IMHA in dogs immune-mediated hemolytic anemia, Has symptoms of exerceise intolerance fatigue, pale gums, decreased appetite. The ow blood count causes oxygen to struggle in the dogs system lowering it making it difficult for the dogs to breathe.
You need to have a blood test done on your dog to see whey type of anemia it has, weather viral, internal bleed, cancer or autoimmune. If the blood count too low the dog can stress no go into shock or die of asphyxiation due to the fact there's not enough cells in the blood to carry oxygen to the lipungs and to the internal organs, the dog will get more and more tired, appear weak and eventiwlly die.
Unfortunately not all dogs show outward signs and symptoms and bthat there is a problem, that's why yearly physicals are important when dogs get their shots update.
We discovered my girl had an issue that went undiagnosed despite many blood tests, she was panting at the end pretty much 24/7 and I insisted on ultrasounds and or x-rays. They found a large tumor filed eith blood aches red to her heart and lungs, it dienypt hurt her but she panted like she was in a 110 degree room al the time. The vet had assumed it was just stress from being at the vets until I videoed her sleeping and trying to eat, having to catch her breath. She's was only 8 years old.
Good luck, please have him checked by the vet, besides anemia the dog could be in pain or have some unseen internal issue going on. Panting not only happens when they are hot but also when they are stressed out, agitated or sick and in pain. More than 24 hours they need to be seen.