Your dog(s) may be turning into this:
"Antivenom (or antivenin or antivenene) is a biological product used in the treatment of venomous bites or stings. Antivenom is created by injecting a small amount of the targeted venom into an animal such as a horse, sheep, goat, or rabbit; the subject animal will undergo an immune response to the venom, producing antibodies against the venom's active molecule which can then be harvested from the animal's blood and used to treat envenomation....."
Doesn't mention dogs but it appears he/they are building up some kind of resistance.
We often hear about that snake hospital in Bangkok which produces antivenom. I've never seen any sheep in Thailand, and few horses, so unknown what animal they use? Dogs are plentiful though.

Pete
EDIT: The second snake looks like the kind SJ and I killed. The first the Thai equiv., of a garter snake.