Sick Chihuahua

Question
I recently adopted 2 chihuahuas, brothers, back in Feb. 2011. I was given a month and a half worth of food which is now all gone. I had to, very unfortunately, change their food quickly. The oldest who is 3 has been acting very strange. He is only laying down and sleeping, unlike his normal enthusiastic self. He is also not opening his eyes fully. He has not thrown up or had any diarrhea. He is responding to his name, he just isn't moving except for the occasional stretches. As for his shots that he has had I am very clueless because I was not given his records. His brother on the other hand is doing fine and acting normal. What is your opinion/advice?

Answer
My guess would be that he is in pain, a lot of it. This is not good because it can mean he is having an acute attack of pancreatitis, common in little dogs and often brought on by a food switch.

I would get him to the vets tomorrow. This can very quickly turn into something very serious. Now granted, most dogs will have diarrhea and/or vomiting, but not all dogs.

I would call your vet tomorrow and ask them but it sounds like a visit is warranted. You need to update their vaccinations when this one feels better.

Get them heartworm tested as well then put on prevention.