This is the time of year and there is a shortage of serum available. People come first, dogs maybe in trouble if bitten. There was a recent newsarticle about a guy in Wrightwood who was bitten and the venom was affecting his nerves instead of his blood cells. This is not the usual reaction to a rattlesnake bite. Instead of swelling, this guy had paralysis.
Be very careful. Rattlesnakes usually hang out in the shade during the heat of the day, such as under a bush. They are more active at sunrise and sunset. A walking stick could help.