​Zoonotic Diseases and the Domestication of Animals

Anshu Sinha is a third-year undergraduate at the Pennsylvania State University studying environmental systems engineering. She is interested in pursuing a career in sustainable design and consulting.

Pets have been a source of happiness, companionship, and affection for centuries – if not millennia. Our relationships with domesticated animals have played a significant role in shaping many aspects of our lives, from impacting the composition of our microbiomes to providing reservoirs for the emergence of pathogens. While the subject of pets and the human microbiome is an article in itself (stay tuned), in this piece we will examine the role pets and other domesticated animals have played in emerging infectious diseases.

The desire for pets is consistently on an upward trend, as more than 50 percent of U.S. households have at least one furry (or not so furry) friend. The American Veterinary Medical Association estimates that between 1996 and 2001, the U.S. population of dogs and cats has grown by 16 percent. Right now, the US is home to over 62 million household dogs and 68 million household cats. With the increase in human-animal contact comes the added potential to transmit certain types of infections.

The most common household pets that can pass infections to people include dogs, cats, birds, tortoises, rodents, some types of fish, and horses. More exotic reptiles can also harbor certain infections. For reasons explained below, such a statement should not be a cause for alarm but, rather, a source for information. Pet owners go through a lot of vet visits that help keep their animals safe, and it’s about time we understand why we are doing what we are doing.

Infections that pass from animals to people are known as zoonoses. Such agents can be bacteria, parasites, viruses, fungi or even more unconventional agents (for instance, Mad Cow Disease is called by a prion, which is essentially a disease-causing protein). Some popular zoonotic diseases that you have probably heard of include rabies, Lyme disease, Salmonella and West Nile Virus. Other more obscure diseases that have been in the news lately that are also zoonoses include anthrax and Ebola.  Zoonotic diseases are nothing new. They date back at least to 430-425 B.C.E. when typhus took over Athens and likely played a pivotal role in the Peloponnesian War. After the Plague of Athens, other zoonoses made their way around the globe: Yellow Fever, malaria and rabies are some of the better-known ones.

The current concern about zoonoses is that they are always lurking in nature. And bringing humans closer to nature allows for these so-called “spillovers,” whereby pathogens make a species jump and successfully infect humans. Nature’s repertoire of microbes can have positive (drug discovery) and negative (disease) implications. AIDS first emerged as a zoonosis, emerging from nonhuman primates, and it is now one of the most devastating infectious diseases in the world. New flu virus strains come from animals like birds and pigs; these organisms act as reservoirs that allow multiple strains of flu to intermingle and exchange genetic information. When such viruses serendipitously gain the ability to cross species barriers and spill into human cells, epidemics (and sometimes pandemics) unfold, especially when pathogens evolve to spread from human to human after they have jumped from animals to humans. AIDS and influenza are both examples of zoonotic diseases that eventually adapted to human populations. Now, of course, both types of virus can spread from person to person and no longer need an animal vector.

Zoonotic diseases spread in many ways. Lyme disease, for instance, stems from the bite of an infected tick. Rabies is transmitted via the bite of infected animals like raccoons and dogs. Meanwhile, Ebola spreads through contact with infected blood, tissues, or excretions: the 2014 Ebola epidemic in West Africa has been traced to fruit contaminated with infected bat feces. Salmonella and Escherichia coli (E. coli) bacteria are instead food-borne and waterborne. You have likely heard of several outbreaks around the world of these bacteria. Finally, as mentioned, Mad Cow Disease is a prion disease contracted from ingestion of contaminated raw meat. This devastating illness has an extremely long incubation and ultimately causes neurological degeneration.

On Infective Perspective, we have already discussed the concept of newly-emerging and re-emerging infections. These themes very much apply to zoonoses as well. The emergence of zoonotic diseases is affected by several factors. Changes in the environment can affect the size and distribution of certain animal species harboring certain types of infections previously unseen by humans. A growing human population leads to increased need for food sources, which leads to higher levels of animal to human contact. It goes without saying that increased contact raises chances of exposure to a zoonosis significantly.

This all makes sense with regards to wild and farm animals. But what about household pets? As mentioned, dogs, cats and reptiles have been implicated as carriers for multiple different diseases. Fortunately, our immune systems are equipped to handle most of them. Furthermore, the majority of household animals in developed countries are extremely unlikely to be harboring the majority of these infections. Dogs have been pinpointed as carriers for zoonotic diseases like brucellosis, Q fever, tularaemia, clostridiosis, leptospirosis, multidrug-resistant infections, and bite wound infections. Cats, meanwhile, are carriers of cat scratch disease, some multidrug-resistant infections and mycobacterial infections, Q fever, tularaemia, several mycoplasma infections, chlamydiosis and bite wound infections. Reptiles are notorious carriers of certain Salmonella bacteria – in fact for this reason, turtle domestication is very controlled in some parts of the world. In addition to the flu, birds are carriers of psittacosis and some mycobacterial infections.

As mentioned, most household pets in a developed country have slim to no chance of actually making their owner sick. People living on farms with livestock – pigs, cows, horses, and chickens – have more health hazards to deal with. Avian flu, for instance, is a serious looming threat and the subject of extensive research and preventive measures. Veterinarians furthermore work hard to keep all kinds of domesticated animals healthy by targeting early stages of zoonotic diseases. Fecal tests, blood tests and vaccinations are such measures that help pets and owners dodge serious viral or bacterial infections. Toxoplasmosis, for instance, is caused by a parasite that lives almost exclusively in cats. Many people are infected with the parasite (in France, it is estimated that a significant portion of the population harbors Toxoplasma gondii), but our immune systems keep it from making us sick. Nonetheless, Toxoplasmosis can be very dangerous for individuals with compromised or underdeveloped immune systems, such as newborns. For this reason, pregnant women are often told to stay away from cat feces to avoid the possibility of passing a parasite on to their child. Fecal tests for cats tell a vet whether a cat is infected with the parasite. If so, there are ways to interfere and clear the parasite.

Veterinarians take precautionary measures themselves by implementing many safety standards including mandating the use of gloves, protective outerwear and other personal protective equipment, sanitation and disinfection, and providing isolation for animals with known zoonoses. In the case of a bird flu outbreak, thousands of birds within a given radius of an index case are slaughtered to prevent spread of the highly virulent and potentially deadly virus. Pet owners are always advised to implement their own safety measures including regular veterinary visits, screenings, and other zoonotic pathogen prevention methods that vary from species to species.
Some vaccines for certain zoonoses are readily available and have therefore helped nearly eliminate many diseases as serious public health problems. Rabies is no longer a public health threat in the majority of the world. Education and awareness on zoonotic diseases is also a key element of prevention. Bats, for instance, harbor a plethora of potentially dangerous mammalian viruses. Knowing this will encourage individuals to avoid coming into contact with or domesticating wild bats. Being bitten by any sort of wild animal should prompt a visit to an urgent care center. Vaccinating pets against rabies should be of second nature to most pet owners. A lot of it is common sense, and the rest of it is covered by public health implementations and educational programs.
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In our interconnected world, the marriage of humans, animals, and the environment is an increasingly important and influential theme in public health. Stay tuned for our upcoming feature on the One Health Initiative, a movement that seeks to promote global collaborations between healthcare providers and scientific researchers in health and environmentally-related disciplines. One Health is a “worldwide strategy for expanding interdisciplinary collaborations and communications in all aspects of health care for humans, animals, and the environment.” This synergism is a unique (and arguably essential) feature of 21st century healthcare that will work to save millions of lives as the link between humans and nature becomes increasingly inextricable. 

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