Their Bugs Are Worse Than Their Bite
Last week, we learned that many children had developed a serious bacterial infection following visits to Florida petting zoos and that at least seven of them went into kidney failure. Meanwhile, half a world away in Angola, more than 100 people have died from the largest outbreak of Marburg hemorrhagic virus, which is also known as green monkey disease because of its source. These dramatic stories are playing out against the backdrop of AIDS (which comes from non-human primates), Lyme disease (from deer), West Nile virus (spread by birds) and SARS (apparently transmitted by civet cats). Topping them all are the recent worries about bird flu, which is currently spreading from chickens to humans and threatens to become the next pandemic.
Such transmissions aren't new. Experts estimate that at least three-quarters of all infectious diseases originally came from animals, and last year Julie L. Gerberding, director of the Centers for Disease Control and Prevention (CDC), noted that "11 of the last 12 emerging infectious diseases that we're aware of in the world, that have had human health consequences, have probably arisen from animal sources." What is worrying is both that these transmissions appear to be on the increase and that many diseases long believed to be noninfectious (such as multiple sclerosis) may in fact be attributable to microbes contracted from animals.
In the face of such threats, we should be taking steps to protect ourselves, ranging from practicing better basic hygiene to realigning our government agencies. Currently, the CDC does a good job of tracking human-to-human disease transmission, and the Animal and Plant Health Inspection Service, under the Department of Agriculture, does a good job of tracking animal-to-animal disease transmission, but no single agency tracks animal-to-human transmission. Responsibility for tracking West Nile virus, for example, is divided among eight federal agencies. Such lack of coordination is evident also at the international level.
A 1992 report by the Institute of Medicine (IOM) noted that "there is little coordination among these agencies and organizations regarding infectious disease surveillance." In 2002, the IOM again expressed the need for improved collaboration. What's more, despite the CDC's efforts to educate the public about these risks, the Bush administration has proposed major cuts in its budget for the coming year.
What we're seeing is the most recent step in a process that has been going on for centuries, ever since people began living in close proximity with animals. Most of the diseases that shaped human history came originally from animals, including yellow fever, plague, tuberculosis, measles, typhoid fever, influenza, smallpox and leprosy. Paradoxically, at a time when fewer and fewer people engage in agricultural work, more and more people are coming into contact with animal diseases, either through house pets, petting zoos, takeout food or the congested nature of modern life, which encourages diseases to spread quickly.
More than 12,000 years ago, though, when humans lived in small groups as hunters and gatherers, they were afflicted with relatively few infectious diseases, or heirloom diseases. These infections, which were passed down the hereditary chain, continue to cause such contemporary illnesses as herpes, hepatitis A and B, and malaria. Early humans acquired a few other infectious diseases, such as anthrax from wild sheep and tularemia from rabbits, as they butchered and ate their kill.
When the relationship between humans and animals changed, so too did human exposure to microbes carried by animals. Animals that early man had watched from afar and occasionally hunted now grazed peacefully nearby. Humans began to share their homes with sheep, goats, chickens and cows -- their most important possessions -- and still do today in many parts of the world.
The spread of microbes from animals to humans was then inevitable. From goats, we acquired the bacteria that cause tuberculosis, which has been identified in human remains more than 7,000 years old and in ancient Egyptian mummies. In 1680, John Bunyan called tuberculosis "the captain of all these men of death," and according to one estimate, in the last century it killed approximately 100 million people.
Measles came to us from cows, a slight modification of the bovine rinderpest virus. Although now regarded as a relatively benign disease, measles devastated the Native Americans, who had never been exposed to it, and, along with smallpox, was a principle reason the invading Spaniards prevailed. It has been estimated that between 1840 and 1990, measles killed about 200 million people worldwide.
Both diseases spread rampantly as people moved into villages, then larger cities. And as humans began traveling between urban areas for trade or warfare, they carried microbes with them. Rats often came along, too, carrying fleas infected with the bacteria responsible for plague. The plague bacteria had existed for thousands of years among marmots and other rodents in central Asia, causing few problems. But when rats started hitchhiking on passing caravans, eventually reaching the Mediterranean region in the 6th century A.D., they boarded ships and initiated the Plague of Justinian, which killed an estimated 100 million people. When the plague returned in the 14th century as the Black Death, it wiped out approximately one-third of Europe's population.
Other human diseases suspected of being the consequence of the domestication of animals include whooping cough (pertussis) from pigs, glanders from horses, typhoid fever from chickens and influenza from ducks. Although the flu virus has existed in water birds for millions of years, when it infects pigs or other mammals it is modified. The modified viruses, if spread to humans, can cause deadly pandemics such as the 1918 influenza outbreak, which killed more than 20 million people worldwide. The current farming system in Southeast Asia, in which ducks, pigs, chickens and humans live beak by jowl, is ideal for fostering such deadly viral strains. This is what is most worrying about bird flu.
Instead of keeping a safe distance from disease-carrying beasts, Americans have literally been inviting them to move in. Americans own about 55 million household dogs, 64 million cats, 31 million caged birds and 7 million reptiles -- and pet food sales suggest that these numbers are increasing. The relationship between humans and their pets has become increasingly intimate, including exchanging kisses, sleeping in the same bed and taking pets along on family trips. Since 1990, the CDC has reported at least 25 separate outbreaks of E. coli 0157 with associated diarrhea and kidney failure in children who probably acquired the bacteria learning how to shear a sheep or milk a cow or simply stroking a goat's back at a petting zoo, just as seems to have happened in Florida, where one of the affected children has since died.
Exotic pets have also become increasingly easy to obtain via the Internet. Anyone with a credit card can order an African pouch rat, bush baby or kinkajou. A buffalo will set you back $2,000, a reindeer $2,700 and a kangaroo $6,000. Cases of sometimes-fatal salmonella disease among infants, transmitted from pet iguanas, are regularly reported. A 2003 outbreak of monkeypox, carried by pet prairie dogs, is another example.
Technological innovations in food production also contribute to the human spread of animal microbes. One example has been the transmission of prions -- the protein-based infectious particles that cause mad cow disease, or bovine spongiform encephalopathy -- following changes in the commercial process of slaughtering cattle and preparing bone meal to feed to other cattle.
Once a microbe spreads from animals to humans, many factors in modern life contribute to its further dissemination. Air travel, for example, caused the rapid spread of SARS in 2003 to 30 countries on five continents. The reuse of disposable plastic syringes without sterilization is common practice in many developing countries and has contributed to the spread of hepatitis, AIDS and Ebola and Lassa fevers.
Add to this ecological and population changes. Global warming and mega-cities of 20 million or more people are both likely to bring about new patterns of disease transmission. In the United States, a cycle of drought and heavy rain in southwestern states led to an outbreak of hantavirus, spread from mice to humans. And the reforestation of northeastern states combined with suburban living has brought a swelling deer population into juxtaposition with humans, resulting in an epidemic of Lyme disease.
Finally, there are changes in human behavior that contribute to the spread of animal microbes. Take our tendency to order in or eat out rather than cooking for ourselves. Salmonella-infected eggs or chicken products can thus affect dozens of households simultaneously.
Still, the major source of concern about animal-associated microbes is not what we know, but what we don't know. There are estimated to be approximately 5,000 different species of viruses and up to 1 million species of bacteria, the vast majority of which have not been identified. They presumably infect all of the known 4,500 species of mammals, of which humans are only one. Further, we know virtually nothing about the microbes that infect birds, reptiles, amphibians, fish or simpler forms of life.
Many surprises await us. There has been speculation for many years that multiple sclerosis may be caused by a virus transmitted from dogs. But what is the relationship between Chlamydia pneumoniae , a bacterium that may have been transmitted to humans from horses, and atherosclerotic heart disease? Our own research is linking schizophrenia to Toxoplasma gondii , a protozoan carried by cats. And what about some cancers? Liver cancer has been linked to hepatitis viruses, an heirloom infection. Stomach cancer has been liked to Helicobacter pylori , which may have been transmitted to humans from sheep.
The most important thing we can do is to educate ourselves and our families about animal pathogens. Pets should not sleep in our beds or sit on the kitchen counter where our food is prepared. Because, yes, Donald Duck really does carry influenza; Mickey Mouse, hantavirus; Pluto, leishmaniasis; Garfield, T. gondii ; Rocky Raccoon, rabies; Bambi, Lyme disease; and Big Bird, West Nile virus. Even beloved Barney, like all reptiles for millions of years, carries salmonella.
The continuing transmission of infectious agents from animals to humans is inevitable. It is not a question of if there will be additional human epidemics, merely when. Only by modifying human behavior and using government resources intelligently can we hope to reduce the incidence and severity of such epidemics.