sudden gerbil lethargic

Question
Regretably the only vet in our area who sees small rodents had an appt today which I was gonna use but poor Chopper passed during the night much to our great sadness and dismay. Seems like he rapidly declined after being perky all day and evening.  I kept a very close eye on him all day and was up most of night with him but not once did he appear to be breathing hard so I never even once thought of pneumonia.  Bless him all he wanted was to be held in my hand all evening cause as soon as I put hand in cage he crawled in and just sat there. We are very devestated over his death here and now uncertain as to whether we ever want another.  After our hamsters living so long, almost 6 yrs, we are a bit leary.  Thanks for all your help and information
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Followup To

Question -
Gerbil was behaving normally all day now this evening midnightish I discovered the gerbil being rather quiet and lethargic acting. Has no real desire to move and keeps eyes closed (no visible redness or swelling). His breathing also appears normal. Cage is relatively clean (clean it twice a week and its due tomorrow evening). He has fresh food and water daily and I hand him a sunflower seed tonight he will eat it.  Discovered that on Monday my husband gave him a small piece of romaine lettuce and was wondering if this may be the cause of his malise.  Chopper is approximately 6 months old and usually very active. Yesterday he was bouncing merrily around his cage.  No wet tail that I can see and droppings all appear normal.

Answer -
He may have a cold or pneumonia at this point there's nothing you can do but hope for the best. You say he seems to be perky during the day but at night he acts stiff and lethargic. He may have pneumonia, that's what it seems like. Take him to a vet asap. Him being cold and stiff at night for two days in a row isn't good. I hope he makes it through the night. You have done everything you can do and you're a good parent for doing it.

Answer
I would not give up on wanting another animal. They bring so much joy and obviously he brought a lot of joy in your life since you are upset over what has happaned. I am so sorry he passed away, but you gave him a wonderful life and his last few memories will be of you holding him and caring for him.