I'm very sorry that your dog is ill. I think the likelihood that he has ingested a toxin is high. Snail bait toxicity comes to mind first and you can read more about that here:
Other considerations include mycotoxins from eating trash, strychnine poisoning, pyrethrin or organophosphate toxicity (insecticides), chocolate/coffee toxicosis, amphetamines and rat poison (bromethalin type).
I would recommend the addition of diazepam intravenously to control muscle
tremors and intravenous fluids to help flush his system. Lactated ringer's would be the fluid of choice. His body temperature needs to be monitored closely as constant/frequent muscle tremors can cause him to develop a dangerous fever and cooling mechanisms should be employed if so. In addition, his liver values should be monitored because if there is a toxin involved, then his liver may become more severely affected as time passes. Rimadyl is contraindicated for dogs with elevated liver values.
I would encourage your veterinarian to call the Animal Poison Control Center at(###) ###-####to speak with a veterinarian who specializes in toxicology. They can help make treatment suggestions such as when to consider the use of corticosteroids as well as if they think it is too late to use activated charcoal (absorbs toxins in the gut) and perform gastric lavage. Recommendations for this will vary among the aforementioned possibilities; however, they may ask your vet specific questions about physical findings, etc. to help them make an educated guess. http://www.aspcapro.org/poison
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