious illness were reported in hospitals in Illinois, Indiana, and Wisconsin. Patients, many of whom had been in close contact with pet prairie dogs, started coming down with skin ulcers and fevers. It was soon discovered that a prairie-dog dealer in Wisconsin had let a number of his animals mix with rodents recently imported from Ghana that happened to be carrying the monkeypox virus. An animal distributor had then sold the infected prairie dogs to pet stores in Milwaukee and at an animal swap meet in northern Wisconsin. Within about a month, 71 human cases of monkeypox in six Midwestern states had been reported to the CDC; luckily, no one died.

It remains unknown how or where waste from the infected prairie dogs was dumped or whether owners released any infected prairie dogs into the wild during the scare. Moreover, U.S. laws remain dangerously lax. At the time of the monkeypox outbreak, it was legal to import any nonendangered African rodent into the United States as a pet -- despite the fact that the risk of bringing in foreign diseases in the process was predictable and could have been avoided through international surveillance and information-sharing programs. (Wildlife health experts and human health workers in central Africa have long associated human monkeypox infections with rodent and squirrel contact.) Since the U.S. outbreak, Washington has imposed restrictions on the import of African rodents, but it remains legal to bring in rodents from other continents, and many other species from around the world continue to be shipped into the United States and many other countries, largely without oversight.