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.

Determining the exact scale of the global wildlife trade is impossible, since the operations range from the extremely local to the international, and are often illegal and informal. Part of the picture, however, can be glimpsed from figures compiled by the Wildlife Conservation Society from a variety of sources. According to these numbers, the annual global trade in live wild animals includes roughly 4 million birds, 640,000 reptiles, and 40,000 primates. Following the SARS outbreak that began in 2002, the Chinese government reportedly confiscated 838,500 wild animals from the markets of Guangdong. But every year, tens of millions of wild mammals, birds, and reptiles continue to flow through these and other trading centers, where they make contact with humans and dozens of other species before being shipped elsewhere, sold locally, or sometimes freed back into the wild -- often carrying new and dangerous pathogens. The number of these animals that end up as food is staggering; indeed, experts estimate that in central Africa alone consumers eat 579 million individual wild animals a year, for a total of more than a billion kilograms of meat. Meanwhile, people in the Amazon basin are thought to consume between 67 and 164 million kilograms of wild animal meat a year, accounting for between 6.4 million and 15.8 million individual mammals alone.

Before these animals (with whatever diseases they may be carrying) are eaten, they encounter -- and possibly transmit pathogens to -- hunters and marketers. They also risk infecting domestic animals and wild scavengers in villages and market areas that consume the remnants and waste of wildlife eaten by humans. All considered, at least a billion direct and indirect contacts among wildlife, humans, and domestic animals result from the handling of wildlife and the wildlife trade annually.

Such contact does not just endanger humans and their pets; the pathogens inadvertently transported around the globe can also devastate local wildlife, disrupting the environment and causing enormous economic harm. In October 2004, avian flu (specifically, the H5N1 type A influenza virus) was detected in two mountain hawk-eagles that were smuggled from Thailand into Belgium in airline carry-on baggage. Last year, another deadly virus entered Italy via a shipment of Pakistani parrots, lovebirds, and finches. Chytridiomycosis, a fungal disease responsible for the extinction of 30 percent of the world's amphibian species, has been spread by the international trade and subsequent release of African clawed frogs (a popular laboratory animal). Tuberculosis originating from domestic cattle has now infected herds of wild bison in Canada, deer in Michigan, and cape buffalo and lions in South Africa. In 1999, rinderpest, a disease originally introduced to Africa by the importation of domestic cattle from India, killed more wild buffalo in Kenya than had been slain by poachers during the previous two decades.

The increasing movement of animals and humans around the world and their greater exposure to the many diseases that dance between them have also placed domesticated livestock at increasing risk. This is especially so since the ravenous international demand for animal meat has turned livestock production into an ultraintensive industry, with swine, poultry, and cattle operations now packing huge numbers of animals into limited spaces. Moreover, projections by the International Food Policy Research Institute indicate a doubling of animal production in developing countries over the next 20 years. Although modern factory-farm practices maximize food production, they also make livestock more susceptible to illness. Infection spreads quickly through crowded animal pens, and growing antibiotic resistance makes fighting disease more difficult. Many farms now routinely mix antibiotics with animal feed to avoid transmitting illnesses, and selective breeding for specific traits often predisposes animals to conditions requiring repeated antibiotic treatment. Such increased antibiotic use is helping to create dangerous drug-resistant superbugs that may endanger both animals and humans.

High-volume food production has also prompted the livestock industry to adopt other dangerous practices, which have already led to at least one high-profile disaster: the outbreak of bovine spongiform encephalopathy (BSE), or mad cow disease, in the United Kingdom. Mad cow disease is a chronic, degenerative disorder that affects the central nervous system of cattle. The disease, known as scrapie in sheep, had existed for hundreds of years without infecting other species. It only crossed over to cattle when British farmers started feeding infected sheep byproducts to their herds in the 1980s. Once BSE jumped to cows it started spreading rapidly, with 182,745 documented cases occurring between 1986 and 2002 in the United Kingdom. In response to the outbreak, European countries banned all imports of British cattle. But BSE has nonetheless been found in Europe, Canada, and the United States since then. It has also jumped to people, and a new human variant of the illness, known as Creutzfeldt-Jakob disease, is believed to be responsible for 150 deaths since 1995.

Malaysia has also fallen victim to a disease spread by new farming techniques: the Nipah virus, which appeared in the country's pig and human populations in 1998, killing 105 people and forcing the Malaysian government to cull more than one million pigs to stop the spread. Five species of fruit bats were later also found to carry the virus, suggesting a wide prevalence of the pathogen among healthy bats. It seems that people had acquired the virus from handling infected pigs, which had contracted the disease from bats feeding in fruit trees standing in newly developed pig farms.

The Nipah outbreak highlights what can happen when people and domestic animals modify previously undisturbed wild habitats. Within natural ecosystems, microbes and wildlife tend to exist more or less in balance. But the introduction of new species -- such as cows, pigs, dogs, or humans -- can allow pathogens to jump into these new hosts, which may have no natural immunity or evolved resistance. The results, predictably, can be devastating.

In addition to the direct health damage they have caused people and animals, animal-related pathogens have destabilized international trade and caused hundreds of billions of dollars of economic damage globally. The report of the U.S. National Intelligence Council's 2020 Project, Mapping the Global Future, has identified a global pandemic as the single most important threat to the global economy. In early 2003, the UN Food and Agriculture Organization reported that more than one-third of the global meat trade was being embargoed as a result of mad cow disease, avian influenza, and other livestock illnesses. According to Bio Economic Research Associates, the rash of emerging or reemerging livestock diseases that have cropped up around the world since the mid-1990s (illnesses that include mad cow disease, foot-and-mouth disease, avian influenza, swine fever, and others) has caused losses of an estimated $100 billion; SARS alone cost the global economy half that amount. The pain caused by such crises, moreover, has spread far beyond those responsible; wildlife market traders were not the ones who paid for the SARS outbreak, and the African rodent importer in Texas did not reimburse the U.S. and local governments for the millions of dollars spent to contain monkeypox in 2003.

Nor can these dollar figures adequately reflect the often devastating effect outbreaks can have on some of the poorest people on the planet. Since 2003, for example, efforts to control the spread of avian influenza in Asia have required the culling of more than 140 million chickens. In countries such as Thailand and Vietnam, the vast majority of these animals were not owned by large, industrial producers but by small farmers and peasants. Losing their livestock was painful indeed, especially since financial compensation schemes for rural poultry owners are rare to nonexistent in much of Southeast Asia. Not only did this lack of compensation increase the damage done by the disease; it also created a serious disincentive for bird owners to report suspicious illnesses among their flocks.