Zoonotic diseases, also called zoonoses, are illnesses caused by germs that are passed between animals and people. Simply put, a zoonotic disease is one that originates in animals and can cause disease in humans. Zoonotic diseases are prevalent throughout the world; they can be caused by viruses, bacteria, parasites, or fungi, and may cause mild or severe illness or death. Experts estimate that about 60% of known infectious diseases in people can be spread by animals, and 3 out of every 4 new diseases in people originated in animals, according to the CDC.
HOW ARE ZOONOTIC DISEASES TRANSMITTED?
Direct contact with animals is the easiest way for diseases to spread from animals to humans, such as through petting, handling, or getting bitten or scratched by an animal. People who work in the livestock industry or in animal care industries (zoos or aquariums, for instance) are more susceptible to exposure to zoonotic diseases because they are often in direct contact with animals. Domestic pets can also be a direct source of exposure, as can wild animals that come into contact with hunters.
Spending time in areas where animals live can lead to indirect exposure to zoonotic disease agents through contact with water or surfaces that infected animals have also come in contact with. Some zoonotic germs can even contaminate the air we breathe. Hantaviruses, for instance, are a family of viruses spread by rodents, but rarely through direct contact. Instead, the viruses are more often spread in aerosolized bits of the rodent's infected fecal matter.
Zoonotic diseases can also be transferred from animals to humans through insects that act as a "middle-man," or vectors for the disease-causing agent. Ticks, for example, transfer bloodborne pathogens, such as the bacteria that causes Lyme disease, from an infected animal to other animals and humans. Mosquitoes and fleas are also common vectors for zoonotic diseases, such as the Zika virus (transmitted by mosquitoes) and the bacterium that causes plague (transmitted by fleas).
People can also catch zoonotic diseases by consuming contaminated food. Eating undercooked meat, eggs or eating unwashed produce contaminated with animal feces can lead to illness from germs carried by an animal. Drinking raw, unpasteurized milk or contaminated water can also cause zoonotic diseases to spread to humans.
EXAMPLES OF ZOONOTIC DISEASES
The World Health Organization works with government and non-government groups around the world to identify and manage the global threat of zoonotic diseases. The eight most prevalent and concerning zoonotic diseases in the United States are:
West Nile virus
Zoonotic influenza is a flu caused by viruses that originate in animals or type A influenza virus. Out of the four types of influenza viruses, type A viruses cause the most severe disease, and are found in ducks, chickens, pigs, whales, horses, seals, and cats, according to the CDC. Only type A flu viruses are known to cause flu pandemics, or global epidemics of the flu. Both type A and type B influenza viruses can cause seasonal flu epidemics, but type B viruses circulate only in humans. Type C flu viruses rarely cause severe illness, while type D infects cattle and is not known to infect humans.
WHY ARE ZOONOTIC DISEASES A GROWING CONCERN?
Zoonotic diseases are a major human health concern for two reasons: Incidences of zoonotic diseases are increasing in frequency, and it is difficult to predict where they are going to show up.
One of the simplest reasons could be that people are invading animal habitats more often, which facilitates more interactions between humans and animals. For example, by deforestation for logging or encroaching on animal habitats, you set up scenarios where you contact wildlife on a more frequent basis.
Certain cultural practices may also be contributing to more zoonotic diseases. For example, many people in the U.S. practice hunting, and in a lot of places in the world, that is the main source of protein. Putting pressure on the environment through hunting and development throws the ecosystem off balance and makes it harder for animals to survive as they were. The animals are forced to travel farther and search harder for food or mates, and in this chronically stressed condition, those animals are more susceptible to disease and more likely to spread the disease to humans. Such hunting practices also provide more opportunities for people to become exposed to zoonotic diseases.
Another potential reason zoonotic diseases are on the rise is that people are more connected to one another now than ever before, providing more opportunities for zoonotic diseases to spread further from their origin. Connectedness is a huge deal because even places that are relatively remote are more connected now than they ever have been in the past.
ZOONOTIC DISEASE PREVENTION
Given the knowledge and evidence that animals can carry these viruses and have coevolved with them over periods of time, and the viruses can spill over to humans, can be very frightening. It can have an outsized impact, albeit psychologically, on our fear of the natural environment and our attitude toward wildlife. Nonetheless, the relative risk of a new zoonotic disease appearing is actually quite low. Moreover, there are clear steps individuals and scientists can take to minimize the risk of exposure and impact of zoonotic diseases.
At the individual level, good hygiene is the best place to start. The CDC recommends always wash your hands with soap and clean water after spending time around animals or in areas where animals live, even if you have not touched the animals. When it comes to your pets, vaccinate your dogs and cats, clean up after them thoroughly, and avoid snuggling pet reptiles or birds as these animals are more likely to spread germs.
In terms of scientific research on zoonotic disease prevention, scientists need to work on figuring out ways for humans to sustainably coexist with our wild neighbors. This means learning more about the animals in the environment — things like what animals live there, where are they going, what are they eating and what are they doing.
If you do not know what is in your backyard, then you do not know what they carry that can be harmful to you. A more thorough understanding of the ecosystem can help scientists produce better ways to predict and prevent zoonotic diseases from popping up. An investment in basic research will be invaluable.