How is it that something called swine flu actually contains genes from swine, avian and human flues? And how are humans able to contract it? Or how do we explain reported infections in turkeys, a cat, and even the death of a ferret from H1N1 passed on by a human?
All these questions are part of the study of zoonotic diseases, which are infectious diseases that can be transmitted between animals and humans. A zoonosis or zoonose can be caused by parasites, fungi, bacteria and viruses and transmitted by many animals (most of which are still being identified). Understanding why these diseases are able to survive in different hosts and how they are transmitted is important since some of the most devastating (and more mundane) agents of disease, including anthrax, cholera, ebola, e.coli, malaria, bubonic plague, rabies, and west nile virus are known zoonoses.
The prevalence of zoonotic diseases has changed along with human history. According to one theory, 10,000 years ago when humans were hunter-gatherers there were less zoonotic diseases. This was because the groups moved frequently and so large sewer and garbage dumps which could attract animals like rats and support diseases did not form. Animals, except for dogs, were also not domesticated and so there was less contact in that way. Furthermore, the small size of the groups lent itself better to parasites and chronic diseases than epidemics that could quickly spread. It was the transition to an agrarian lifestyle that allowed zoonotic diseases greater opportunity to jump species to the increased crowding and interaction with more domesticated animals. As time went on and there was trade over longer distances and with more groups, diseases spread, evolved and found new hosts. Even as more vaccines, treatments and preventative measures have developed, diseases have evolved and the continued interaction of people with animals and other humans has aided this adaptation by providing new combinations. For example, the H1N1 virus is believed to be a mix of flus, rather than the evolution of a single virus. As a Washington Post reporter explains “Scientists suspect that two influenza viruses common in swine, one rooted in Eurasia and the other in North America, came together in a single cell within a pig. The two viruses exchanged their genes like a couple of kids swapping school clothes. The result was a novel strain of virus, with, according to scientists, two genes from the Eurasian virus and six genes from the North American virus.” The virus then jumped to humans, who are biologically similar to pigs, and off to other humans and in some cases, back to pigs. As long as the virus, parasite, etc. has what it needs to reproduce, it can find a home in multiple hosts. According to Science Daily, currently sixty-one percent of pathogens are zoonotic.
Management of these diseases is often part a new field, conservation medicine, which studies the relationship between the environment and human and animal health. While basic steps such as hand washing, limiting contact with carrier animals, etc. can prevent the spread of zoonotic diseases that we are already aware of, what may be more important is monitoring their development and when they first spread from species to species. For example, bird flu (H5N1) is being watched for epidemic potential if it mutates in a way that allows easier infection of humans. There are also programs like the PREDICT initiative, which is funded by The United States Agency for International Development (USAID). The goal of PREDICT is to establish a global early warning system for disease emergence that is capable of detecting, tracking and predicting the emergence of new infectious diseases in high-risk wildlife (like bats, rodents, and non-human primates) that could pose a major threat to human health.
Zoonotic diseases are not special super-viruses or unique parasites, but actually account for the majority of human diseases. While they usually don’t come from pets, with travel and international interactions, even those who don’t catch diseases from an animal risk catching it from other humans. Monitoring the genetic and host changes of these diseases is the best way to eventually stop them, but since zoonoses account for so many diseases, there is no one solution or way to stop all of them. Accomplishing that would imply that we have found a way to prevent all disease, and that is something that doesn’t seem likely of ever happening.