The Sharing of Disease
by Abigail Garnett
The media frenzy which resulted from the recent outbreaks of West Nile virus, Avian flu, and SARS was only one symptom of the growing risk of a global pandemic. Research regarding the causes and spreading of these diseases has forced scientists to acknowledge not only our susceptibility to new diseases, but also our ignorance as to their roots. Research has shown that these three diseases, along with HIV, ebola, yellow fever, and Lyme disease, are just a few examples of what are known in the scientific community as zoonoses. A zoonosis is a disease which results from an aberrational interspecies leap-in other words, a pathogen which previously was found only in animals that begins to target humans.
While most remain unfamiliar with this term, it has been the source of a rapidly increasing number of new diseases in the past few decades-illnesses which, along with viral mainstays like HIV and yellow fever, are proving impossible to eradicate. With each outbreak it has become clearer that even in an age of terrorism, nuclear weaponry, and global warming, one of the top threats to humanity is microscopic, nearly impossible to track, and could be lying dormant anywhere in the world.
Most of the great epidemics of the modern age have been the result of zoonotic pathogens. Some of the most hard-to-destroy zoonoses, such as rabies, continue to kill thousands each year and maintain a rate of nearly 100 percent mortality when left untreated. In contrast, some "spillovers" result in relatively small and violent outbreaks, as in the case of Hendra, which killed several horses and their trainer within a few days of its outbreak near Brisbane, Australia. The variability of the forms that zoonotic diseases can take is not surprising given their common occurrence. It is estimated that around 60 percent of infectious diseases known today are the result of some form of zoonotic pathogen.
Even though zoonoses have been affecting us for many years, the reasons for their occurrence remain unclear. The website for the Special Pathogens Branch of the Center for Disease Control and Prevention lists the cause of Hendra as "direct exposure to tissues and secretions from infected horses." However, the truth is that scientists know very little about the reasons for Hendra's sudden attack, which had never been seen in either horses or humans before 1994. Zoonotic pathogens can lie dormant in a host species for thousands of years showing no symptoms before a "spillover" occurs. Researchers in Australia eventually found traces of the Hendra virus in the blood of a species of fruit bat, but they still are unsure why the virus chose to lash out, or indeed whether it might happen again.
The avian influenza strain classified as H5N1 (we know it as "bird flu") has, despite affecting humans in fourteen different countries and being touted as the world's top pandemic threat, resulted in 216 fatalities-significant, naturally, but not something threatening at the global level, at first glance-since 2003, according to data from the World Health Organization. The vast majority of cases have, in fact, occurred in animals. Why then, has it caused such panic? The severity of the threat results from its unique characteristics as a zoonotic disease. As pathogens lurk within their reservoir hosts without causing illness, it is impossible to tell whether a disease has been eradicated or is just "waiting" for its next opportunity to lash out. Because we know so little about the actual reasons why pathogens cross over, we cannot predict when new strains will emerge, or whether they may have been mutating within their host species into lethal, unprecedented varieties. David Quammen of National Geographic writes, "Experts fear that H5N1 may mutate to a virulent form able to wipe out millions." If it does, the consequences will be vast, but there is almost no way to predict when or where such an outbreak might occur.
Outbreaks of recent years have caused scientists to intensify their search for answers. Experimentation with animal genomes has proved promising. Kurt A. Zuelke of the Agricultural Research Journal reports that at the National Animal Disease Center in Ames, Iowa, researchers have altered the genomes of cattle to keep them from producing certain proteins which are believed to be the cause of bovine spongiform encephalopathies, or mad cow disease. Zuelke writes that NADC's work marks "an increase in collaborative research between veterinary and human medical researchers to combat potential emerging zoonotic diseases."
This approach, which considers the health of animals and humans together, is especially valuable when considering that many diseases are actually anthropozoonotic, traveling between humans and other species in both directions. Clearly, human safety relies on a global health derived from ecology in general, rather than the incomplete subdivisions of human and veterinarian medicine. Zoonosis is merely a colorful example of a basic truth: that health is a global concept, and must be considered as such.
