All mammals with hair (I'm going to exclude whales in that *smile*) have demodex mites -- they're what you'd call 100% common.
No, demodex isn't 'contagious' in that sense. All of a sudden we have a bunch of demodex threads going -- but demodex is essentially a genetic flaw in the immune system that allows the mite to run amok and cause serious skin problems, infections, etc. It's worsened by stuff like 'stress' (just being a puppy can be stressor enough, but you have to include stuff like sexual maturation, vaccines, weather changes and just all kinds of stuff depending on the dog and it's own unique 'self' and situation). So an animal thrown into a shelter situation -- yep, that's gonna worsen it, and often they wind up with a ton of vaccines during their stay or just prior to and that worsens it.
If you have an elderly animal that has a really severely compromised immune system and that dog is already pre-disposed to demodex in its history, then you could have some transferrence between a dog with severe demodex and that severely compromised elderly or sick dog. But generally speaking, no -- demodex isn't contagious.
Bottom line -- demodex mites don't bite the dog. If the dog itches its because it's sensitive to the mites' presence in the skin (actually it's almost like an allergy - the dog is super irritated by the 'poop' the mites leave behind in the hair follicles and skin pores).
However, when you talk about demodex and 'mange' you also then have to talk about sarcoptic mange -- particularly in shelters. THAT is the contagious one. (A lot of shelters will simply euthanize an animal if they suspect it because the contagion is so feared)
It's sarcoptic mange that is contagious. In people we call that "scabies" (the bum on the corner?). Typically sarcops are associated with the extreme, unrelenting "ITCH" - the 'mangey dog' that has skin like bald leather who is hairless from scratching for months/years on end??
Each species of warm-blooded animals (this goes beyond just mammals) pretty much has its own strain of sarcops. If they get an easy life, a sarcop prefers to stay on its preferred species. that means the bum on the corner may have scabies ... and they might jump on the dog to feed, but they'd try to jump back on the human to reproduce.
That line, in modern culture, is getting really blurry. Because we've driven nature out of 'suburbia' sarcops mites have learned to be VERY resillient -- so you will find dogs can actually get infected by wildlife sarcops (fox, deer, coon, squirrel, etc.)
ALSO -- and for shelter workers this is a small, but unlikely, caution -- the one place you need to be concerned is if you see a dog with "demodex" that is just plain completely focused on scratching. The itch just doesn't stop. They wake up to scratch, they spend virtually every waking moment scratching ... they can't stop scratching.
It's possible, that dog **may** have both demodex and sarcops. It happens. It's **not** common but it does happen. I've seen it in feral pups that may have started out with just demodex but then got tossed out on the street and picked up sarcops along the way. Generally they are in really poor shape. Probably in the thousands of dogs I've helped, I've maybe seen it occur 5-10 times. Vets often don't pick up on it because once they smell the demodex they assume that's it. Or maybe they see the scratching and then just treat for sarcops and don't look further.
They're a challenge because the typical 'dips' that they use on sarcops will make demodex worse, and the typical 'dips' used on demodex can allow the sarcops to get worse. It surely can be treated, but it takes a bit of concern and someone actually recognizing the problem.
You're not going to find this in the little dog with a couple of spots of demodex on its face -- but it's simply that a dog with demodex has a really compromised immune system, and sarcops can be really attracted to a sick animal.
I'm stressing tho -- this is NOT common. But if you see a dog labeled as 'demodex' but it just plain can NOT stop scratching ... not even to eat or sleep ... it's possible it may have both. Pretty darned rare, but can happen.
Sunshinegirl -- if you want my article on demodex email me. I'm happy to have you give it to the shelter "powers that be" and if they think it helpful, they can hand it out to people who might adopt a demodex dog - just make sure you give them my email so I can help them with it if they want. I've done that lots of times with shelters.
But generally speaking, no -- you surely don't need to be afraid of a dog with demodex. All dogs have demodex mites on them (just like we do) and in the proper place they are literally somewhat beneficial in the body's operation.
Darned good question tho -- so many who work in shelters don't even know there ARE two different types of mange and they are so completely different.