We live in the southern sierras and we had our dog vaccinated with the rattlesnake vaccine. Yes, I know there are contraindications, but I'm willing to take that chance. I have to make these same decisions with my horses. Most any vaccine has a risk when given. My son is involved with over 50 vet clinics in Southern California; and they are all recommending that the dogs be vaccinated. As noted in a prior message, it's a 2 stage process for the first year, after that a yearly booster. You will still have to take the dog in for vet care if bitten, but it will be much less costly and less tramatic on the dog; survival rate is very high. A dog that is bitten and does not have the vaccine can costs between $1500 to $3,000 for medical care. We had a small doxie dog bitten many years ago by a young rattler and that cost us close to $2,000 at that time, and he almost didn't make it.
There also is a second option available. You can have your dog "snake" trained via snake avoidance techniques. It's offered in some areas by dog trainers. If you would like to go that route, contact a local vet or a training facility and see they have any information. This training will keep the dog from any snake, not just the rattler. It's a one time thing, so it could be done when you get to the Southwest.
BTW, the green mojave's venom is fast acting and most animals, including humans, will generally die from it before reaching help.
Also be aware that besides encountering the sidewinders, you can also stumble onto what we call out here "Mexican Reds". They are very aggressive and are larger than sidewinders or diamondbacks. Their heads are large and they are distinctivly "red" color.
I have horses and I have cantered (galloped) over a couple by mistake (I thought they were branches) and by the time they realize I've jumped across them, the horse and I are down the trail..... But I did know of a horse that was bitten in the chest. It took months to recover and the flesh around the bite died and never grew back properly. All the horse was doing was grazing and came across the snake. The biggest fear with horses is that they get bitten in the nose then the nose swells causing difficult breathing. When Tim and I were horse packing in the wilderness, I carried short pieces of PVC pipe, so that if the horse got bitten in the nose, I could insert the PVC pipe to keep the air passage open.
Probably more than any body needed to know... ;D
Marsha~
Marsha~