I live in the suburbs and One Sunday afternoon about 4 years ago, I was cleaning out my car when my Rotty came running up to me barking and whimpering, and she starting running off towards the back yard, so I followed her, there I found my Big beautiful British Shorthair Cat laying on the concrete, saliva everywhere, barely breathing, and completely paralized, I rushed him to the nearest Vet that was opened on a Sunday, but unfortunately his lungs had filled with fluid (pulmonary Edema) and he died. The Vets were unsure why.
The next day I took his body to MY Vet, where they did a Post Mortem to determine death, they told me he was bitten by a snake on the pad of his foot and it was likely he had a massive allergic reaction to the venom.
We searched everywhere possible for the snake in the back yard, but never found it, I was that worried my Rotty would get bitten, I kept her inside my house, only letting her out for toilet breaks, never the less, she didn't get bitten and is still with me today. I hope your dogs are o.k