Zoonoses are diseases which are transferrable between humans and other kinds of animals. Though there are many zoonoses, some of which are fairly common amongst humans, the actual occurrence of inter-species transference to people from pets is quite rare. When one narrows the question a little further, and asks the likelihood of catching a deadly disease from a dog, it is discovered that one is more likely to be struck by lightning.
This essay is not meant to suggest there is no danger at all. In undeveloped nations, or in any human/canine population which does not take advantage of modern hygiene and vaccinations, there is a risk of disease transference between dogs and people. Even in developed nations, fungal infections such as ringworm can be passed back and forth between humans and dogs, but it is much more common for humans to catch it from other humans than from dogs. There are many diseases which humans can catch from dogs and among them are giardia, salmonella, bordatella, roundworm, and listeriosis.
Dogs are so closely related to wolves that it is very difficult to tell them apart genetically. Chihuahua DNA looks more like timber wolf DNA than human DNA looks like chimpanzee DNA. While some of the dog’s biological functions have been changed by human interference (particularly their reproductive biology; dogs are breeding machines, compared to wolves), dogs still possess much of the same gastro-intestinal fortitude that wolves do. The canine gastro-intestinal system is physiologically similar to the human system, but chemically much more extreme. We both have teeth, an esophagus, intestines, and only one stomach. We have the ability to vomit but don’t always do so, and our digestion process begins with saliva. In both canine and human milk, tears, and saliva, there is an enzyme called lysozome. Lysozome is a powerful enzyme which naturally attacks bacteria and kills it by separating its water molecules nearly instantly upon contact. Lysozome is a natural and effective way to combat salmonella, e. coli, giardia and other diseases; it is one of the chemicals in a mother’s milk which inherently protect newborn infants and puppies. Dog saliva contains about 12 times more Lysozome than human saliva does. If both a dog and a human eat the same things, a dog’s mouth actually is much cleaner than a human’s.
Both humans and dogs digest food in the stomach through an immersion in gastric fluid, which in both species is a combination of water, hydrochloric acid, potassium chloride, and sodium chloride. The concentration of these acids in dogs, however, is about four times as strong as it is in humans, and because wild canines are gorgers (often eating once every few days), most breeds of dog have stomachs twice the relative size of people. This means twice the amount of gastric fluid, which we already established is four times as strong. Wolves and dogs both have the ability to eat all kinds of horrible things and not only survive, but thrive. Their digestive systems are so alkaline and caustic that they can live on garbage and scraps. An interesting experiment with a dog is to give them a marrow bone approximately four times longer than their tongue. An experienced dog will eat the marrow out of each end, then mouth one end at a time, letting its saliva gather on top of the marrow. The saliva is so alkaline it melts the marrow, letting the dog drink it from the end of the bone. In a dog’s mouth, most bacteria and disease never have a chance.
The environment of the dog’s mouth is only as clean is what is currently in it, of course; it is easy to understand why some folks are squeamish about kissing dogs, or letting them lick us. Dogs eat things that we see as disgusting and even dangerous, and many of them are. Once the offending morsel has cleared the dog’s esophagus, however, it only takes a few minutes for the dog’s mouth to be clean again.
Statistically, hard data is difficult to find. The few cases of zoonotic transmissions from pet dogs to humans that are well documented occurred when people were careless with their own hygiene, and ingested dog feces off their own hands after carrying their pets. A chart is included here that shows frequency of zoonotic infections for 2007 in the European Union. It does not break out the frequency by species, but with the exception of three rabies cases, these are all diseases contracted by eating infected meat or processed vegetables. During that same year in the United States, 278 people were struck by lightning, 47 of who died. For those folks who practice normal daily hygiene rituals, and whose dogs are on a standard prescribed vaccination schedule, we are literally more likely to be struck by lightning than we are to catch a deadly disease from kissing a dog, or letting a dog lick us.