We have about 5 stairs in the front of our house and I have always carried them down when they were very young puppies, until about 10 to 16 weeks (depending on the pup). I always let them walk up the stairs with close supervision from me.
As far as I am aware there is no direct link to getting HD/ED from going up and down stairs. Dysplasia is an inherited disease and environmental issues may play a part in the severity of dysplasia. There has to be a dog in your dogs line that had/has dysplasia, why it occurs in some litter mates or just one of the litter and not the others is anyone's guess. Dysplasia is so prominent in GSDs that there will be a dog in your dogs lines with HD or ED. There is evidence that if your dog has the gene and your dog is overfed - way overweight or over exercised - excessive running on hard surfaces, excessive jumping then these environmental factors will play a part in the dysplasia development.
In terms of your stairs and whilst your puppy is very young, you could allow him to go up and then judge whether he is ready to walk down.
if your GSD is eating and eating and eating and losing weight - please consider testing for EPI.
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