Lethargic Rat

Question
My pet rat Basil is 1.5 years old and has always been active and healthy (with exception for a minor mite infestation about 6 months ago). He tends to be more active in the evenings, a time which he uses to run around, explore, play and wrestle with his cagemate. However, he has recently started showing less interest in everything (except food) and has become much less active than usual. The symptoms began after I cleaned his cage a few days ago and refilled the food bowl with the same rat mix that he has always been fed. As usual, he nearly emptied the bowl. It's hard to tell how much of it he is hiding vs. the amount he is actually consuming. Anyways, after about an hour of excitement and activity, he quieted down. This is unusual for him; it usually takes a few hours for him to get tired. I found him curled up in a corner of my room under my dresser. He stayed there for several hours before eventually going back to his cage, where he slept till the next morning. Since then, He has been sleeping a lot more than usual.
He doesn't seem to be having any trouble breathing. I have seen no excessive scratching or hair loss. He is still eating and drinking as normal. Really, the only change is his level of activity. He seems to respond only to food, and has not been playing with his cagemate, or even me, as much as he used to.
I'm not sure how much of this is worth worrying about. I suppose he could just be getting older and is thus less active, but the change seemed pretty sudden. The fact that he hasn't been as social or active makes me wonder if there is some sort of internal problem. Is there anything in particular that I should be looking out for? Should I make any changes to his diet? And most importantly, should I schedule an appointment with the vet?

I hope to hear from you soon. Thanks in advance!!

-Lauren and Basil

Answer
Hi Lauren, first I truly apologize for the long delay in responding as I have been ill for over a week.  I hope that it is not too late and that you made a decision already on whether to see the vet.  I will answer your question regardless if you've already sought help elsewhere.

An 18 month old rat is far from old.  Many rats remain quite active past the age of 2 or even 3.  As long as they are in good health, then they should continue to be active. Of course, as you may know, male rats tend to be far lazier than females and after they reach about 6 months old, they tend to laze around and sleep and cuddle.  Therefore, the sudden change doesn't make sense in this regard.

I am inclined to believe that Basil is ill.  There are many illnesses that may cause lethargy.  Many of these illnesses also cause appetite loss, and because you say Basil has a good appetite, this leaves me hopeful that he does not have a serious illness.  That is not to say you should not worry or leave it be because if an illness goes undiagnosed and untreated, it may turn serious.

One serious illness that comes to mind that is fairly common in rats is a pituitary tumor (PT).  Sleeping more is one symptom of a PT but you should also see others in this case including loss of balance, one bulging eye, and eventual loss of use of front limbs.  A stroke is also possible.  I am not a vet and am just giving you some ideas that comes to mind from my experience.  If you have not already done so, I highly recommend that you take Basil to a good vet that has experience with exotic pets, not just a regular vet.  Then you will have a more accurate diagnosis and perhaps be given medication.

I feel terrible that I could not respond sooner.  I hope your little boy will be ok.  I would love an update on how he is doing and to find out if you did get a diagnosis from a vet.