By Dr. Karen Shaw Becker
It's fairly common knowledge that animals can make humans sick. From salmonella transmitted by turtles to roundworms from your new puppy or kitten, such zoonotic diseases are a fact of life. Lesser known, however, is the fact that people can make animals sick, too. It's becoming a more important issue as the world becomes increasingly connected.
"A pathogen that emerges today in one country can easily be transported unnoticed in people, animals, plants, or food products to distant parts of the world in less than 24 hours," researchers wrote in a comprehensive review published in PLOS One.1
Understanding emerging diseases is therefore crucial to public health, but this is becoming increasingly complex due to increases in concentrated animal feeding operations (CAFOs), interactions between humans and wild animals and rapid movement of humans and animals around the globe.
It's relatively common to hear about zoonotic diseases transmitted from animals to people, such as bird flu and swine flu, but so-called "reverse zoonosis," in which humans transmit disease to animals, is not uncommon. Researchers screened 56 articles from 56 countries, spanning a period of three decades, to document such human-to-animal disease transmissions.
What Types of Diseases Can You Transmit to an Animal?
It's not a stretch to believe that a disease that makes you sick could make your pet sick too. In fact, 90 percent of the pathogens known to affect dogs and cats are "multiple species pathogens."2 The featured systematic review revealed reports of numerous types of human-to animal disease transmissions, including bacterial pathogens, viral pathogens, human parasites, fungi and more.
"Bacterial pathogen reports were centered in North America and Europe. Viral studies were well-distributed globally. Parasitic disease reports were conducted chiefly in Africa. Fungal studies were conducted almost exclusively in India," the researchers noted. As for which types of animals were affected with "human diseases," the reports included wildlife, livestock and pets, with a variety of conditions. For example:3
Giardia duodenalis (parasite): This parasite, which is common in domestic dogs and cats and humans, was found in the feces of endangered African painted dogs.4 The strain detected was a subtype associated with humans, and researchers believe the animals may have been exposed due to tourists and local residents defecating in and around national parks.
Influenza: Widespread human-to-animal transmission of H1N1 flu was detected in South Korea in 2009,5 with the PLOS One researchers noting, "[T]he novel virus was able to travel across the globe and from humans to swine in less than two months."6 The first case of a human transmitting H1N1 to her cat also occurred in 2009,7 with additional human-to-animal cases reported in cats and dogs since.
Respiratory illness: In 2009, seven captive chimpanzees in Chicago, Illinois got sick from human metapneumovirus (HMPV), a respiratory infection. One died as a result. According to researchers reporting on the outbreak:
Tuberculosis: In 2004, a Yorkshire terrier died after contracting tuberculosis from his owner,8 and there are also reports of captive elephants contracting tuberculosis from their handlers.9
MRSA: Methicillin-resistant Staphylococcus aureus (MRSA) is a superbug that can lead to minor skin infections to life-threatening infections. In a study of the homes of 49 children with MRSA infections, 11 family pets were also colonized with the bacteria.10 What's more, since many dogs and cats carrying MRSA show no symptoms, it's possible for the bacteria to "hide" on pets and re-infect human family members.
E. coli: People carry more drug-resistant strains of E. coli bacteria than their dogs, according to a study conducted in 2009,11 indicating owners are more apt to transmit multiple-drug resistant E. coli to their dogs than vice versa.
Toward a 'One Health' Approach to Health and Disease
There's increasing recognition that a "One Health" approach to health is necessary. It recognizes that people, animals and the environment are interconnected, with the health of each affecting the health of the others. But just as there is awareness that people can get sick from animals, hence the recommendations to wash your hands after playing with your pet, there needs to be increased awareness that people pose risks to animals, too.
"Public education and awareness should be augmented to include the potential health threats inflicted on a susceptible animal by an unhealthy human," researchers wrote in PLOS One, referring not only to animals in zoos and aquariums, which are exposed to more than 700 million visitors a year, collectively, but also to those in national parks and other wild settings, animals in the exotic pet trade and even domestic pets. They concluded:12
"As pet ownership seems to be increasing worldwide and more exotic pets are being introduced to private homes, the potential for disease transmission between humans and animals will continue to increase. Veterinarians must more fervently protect animals under their care from human disease threats.
Adopting a One Health strategy for emerging disease surveillance and reporting will benefit both humans and animals and produce a more collaborative response plan."