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I'm sorry to hear that your dog is not feeling well, but hopefully he'll be back to himself soon.
Unfortunately, panting and lethargy and not specific for any certain disease or condition. When dogs are overheated or are experiencing an elevation in body temperature (fever) they compensate through panting in order to compensate and lower their core temperature because they can't sweat like humans. Depending on the region of the country you live in, your dog may have overdone it outside to the point where he is experiencing heat exhaustion or heat stroke. This can lead to extreme lethargy and sometimes shock and death. That being said, your dog may be experiencing lethargy and panting from a large variety of other problems or conditions, as these symptoms are not specific for any certain illness.
I would recommend monitoring his vital signs and overall mentation for improvement and then seek veterinary care if you feel he is not looking better soon. You can measure his body temperature rectally with a thermometer, and normal ranges from 99.5-102.5. If there is no underlying reason for the panting to continue other than he may be overheated, his respiratory rate should improve, and normal respiratory rate is about 12-36 breaths per minute. Also, you can take his heart rate by placing a cupped hand over his chest underneath one of his armpits. The number of normal beats per minute should be between 70-120. If you notice that his vital signs do not improve and his overall mental alertness seem to be continually declining despite getting him into a comfortable environment indoors, I would recommend seeking emergency veterinary care.
I hope this answer was helpful, I wish you and your dog the best.
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Thanks for the additional information.
As stated previously, lethargy is unfortunately not specific for any certain illness. However, that being said I would be particularly curious about tick borne diseases (lyme, ehrlichia, rocky mountain spotted fever) since these are relatively common in your region of the country and can commonly cause lethargy in dogs. The most common presenting symptom I see in dogs that have tick borne illness is lethargy and inappetance, and usually owners report that their dog just doesn't seem like theirself.
I would continue to monitor his vital signs (and the color of his gums; they should be pink) as I previously stated, and you can probably avoid an emergency visit if he seems to normal other than the lethargy. Your veterinarian can hopefully see him tomorrow, and usually it tick borne diseases can be screened for by a simple in-house test that takes about 5 minutes to perform.
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