Would it surprise you to know that most infectious diseases have some sort of association with non-human animals? Measles, for example, is thought to have diverged from rinderpest sometime between the year 1000 and 1100 of the common era. This is because the two viruses are the same genetically, so scientific techniques allow us to see how far back in time we would have to go to see them diverge. As to why they diverged, the theory is the same as it is from other viruses who share a common ancestor in a virus that causes disease in animals. Namely, a person caught the virus from an animal and the virus adapted to humans and started spreading among the human population.
There are other zoonotic diseases that don’t spread easily from person to person but are still a burden on public health. Rabies is transmitted from animal bites to humans, but there are no documented cases of rabies being passed from one person to another directly. The only thing close to a person-to-person infection is an indirect transmission from exposure to wounds or from organ transplants.
The zoonoses that usually get our attention are the rare ones that seem exotic and sound more like the plots of science fiction — or horror — stories. For example, Ebola is believed to be a zoonotic infection occurring more and more often on the African Continent because people are building cities into the jungles. This has exposed more people to animals like bats that may be the natural hosts of the disease. The result has been large epidemics with very high numbers of cases and deaths.
One zoonotic virus that has caused a pandemic, other than influenza, is the Human Immunodeficiency Virus (HIV). The virus shares many of the genetic characteristics as Simian Immunodeficiency Virus (SIV), a virus that causes a similar syndrome in non-human primates. The theory is that someone came into contact with a non-human primate and contracted a mutated version of SIV — or a virus that is ascendant to both HIV and SIV — and developed HIV. That new virus then adapted to humans and has caused death and disease the world over. Since the HIV pandemic began, it is estimated that over 32 million people have died and millions more have been infected.
Then there are the zoonotic infections that we don’t think about as much because they don’t cause large epidemics or even pandemics. These are also easily preventable because they just require common-sense actions like washing our hands or preparing and cooking food properly to avoid them. In this video, Kerry Rood, DVM, a veterinarian from Utah State University gives a talk about zoonotic infections we see every day from handling animals at state fairs or at home, or from eating foods contaminated with viruses and bacteria from animals we may come into contact with:
Going back to the days of Dr. Edward Jenner and his smallpox vaccine, we have learned about how understanding zoonotic infections can help us understand human immunity and how we can harness that understanding into action. When Dr. Jenner saw that milkmaids caught cowpox from milking cows, and that the cowpox sores were very similar to those caused by the much deadlier smallpox, he correctly theorized that being protected against cowpox meant that the immune system would also be activated against smallpox.
Later on, Louis Pasteur applied similar principles of exposing people to a milder version of a virus to protect them against a stronger version of the virus. Pasteur and others also experimented on bacteria. Little by little, over the course of just over 200 years, these series of experiments on zoonotic infections has lead to the current schedule of vaccination that protects most of us from some very serious infections. It is this same science that is leading the current search for a vaccine against the coronavirus that has recently emerged in China.
As the presentation by Dr. Kerry shows, we need to be aware of many infections if we are in close contact with animals of any kind. Our house pets need to be immunized so that a random encounter with a rabid raccoon or fox doesn’t mean their death nor puts us in danger. We need to drink pasteurized milk to make sure that the bacteria that are found in cows don’t make us extremely sick. And we need to publicly fund and support surveillance systems that keep an eye on the patterns of infection and can serve as early warning systems for the next zoonoses coming over the horizon.
When we go to the farm shows and petting zoos, we really need to wash our hands with soap and water. As Dr. Kerry explains, hand sanitizer is not enough if there is “organic matter,” like feces or dirt, on our hands. Even when handling common household pets, it is necessary to wash our hands and avoid having them around food preparation areas. Just last year, pet turtles caused a disseminated Salmonella epidemic in several US states.
The coronavirus in China likely originated at a market where animals and animal products are in close proximity with people. In 2003, it was close proximity with civets (cats) that was the main theory about where SARS jumped from animals to humans. For the 2012 MERS virus, the evidence is there that it may have been milk from camels. While we are still in early phases of the 2019-2020 coronavirus epidemic, the clues are there that it is also an animal-to-human epidemic, a zoonotic.
For more information about zoonotic diseases and how to prevent them, take a quick look at this page from the Centers for Disease Control and Prevention (CDC). And don’t forget to wash your hands early and often.