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From what I've heard, vets are also a little reluctant to use antivenin because the allergic reaction can be worse than the initial problem. Most reasonably sized animals (including humans) are likely to pull through without help, if it isn't a rattlesnake subspecies that has picked up the neurotoxin.
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There is a relatively high reaction rate to antivenom. Many times that is due to the animal it was produced in. In other words, it is produced by injecting either horses or sheep to produce antibodies (antivenom), but they can't get all the sheep/horse impurities out. Even so, the reaction rate is not THAT high, so I have no qualms of using antivenom if the dogs life is really in danger. There are drugs to counteract most reactions
But for a dog who is bitten on the head, the reaction rate is probably higher than the risk to the dog, so I don't recommend using it. For the others, I think the real issue is cost. If it were $50 to treat a dog with antivenom, you would see its use skyrocket.
And, yes, most dogs bitten on the face are likely to pull through, but I would always recomend treatment. The one dog I have seen die bitten in the face was NOT brought in for treatment because he was a big dog and the owner heard that he would be fine. That dog got over the venom part of it, but died from a bacterial infection that spread from the bite area.
As an interesting sidenote, you can now produce your dogs own antivenom the same way the get it from sheep/horses by vaccinating the dog for snakebite. Two intitial vaccines and then boosters 1-2 times a year depending on exposure is required. I have used it in probably 400 dogs so far. Seems to be safe as any vaccine. Real question is: does it work?
I can see no reason why you would be able to create the antivenom in a sheep/horse and not the dog, so that makes sense. The company that makes it claims it lasts for 6-9 months, thus why some dogs are vaccinated twice yearly. Vaccinating a dog essentially keeps a store of antivenom inside the dog to be used if needed. I have had two dogs bit on the face that were vaccinated. Normally, the swelling lasts at least 4-6 days and some of the tissue around the bite dies and sloughs off. Both of these dogs seem to have less swelling which was gone within two days and no sloughing of tissue occurred. I have my own dog vaccinated.