The origin of animal husbandry by humans dates back to the Stone Age  and is considered a key step in human cultural evolution. Wild animals were domesticated for use as a reliable food source, as supporters during hunt or as working animals. However, living in close contact with animals, pets or otherwise, such as dogs, cats, horses, cows, goats, sheep, birds and so on, also meant living in close contact with their affiliated microorganisms, including those with pathogenic potential. Unfortunately, some of these microbes consider humans as attractive hosts, too.
An infectious disease that can be transmitted from animals to humans, and vice versa, is known as a zoonosis. Plague, malaria, rabies, bird and swine flu, and Ebola fever are prominent examples. In fact, many infectious diseases that are nowadays transmitted between humans have an animal origin. Measles probably originated from a rinderpest (“cattle plague”) virus that mutated in the 11th or 12th century . The HI virus (HIV) that causes AIDS is known to stem from a simian immunodeficiency virus that infects apes, which adapted to humans in the early 20th century in central Africa . Mycobacterium tuberculosis, the causative agent of tuberculosis, one of the most widespread human infectious diseases on Earth, probably relocated from animals to the early ancestors of humans more than a million years ago . Finally, the SARS-CoV-2 virus causing COVID-19 is (most likely) also of zoonotic origin, believed to originate from a bat-borne virus .
In view of approximately 200 pathogens causing zoonoses, 2.5 billion cases of human illness and 2.7 million human deaths worldwide, caused by zoonotic pathogens , it appears somewhat ridiculous when some scientists suggest animals, in particular pets, can act as a “novel microbiome-based therapy” . Are these people crazy?
Maybe not. Their suggestion is based on the observation that humans and their accompanying animals not only share pathogenic microorganisms, but also a huge diversity of commensal or maybe even symbiotic microbes that impart positive effects on human health and well-being. This finding appears to be of particular relevance to humans living in industrialized countries and urban environments, who often lack contact with a natural, diverse and healthy environmental microbiota. This lack of contact has in fact been identified as a reason for the increasing prevalence of diseases such as asthma, atopic dermatitis, or rhinitis . Indeed, there is a growing body of evidence that microbial exchange between animals, humans, and their homes might be beneficial for humans, in particular for children.
In a landmark study , asthma prevalence was compared between Amish and Hutterite children. The major difference between these otherwise strikingly similar US religious groups (in terms of genetics and lifestyle) is the environment in which the children develop. While the Amish practice traditional farming (including close contact with farm animals), the Hutterites use industrialized farming practices. The prevalence of asthma and allergic sensitization was 4 to 6 times lower in the Amish population when compared to the Hutterites, whereas median endotoxin levels in Amish house dust was 6.8 times as high. Endotoxin is a special structure in the outer membrane of Gram-negative bacteria that is known to elicit many physiological reactions inside the human body. Injected intranasally in mice, dust from Amish houses inhibited airway hyperreactivity and eosinophilia ─ typical symptoms of allergic reactions. The results indicate that animals shape the microbial community in which children are developing in a positive, health-promoting way.
On average, one third (33%) of households globally have a dog. Cats are the second most popular choice and account for nearly a quarter (23%) of pet ownership . As early as the 2000s , researchers were investigating the effect of the presence or absence of dogs and cats in a household and the development of atopy. Parents of 1-year olds were asked how many pets were in the home from birth to the age of 1 year. The children were then tested throughout childhood until the age of 6−7 years for both pet and non-pet allergies. As the number of pets such as dogs and cats increased, the development of atopy to indoor and outdoor allergens decreased. Notably, exposure to pets at 6 or 7 years old had no effect on allergies.
From a mechanistic point of view, an increased and immune-stimulating microbial diversity due to microbial exchange between pets, their owners and their environment, is seen as one reason for the health benefits conferred by pets . For instance, the exchange between dogs and their owners is so strong that dog owners share as much microbial diversity with each other as that of cohabiting partners, even if the two dog owners do not cohabit . If a mother is sharing a microbial community with an animal throughout her pregnancy, the newborn child will be exposed to a higher diversity of microbes compared with a child born to a mother without pets . Such an increase in diversity is considered protective against the development of asthma and allergy, at least when encountered during childhood.
Coming back to the question in hand. Are pets probiotic? Probiotics are living microorganisms that, when consumed, confer a health benefit to their host. Among others, Lactobacillus bacteria are well known probiotics, included in many dairy products. Fujimura and colleagues  could show that mice exposed to dog-associated house dust were protected against airway allergens and viruses. The exposed mice showed a distinct gut microbiome composition, highly enriched for Lactobacillus johnsonii, which itself also conferred airway protection when taken as an oral supplement as a single species. Such data suggest that some beneficial and immunostimulatory effects of animals might indeed be based on the transfer of probiotic bacteria.
What does this mean in practice? Should you kiss your dog twice a day instead of eating probiotic dairy products? Clearly no! The beneficial effects of pets (transfer of immune-stimulating microbes, mental support and so on) always need to be balanced with the risk of zoonotic infections . All scientists, including those propagating pets as a novel microbiome-based therapy , agree that basic hygiene practices must be followed for a secure and overall health-promoting relationship between humans and pets. These practices include proper hand hygiene after contact with pets and their equipment, safe management and disposal of pet faeces, no licking of the face or open wounds and no pets in the bedroom or kitchen, as well as regular checks and treatments (vaccinations, deworming) by a veterinarian. For a more comprehensive overview, take a look at ref. 14.
As we come to the end of this article, some readers might be disappointed, because the microbiome-mediated immune-strengthening health aspects of pets and animals are probably most effective when experienced during babyhood. So, buying a dog at the age of 50 years might not help you with your hay fever or atopic dermatitis. However, the active lifestyle that comes with being a dog owner might itself have a positive impact on your gut ecosystem. Moderate exercise leads to reduction in inflammation and intestinal permeability and an improvement in body composition, and positive changes in the gut microbiota composition and in microbial metabolite production . So, it´s never too late to buy a dog.
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