Seems like a niche just ripe for bacterial and viral exploitation. Is it perhaps because the density of the dog population isn’t high enough to get a dog to transmit to another to “refine” any viral/bacterial mutations?
This is a total WAG (and maybe shouldn’t even appear in the erudite space of GQ). But what the heck . . .
It may be that most of the bacteria and viruses affecting dogs (and other species) have evolved to (co)-exist in/on that host. So, if they try to make the jump to humans, they find themselves in an ecologically unfriendly zone, fail to thrive, and fade away.
All that being said, there are definitely micro-organisms that can make the leap especially when the human host’s immune system is weak and/or when the amount of infective agents making the jump is large, i.e. a large bolus of infective agents. For example, there are whole classes of zoonoses.
Another WAG, but could it be because dogs have a higher natural body temperature than humans? I think a lot of pathogens have only quite a narrow range of temperatures that they like.
There are more bacteria that can make the leap than viruses which has largely to do with the way those pathogens cause disease. Viruses need to be able to get into the host’s cells in order to replicate, unlike bacteria. To do this, viruses use receptors on their outsides that exploit receptors on the host’s cells. It’s like having one key and you’re trying to find the one model of car on the planet it opens. A virus that has a “dog cell” key isn’t likely to find a “human cell” lock that opens as reliably as a dog lock. Since bacteria don’t need to get into the cells, many of the ones that make dogs sick also make humans sick.
This works for any two species you might pick, dog to cat, mouse to hedgehog, fish to bird. Bacteria are easier to share than viruses.
The real question is why parasites aren’t more catching between species. Horses, for example, rarely get fleas. Humans seem to never get ear mites. And, although everything gets lice, no species gets lice from another species. Although, the coolest parasites require several different species to complete their life cycles.
The virus that causes measles in humans is similar to the one that causes distemper in dogs. Biologists believe that the disease jumped species (from dogs to humans) in prehistoric times.
This is a good question - one I hadn’t thought of before. Since may human diseases originated in domestic animals, and dogs are the oldest and most wide-spread of domestic animals, it does seem strange that we’re not facing yearly outbreaks of canine flu. Maybe the pathogens that make the species jump are transmitted during the slaughtering process, and dogs aren’t usually slaughtered.
Nice unintentional pun.
There aren’t that many diseases communicable between humans and any other animals, not just dogs. Cows and sheep have been domesticated for a long time, and there aren’t many diseases transmitted between them and us either - nor between cows and dogs or dogs and sheep and so on. Those diseases that can get passed on (like the black death) are not about the animals themselves so much as their parasites - and even then that’s relatively rare. We’re all just too different.