In a Special to the Washington Post, E. Fuller Torrey and Robert H. Yolken point out that human infection via bacterial and viral transmission from animals have been around for a long time and does not discriminate. In Angola, about 150 people have died from the largest outbreak of Marburg hemorrhagic virus, also known as green-monkey disease because of its source. There are also international cases of AIDS (which comes from nonhuman primates), Lyme disease (from deer), West Nile virus (spread by birds) and SARS (apparently transmitted by civet cats), and bird flu. In the US, Florida recently had an outbreak of E.coli O157:H7 due to animals in petting zoos.
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, 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.”
More than 12,000 years ago, when humans lived in small groups as hunters and gatherers, they were afflicted with relatively few infectious diseases. 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.
Most diseases come originally from animals, including yellow fever, plague, tuberculosis, measles, typhoid fever, influenza, smallpox and leprosy. 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 allows diseases to spread quickly.
Other human diseases suspected of being the consequence animals include whooping cough 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, is ideal for fostering such deadly viral strains.
There are estimated to be about 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.
The most important thing we can do is to educate ourselves about animal pathogens. The continuing transmission of infectious agents from animals to humans, as well as the possibilities of epidemics, are inevitable. Only by educating ourselves, modifying our behavior and using available resources intelligently can we hope to reduce the incidence and severity of such epidemics.