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Thank you for using Just Answer. My name is Rebecca; I am a veterinarian. Please give me a moment to type my answer.
I am sorry to hear you are worried about your dog's diarrhea.
Have you had a stool sample tested for internal parasites? He might have Coccidia, the only way to know is to look for these microscopic parasites in a stool sample. Bactrim is an antibiotic, so if he improves with this medication it may be the anti bacterial properties that are helping.
I might try metronidazole in case he has Giardia (fenbendazole can treat some cases of Giardia but is not always effective).
How much ivermectin did you give him?
Ivermectin has to be given in tiny doses; it can cause blindness, seizures, and other neurologic signs. If you use the cattle Ivermectin, you would give a 60 pound dog about 9 mg which would be less than 1 ml (1 cc) of Ivomec cattle wormer. Some dogs that have a genetic mutation can't even take that much.
I hope you were able to see my responses; I have not heard back from you! How is he doing? Do you have any other questions?