Thanks for your question regarding your boy Max who has been lethargic and off food for the last little while. Given his symptoms and age, there are quite a number of possible causes for Max's symptoms including anything from musculoskeletal pain (due to arthritis for example), all the way through to a toxicity (hopefully there isn't anything he could have gotten in to, in the last day or two), early gastroenteritis, or even an internal organ or endocrine issue. You will need to continue to keep a very close eye on Max, and if he seems particularly lethargic or painful right now, you really are best to get him seen straight away if you can.
For now, keep Max quiet and comfortable inside if that is possible. Make sure he has plenty of water available and encourage him to drink. Don't panic too much about him not eating today, but perhaps you could try him with some freshly cooked, boneless, skinless chicken breast later on tonight. Over the next few hours, please keep a close eye on his mucus membranes, capillary refill time and respiratory rate as follows:
Mucus membranes - flip his lip and look at the color of his gums. They should maintain a nice salmon pink color. Get him to the emergency Vet if they appear white or very pale pink, or if they are a dark deep red color.
Capillary Refill time - this measures blood perfusion and test this by putting your thumb on his gum to apply pressure. After you release your thumb you will see the gum blanch. Capillary refill time is the amount of time it takes (in seconds) for the gum to return to a healthy pink color from the blanched white color. If 2 seconds or less don't worry - if it is taking significantly more time, again - off to the emergency Vet.
Respiratory Rate - if he is continuously panting throughout the night, this is a sign of shock and or pain and a signal for a trip to the emergency Vet.
I hope all of the above makes sense? If you have more questions or if I can help in any other way, please do not hesitate to ask! If you would like to accept my answer, please press RATE OUR CONVERSATION (I am not compensated in any other way). Bonuses are always welcome. Thanks! I hope to work with you again soon!
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