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.
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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.
Author's e-mail:
torreyf@stanleyresearch.org
E. Fuller Torrey, an associate director of the Stanley Medical Research Institute in Bethesda, and Robert H. Yolken, a neurovirologist at Johns Hopkins University School of Medicine in Baltimore, are co-authors of "Beasts of the Earth: Animals, Humans, and Disease" (Rutgers University Press).
