Your dogs are much more at risk of being bitten by a rattlesnake on the trail than is your horse. So, I suggest sending your dog through a snake avoidance clinic. We send all our search & rescue dogs through these clinics so that we can avoid the issue of a dog bitten by a snake.
As for your horse, unless it is bitten in the face (which should not happen on the trail), it should recover with no issues. And even if bitten on the nose, the issue is swelling that might block breathing (which can be easily handled by putting an 18" length of regular garden hose up their nostril to permit breathing for the few hours of the swelling).
Here is the website for the major snake avoidance clinic in California.
I live south of Mojave in an area that is supposed to be rampant with Mojave Green's and I ride the trails constantly for hours at a time. I've only seen one Mojave Green in four years and it rattled at me in plenty of time for my horse to step out of it's way. I try to be extra vigilant if riding early morning or dusk, but I've still only seen that one. Life is too short to avoid what you love due to fear of the unknown. Everything we do on horseback is potentially life-threatening in one way or another.
Allah took a handful of southerly wind, blew His breath over it, and created the horse. Thou shall fly without wings, and conquer without any sword, O, Horse! Anonymous Bedouin legend
Rattlesnakes physically can't strike more than half their body length, but they can advance toward you while striking. Run away!
We have the surliest of North American rattlers here in Cottonmouth County, the irascible Western Diamondbacks. I see maybe 5 or 6 a year, in spring and early summer. The most recent one was cornered against a boulder by my doofy yella lab puppy, who thought the rattling was just awesome. Rattlesnakes are thrifty with the venom and seem somewhat reluctant to deploy it in these encounters, possibly because dogs and horses and humans are too big to eat. This one didn't strike the puppy, although it easily could have.
After each sighting I completely freak out, avoiding the trail in question, spooking at every little S-shaped twig on the ground, keeping the dogs in, etc. This is totally irrational behavior, I realize. Fortunately, the half-life of my PTSD (post-traumatic snake disorder) is pretty short, and I'm able to resume my normal routine (sitting on the porch with a pitcher of margs while complaining about the weather) within a few days.
I was the OP. My dog has survived but it was "touch and go" for a while. Our vet was recently out to do some things with the horses and she said she was glad to see Kelly as she had some sleepless nights thinking she would not make it. Kelly is 15 and a shepherd mix. She now has two big scars on the side of her face where she was bitten (two bites).
Kelly is extra vigilent now. My husband likes to take her for walks on the trails in the morning. She walks down the center of the wide trails always watching for movement. When he tries to get her to take a different trail that is narrower, she sits down and refuses to move. Smart dog!
To the poster who asked about the vaccine, there is a shot that "helps" the dogs survive a rattlesnake bite. The first vaccination comes in two parts to help build immunity. Then its followed by annual revaccinations. Some of our neighbors give the anuual updated shot twice in the summer.
I have continued to ride and just try to stick to wide trails when we can. I also try to go in the heat of the day. And I keep my eyes open looking for anything that might resemble a snake. Of course there are no guarentees, but I do what I can to make our rides safe. So far, we are fine!
I'm in AZ and saw my first rattler out on the trails last weekend. It was mid morning and it warned us as we approached. I may not have even noticed had the other horse not spooked a bit at it.
It made me realize I need to be more vigilant while out in the trails.
Last week a horse where I board was bit in the face by a rattler. Thankfully one of the farm workers noticed his head was much larger than normal and called the barn owner. He is recovering, but the vet bill including hospitalization is in the thousands.
I am in Central GA, it has been a very snakey year this year. We have found loads of them (non venomous) in our barn. There have been several timber rattlers sighted on the trails. We see snakes all the time sunning on the roads. I can't stand them
the half-life of my PTSD (post-traumatic snake disorder) is pretty short, and I'm able to resume my normal routine (sitting on the porch with a pitcher of margs while complaining about the weather) within a few days.